Patent Publication Number: US-9894730-B2

Title: Apparatus and system for providing power to solid state lighting

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 14/293,975, filed Jun. 2, 2014, now U.S. Pat. No. 9,408,259, which is a continuation of U.S. application Ser. No. 13/572,499, filed Aug. 10, 2012, now U.S. Pat. No. 8,742,679, which is a continuation of U.S. application Ser. No. 12/207,353, filed Sep. 9, 2008, now U.S. Pat. No. 8,242,704, the disclosures of which are incorporated by reference herein in their entirety. 
    
    
     BACKGROUND 
     Arrays of light emitting diodes are utilized for a wide variety of applications, including for ambient lighting and displays. For driving an array of LEDs, electronic circuits typically employ a power converter or LED driver to transform power from an AC or DC power source and provide a DC power source to the LEDs. When multiple LEDs are utilized, LED arrays may be divided into groups or channels of LEDs, with a group of LEDs connected in series typically referred to as a “string” or channel of LEDs. 
     Multichannel power converters are known, for example Subramanian Muthu, Frank J. P. Schuurmans, and Michael D. Pashly, “Red, Blue, and Green LED for White Light Illumination,”  IEEE Journal on Selected Topics in Quantum Electronics,  8(2):333-338, March/April 2002. Such prior art multistring LED drivers may utilize redundant power conversion modules, with a separate power module used for each LED string and typically comprising a driver, a transformer, a sensor, a controller, etc., for example. A similar approach is suggested in Chang et al., U.S. Pat. No. 6,369,525, entitled “White Light-Emitting-Diode Lamp Driver Based on Multiple Output Converter with Output Current Mode Control,” which utilizes multiple redundant power conversion modules, with each power conversion module configured to provide power for a corresponding LED string. Providing redundant elements such as a redundant power module for each channel may increase the number of components and may increase the size and weight of the power converter. Such utilization of relatively many components may also increase costs, such as component costs and manufacturing costs, or reduce reliability. For prior art power converters utilizing redundant power modules, a fault in a power module, such as if one or more components in the power module fail, may result in the power module no longer providing power or providing power at a reduced level and may cause a corresponding channel of LEDs to lose power. 
     Another prior art method (Supertex data sheets LV 9120/9123 and Application Note AN-H13) arranges LED strings in series and utilizes a power converter to provide power to the series arrangement of LED strings. In such an arrangement, the voltage level across the series of strings may be substantially equal to the sum of each voltage level across each of the multiple strings, resulting in an accumulated, total voltage level across multiple strings that may reach significantly high levels.  FIG. 1  is a voltage map illustrating such voltage levels at the output of a prior art power converter and across a plurality of LED strings, for an example configuration in which the power converter drives four LED strings coupled in series. The vertical axis represents voltage “V.” Points along the horizontal axis represent corresponding points in the series configuration of LED strings. The first voltage level  20  for the “POWER CONVERTER OUTPUT,” marks the voltage rise across the output of the prior art power converter from substantially zero volts at the negative output terminal of the power converter to a total voltage VT at the positive output terminal of the power converter. The second voltage level  21  for an LED “FIRST STRING” illustrates the voltage drop across the first string of LEDs, the third voltage level  22  for an LED “SECOND STRING” illustrates the voltage drop across the second string of LEDs, and so on. As illustrated, the voltage level drops substantially to zero (24) across the fourth string. If the voltage across each string is 50V, for example, the total voltage level VT across the four strings or across the prior art power converter output is substantially equal to the sum of the voltage levels across each string, or 200V. Such relatively high voltage levels may make such a series arrangement unsuitable for some applications, such as where people may possibly come in contact with power provided to LED arrays. Operating at relatively high voltage levels may also incur additional costs for an apparatus, such as costs for components adapted to operate with such high voltage levels and for additional insulation and other safety equipment, such as to protect people and property. This prior art approach of providing power to a series of LED strings also does not provide a means for a controller to independently control the brightness of each string or to independently turn individual strings on or off. 
     Other prior art power converters with multiple power modules for multiple LED strings typically couple each load (e.g., channel or string of LEDs) to one of a plurality of power modules in a parallel configuration, i.e., a first terminal of the load is coupled to a first terminal of the power module and a second terminal of the load is coupled to a second terminal of the same power module. With such an arrangement, if one or more components in the power module fail, the load may lose power. Also, such an arrangement, in which each power module is coupled in parallel to a load, typically utilizes redundant circuitry, such as multiple sensors and multiple controllers, to provide a desired current level to multiple loads. 
     Accordingly, a need remains for a multichannel power converter that provides power to a plurality of LEDs, such as multiple strings or channels of LEDs, at comparatively low overall voltage levels, and that provides an overall reduction in size, weight, and cost of the LED driver, such as by sharing components across channels. Such a converter may further provide selected or predetermined power levels to the LEDs and may also compensate for variations in circuit parameters such as manufacturing tolerances, input voltage, temperature, etc. The power converter should be fault tolerant. For example, in the event that one or more power modules or channels fail, the power converter should continue to provide power to operational channels. Also, it would be desirable to provide a power converter adapted for providing independently selected power levels for each LED channel and for independently turning LED channels on or off. 
     SUMMARY 
     The exemplary embodiments of the present disclosure provide numerous advantages for supplying power to loads such as LEDs. The various exemplary embodiments are capable of sustaining a plurality of types of control over such power delivery, such as providing a substantially constant or controlled current output to a plurality of groups or channels of LEDs. The exemplary embodiments may be provided which share power converter components across multiple channels, providing advantages such as relatively smaller size, less weight, lower cost, and higher reliability, compared to prior art power converters. The exemplary embodiments utilize a transformer with a plurality of secondary windings and a plurality of power modules, with each power module coupled to a group of LEDs in an alternating series arrangement, and shared regulation circuitry such as one or more common sensors, a common controller, a common transformer primary, etc. The exemplary embodiments may utilize bypass circuits to redirect current flow in the event that one or more channels or power modules become inoperative, such as during short circuit or open circuit conditions, with the bypass circuits enabling the power converter to provide power to remaining operational channels. 
     A first exemplary apparatus embodiment for power conversion, in accordance with the teachings of the present disclosure, is couplable to a power source, with the exemplary apparatus comprising: a primary module comprising a transformer having a transformer primary; a first secondary module couplable to a first load, with the first secondary module comprising a first transformer secondary magnetically coupled to the transformer primary; and a second secondary module couplable to a second load, with the second secondary module comprising a second transformer secondary magnetically coupled to the transformer primary, the second secondary module couplable in series through the first or second load to the first secondary module. 
     Typically, when energized by the power source, the first secondary module has a first voltage polarity and is couplable in a series with the first load configured to have an opposing, second voltage polarity. In an exemplary embodiment, a resultant voltage of the first voltage polarity combined with the second voltage polarity is substantially less than a magnitude of the first voltage polarity or the second voltage polarity. In another exemplary embodiment, the first voltage polarity and the second voltage polarity substantially offset each other to provide a comparatively low resultant voltage level. 
     Typically, when energized by the power source, the second secondary module has a third voltage polarity and is couplable in a series with the second load configured to have an opposing, fourth voltage polarity. In an exemplary embodiment, a resultant voltage of the combined first voltage polarity, the second voltage polarity, the third voltage polarity and the fourth voltage polarity is substantially less than a magnitude of the first voltage polarity, or the second voltage polarity, or the third voltage polarity, or the fourth voltage polarity. In another exemplary embodiment, the first voltage polarity, the second voltage polarity, the third voltage polarity, and the fourth voltage polarity substantially offset one another to provide a comparatively low resultant voltage level. 
     An exemplary apparatus may further comprise: a current sensor coupled to the first secondary module or the second secondary module and adapted to sense a current level; and a controller coupled to the current sensor and to the primary module, the controller adapted to regulate a transformer primary current in response to the sensed current level. 
     Another exemplary apparatus may further comprise: a first bypass circuit coupled to the first secondary module; and a second bypass circuit coupled to the second secondary module. An exemplary first bypass circuit is adapted to bypass the first secondary module and the first load in response to a detected fault, such as an open circuit. 
     In an exemplary embodiment, the first and second load each comprise at least one light emitting diode, and the controller is further adapted to provide dimming of light output by regulating the first bypass circuit or the second bypass circuit. For example, the controller may be further adapted to provide pulse width modulation to regulate the first bypass circuit or the second bypass circuit. Also for example, the controller may be further adapted to turn a corresponding switch into an on state or an off state to regulate the first bypass circuit or the second bypass circuit. Also for example, the first and second load each comprise at least one light emitting diode, and the controller may be further adapted to provide dimming of light output by regulating the transformer primary current. 
     In another exemplary embodiment, the first load comprises at least one first light emitting diode having a first emission spectrum (such as an emission spectrum in the red, green, blue, white, yellow, amber, or other visible wavelengths), and the second load comprises at least one second light emitting diode having a second emission spectrum. For example, a first LED may provide emission in the red visible spectrum, a second LED may provide emission in the green visible spectrum, and a third LED may provide emission in the blue visible spectrum. In such an exemplary embodiment, the controller may be further adapted to regulate an output spectrum by regulating the first bypass circuit, or the second bypass circuit, or a third bypass circuit, such as by dimming or bypassing a corresponding LED string, to modify the overall emitted light spectrum, such as to increase or decrease corresponding portions of red, green, or blue, for example. 
     In an exemplary embodiment, the controller may be electrically isolated from the primary module. For example, the controller may be coupled optically to the primary module. 
     In exemplary embodiments, the first secondary module and the second secondary module may be configured to have at least one of the following circuit topologies: a flyback configuration, a single-ended forward configuration, a half-bridge configuration, a full-bridge configuration, or a current doubler configuration. 
     Also in exemplary embodiments, the first secondary module may further comprise a first rectifier and a first filter, with the first rectifier coupled to the first transformer secondary, and the second secondary module may further comprise a second rectifier and a second filter, with the second rectifier coupled to the second transformer secondary. 
     An exemplary lighting system is also disclosed, with the system couplable to a power source, and with the system comprising: a primary module comprising a transformer having a transformer primary; a first light emitting diode; a second light emitting diode; a first secondary module coupled in series to the first light emitting diode, the first secondary module comprising a first transformer secondary magnetically coupled to the transformer primary; a second secondary module coupled in series to the second light emitting diode, the second secondary module comprising a second transformer secondary magnetically coupled to the transformer primary, the second secondary module coupled in series through the first or second light emitting diode to the first secondary module; a current sensor adapted to sense a current level; and a controller coupled to the current sensor and to the primary module, with the controller adapted to regulate a transformer primary current in response to the sensed current level. 
     Another exemplary apparatus for power conversion is also disclosed, with the apparatus couplable to a power source and to a plurality of light emitting diodes, and with the apparatus comprising: a primary module comprising a transformer having a transformer primary; a first secondary module couplable in series to a first light emitting diode of the plurality of light emitting diodes, the first secondary module comprising: a first transformer secondary magnetically coupled to the transformer primary, a first rectifier coupled to the first transformer secondary, and a first filter coupled to the first rectifier; a second secondary module couplable in series to a second light emitting diode of the plurality of light emitting diodes, the second secondary module couplable in series through the first or second light emitting diode to the first secondary module, the second secondary module comprising: a second transformer secondary magnetically coupled to the transformer primary, a second rectifier coupled to the second transformer secondary, and a second filter coupled to the second rectifier; a current sensor adapted to sense a current level; a controller coupled to the current sensor and to the primary module, the controller adapted to regulate a transformer primary current in response to the sensed current level; a first bypass circuit coupled to the first secondary module; and a second bypass circuit coupled to the second secondary module. 
