Abstract:
Solid state light source driving and dimming systems are provided that enable a plurality of solid state light source (e.g., LED) driver circuits to be coupled to a single AC voltage source. The driver circuits may include constant current circuitry configured to generate a constant AC current from the AC voltage source, and rectifier circuitry configured to generate a DC current to drive the solid state light source (e.g., LEDs). Dimming control includes shunt circuitry operable with a PWM switch to shunt the AC voltage source during certain portions of a PWM signal and to decouple the shunt circuitry from the AC voltage source during other portions of the PWM signal. Shunting the AC voltage source causes the interruption of the DC current to effectively turn off the LEDs. Decoupling the shunt circuitry may improve overall efficiency of power transfer to the LEDs.

Description:
TECHNICAL FIELD 
       [0001]    The present application relates to driving and dimming solid state light sources using an AC voltage source, and more particularly, to driving multiple solid state light source strings using an AC voltage source. 
       BACKGROUND 
       [0002]    Conventional driving systems for solid state light sources, such as but not limited to light emitting diodes (LEDs), typically utilize DC/DC converter circuits to generate a constant DC current to drive the LEDs. Power to a DC/DC converter is typically supplied from an AC voltage source. 
       SUMMARY 
       [0003]    Conventional driving systems for solid state light sources, such as those described above, while typically offering stable drive current, unnecessarily increase electronic component count. This may degrade the efficiency of power transfer to the LEDs. In addition, these conventional driving systems are typically ill-suited to supply power to a plurality of LED strings, since there is no guarantee that the individual channels will remain isolated and/or grounded (non-floating) during operation. 
         [0004]    In an embodiment, there is provided a solid state light source driving and dimming system. The solid state light source driving and dimming system includes a plurality of solid state light source driver circuits configured to be coupled to an AC voltage source. Each driver circuit includes: a constant current circuitry coupled to the AC voltage source, wherein the constant current circuitry is configured to generate a constant AC current from the AC voltage source; rectifier circuitry coupled to the constant current circuitry and configured to generate a DC current to drive at least one solid state light source; shunt circuitry coupled to a negative voltage rail and a positive voltage rail of the AC voltage source; switch circuitry coupled to the shunt circuitry; and pulse width modulation (PWM) circuitry configured to generate a PWM signal to control a conduction station of the switch circuitry; wherein when the switch circuitry is closed, a conduction path exists between the AC voltage source and the shunt circuitry through the switch circuitry to discontinue the DC current, and when the switch circuitry is closed, the shunt circuitry is electrically decoupled from the AC voltage source. 
         [0005]    In a related embodiment, the constant current circuitry may include a ballast capacitor coupled to the positive rail of the AC voltage source. In another related embodiment, the shunt circuitry may include a first diode coupled to the positive voltage rail and in forward bias toward the switch; and a second diode coupled to the negative voltage rail and in forward bias toward the switch; wherein when the switch is closed, the AC voltage source may be shunted through the first and second diodes to discontinue the DC current to the at least one solid state light source. 
         [0006]    In yet another related embodiment, the shunt circuitry may include a first diode coupled to the negative voltage rail and in forward bias toward the positive voltage rail; a second diode coupled to the first diode and the positive voltage rail and in forward bias toward the switch; and a third diode coupled to the negative voltage rail and in forward bias toward the switch; wherein when the switch is closed, the AC voltage source may be shunted through the first, second and third diodes to discontinue the DC current to the at least one solid state light source. 
         [0007]    In still another related embodiment, the rectifier circuitry may include full wave bridge rectifier circuitry configured to generate a full wave rectified AC current from the AC current and a filtering capacitor in parallel with the at least one solid state light source; and wherein the filtering capacitor may be configured to filter the full wave rectified AC current into the DC current to drive the at least one solid state light source. 
         [0008]    In yet still another related embodiment, the rectifier circuitry may include three diodes configured to generate a rectified AC current from the AC current and a filtering capacitor in parallel with the at least one solid state light source; and wherein the filtering capacitor may be configured to filter the rectified AC current into the DC current to drive the at least one solid state light source. In still yet another related embodiment, the solid state light source driving and dimming system may further include a return diode shared by the driver circuits, wherein the return diode may be coupled to the switch and the shunt circuitry and in forward bias toward the negative voltage rail; wherein when the switch is closed, the return diode may provide a current path from the positive voltage rail, through the shunt circuitry and the switch and to the negative voltage rail. 
         [0009]    In yet still another related embodiment, the solid state light source driving and dimming system may further include first and second return diodes shared by the driver circuits, wherein the first return diode may be coupled to the switch and the shunt circuitry and in forward bias toward the negative voltage rail, and the second return diode may be coupled to the rectifier circuitry and the solid state light source and in forward bias toward the negative voltage rail; and wherein when the switch is closed, the first return diode may provide a current path from the positive voltage rail, through the shunt circuitry and the switch and to the negative voltage rail, and wherein when the switch is opened, the second return diode may provide a current path from the solid state light source to the negative voltage rail. 
         [0010]    In still yet another related embodiment, the switch circuitry and the PWM circuitry may be coupled to a common ground. In yet still another embodiment, the rectifier circuitry and the at least one solid state light source may be coupled to a common ground. In still another related embodiment, the switch circuitry, the PWM circuitry, the rectifier circuitry and the at least one solid state light source may be coupled to a common ground. 
         [0011]    In yet another related embodiment, each driver circuit may further include isolation circuitry coupled to a negative voltage rail of the AC current source and configured to electrically isolate each driver circuit from each other. In still another related embodiment, the solid state light source driving and dimming system may further include an isolation transformer having a primary winding and a plurality of secondary windings, wherein the primary winding may be coupled to the AC voltage source and each driver circuit may be coupled to a respective secondary winding, and wherein the isolation transformer may be configured to electrically isolate each driver circuit from each other. 