     An exemplary method of providing power to a plurality of light emitting diodes is also disclosed. The exemplary method comprises: routing current from a first secondary module to a first light emitting diode coupled in series to the first secondary module to generate a first voltage across the first light emitting diode having an opposing polarity to a second voltage across the first secondary module; routing current from the first light emitting diode to a second secondary module coupled in series to the first light emitting diode; routing current from the second secondary module to a second light emitting diode coupled in series to the second secondary module to generate a third voltage across the second light emitting diode having an opposing polarity to a fourth voltage across the second secondary module; and routing current from the second light emitting diode to the first secondary module or to a third secondary module coupled in series to the second light emitting diode. 
     In an exemplary embodiment, the method further comprises: detecting a fault in the first secondary module or the first light emitting diode; and in response to the detected fault, providing a current bypass around the first secondary module and the first light emitting diode from a third light emitting diode to the second secondary module. The exemplary steps of detecting a fault and providing a current bypass may further comprise: sensing a first parameter; comparing the first parameter to a first threshold; and when the first parameter is greater than or substantially equal to the first threshold, switching current from the third light emitting diode to the second secondary module. For example, the detected fault may be a short circuit or an open circuit. 
     In another exemplary embodiment, the method further comprises: detecting a fault in the first secondary module or the first light emitting diode; and in response to the detected fault, interrupting the current from the first secondary module to the first light emitting diode. The exemplary steps of detecting a fault and interrupting the current may further comprise: sensing a second parameter; comparing the second parameter to a second threshold; and when the second parameter is greater than or substantially equal to the second threshold, creating an open circuit in the series path of the first secondary module and the first light emitting diode. 
     In another exemplary embodiment, the method further comprises: routing current from the first secondary module to the first light emitting diode for a first predetermined on-time duration at a first frequency; and routing current from the second secondary module to the second light emitting diode for a second predetermined on-time duration at a second frequency. 
     Numerous other advantages and features of the present disclosure will become readily apparent from the following detailed description of the disclosure and the embodiments thereof, from the claims and from the accompanying drawings. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The objects, features and advantages of the present disclosure will be more readily appreciated upon reference to the following when considered in conjunction with the accompanying drawings, wherein like reference numerals are used to identify identical components in the various views, and wherein reference numerals with alphabetic characters are utilized to identify additional types, instantiations or variations of a selected component embodiment in the various views, in which: 
         FIG. 1  is a graphical diagram illustrating a voltage map of voltage levels at the output of a prior art power converter and across corresponding loads; 
         FIG. 2  is a block diagram illustrating a first exemplary system and a first exemplary apparatus in accordance with the teachings of the present disclosure; 
         FIG. 3  is a block diagram illustrating a second exemplary system and second exemplary apparatus in accordance with the teachings of the present disclosure; 
         FIG. 4  is a block diagram illustrating a third exemplary system and third exemplary apparatus in accordance with the teachings of the present disclosure; 
         FIG. 5  is a graphical diagram illustrating a voltage map of voltage levels across power modules and LEDs in accordance with the teachings of the present disclosure; 
         FIG. 6  is a graphical diagram illustrating a voltage map of voltage levels during a bypass of a component fault in accordance with the teachings of the present disclosure; 
         FIG. 7  is a flow diagram illustrating a first exemplary method of bypassing a component fault in accordance with the teachings of the present disclosure; 
         FIG. 8  is a block and circuit diagram illustrating a fourth exemplary system and fourth exemplary apparatus in accordance with the teachings of the present disclosure; 
         FIG. 9  is a flow diagram illustrating a second exemplary method of bypassing a component fault in accordance with the teachings of the present disclosure; 
         FIG. 10  is a block and circuit diagram illustrating a fifth exemplary system and fifth exemplary apparatus in accordance with the teachings of the present disclosure; 
         FIG. 11  is a flow diagram illustrating a method of adjusting LED brightness or emission levels in accordance with the teachings of the present disclosure; 
         FIG. 12  is a block and circuit diagram illustrating a sixth exemplary system and sixth exemplary apparatus in accordance with the teachings of the present disclosure; and 
         FIG. 13  is a circuit diagram illustrating an example of a secondary module with bypass circuitry and coupled to an LED channel in accordance with the teachings of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     While the present disclosure illustrates embodiments in many different forms, there are shown in the drawings and will be described herein in detail specific exemplary embodiments thereof, with the understanding that the present disclosure is to be considered as an exemplification of the principles of the claimed subject matter and is not intended to limit the claimed subject matter to the specific embodiments illustrated. In this respect, before explaining at least one embodiment consistent with the present invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and to the arrangements of components set forth above and below, illustrated in the drawings, or as described in the examples. Methods and apparatuses consistent with the present invention are capable of other embodiments and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein, as well as the abstract included below, are for the purposes of description and should not be regarded as limiting. 
       FIG. 2  is a block diagram illustrating a first exemplary system  100  and a first exemplary apparatus  101  in accordance with the teachings of the present disclosure. The system  100  comprises the apparatus  101  and a plurality of loads  130   1 ,  130   2 ,  130   3 , through  130   N , and is couplable to receive input power, such as an AC or DC input voltage, from power source  110 . (AC and DC input voltages as referred to herein and within the scope of the present disclosure are discussed in greater detail below.) The apparatus  101  comprises a primary module (or primary power module)  515 , a controller  125 , and a plurality of “N” secondary modules  520   1 ,  520   2 ,  520   3 , through  520   N , which may be referred to collectively herein as secondary modules  520 . Primary module  515  is coupled to secondary modules  520  magnetically, with the magnetic coupling illustrated as dashed lines. The primary module  515  comprises at least one transformer primary, and each secondary module  520  comprises a corresponding transformer secondary magnetically coupled to the transformer primary, such as by being wound on a common magnetic core or otherwise in magnetic or close proximity. In exemplary embodiments, as described in greater detail below, a secondary module may comprise a power module (having the transformer secondary) and, as an option, a bypass circuit. As illustrated, loads  130  comprise a plurality of “N” individual loads  130   1 ,  130   2 , through  130   N . 
     Primary module  515  is couplable to power source  110  and provides power to secondary modules  520 . Power source  110  may provide, for example, AC, DC, chopped DC, or another form of power. In an exemplary embodiment, primary module  515  provides power in the form of magnetic energy via a transformer primary (also referred to as a primary winding) and each secondary module  520  receives the magnetic energy via a corresponding transformer secondary (also referred to as a secondary winding). Primary module  515  may comprise, for example and without limitation, an AC-to-DC converter, such as a rectifier, and a switch adapted to conduct or otherwise apply power in the form of a current or voltage to a transformer primary. The power applied to the transformer primary may comprise a power signal such as a sine wave, a square or rectangular wave, a series of pulses, etc. The power signal may vary, such as in terms of amplitude and/or wave shape, in response to a control signal from controller  125 . Those having skill in the electronic arts will recognize that numerous techniques are available for providing power to a transformer primary, and that primary module  515  may have innumerable implementations and configurations, any and all of which are considered equivalent and within the scope of the present disclosure. 
     In an exemplary embodiment, a first terminal of a first load  130   1  is coupled to a first secondary module  520   1  and a second terminal of first load  130   1  is coupled to a second secondary module  520   2 . A first terminal of a second load  130   2  is coupled to second secondary module  520   2  and a second terminal of second load  130   2  is coupled to a third secondary module  520   3 . Other loads  130  and secondary modules  520  are similarly coupled (i.e., each load is coupled to two (electrically adjacent) secondary modules) up through load  130   N , where a first terminal of an N th  load  130   N  is coupled to an N th  secondary module  520   N  and a second terminal of N th  load  130   N  is coupled to first secondary module  520   1 . Such an arrangement places secondary modules  520  and loads  130  in series, with a load between each pair of adjacent secondary modules  520 . Such an arrangement may be referred to herein as an “alternating series” arrangement in two ways, with a secondary module  520  alternating with a load  130  in series, and as discussed below, with corresponding voltages across a secondary module  520  and a load  130  alternating in polarities. (The term “adjacent” may refer to sequential components in a series circuit. For example, secondary module  520   N  may be considered to be adjacent to secondary module  520   N−1  and secondary module  520   1 .) In an exemplary embodiment, secondary modules  520  and loads  130  are coupled in series so that current flows through a secondary module  520  and a load  130 , then another secondary module  520  and a load  130 , and so on, in a complete circuit. 
     In an exemplary embodiment, the secondary modules  520  and loads  130  are arranged such that each output voltage level provided by a secondary module  520  is substantially compensated by a corresponding voltage drop across a corresponding load  130 . For example, a voltage rise with a first voltage polarity, such as a positive voltage across first secondary module  520   1  which provides power to first load  130   1  is substantially offset by a corresponding voltage drop across the first load  130   1  having a second, opposing voltage polarity, such as a negative voltage. A similar pattern holds for other secondary modules  520  and loads  130 , wherein the voltage rises across each secondary module and then drops across each corresponding load, providing a resultant, overall voltage that is substantially less than the magnitude of the voltage rise or the voltage drop, and may even be relatively or substantially close to zero (depending upon whether the opposing voltage polarities are closely matched). As a result, overall voltage levels at the terminals of loads  130  remain within predetermined and comparatively lower limits. This novel feature of the present disclosure is discussed below in greater detail with reference to  FIG. 5 . 
     Controller  125  may be adapted to sense one or more parameters from one or more secondary modules  520  or loads  130 . Sensed parameters, for example, may comprise a current level or a voltage level, such as a current level through or voltage level of one or more loads  130  or secondary modules  520 . The sensed current or voltage level may be utilized by controller  125  and primary module  515  to directly or indirectly regulate current through loads  130 , such as to provide substantially stable current levels or current levels at or near selected or predetermined values. For example, in response to a sensed parameter, the controller  125  may increase or decrease the current through the transformer primary of the primary module  515 , and/or may separately modify current or voltage provided by a secondary module  520 , such as by using the bypass circuitry discussed below (not separately illustrated in  FIG. 2 ). 
     For example, and among other things, the controller  125  utilizes one or more sensed parameters, as feedback signals, to output a control signal to primary module  515 , such as to regulate power levels to loads  130 . The control signal may be utilized by primary module  515  to determine a power level to be provided to secondary modules  520 . In an exemplary embodiment, the controller  125  may utilize a sensed parameter to cause primary module  515  to reduce the level of power or current provided to secondary modules  520  if current to loads  130  exceeds a first predetermined threshold or to increase the level of power or current provided to secondary modules  520  if current to loads  130  falls below a second predetermined threshold. 
     Controller  125  may also be adapted to supply control signals to secondary modules  520  to independently adjust power or current levels to loads  130   1 ,  130   2 ,  130   3 , through  130   N , such as for dimming or turning on or off one or more channels. In an exemplary embodiment, a temperature sensor (not separately illustrated in  FIG. 2 ), is adapted to determine a parameter in response to a temperature such as LED temperature, and provides feedback to controller  125  for thermal regulation, such as adjusting output power levels in response to one or more sensed temperature values. For example, controller  125  may be configured to reduce the power level to loads  130  if a sensed temperature value rises above a predetermined level. Other forms of control of power levels provided to an individual secondary module  520  and/or a load  130  is discussed in greater detail below. 
     Secondary modules  520  may be configured to bypass or shunt current past one or more loads  130  in the event of one or more faults, such as short circuits or open circuits in one or more secondary modules  520  or loads  130 . As illustrated in  FIG. 2 , secondary modules  520  are each coupled to two adjacent secondary modules  520 , thereby providing a path for such current bypass. For example, in the event of a detected fault in load  130   1 , secondary module  520   1  may redirect current to secondary module  520   2  that would otherwise be provided to load  130   1 . 
     Controller  125  may comprise analog circuitry such as amplifiers, comparators, integrators, etc. and/or digital circuitry such as processors, memory, gates, A/D and D/A converters, etc. Those having skill in the electronic arts will recognize that numerous techniques are known for regulating power to one or more loads and that controller  125  may have innumerable implementations and configurations, any and all of which are considered equivalent and within the scope of the present disclosure. 
       FIG. 3  is a block diagram illustrating a second exemplary system  100 A and second exemplary apparatus in accordance with the teachings of the present disclosure. The system  100 A is couplable to a power source  110  and the system  100 A comprises a primary module  515 A (as an example of a primary module  515 ), a plurality of secondary (power) modules  520 A (as examples of secondary modules  520 ), a controller  125 , a sensor  165 , an optional isolator  120 , and loads  130 . The apparatus (also couplable to a power source  110 ) is illustrated generally and may be considered to comprise the primary module  515 A, the plurality of secondary modules  520 A, the controller  125 , the sensor  165 , and optionally the isolator  120 . In this exemplary embodiment, the primary module  515 A comprises a driver (circuit)  115  and a transformer primary  105  (of transformer  155 ). In this exemplary embodiment, each secondary module  520 A comprises a corresponding power module  140  and, as an option, a corresponding bypass circuit  145 . Each power module  140  comprises a transformer secondary  150  (of transformer  155 ) and other circuitry, such as a rectifier  135  and a filter  195 . The optional isolator  120  also may be considered to be contained within the primary module  515 A. 
     Stated another way, the system  100 A comprises a driver  115 , a controller  125 , a transformer  155 , a sensor  165 , a plurality of secondary power modules  140   1 ,  140   2 , through  140   N , and a plurality of loads  130   1 ,  130   2 , through  130   N . In exemplary embodiments, the system  100 A may further comprise a plurality of bypass circuits  145   1 ,  145   2 , through  145   N . In exemplary embodiments, system  100 A may further comprise an isolator  120  configured to, for example, electrically isolate the driver  115  from the controller  125 . (AC and DC input voltages as referred to herein and within the scope of the present disclosure are discussed in greater detail below). In an exemplary embodiment, each power module  140   1 ,  140   2 , through  140   N  comprises a corresponding transformer secondary ( 150   1 ,  150   2 , through  150   N ), a corresponding rectifier ( 135   1 ,  135   2 , through  135   N ), and a corresponding filter ( 195   1 ,  195   2 , through  195   N ), respectively. In an alternative exemplary embodiment, filters  195  may be omitted or combined with rectifiers  135 . 
     As illustrated, loads  130  comprise a plurality of “N” individual loads  130   1 ,  130   2 , through  130   N . Components with a plurality of instantiations may be referenced herein collectively without subscripts or individually with subscripts. For example, loads  130  may be referred to equivalently as loads  130   1 ,  130   2 , through  130   N . Similar notation applies to power modules  140 , secondaries  150 , rectifiers  135 , filters  195 , bypass circuits  145 , etc. 
     In  FIG. 3 , transformer  155  is illustrated with a split secondary configuration and comprises a transformer primary  105  and a plurality of transformer secondaries  150   1 ,  150   2 , through  150   N . Primary  105  is magnetically coupled to secondaries  150   1 ,  150   2 , through  150   N , such as through a transformer core  156 . Transformer  155  may be configured, using any of various methods known in the electronic arts, for example and without limitation as a forward transformer, a flyback transformer, a flyback or forward transformer with active reset, etc. Those having skill in the electronic arts will recognize that alternate transformer configurations may be utilized. For example transformer  155  may also be implemented with a plurality of primaries or as a plurality of transformers, such as with primaries coupled in parallel. 
     As illustrated, a power source  110  provides AC or DC power to driver  115 . As mentioned above, such AC or DC power may be, for example, single phase or multiphase AC, DC or chopped DC power, such as from batteries or from an AC to DC converter, or any other form of electrical power. Driver  115  receives power from power source  110 , converts received power to DC if appropriate, receives control signals from controller  125  (optionally via isolator  120 ), and provides a driving signal to primary  105 . Driver  115  may, for example, provide a PWM (pulse width modulated) signal, and may use any of various modes of operation such as continuous conduction mode (CCM), discontinuous conduction mode (DCM), and critical conduction mode. Driver  115  may comprise one or more stages such as power conversion stages. Those having skill in the electronic arts will recognize that there are numerous methods for utilizing a controller  125  and a driver  115  for providing driving signals, any and all of which are considered equivalent and within the scope of the present disclosure. 
     Transformer secondaries  150   1 ,  150   2 , through  150   N  are coupled to and provide power to rectifiers  135   1 ,  135   2 , through  135   N , respectively. In an exemplary embodiment, rectifiers  135   1 ,  135   2 , through  135   N  convert AC power from secondaries  150   1 ,  150   2 , through  150   N , respectively, into DC power. Filters  195   1 ,  195   2 , through  195   N  smooth the DC power from rectifiers  135   1 ,  135   2 , through  135   N , respectively, to provide a relatively or comparatively stable DC power level. 
     In the exemplary embodiment as illustrated in  FIG. 3 , the power modules  140   1 ,  140   2 , through  140   N  and loads  130   1 ,  130   2 , through  130   N  are provided in an “alternating series” configuration, wherein the loads  130  and power modules  140  are in series, with loads  130  alternatingly interspersed between power modules  140 . As illustrated, loads  130  and power modules  140  form a ring-like arrangement, with current passing alternately through loads  130  and power modules  140  in a complete circuit. 
     In an exemplary embodiment, a first terminal of a first load  130   1  is coupled to a second terminal of a first power module  140   1  and a second terminal of the first load  130   1  is coupled to a first terminal of a second power module  140   2 . Other cells may be coupled similarly, i.e., a first terminal of “K th ” load  130   K , 1≦K&lt;N, is coupled to a second terminal of K th  power module  140   K  and a second terminal of K th  load  130   K  is coupled to a first terminal of a K+1 th  power module  140   K+1 . In an exemplary embodiment, a first terminal of N th  load  130   N  is coupled to a second terminal of N th  power module  140   N  and a second terminal of N th  load  130   N  is coupled to a first terminal of sensor  165 . A second terminal of sensor  165  is coupled to a first terminal of first power module  140   1 . In an alternative embodiment (not illustrated in  FIG. 3 ), the first terminal of N th  load  130   N  is coupled to the second terminal of N th  power module  140   N  and the second terminal of N th  load  130   N  is coupled to the first terminal of first power module  140   1 . 
     In an exemplary embodiment, a sensor  165  determines a sensed parameter such as a current level. Controller  125  receives the sensed parameter information or signal from sensor  165  and utilizes the sensed parameter information to provide one or more control signals (such as a series of control signals) for driver  115 . 
     While  FIG. 3  and other Figures herein illustrate embodiments with exemplary sensor locations, those having skill in the electronic arts will recognize that there are innumerable other sensor locations, implementations and configurations, any and all of which are considered equivalent and within the scope of the present disclosure. For example, sensor  165  may be placed in series with any of loads  130  or power modules  140 . As another example, one or more sensors may be incorporated into one or more loads  130 , power modules  140 , or bypass circuits  145 . Sensors may comprise various types of sensing components such as optical sensors, temperature sensors, voltage sensors, current sensors, etc. For example, sensor  165  may comprise one or more optical components adapted to utilize LED brightness to determine one or more sensed parameters. 
       FIG. 3  and other Figures herein illustrate exemplary arrangements wherein loads  130  and power modules are coupled in alternating series in a ring-like arrangement to form a complete circuit; however, it is to be understood that loads  130  and power modules  140  may be arranged in innumerable configurations, including without limitation arrangements comprising a plurality of rings, arrangements wherein a plurality of power modules  140  are coupled between loads  130 , arrangements wherein a plurality of loads  130  are coupled between power modules  140 , etc., any and all of which are considered equivalent and within the scope of the present invention. 
     In an exemplary embodiment, bypass circuits  145  provide a switchable current (or voltage) path around loads  130  and power modules  140 . Bypass circuits  145  may be utilized to provide current flow in the event of detected faults or to provide a means for reducing or increasing current flow through individual loads  130 , such as for light dimming and for turning individual loads  130  on or off. Bypass circuits  145  are described in further detail below. 
     In an exemplary embodiment, current levels in power modules  140  and loads  130  may be substantially the same (since they are coupled in series), so current sensing and corresponding control may be accomplished with fewer components, compared to prior art multichannel LED drivers where power to individual channels is separately regulated for each channel. More particularly, in the exemplary embodiment illustrated in  FIG. 3 , current provided to multiple loads  130  may be regulated by shared components such as sensor  165 , controller  125 , isolator  120 , driver  115 , and transformer  155 , which may be shared across a plurality of channels. Compared to prior art multichannel LED drivers in which current to each load is regulated by a separate and redundant set of components such as redundant sensors, controllers, isolators, and drivers, exemplary embodiments of the present invention may provide numerous advantages such as fewer components, lower component and manufacturing costs, reduced size and weight, and higher reliability. 
     In an exemplary embodiment, as mentioned above, the power modules  140  (of the secondary modules  520 ) and loads  130  are arranged such that each output voltage level provided by a power module  140  (of a corresponding secondary module  520 ) is substantially compensated by a corresponding voltage drop across a corresponding load  130 . For example, a voltage rise with a first voltage polarity, such as a positive voltage across first power module  140   1  which provides power to first load  130   1 , is substantially offset by a corresponding voltage drop across the first load  130   1  having a second, opposing voltage polarity, such as a negative voltage. A similar pattern holds for other power modules  140  and loads  130 , wherein the voltage rises across each power module  140  and then drops across each corresponding load, providing a resultant, overall voltage that is substantially less than the magnitude of the voltage rise or the voltage drop, and may even be relatively or substantially close to zero (depending upon whether the opposing voltage polarities are closely matched). As a result, overall voltage levels at the terminals of loads  130  remain within predetermined and comparatively lower limits, as described above. 
       FIG. 4  is a block diagram illustrating a third exemplary system  100 B and third exemplary apparatus in accordance with the teachings of the present invention. For ease of reference and visual clarity, the apparatus, primary module and secondary module divisions of the system  100 B are not separately demarcated or otherwise separately illustrated in  FIG. 4 . The system  100 B also is couplable to receive input power, such as an AC or DC input voltage, from power source  110 , and the system  100 B comprises a plurality of loads, illustrated as LEDs  170 , a driver  115 , an optional isolator  120 A, a controller  125 A, a plurality of power modules  140 A 1 ,  140 A 2 , through  140 A N , a plurality of bypass circuits  145 A 1 ,  145 A 2 , through  145 A N , a transformer  155 , and a sensor  260 . (An apparatus portion of system  100 B is not separately illustrated, but may be considered to comprise driver  115 , optional isolator  120 A, controller  125 A, sensor  260 , power modules  140 A, transformer  155 , and bypass circuits  145 A. In this exemplary embodiment, a primary module is not separately illustrated, but may be considered to comprise driver  115  and transformer primary  105  (of transformer  155 ). Also in this exemplary embodiment, a secondary module is not separately illustrated, but may be considered to comprise a corresponding power module  140 A and, as an option, a corresponding bypass circuit  145 A. Each power module  140 A comprises a transformer secondary  150  (of transformer  155 ) and other circuitry as illustrated. The optional isolator  120 A also may be considered to be contained within the primary module.)  FIG. 4  provides an example of the power modules  140 A (of a corresponding secondary module) and transformer primary  105  (of a primary module) having a flyback configuration. 
     Each power module ( 140 A 1 ,  140 A 2 , through  140 A N ) comprises a corresponding transformer secondary ( 150   1 ,  150   2 , through  150   N ), a corresponding diode ( 225   1 ,  225   2 , through  225   N ), and a corresponding capacitor ( 220   1 ,  220   2 , through  220   N ), respectively. Each bypass circuit ( 145 A 1 ,  145 A 2 , through  145 A N ) comprises a switch, illustrated as a silicon controlled rectifier (SCR) ( 230   1 ,  230   2 , through  230   N ) and a voltage sensor, illustrated as a zener diode ( 235   1 ,  235   2 , through  235   N ), respectively. Transformer  155  comprises primary  105  and a plurality of secondaries  150   1 ,  150   2 , through  150   N . Isolator  120 A comprises a first optical isolator  210  and a second optical isolator  215 . One skilled in the electronic arts will recognize that isolator  120 A, illustrated in  FIG. 4  and elsewhere herein, may be, in various exemplary embodiments, omitted or implemented using any of numerous methods, such as utilizing various types of isolators such as optical isolators, transformers, differential amplifiers, etc., any and all of which are considered equivalent and within the scope of the present invention. 
     In  FIG. 4  and elsewhere herein, the exemplary configuration of LEDs as strings is illustrative. As discussed in greater detail below, other arrangements are possible, any and all of which are considered equivalent and within the scope of the present invention, 
     In the following discussion, operation of power modules  140 A will be described using power module  140 A 1  as an example. Operation of power modules  140 A 2  through  140 A N  is similar. As illustrated, power module  140 A 1  comprises a transformer secondary  150   1 , a diode  225   1 , and a capacitor  220   1 . The secondary  150   1  provides power to diode  225   1 . Diode  225   1  acts as a half-wave rectifier to provide DC power to a DC smoothing filter, illustrated as capacitor  220   1 . In  FIG. 4  and elsewhere herein, capacitors may be polarized or non-polarized. The secondary  150   1  charges capacitor  220   1  through diode  225   1 . Capacitor  225   1  and secondary  150   1  (via diode  225   1 ) provide DC power to LED string  170   1 . 
     As with  FIG. 3 , power modules  140 A and LED strings  170  may be coupled in alternating series, with a first terminal of each LED string  170   K , 1≦K&lt;N, coupled to a second terminal of power module  140 A K  and a second terminal of each LED string  170   K  coupled to a first terminal of a second power module  140 A K+1 . The first terminal of LED string  170   N  is coupled to a second terminal of power module  140 A N  and a second terminal of LED string  170   N  is coupled through a first sensor, illustrated as resistor  260 , to a first terminal of power module  140 A 1 . 
     As illustrated in  FIG. 4 , power modules  140 A and LEDs  170  are arranged as alternating in series in a ring-like arrangement so that current flows alternately through a power module  140 A and LEDs  170 . Current flowing out of power module  140 A 1  flows in sequential order through LEDs  170   1 , power module  140 A 2 , LEDs  170   2 , etc., then through power module  140 A N , LEDs  170   N , resistor  260 , and back to power module  140 A 1 . This novel current path allows overall, resulting voltage levels to remain relatively low compared to prior art systems. In particular, a voltage rise across a given power module  140 A K  is substantially matched by a corresponding voltage drop across a corresponding LED string  170   K , as illustrated in  FIG. 5 . 
     More particularly, in an exemplary embodiment, as mentioned above, the power modules  140 A and LEDs  170  (as loads  130 ) are arranged such that each output voltage level provided by a power module  140 A (of a corresponding secondary module) is substantially compensated by a corresponding voltage drop across corresponding LEDs  170 . For example, a voltage rise with a first voltage polarity, such as a positive voltage across first power module  140 A 1  which provides power to first LEDs  170   1 , is substantially offset by a corresponding voltage drop across the first LEDs  170   1  having a second, opposing voltage polarity, such as a negative voltage. A similar pattern holds for other power modules  140 A and LEDs  170 , wherein the voltage rises across each power module  140 A and then drops across each corresponding string of LEDs  170 , providing a resultant, overall voltage that is substantially less than the magnitude of the voltage rise or the voltage drop, and may even be relatively or substantially close to zero (depending upon whether the opposing voltage polarities are closely matched). As a result, overall voltage levels at the terminals of LEDs  170  remain within predetermined and comparatively lower limits, as described above. 
       FIG. 5  is a graphical diagram illustrating a voltage map of voltage levels across power modules  140 A and LEDs  170  in accordance with the teachings of the present invention. The voltage map illustrates voltage levels for an example configuration wherein four power modules  140 A 1 ,  140 A 2 ,  140 A 3 , and  140 A 4  drive four LED strings  170   1 ,  170   2 ,  170   3 , and  170   4 . The vertical axis represents voltage levels. Points along the horizontal axis represent corresponding points in the circuit topology. The first voltage level  25  for “FIRST POWER MODULE” illustrates the voltage rise with a first voltage polarity across the first power module  140 A 1  from substantially zero volts at a first terminal of first power module  140 A 1  to a voltage level of approximately (or slightly greater than) V 1  at a second terminal of the first power module  140 A 1 . The second voltage level  26  for a “FIRST LOAD” illustrates the voltage drop with a second, opposing voltage polarity across a first and second terminal of the first LED string  170   1  to a level relatively near zero. Accordingly, the voltage rise across first power module  140 A 1  is substantially offset by the voltage drop across first LED string  1701   1  so that the overall or resultant voltage (of the voltage rise (or first voltage polarity) combined with the voltage drop (or second voltage polarity)) is substantially less than a magnitude of the first voltage polarity or the second voltage polarity, and as illustrated, is substantially close to zero volts. 
     In the example illustrated in  FIG. 5 , the voltage across first LED string  170   1  drops to a level slightly below zero, a situation that may occur, for example, if there is a difference between the voltage rise and the voltage drop. The voltage drop across LEDs  170  may substantially match the corresponding voltage rise across power modules  140 , though there may be some difference between the voltage rise and the voltage drop due to factors such as variations in characteristics of power modules  140 A and LEDs  170 . In practice, the voltage across each load may drop to a level slightly above or slightly below zero. Such differences may arise as a result of numerous factors such as manufacturing tolerances, temperature, device aging, engineering approximations, variability of the power source  110 , etc. It should be understood that the voltage maps shown in  FIG. 1 ,  FIG. 5 , and  FIG. 6  (described later) are exemplary and approximate, that the illustrations herein represent an idealized example for purposes of explication and should not be regarded as limiting, and that actual measurements in practice may and likely will deviate from these representations. 
     The third voltage level  27  for “SECOND POWER MODULE” shows the voltage rise (i.e., a third voltage polarity) across second power module  140 A 2 . The fourth voltage level  28  for “SECOND LOAD” shows the subsequent voltage drop (i.e., a fourth voltage polarity) across the second LED string  170   2  to a level relatively near zero. Such a pattern of voltage rising across power modules  140 A and falling by approximately the same amount across LEDs  170  continues through to the fourth load, where the voltage level falls across the fourth load to a value relatively near zero (29). In other words, the voltage rise across power modules  140 A may be approximately proportional to the voltage drop across LED strings  170 , with the voltage level returning to a value relatively near or about zero volts after each voltage drop. The voltage map of  FIG. 5  illustrates how an exemplary embodiment with an alternating series configuration may provide power conversion where the maximum voltage level is approximately that of a voltage level across a single LED string  170   K , 1≦K≦N. Compared to a prior art power converter such as a system with a voltage map as illustrated in  FIG. 1 , or where the maximum voltage may be substantially equal to the sum of voltage levels across multiple strings, exemplary embodiments of the current invention may operate with relatively lower voltage levels. In addition, with relatively lower voltage levels, expenses such as costs for components adapted to operate with relatively high voltage levels and for additional insulation and other safety equipment may be reduced or substantially eliminated. 
     Referring again to  FIG. 4 , bypass circuits  145 A provide switchable current paths around power modules  140 A and LEDs  170 . In an exemplary embodiment, bypass circuits  145 A may provide one or more alternate current (or voltage) paths in the event of a fault, such as a short circuit or an open circuit condition. Such a fault may occur, for example, in one or more of power modules  140 A or LEDs  170 . In an alternative embodiment, bypass circuits  145 A provide for reducing or increasing power levels to one or more of LED strings  170 , for example to selectively reduce or increase brightness levels, or to change or modify the overall emitted spectrum, as mentioned above. 
     The operation of bypass circuits  145 A in an exemplary embodiment is described utilizing an example of a first bypass circuit  145 A 1 , a first power module  140 A 1 , and a first LED string  170   1 . Operation of bypass circuits  145 A 2  through  145 A N  is similar. Transformer  155  provides power to diode  225   1  via secondary  150   1 . Diode  225   1  is configured as a half-wave rectifier and converts power from secondary  150   1  to DC power. Capacitor  220   1  acts as a filter to smooth the DC power and provide a relatively constant DC power level. As illustrated in  FIG. 4  and elsewhere herein, the first power module  140 A 1  comprises a DC smoothing filter, illustrated as capacitor  220   1 ; however, in various embodiments, power modules  140 A may be configured with or without DC smoothing filters. Since the voltage rise across power module  140 A 1  may be substantially offset by the voltage drop across LED string  170   1 , the voltage across bypass circuit  145 A 1 , absent faults, may be close to zero. 
     An exemplary embodiment of the present invention provides continued operation for one or more channels in the event of any of several fault modes. An example of a first fault mode is where an LED string becomes substantially nonconducting. In an exemplary embodiment, if LED string  170   1  becomes a relatively high impedance or open circuit (i.e. enters a state where it is substantially nonconducting), such as due to a failed LED or a broken connection, the voltage level across bypass circuit  145 A 1  may increase. The voltage level increase may be caused by current from other power modules  140 A 2 ,  140 A 3 , etc., providing power to a relatively high impedance circuit comprising LED string  170   1 . When the voltage level across bypass circuit  145 A 1  reaches or exceeds a predetermined level, such as a threshold voltage, bypass circuit  145 A 1  detects a fault. (Other examples of detecting faults by comparing parameter values to thresholds are described below.) After the voltage level across bypass circuit  145 A 1  reaches or exceeds a predetermined level (such as a predetermined level determined, in part, by a threshold (or breakdown) voltage of zener diode  235   1 ), zener diode  235   1  conducts current into the gate of SCR  230   1  and causes SCR  230   1  to switch on (i.e. switch to a conducting state). With SCR  230   1  switched on, SCR  230   1  shunts current past power module  140 A 1  and LED string  170   1  to other power modules  140 A and LEDs  170 . By thus shunting current around the open circuit (as an example of a detected fault), bypass circuit  145 A 1  provides an alternate path for current to flow to power modules  140 A 2  through  140 A N  and LEDs  170   1  through  170   2  in the event of an open circuit (or high impedance) condition in power module  140 A 1  or LED string  170   1 . Likewise, bypass circuits  145 A 2  through  145 A N  provide alternate current paths in the event of open circuit conditions in power modules  140 A 1  through  140 A N  or LED strings  170   1  through  170   N , respectively. 
       FIG. 6  is a graphical diagram illustrating a voltage map of voltage levels during a component fault in accordance with the teachings of the present invention.  FIG. 6  illustrates how voltage levels may change from those illustrated in  FIG. 5  in the event of a fault, such as an open circuit in the second power module or the second load as illustrated. During a fault condition, such as a second fault mode where second power module  140 A 2  stops providing power and becomes an open circuit, a second bypass circuit  145 A 2  may shunt current around power module  140 A 2  and LED string  170   2 . With second power module  140 A 2  providing substantially no power, the voltage rise across second power module  140 A 2  may be substantially zero. With substantially no current flowing through the second load LED string  170   2  (due to the fault in power module  140 A 2  and current shunted by second bypass circuit  145 A 2 ), the voltage drop across the second load may be substantially zero. The voltage rise and drop of substantially zero are illustrated in  FIG. 6  and appear as a substantially flat voltage level  30  from the point labeled “SECOND POWER MODULE” to the point labeled “SECOND LOAD.” As described and illustrated in the example of  FIG. 6 , a fault in the second power module  140 A 2  may affect the associated load, LED string  170   2 , but the second bypass circuit  145 A 2  provides an alternate current path so that operational channels such as the first load, third load, and fourth load may receive power. 
     Returning to  FIG. 4 , zener diode  230   1  effectively operates as and may be considered to be a sensor, since it senses and responds to a parameter such as voltage across power module  140 A 1  and LED string  170   1 . Operation of first bypass circuit  145 A 1  may be described as a method of sensing a parameter such as a voltage level, comparing the sensed parameter to a threshold such as the first zener diode  230   1  breakdown voltage level, and, when the sensed parameter is greater than the threshold, redirecting current from LED string  170   N  (via resistor  260 ) around first power module  140 A 1  and first LED string  170   1  to a second power module  140 A 2  and LED string  170   2 . 
       FIG. 7  is a flow diagram illustrating a first exemplary method of bypassing a component fault in accordance with the teachings of the present invention. For ease of explanation, the circuit topology of  FIG. 4  will be utilized in the following discussion of  FIG. 7 , with the understanding that the derived bypass methodology of the exemplary embodiments is applicable to numerous bypass topologies, including (without limitation) those illustrated in  FIG. 3 ,  FIG. 4 ,  FIG. 8 ,  FIG. 10 ,  FIG. 12 , and  FIG. 13 , and is not limited to those specifically illustrated herein. The method illustrated in  FIG. 7  may utilize, as an example, a first power module  140 A 1 , a first load, illustrated in  FIG. 4  as LED string  170   1 , a first bypass circuit  145 A 1 , and a second load, illustrated as LED string  170   2 . 
     Beginning with start step  600 , a first power module  140 A 1  provides power to a first load, implemented as LED string  170   1 . In step  610 , a bypass circuit  145 A 1  determines a first sensed parameter, such as a voltage level across the first power module  140 A 1  and the first load, LED string  170   1 . Typically, the first sensed parameter will be measured continuously or periodically (e.g., sampled), for ongoing use in a plurality of comparison steps. In step  615 , the first sensed parameter is compared to a first threshold such as a first predetermined value substantially proportional to the breakdown voltage of the zener diode  235   1 , plus the gate voltage of SCR  230   1  (the voltage applied to the gate that turns on SCR  230   1 ). In step  620 , when the value of the first sensed parameter is greater than or substantially equal to the first threshold, the method proceeds to step  625  and bypasses the detected fault (illustrated in two steps), where the first switch, SCR  230   1  is turned on (step  625 ), for example by zener diode  235   1  then to step  630 , where due to the conducting SCR  230   1 , the bypass circuit  145 A 1  reroutes current around the first power module  140 A 1  and the first load, LED string  170   1  and provides current to the second load, LED string  170   2 . In one embodiment of the present invention, the first switch may remain in an on state until power is removed from power modules  140 A. As other faults may occur, following step  630 , when the method is to continue (i.e., as long as input power is available to the converter), step  635 , the method returns to step  610  for ongoing monitoring, and otherwise may end, return step  640 . When the value of the first sensed parameter is not greater than or substantially equal to the first threshold in step  620 , and also when the method is to continue in step  635 , the method also returns to step  610 . 
     Referring again to  FIG. 4 , an example of a second fault mode is where power module  140 A 1  stops providing power and becomes an open or relatively high impedance circuit. In an exemplary embodiment, this second fault mode results in a sequence of events similar to those of the first fault mode and as described above and illustrated in  FIG. 7 , i.e. voltage increases across bypass circuit  145 A 1 , zener diode  235   1  trips, triggering SCR  230   1 , and SCR  230   1  shunts power around power module  140 A 1  and LED string  170   1 . 
     An example of a third fault mode is where LED string  170   1  substantially becomes a short circuit (i.e. is set to a relatively low impedance state). In an exemplary embodiment, if LED string  170   1  substantially becomes a short circuit, LED string  170   1  continues to conduct current, thus providing a path for current to flow to other channels. Power module  140 A 1  may continue to provide power, which may be utilized by other LED channels. 
     An example of a fourth fault mode is where power module  140 A 1  becomes a short circuit (i.e. enters a relatively low impedance state), such as if power module  140 A 1  stops providing power or provides power at a reduced level, yet continues to conduct current. In an exemplary embodiment, current may continue to flow through power module  140 A 1  and LED string  170   1 . If the breakdown voltage of zener diode  235   1  is set to a relatively high voltage level, such as a value greater than the operational forward voltage across LED string  170   1 , then zener diode  235   1  and SCR  230   1  may remain in a nonconducting state and LED string  170   1  may continue to receive power. At least some of the power provided to LED string  170   1  during this fourth fault mode may be provided by one or more of power modules  140 A 2  through  140 A N . In such an exemplary embodiment, LED string  170   1  may remain lit while its corresponding power module  140 A 1  fails, which is a significant improvement, compared to prior art where an LED channel may lose power if its corresponding power converter fails. In an alternative exemplary embodiment, the breakdown voltage of zener diode  235   1  is set to a relatively low voltage level, such as significantly less than the operational forward voltage across LED string  170   1 . In this alternative exemplary embodiment, in the fourth fault mode, zener diode  235   1  trips, triggering SCR  230   1 , which shunts current around power module  140 A 1  and LED string  170   1 . 
     As described above, in the event of a fault in a representative power module  140 A 1  or LED string  170   1 , under the fault modes described herein, other LED strings (i.e., LED strings  170   2 ,  170   3 , through  170   N ) may continue to receive power. This desirable feature, described herein with respect to power module  140 A 1 , LED string  170   1 , and bypass circuit  145 A 1 , as an example, may apply also to other LED strings  170   2  through  170   N  and their corresponding bypass circuits  145 A 2  through  145 A N  and power modules  140 A 2  through  140 A N , respectively. A fault in circuitry associated with one or more channels may tend to increase or decrease power levels in other channels. Controller  125 A may compensate for such a power level change, such as by utilizing a sensed parameter from resistor  260  and adjusting a power output level from driver  115  to primary  105  to bring levels of power provided to LED strings  170  closer to selected or predetermined values using feedback and control methods known in the electronic arts. 
     Continuing with  FIG. 4 , resistor  260  acts as a current sensor, placed in series with power modules  140 A and LED strings  170  and provides a sensed parameter value to controller  125 A via a first input  310  and a second input  315 . Controller  125 A utilizes the sensed parameter value to provide a control signal, such as via a first output  350 , a second output  355 , and a first optical isolator  210  to driver  115  for maintaining current levels through LED  170  within a predetermined range. 
     A third output  360  and a fourth output  370  of controller  125 A may be utilized to provide an over-voltage signal via optical isolator  215  to driver  115 . An over-voltage condition may comprise, for example, a state where a voltage level across one or more components, such as LED strings  170  or power modules  140 A, rises above a predetermined level. This predetermined level may, for example, correspond to a voltage level deemed to be unsafe or correspond to a condition where LEDs  170  may no longer be receiving useful amounts of power, in which case it may be desirable to discontinue providing power to power modules  140 A. Such an over-voltage condition may cause current through resistor  260  to decrease, so voltage across resistor  260  may be utilized in determining an over-voltage condition. In an exemplary embodiment, the value of a sensed parameter such as LED current may be determined utilizing resistor  260  and compared to a predetermined threshold by controller  125 A. If the value of the sensed parameter is less than the predetermined threshold, controller  125 A may output an over-voltage signal (optionally via optical isolator  215 ) to driver  155 , causing driver  115  to discontinue providing power to primary  105 . 
     In the exemplary embodiment illustrated in  FIG. 4  and elsewhere herein, it may be desirable to protect LEDs  170  from power surges at startup and to provide a “soft start,” where power to LEDs  170  may be increased at a controlled rate, when power is first applied. In an exemplary embodiment, controller  125 A provides a “soft start” at power-up. For example, when power source  110  first provides power to driver  115 , controller  125 A may provide a set of control signals to driver  115 , wherein the control signals may be adapted to cause power to LEDs  170  to increase gradually to operational levels and to maintain output power levels below predetermined levels such as maximum rated power for LEDs  170 . Other controllers (such as controllers  125 ,  125 A,  125 B,  125 C, and  125 D) described and illustrated herein may also be adapted to provide a soft start. Those having skill in the electronic arts will recognize that numerous methods are known for generating control signals to provide a soft start, any and all of which are considered equivalent and within the scope of the present invention. 
       FIG. 8  is a block and circuit diagram illustrating a fourth exemplary system  100 C and fourth exemplary apparatus in accordance with the teachings of the present invention. As illustrated, the fourth exemplary system  100 C differs from the respective third exemplary system  100 B insofar as system  100 C utilizes multiple sensors, comprising resistors  260 , buck-based rectifiers for DC power conversion, diacs  180  for bypass, and fuses  190  for current protection, and otherwise functions similarly as described above for system  100 B. Each power module ( 140 B 1 ,  140 B 2 , through  140 B N ) comprises a corresponding first diode ( 240   1 ,  240   2 , through  240   N ), a corresponding second diode ( 245   1 ,  245   2 , through  245   N ), and a corresponding inductor ( 250   1 ,  250   2 , through  250   N ), respectively. Controller  125 B is configured with one or more inputs, illustrated as inputs  310   1 ,  310   2 , through  310   N  and  315   1 ,  315   2 , through  315   N . An apparatus portion of system  100 C is not separately illustrated, but may be considered to comprise driver  115 , isolator  120 A, controller  125 B, resistors  260 , power modules  140 B, transformer  155 , and bypass circuits  145 B. In this exemplary embodiment, a primary module is not separately illustrated, but may be considered to comprise driver  115  and transformer primary  105  (of transformer  155 ). Also in this exemplary embodiment, a secondary module is not separately illustrated, but may be considered to comprise a corresponding power module  140 B and, as an option, a corresponding bypass circuit  145 B. Each power module  140 B comprises a transformer secondary  150  (of transformer  155 ) and other circuitry as illustrated. The optional isolator  120 A also may be considered to be contained within the primary module.  FIG. 8  provides an example of the power modules  140 B (of a corresponding secondary module) and transformer primary  105  (of a primary module) having a single-ended forward configuration. 
     Fuses  190  may be any of a wide variety of devices known to limit current or provide current protection, as known or becomes known to those having skill in the electronic arts, such as resettable fuses, non-resettable fuses, resistors, voltage dependent resistors such as varistors or metal oxide varistors, circuit breakers, thermal breakers such as bimetallic strips and other thermostats, thermistors, positive temperature coefficient (PTC) thermistors, polymeric positive temperature coefficient devices (PPTCs), switches, sensors, active current limiting circuitry, etc. Depending upon the selected embodiment, with the diacs  180  considered first switches, the fuses  190  may function as and be considered second “switches” in accordance with the present invention. 
     Operation of power modules  140 B, fuses  190 , resistors  260 , and bypass circuits  145 B will be described herein utilizing power module  140 B 1 , fuse  190   1 , resistor  260   1 , and bypass circuits  145 B 1  as examples. Operation of power modules  140 B 2  through  140 B N , fuses  190   2  through  190   N , and bypass circuits  145 B 2  through  145   N  is similar. Power module  140 B 1  comprises a transformer secondary  150   1 , a first diode  240   1 , a second diode  245   1 , an inductor  250   1 , and a capacitor  220   1 . The transformer secondary  150   1  provides power through first diode  240   1  to inductor  250   1 . First diode  240   1 , second diode  245   1 , and inductor  250   1  form a buck-based rectifier to convert power from secondary  150   1  to DC. Inductor  250   1  and a DC smoothing filter, illustrated as capacitor  220   1 , provide power to LED string  170   1 . As illustrated, bypass circuit  145 B 1  differs from the respective exemplary bypass circuit  145 A 1  in  FIG. 4  insofar as bypass circuit  145 B 1  is implemented utilizing a diac  180   1 . In alternative embodiments (not separately illustrated), the diac  180   1  may be replaced with another switch such as a thyristor (e.g., a Sidac). Diac  180   1  senses a parameter such as a voltage level across bypass circuit  145 B 1 . If the sensed parameter value is greater than a predetermined threshold, the diac trips, i.e., enters a closed or “on” or conducting state, and shunts current past fuse  190   1 , LED string  170   1 , and power module  140 B 1 . 
     In an exemplary embodiment, operation of the topology illustrated in  FIG. 8  under various fault modes is similar to that described above with reference to  FIG. 4 . In an alternative embodiment illustrated in  FIG. 9  (below), operation of the embodiment illustrated in  FIG. 8  differs from that of  FIG. 4  insofar as fuses  190  may be utilized to interrupt current during one or more short circuits in LED strings  170  or when current levels through any of LED strings  170  are greater than a predetermined threshold. 
     Controller  125 B functions similarly to controller  125 A, as described above, but is able to utilize additional signals from the additional sensors  260  to provide more fine-tuned control over the driver  115 . Feedback signals from any of the sensors  260  may be utilized, for example, to control the voltage or current levels of the driver  115  (and/or transformer primary  105 ) and/or to control various switches (e.g., as illustrated separately in  FIG. 10 ). 
       FIG. 9  is a flow diagram illustrating a second exemplary method of bypassing a component fault in accordance with the teachings of the present invention. In the discussion below,  FIG. 8  is utilized as a reference, however it is to be understood that the exemplary method illustrated in  FIG. 9  is applicable to numerous topologies, including without limitation those illustrated in the Figures herein. Beginning with start step  645 , a power module ( 140 B 1 ) provides power to a corresponding first load, implemented as LED string  170   1 . Depending upon the type of switching utilized, initially at start up, a first switch (such as an SCR  230   1  or a diac  180   1 ), may be set to an off state, and a second switch, such as a fuse  190   1 , may be set to an on state (such as when a fuse is closed or in a conducting state). 
     In step  650 , a first parameter is determined, such as a voltage level across the bypass circuit  145 B 1  or other circuit parameter, such as by the bypass circuit  145 B 1  (comprising a first switch, such as an SCR  230   1  or a diac  180   1 , and a first sensor, such as a zener diode  235   1  or the diac  180   1 ). In step  655 , a second parameter is determined, such as current through the first corresponding load, LED string  170   1 , typically by a fuse  190   1 , functioning as both a second switch and a sensor. Typically, the first and second parameters will be measured continuously or periodically (e.g., sampled), for ongoing use in a plurality of comparison steps. 
     In step  660 , the magnitude of the first parameter (e.g., (1) the voltage level across bypass circuit  145 B 1  or (2) the voltage level across first power module  140 B 1 , fuse  190   1 , and the first load, LED string  170   1 ) is compared to a first threshold, such as the diac  180   1  trip voltage. (The comparison in step  660  is a magnitude comparison, comparing the magnitude of the first parameter with the magnitude of the first threshold, since the polarities of the first parameter and the first threshold may be reversed.) If LED string  170   1  becomes an open circuit or enters a relatively or substantially high impedance state, the voltage rise across power module  140 B 1  may be substantially greater than the (otherwise offsetting) voltage drop across LED string  170   1 , and the voltage level across bypass circuit  145 B 1  may be greater than or substantially equal to a first threshold, such as a diac  180   1  trip voltage level. Similarly, if LED string  170   1  becomes a short circuit or enters a relatively or substantially low impedance state, such that it no longer provides an offsetting voltage, the voltage rise across power module  140 B 1  may be substantially greater than the (otherwise offsetting) voltage drop across LED string  170   1 , and the voltage level across bypass circuit  145 B 1  may be greater than or substantially equal to a first threshold, such as a diac  180   1  trip voltage level. Accordingly, in step  670 , when the value of the first parameter is greater than or substantially equal to the first threshold, the method proceeds to step  680  and bypasses or reroutes current around the power module and corresponding load, e.g., reroutes current to a next power module and a next load. In exemplary embodiments, step  680  is accomplished by turning on a first switch (i.e., setting the first switch to a conducting state), such as SCR  230   1  or diac  180   1 . In addition, in exemplary embodiments, the second switch (e.g., fuse  190 , or other type of second switch) may be open circuited or otherwise rendered substantially non-conducting. When the value of the first parameter is not greater than or substantially equal to the first threshold, the method proceeds to step  685 . 
     It should be noted that, in the embodiments illustrated in  FIG. 8  and  FIG. 9  and elsewhere herein, the breakdown voltage or trip voltage of bypass circuits  145 B (and variations  145 ,  145 A, etc.) may be symmetrical or asymmetrical. For example, the bypass circuits may be configured to trigger at a first voltage threshold in a positive direction and at a second voltage threshold in a negative direction. 
     Similarly, in step  665 , the magnitude of the second parameter is compared to a second threshold, such as the rated current or break point of fuse  190   1 . If LED string  170   1  becomes a short circuit or enters a relatively low impedance state (as with the third fault mode described above), power module  140 B 1  may provide a relatively high level of current through fuse  190   1  that is greater than the second threshold. In step  675 , when the magnitude (or value) of the second parameter is greater than or substantially equal to a second threshold, such a fuse  190   1  or other similar device will become non-conducting or otherwise turn off, creating an open circuit, which will have the ultimate effect of bypassing or rerouting current around the power module and corresponding load, e.g., reroutes current to a next power module and a next load, step  680  (via steps  650 ,  660 ,  670  and  680  discussed above). More particularly, if the portion of the circuit having the LED string  170   1  becomes an open circuit via a non-conducting fuse  190   1  or enters a relatively or substantially high impedance state, the voltage rise across power module  140 B 1  may be substantially greater than the (otherwise offsetting) voltage drop across LED string  170   1 , and the voltage level across bypass circuit  145 B 1  may be greater than or substantially equal to a first threshold, such as a diac  180   1  trip voltage level, which will reroute current as previously discussed. In an exemplary embodiment (not shown in  FIG. 9 ), depending on how the first switch (e.g., SCR  230   1  or a diac  180   1 ) is implemented, if fuse  190   1  is resettable, it may close after the rerouting of step  680 . When the value of the second parameter is not greater than or substantially equal to the second threshold in step  675 , the method proceeds to step  685 . In an exemplary embodiment of the present invention, the first switch may remain in an on state until power is removed from the power module  140 B 1 . Following steps  670 ,  675  or  680 , when the method is to continue, e.g., until power is removed from power module  140 B 1 , the method returns to steps  650  and  655 , and otherwise may end, return step  690 . 
       FIG. 10  is a block and circuit diagram illustrating a fifth exemplary system  100 D and fifth exemplary apparatus in accordance with the teachings of the present invention. As illustrated, the fifth exemplary system  100 D differs from the exemplary systems previously discussed insofar as power modules  140 C utilize a half-bridge configuration and in the addition of first switches  275 , second switches  270 , and inverters  280  to bypass circuits  145 C. Bypass circuits  145 C 1 ,  145 C 2 , through  145 C N  comprise SCRs  230   1 ,  230   2 , through  230   N , zener diodes  235   1 ,  235   2 , through  235   N , first switches  275   1 ,  275   2 , through  275   N , second switches  270   1 ,  270   2 , through  270   N , and inverters  280   1 ,  280   2 , through  280   N , respectively. Power modules  140 C 1 ,  140 C 2 , through  140 C N  comprise center-tapped transformer secondaries  150   1 ,  150   2 , through  150   N , first diodes  255   1 ,  255   2 , through  255   N , second diodes  285   1 ,  285   2 , through  285   N , inductors  151   1 ,  151   2 , through  151   N , and capacitors  220   1 ,  220   2 , through  220   N , respectively. (An apparatus portion of system  100 D is not separately illustrated, but may be considered to comprise driver  115 , isolator  120 A, controller  125 C, resistor  260  (as a sensor), power modules  140 C, transformer  155 , and bypass circuits  145 C. In this exemplary embodiment, a primary module is not separately illustrated, but may be considered to comprise driver  115  and transformer primary  105  (of transformer  155 ). Also in this exemplary embodiment, a secondary module is not separately illustrated, but may be considered to comprise a corresponding power module  140 C and, as an option, a corresponding bypass circuit  145 C. Each power module  140 C comprises a transformer secondary  150  (of transformer  155 ) and other circuitry as illustrated. The optional isolator  120 A also may be considered to be contained within the primary module.)  FIG. 10  provides an example of the power modules  140 C (of a corresponding secondary module) and transformer primary  105  (of a primary module) having a half-bridge configuration. 
     The system and apparatus illustrated in  FIG. 10 , as discussed in greater detail below, is particularly useful for dimming applications in LED lighting, for example, along with control over the emitted spectrum of such lighting. In addition, in the event the system  100 D and corresponding apparatus may be utilized in dynamic or addressable displays, control is provided for individual on, off, and emission scaling (e.g., brightness scaling) for pixel addressability (e.g., when an LED  170  or string of LEDs  170  forms a pixel for an addressable display). 
     Operation of bypass circuits  145 C and power modules  140 C in an exemplary embodiment will be described utilizing, as an example, a first bypass circuit  145 C 1 , a first power module  140 C 1 , and a first LED string  170   1 . Operation of other bypass circuits  145 C 2  through  145 C N  and power modules  140 C 2  through  140 C N  is similar. Secondary  150   1 , first diode  255   1  and second diode  285   1  form a full-wave, half-bridge rectifier and provide power to inductor  151   1  and capacitor  220   1 , which in turn provide power to LED string  170   1 . SCR  230   1  and zener diode  235   1  provide a bypass function similar to that illustrated in  FIG. 4 . A first switch  275   1 , with its source and drain coupled in parallel with the anode and cathode of SCR  230   1 , provides an additional bypass function in response to first output signal (on output  370   1 ) from controller  125 C to the gate of first switch  275   1 . In an exemplary embodiment, the gate of a second switch  270   1  receives a complement of the first output signal via inverter  280   1  so that the second switch  270   1  turns off at generally or substantially the same time as first switch  275   1  turns on and second switch  270   1  turns on at generally or substantially the same time as first switch  275   1  turns off. (It is to be understood that there may be some switching delay such as due to component response times and the intervening inverter  280 .) In an alternative embodiment, inverter  280   1  may be replaced with a dual output buffer (not separately illustrated) with a first output such as a non-inverting output and a second output such as an inverting output, wherein the first output is coupled to the gate of the first switch  275   1  and the second output is coupled to the gate of the second switch  270   1 . The buffer may be part of or separate from controller  125 C. In the exemplary embodiment illustrated in  FIG. 10 , second switch  270   1  is shown in a low-side location. Alternative positions are possible, such as high-side locations, such as (not separately illustrated) in series with LEDs  170 . 
     With first switch  275   1  in an off state and second switch  270   1  in an on state, power module  140 C 1  provides power to LED string  170   1 . With first switch  275   1  in an on state and second switch  270   1  in an off state, power module  140 C 1  is disconnected from LED string  170   1  and bypass circuit  145 C 1  shunts current around power module  140 C 1  and LED string  170   1 . Controller  125 C may thus utilize first output signal  370   1  to turn LED string  170   1  off and on. Similarly, controller  125 C may turn LED strings  170   2  through  170   N  on and off independently via additional output signals on outputs  370   2  through  370   N , respectively. Such a capability may be utilized, for example, for controlling LED displays or lighting where it may be desired to turn individual LEDs or channels of LEDs on and off, entirely, periodically, or otherwise selectably. In an exemplary embodiment, controller  125 C may also effectively reduce or increase the average power level provided to individual LED strings  170 , such as for setting apparent brightness (as perceived by the human eye) to a selected or predetermined level (i.e., dimming), utilizing pulse wave modulation (PWM). By rapidly (relative to the response time of the human eye) turning individual LED channels  170  off and on and by adjusting the ratio of “on” time t ON  to “off” time t OFF , the LED channels  170  may appear to independently dim or brighten in response to corresponding output signals on outputs  370   1  through  370   N  from controller  125 C. In addition, controller  125 C may also increase or decrease the brightness, such as average brightness, of LED strings  170  as a group by providing signals to driver  115  adapted to cause driver  115  to increase or decrease the amount of power or current provided to primary  105 . 
     In another exemplary embodiment, a first load comprises at least one first LED  170   1  having a first emission spectrum (such as an emission spectrum in the red, green, blue, white, yellow, amber, or other visible wavelengths), and a second load comprises at least one LED  170   2  having a second emission spectrum. For example, a first LED may provide emission in the red visible spectrum, a second LED may provide emission in the green visible spectrum, and a third LED may provide emission in the blue visible spectrum, and so on. In such an exemplary embodiment, the controller  125 C may be further adapted to regulate an output spectrum by regulating the first bypass circuit, or the second bypass circuit, or a third bypass circuit, such as by dimming or bypassing a corresponding LED string, to modify the overall emitted light spectrum, such as to increase or decrease corresponding portions of red, green, or blue emitted light, for example. This type of control may be utilized to provide any type of architectural or other ambient lighting effect. 
       FIG. 11  is a flow diagram illustrating a method of adjusting LED brightness or emission levels, including turning or pulsing on or off strings of LEDs  170 , independently or non-independently, in accordance with the teachings of the present invention. This method may include determining a pulse width for the duration of switching on (or on-time duration) for each LED channel  170   1 ,  170   2 , through  170   N  and/or an overall power level or emission spectrum for a plurality of LED channels  170 . These types of parameters may also be predetermined or stored in any associated memory of controller  125 C. Beginning with start step  710 , controller  125 C determines (or obtains from a memory circuit) one or more reference levels, corresponding to desired (e.g., selected or predetermined) brightness or emission spectrum of LED channels  170 , in step  715 . Reference levels may, for example, be read from a memory or from a processor or other device and may be predetermined or dynamically determined. In an exemplary embodiment, reference levels represent a selected or predetermined brightness for each LED channel  170   1 ,  170   2 , through  170   N . In another exemplary embodiment, reference levels may be varied dynamically during operation (e.g., by the user) and represent a user-selected or predetermined brightness for each LED channel  170   1 ,  170   2 , through  170   N . In another exemplary embodiment, reference levels may be varied dynamically during operation (e.g., by the user) and represent a user-selected or predetermined color brightness for each LED channel  170   1 ,  170   2 , through  170   N , where the various LED channels have different emission spectra, such as red, green, blue, amber, white, etc. 
     In step  720 , a primary power or current level is determined, for example by controller  125 C. The primary power or current level may, for example, be determined as a function of a general power setting such as average desired brightness, emission spectra (desired output color), which also may be averaged over LED channels  170  or total selected or predetermined output power for power modules  140 C 1 ,  140 C 2 , through  140 C N . In step  725 , the determined primary power or current level is utilized to provide power to transformer primary  105 . 
     In step  730 , a pulse width or a pulse “on” time t ON  and “off” time t OFF  are determined for each channel. The value of t ON  and t OFF  may be different for each channel. In an exemplary embodiment, t ON  may be substantially proportional to the selected or predetermined brightness of the corresponding channel. The “off” time t OFF  may be determined utilizing any of various methods such as determining t OFF  to be substantially proportional to a predetermined pulse interval (i.e. the period of time between the start of two adjacent pulses) minus t ON . A pulse interval may, for example, be predetermined such that the action of LEDs  170  turning on and off is substantially imperceptible to the human eye. 
     The perceived brightness of each channel may be substantially proportional to both the corresponding pulse width determined in step  730  for the corresponding channel and the primary power or current level determined in step  720 . In an exemplary embodiment, each LED channel is turned on in step  735  for an “on” time t ON  and turned off in step  740  for an “off” time t OFF . When the method is to continue, step  745 , the method returns to step  715 , and otherwise may end, return step  750 . 
       FIG. 12  is a block and circuit diagram illustrating a sixth exemplary system  100 E and sixth exemplary apparatus in accordance with the teachings of the present invention. As illustrated, the sixth exemplary system  100 E differs from the previously discussed systems insofar as power modules  140 D utilize a current doubling circuit configuration and in changes to the bypass circuits, denoted in  FIG. 12  as bypass circuits  145 D 1 ,  145 D 2 , through  145 D N . (An apparatus portion of system  100 E is not separately illustrated, but may be considered to comprise driver  115 , isolator  120 A, controller  125 D, resistor  260  (as a sensor), power modules  140 D, transformer  155 , and bypass circuits  145 D. In this exemplary embodiment, a primary module is not separately illustrated, but may be considered to comprise driver  115  and transformer primary  105  (of transformer  155 ). Also in this exemplary embodiment, a secondary module is not separately illustrated, but may be considered to comprise a corresponding power module  140 D and, as an option, a corresponding bypass circuit  145 D. Each power module  140 D comprises a transformer secondary  150  (of transformer  155 ) and other circuitry as illustrated. The optional isolator  120 A also may be considered to be contained within the primary module.)  FIG. 12  provides an example of the power modules  140 D (of a corresponding secondary module) and transformer primary  105  (of a primary module) having a current doubler configuration. 
     Power modules  140 D 1 ,  140 D 2 , through  140 D N  comprise transformer secondaries  150   1 ,  150   2 , through  150   N , first diodes  410   1 ,  410   2 , through  410   N , second diodes  415   1 ,  415   2 , through  415   N , first inductors  430   1 ,  430   2 , through  430   N , and second inductors  435   1 ,  435   2 , through  435   N , respectively. Bypass circuits  145 D 1 ,  145 D 2 , through  145 D N  comprise third diodes  420   1 ,  420   2 , through  420   N , diacs  180   1 ,  180   2 , through  180   N , and switches  275   1 ,  275   2 , through  275   N , respectively. 
     Operation of bypass circuits  145 D and power modules  140 D in an exemplary embodiment is described utilizing, as an example, a first bypass circuit  145 D 1 , a first power module  140 D 1 , and a first LED string  170   1 . Operation of other bypass circuits  145 D 2  through  145 D N  and power modules  140 D 2  through  140 D N  is similar. Secondary  150   1  provides power to a rectifier circuit, configured as a current doubler and comprising first diode  410   1 , second diode  415   1 , first inductor  430   1 , and second inductor  435   1 . The first power module  140 D 1  provides power to LED string  170   1 . 
     Bypass circuit  145 D 1  comprises third diode  420   1 , diac  180   1 , and switch  275   1 . Third diode  420   1  provides current bypass for power module  140 D 1 , while diac  180   1  and switch  275   1  provide current bypass for LED string  170   1 . If LED string  170   1  becomes an open or relatively high impedance circuit, a voltage level across diac  180   1  may increase to a value greater than or substantially equal to a predetermined threshold, causing diac  180   1  to trip and bypass (i.e., shunt current around) the LED string  170   1 . Third diode  420   1  is coupled in parallel with power module  140 D 1  and may shunt current around power module  140 D 1  to LED string  170   1  and to other channels in the event of a fault in power module  140 D 1 . That LED string  170   1  may continue to receive power despite a fault in the corresponding power module  140 D 1  is a significant advantage of exemplary embodiments of the present invention over prior art power converters. Third diode  420   1  may be considered optional because, in various exemplary embodiments, other components in the rectifier circuit may shunt power past power module  140 D 1  in the event of a fault in power module  140 D 1 . For example, if secondary  150   1  becomes an open circuit, diode  410   1  and inductor  430   1  may provide a current path through power module  140 D 1 . Third diode  420   1 , placed across a power module, may also be utilized in conjunction with alternate embodiments such as those illustrated in  FIG. 2 ,  FIG. 3 ,  FIG. 4 ,  FIG. 8 , and  FIG. 10  to bypass power module  140 D 1  (or variations) in the event of a power module fault. 
     Switch  275   1 , placed in parallel with LED string  170   1 , may serve as a current shunt to substantially stop current flow through LED string  170   1  and set LED string  170   1  to an “off” state in response to a control signal on output  370   1  of controller  125 D, as previously discussed. Similarly, controller  125 D may independently control LED strings  170   2  through  170   N  by providing output signals (on outputs  370   2  through  370   N ) to the respective gates of switches  275   2  through  275   N . Such control may be separate and independent or may be coordinated, such as for brightness control or architectural lighting effects. As with the exemplary embodiments illustrated in  FIG. 10  and  FIG. 11 , controller  125 D may turn LED strings  170   1 ,  170   2 , through  170   N  on and off independently or may dim or brighten individual channels, for example by utilizing PWD methods such as the method described in  FIG. 11 . 
       FIG. 13  is a circuit diagram illustrating an example of a secondary module with bypass circuitry and coupled to an LED channel in accordance with the teachings of the present invention, comprising a power module  140 A N , a bypass circuit  145 A N , and an LED string  170   N . Components illustrated in  FIG. 13  correspond to components associated with an N th  channel as illustrated in  FIG. 4 . The topology further comprises a first terminal  545 , which may be coupled to an adjacent LED channel and associated circuitry, and a second terminal  540 , which may be coupled to an adjacent, N−1 th  secondary module and associated circuitry. Power module  140 A N  comprises a transformer secondary  150   N , diode  225   N , and capacitor  220   N . Bypass circuit  145 A N  comprises a switch, illustrated as an SCR  230   N , and a sensor, illustrated as zener diode  235   N . Secondary  150   N  provides power through diode  225   N  to capacitor  220   N . Diode  225   N  and capacitor  220   N  provide power to LED string  170   N . If voltage across bypass circuit  145 A N  increases to a point greater than or substantially equal to a predetermined threshold, zener diode  235   N  conducts, turning on SCR  230   N . With SCR  230   N  in an “on” state, current is bypassed around power module  140 A N  and LED string  170   N . In particular, SCR  230   N  shunts current from an associated secondary module and LED channel via first terminal  545 , to an adjacent secondary module and LED channel via second terminal  540 . 
     The controller  125  (including variations  125 A,  125 B,  125 C, and  125 D) may be any type of controller or processor, and may be embodied as any type of digital logic or analog circuitry or combination thereof or any other circuitry adapted to perform the functionality discussed herein. The controller (including variations) may have other or additional outputs and inputs to those described and illustrated herein, and all such variations are considered equivalent and within the scope of the present invention. Similarly, not all inputs and outputs may be utilized for a given embodiment of the present invention. As the term controller, processor or control logic block is used herein, a controller or processor or control logic block may include use of a single integrated circuit (“IC”), or may include use of a plurality of integrated circuits or other components connected, arranged or grouped together, such as controllers, microprocessors, digital signal processors (“DSPs”), parallel processors, multiple core processors, custom ICs, application specific integrated circuits (“ASICs”), field programmable gate arrays (“FPGAs”), adaptive computing ICs, associated memory (such as RAM, DRAM and ROM), discrete components, and other ICs and components. As a consequence, as used herein, the term controller, processor or control logic block should be understood to equivalently mean and include a single IC, or arrangement of custom ICs, ASICs, processors, microprocessors, controllers, FPGAs, adaptive computing ICs, or some other grouping of integrated circuits or electronic components which perform the functions discussed herein, with any associated memory, such as microprocessor memory or additional RAM, DRAM, SDRAM, SRAM, MRAM, ROM, PROM, FLASH, EPROM, or E 2 PROM. A controller or processor (such as controller  125 ,  125 A,  125 B,  125 C, and  125 D), with its associated memory, may be adapted or configured (via programming, FPGA interconnection, or hard-wiring) to perform the methodology of the invention, as discussed above and below. For example, the methodology may be programmed and stored, in a controller  125  and other equivalent components, as a set of program instructions or other code (or equivalent configuration or other program) for subsequent execution when the controller or processor is operative (i.e., powered on and functioning). Equivalently, the controller may be implemented in whole or part as FPGAs, digital logic such as registers and gates, custom ICs and/or ASICs, the FPGAs, digital logic such as registers and gates, custom ICs or ASICs, also may be designed, configured and/or hard-wired to implement the methodology of the invention. For example, the controller or processor may be implemented as an arrangement of controllers, microcontrollers, microprocessors, state machines, DSPs and/or ASICs, which are respectively programmed, designed, adapted or configured to implement the methodology of the invention. 
     The controller  125  (and variations) may comprise memory, which may include a data repository (or database) and may be embodied in any number of forms, including within any computer or other machine-readable data storage medium, memory device or other storage or communication device for storage or communication of information, currently known or which becomes available in the future, including, but not limited to, a memory integrated circuit (“IC”), or memory portion of an integrated circuit (such as the resident memory within a controller or processor IC), whether volatile or non-volatile, whether removable or non-removable, including without limitation RAM, FLASH, DRAM, SDRAM, SRAM, MRAM, FeRAM, ROM, EPROM, or E 2 PROM, or any other form of memory device, such as a magnetic hard drive, an optical drive, a magnetic disk or tape drive, a hard disk drive, other machine-readable storage or memory media such as a floppy disk, a CDROM, a CD-RW, digital versatile disk (DVD) or other optical memory, or any other type of memory, storage medium, or data storage apparatus or circuit, which is known or which becomes known, depending upon the selected embodiment. In addition, such computer readable media includes any form of communication media, which embodies computer readable instructions, data structures, program modules or other data in a data signal or modulated signal. The memory may be adapted to store various look up tables, parameters, coefficients, other information and data, programs or instructions (of the software of the present invention), and other types of tables such as database tables. 
     As indicated above, the controller may be programmed, using software and data structures, for example, to perform the methodology of the present disclosure. As a consequence, systems and methods may be embodied as software, which provides such programming or other instructions, such as a set of instructions and/or metadata embodied within a computer readable medium, discussed above. In addition, metadata may also be utilized to define the various data structures of a look up table or a database. Such software may be in the form of source or object code, by way of example and without limitation. Source code further may be compiled into some form of instructions or object code (including assembly language instructions or configuration information). The software, source code or metadata may be embodied as any type of code, such as C, C++, C#, SystemC, LISA, XML, Java, ECMAScript, JScript, Brew, SQL and its variations (e.g., SQL 99 or proprietary versions of SQL), DB2, Oracle, or any other type of programming language which performs the functionality discussed herein, including various hardware definition or hardware modeling languages (e.g., Verilog, VHDL, RTL) and resulting database files (e.g., GDSII). As a consequence, a “construct”, “program construct”, “software construct” or “software”, as used equivalently herein, means and refers to any programming language, of any kind, with any syntax or signatures, which provides or can be interpreted to provide the associated functionality or methodology specified (when instantiated or loaded into a processor or computer and executed, including the controller  125 , for example). 
     The software, metadata, or other source code and any resulting bit file (object code, database, or look up table) may be embodied within any tangible storage medium, such as any of the computer or other machine-readable data storage media, as computer-readable instructions, data structures, program modules or other data, such as discussed above, e.g., a floppy disk, a CDROM, a CD-RW, a DVD, a magnetic hard drive, an optical drive, or any other type of data storage apparatus or medium, as mentioned above. 
     In some exemplary embodiments, control circuitry may be implemented using digital circuitry such as logic gates, memory registers, a digital processor such as a microprocessor or digital signal processor, I/O devices, memory, analog-to-digital converters, digital-to-analog converters, FPGAs, etc. In other exemplary embodiments, this control circuitry may be implemented in analog circuitry such as amplifiers, resistors, integrators, multipliers, error amplifiers, operational amplifiers, etc. For example, one or more parameters stored in digital memory may, in an analog implementation, be encoded as the value of a resistor or capacitor, the voltage of a zener diode or resistive voltage divider, or otherwise designed into a circuit. It is to be understood that embodiments illustrated as analog circuitry may alternatively be implemented with digital circuitry or with a mixture of analog and digital circuitry and that embodiments illustrated as digital circuitry may alternatively be implemented with analog circuitry or with a mixture of analog and digital circuitry within the scope of the present disclosure. 
     Controller  125  executes methods of control as described in the exemplary embodiments. Methods of implementing, in software and/or logic, a digital form of the embodiments shown herein is well known by those skilled in the art. The controller  125  may comprise any type of digital or sequential logic for executing the methodologies and performing selected operations as discussed above and as further described below. For example, the controller  125  may be implemented as one or more finite state machines, various comparators, integrators, operational amplifiers, digital logic blocks, configurable logic blocks, or may be implemented to utilize an instruction set, and so on, as described herein. 
     Switches illustrated and described herein, such as fuses  190  and switches shown in the Figures, are illustrated as SCRs, diacs, MOSFETs, diodes, fuses, etc., and may be implemented as any type of power switch, in addition to those illustrated, including without limitation a thyristor such as a diac, sidac, SCR, triac, or quadrac, a bipolar junction transistor, an insulated-gate bipolar transistor, a N-channel or P-channel MOSFET, a relay or other mechanical switch, a vacuum tube, various enhancement or depletion mode FETs, fuses, diodes, etc. A plurality of power switches may be utilized in the circuitry. 
     Numerous advantages of the exemplary embodiments, for providing power to loads such as LEDs, are readily apparent. The exemplary embodiments provide power conversion for multiple channels of LEDs at comparatively low voltage levels. The exemplary embodiments provide an overall reduction in size, weight, and cost of the power converter by sharing components across channels. The exemplary embodiments provide increased reliability by providing continued operation of one or more channels in the event of faults. The exemplary embodiments further provide stable output power levels and compensate for factors such as temperature, component aging, and manufacturing tolerances. Exemplary embodiments provide independent control over individual channels such as dimming, emission spectra, and turning channels on or off. 
     Although various methods, systems and apparatuses have been described with respect to specific embodiments thereof, these embodiments are merely illustrative and should not be considered restrictive in any manner. In the description herein, numerous specific details are provided, such as examples of electronic components, electronic and structural connections, materials, and structural variations, to provide a thorough understanding of embodiments disclosed. One skilled in the relevant art will recognize, however, that an embodiment can be practiced without one or more of the specific details, or with other apparatus, systems, assemblies, components, materials, parts, etc. In other instances, well-known structures, materials, or operations are not specifically shown or described in detail to avoid obscuring aspects of embodiments disclosed herein. In addition, the various Figures are not drawn to scale and should not be regarded as limiting. 
     Reference throughout this specification to “one embodiment,” “an embodiment,” or a specific “embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment and not necessarily in all embodiments, and further, are not necessarily referring to the same embodiment. Furthermore, the particular features, structures, or characteristics of any specific embodiment may be combined in any suitable manner and in any suitable combination with one or more other embodiments, including the use of selected features without corresponding use of other features. In addition, many modifications may be made to adapt a particular application, situation or material to the essential scope and spirit of the claimed subject matter. It is to be understood that other variations and modifications of the embodiments described and illustrated herein are possible in light of the teachings herein and are to be considered part of the spirit and scope of the appended claims. 
     It will also be appreciated that one or more of the elements depicted in the Figures can be implemented in a more separate or integrated manner, or even removed or rendered inoperable in certain cases, as may be useful in accordance with a particular application. Integrally formed combinations of components are also within the scope of the claimed subject matter, particularly for embodiments in which a separation or combination of discrete components is unclear or indiscernible. In addition, use of the term “coupled” herein, including in its various forms such as “coupling” or “couplable,” means and includes any direct or indirect electrical, structural or magnetic coupling, connection or attachment, or adaptation or capability for such a direct or indirect electrical, structural or magnetic coupling, connection or attachment, including integrally formed components and components which are coupled via or through another component. 
     As used herein for purposes of the claimed subject matter, the term “LED” and its plural form “LEDs” should be understood to include any electroluminescent diode or other type of carrier injection- or junction-based system which is capable of generating radiation in response to an electrical signal, including without limitation, various semiconductor- or carbon-based structures which emit light in response to a current or voltage, light emitting polymers, organic LEDs, and so on, including within the visible spectrum, or other spectra such as ultraviolet or infrared, of any bandwidth, or of any color or color temperature. 
     Channels of LEDs may have the same or different numbers of LEDs. Channels of LEDs may be illustrated and described herein utilizing LED strings as exemplary embodiments, however it is to be understood that LED channels may comprise one or more LEDs in innumerable configurations such as a plurality of strings in series or parallel, arrays of LEDs, LEDs of various types and colors, and LEDs combined with other components such as diodes, resistors, fuses, positive temperature coefficient (PTC) fuses, sensors such as optical sensors or current sensors, switches, etc., any and all of which are considered equivalent and within the scope of the present disclosure. Although, in an exemplary embodiment, the power converter drives one or more LEDs, the converter may also be suitable for driving other linear and nonlinear loads such as computer or telephone equipment, lighting systems, radio transmitters or receivers, telephones, computer displays, motors, heaters, etc. Where reference is made herein to a load or group of LEDs, it is to be understood that a load (such as LEDs) may comprise a plurality of loads. 
     In the foregoing description and in the Figures, sense resistors are shown in exemplary configurations and locations; however, those skilled in the art will recognize that other types and configurations of sensors may also be used and that sensors may be placed in other locations. Alternate sensor configurations and placements are within the scope of the present disclosure. 
     It is to be understood in discussing fault modes that the terms “short circuit” and “open circuit” are used herein as examples of types of component failures. The term “short circuit” may include partial short circuit conditions where impedance or voltage drops to a level lower than normal (i.e., absent faults) operational level, such as below a predetermined threshold. The term “open circuit” may include partial open circuit conditions where impedance or voltage increases to a level higher than during normal operation, such as above another predetermined threshold. 
     As used herein, the term “DC” denotes both fluctuating DC (such as is obtained from rectified AC), chopped DC, and constant voltage DC, such as is obtained from a battery, voltage regulator, or power filtered with a capacitor. As used herein, the term “AC” denotes any form of alternating current, such as single phase or multiphase, with any waveform (sinusoidal, sine squared, rectified sinusoidal, square, rectangular, triangular, sawtooth, irregular, etc.), and with any DC offset and may include any variation such as chopped or forward- or reverse-phase modulated alternating current, such as from a dimmer switch. 
     In the foregoing description of illustrative embodiments and in attached figures where diodes are shown, it is to be understood that synchronous diodes or synchronous rectifiers (for example relays or MOSFETs or other transistors switched off and on by a control signal) or other types of diodes may be used in place of standard diodes within the scope of the present disclosure. Exemplary embodiments presented here typically generate positive voltages with respect to ground potential; however, the teachings of the present disclosure apply also to power converters that generate positive and/or negative voltages, where mixed or complementary topologies may be constructed, such as by reversing the polarity of semiconductors and other polarized components or by swapping positive and negative terminals on power modules, bypass circuits, loads, etc. 
     Furthermore, any signal arrows in the drawings/Figures should be considered only exemplary, and not limiting, unless otherwise specifically noted. Combinations of components of steps will also be considered within the scope of the present disclosure, particularly where the ability to separate or combine is clear or foreseeable. The disjunctive term “or,” as used herein and throughout the claims that follow, is generally intended to mean “and/or,” having both conjunctive and disjunctive meanings (and is not confined to an “exclusive or” meaning), unless otherwise indicated. As used in the description herein and throughout the claims that follow, “a,” “an,” and “the” include plural references unless the context clearly dictates otherwise. Also as used in the description herein and throughout the claims that follow, the meaning of “in” includes “in” and “on” unless the context clearly dictates otherwise. 
     The foregoing description of illustrated embodiments, including what is described in the summary or in the abstract, is not intended to be exhaustive or to limit the claimed subject matter to the precise forms disclosed herein. From the foregoing, it will be observed that numerous variations, modifications and substitutions are intended and may be effected without departing from the spirit and scope of the novel concepts described here. It is to be understood that no limitation with respect to the specific methods and apparatus illustrated herein is intended or should be inferred. It is, of course, intended to cover by the appended claims all such modifications as fall within the scope of the claims.