         [0012]    In another embodiment, there is provided a solid state light source driving and dimming system. The solid state light source driving and dimming system includes: a plurality of solid state light source driver circuits configured to be coupled to an AC voltage source, each driver circuit including: constant current circuitry coupled to an AC voltage source, the constant current circuitry is configured to generate a constant AC current from the AC voltage source; isolation circuitry coupled to the AC voltage source and configured to electrically isolate each driver circuit from each other; rectifier circuitry coupled to the constant current circuitry and configured to generate a DC current to drive at least one solid state light source; shunt circuitry coupled to a negative and positive voltage rails of the AC voltage source; switch circuitry coupled to the shunt circuitry; and pulse width modulation (PWM) circuitry configured to generate a PWM signal to control a conduction station of the switch circuitry; wherein when the switch circuitry is closed, a conduction path exists between the AC voltage source and the shunt circuitry through the switch circuitry to discontinue the DC current, and when the switch circuitry is closed, the shunt circuitry is electrically decoupled from the AC voltage source. 
         [0013]    In a related embodiment, the shunt circuitry may include: a first diode coupled to the negative voltage rail and in forward bias toward the positive voltage rail; a second diode coupled to the first diode and the positive voltage rail and in forward bias toward the switch; and a third diode coupled to the negative voltage rail and in forward bias toward the switch; wherein when the switch is closed, the AC voltage source may be shunted through the first, second and third diodes to discontinue the DC current to the at least one solid state light source. 
         [0014]    In another related embodiment, the isolation circuitry may include a capacitor coupled to the negative voltage rail and the constant current circuitry may include a capacitor coupled to the positive voltage rail, and wherein the capacitance of the isolation circuitry and the constant current circuitry may be approximately equal. In yet another related embodiment, the switch circuitry, the PWM circuitry, the rectifier circuitry and the at least one solid state light source may be coupled to a common ground. 
         [0015]    In another embodiment, there is provided a solid state light source driving and dimming system. The solid state light source driving and dimming system includes: an isolation transformer having a primary winding coupled to an AC voltage source and a plurality of secondary windings, wherein the isolation transformer is configured to electrically isolate each respective secondary winding from each other; a plurality of solid state light source driver circuits configured to be coupled to a respective secondary winding, each driver circuit including: constant current circuitry coupled to a secondary winding, the constant current circuitry is configured to generate a constant AC current from the AC voltage source; rectifier circuitry coupled to the constant current circuitry and configured to generate a DC current to drive at least one solid state light source; shunt circuitry coupled to a negative and positive voltage rails of the secondary winding; switch circuitry coupled to the shunt circuitry; and pulse width modulation (PWM) circuitry configured to generate a PWM signal to control a conduction station of the switch circuitry; wherein when the switch circuitry is closed, a conduction path exists between the secondary winding and the shunt circuitry through the switch circuitry to discontinue the DC current, and when the switch circuitry is closed, the shunt circuitry is electrically decoupled from the secondary winding. 
         [0016]    In a related embodiment, the shunt circuitry may include: a first diode coupled to the negative voltage rail and in forward bias toward the positive voltage rail; a second diode coupled to the first diode and the positive voltage rail and in forward bias toward the switch; and a third diode coupled to the negative voltage rail and in forward bias toward the switch; wherein when the switch is closed, the secondary winding may be shunted through the first, second and third diodes to discontinue the DC current to the at least one solid state light source. In another related embodiment, the switch circuitry, the PWM circuitry, the rectifier circuitry and the at least one solid state light source may be coupled to a common ground. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    The foregoing and other objects, features and advantages disclosed herein will be apparent from the following description of particular embodiments disclosed herein, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles disclosed herein. 
           [0018]      FIG. 1  is a circuit diagram of one exemplary LED driver system consistent with one embodiment of the present disclosure. 
           [0019]      FIG. 2  is a circuit diagram of another exemplary LED driver system consistent with one embodiment of the present disclosure. 
           [0020]      FIG. 3  is a circuit diagram of another exemplary LED driver system consistent with one embodiment of the present disclosure. 
           [0021]      FIG. 4  is a circuit diagram of another exemplary LED driver system consistent with one embodiment of the present disclosure. 
           [0022]      FIG. 5  is a circuit diagram of another exemplary LED driver system consistent with one embodiment of the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0023]    Embodiments described herein concern driving and dimming solid state light sources, such as but not limited to light emitting diode (LED) strings. Solid state light sources may include, in addition to LEDs and among other things, organic LEDs (OLEDs), as well as other LED-based light sources. The drive current for an LED string may be derived, for example, from a conventional AC power source and/or an instant start ballast conventionally used to drive one or more linear fluorescent lamps. Thus, embodiments disclosed herein may be used as a direct retrofit to replace conventional fluorescent lamps with LED-based lightning, and in some embodiments, the need for DC/DC converter circuitry may be eliminated. PWM dimming techniques may be employed to control the brightness and/or color of individual LED strings. Advantageously, embodiments disclosed herein may offer reduced component count which may translate to increased power factor efficiency and significant cost savings over conventional LED driving systems. 
         [0024]      FIG. 1  is a circuit diagram of a solid state light source driver system  100  according to embodiments described herein. In  FIG. 1 , the solid state light sources are a string of LEDs. The solid state light source driver system  100  includes an AC voltage source  102 , current source circuitry  104 , rectifier circuitry  110 , and an LED string  112 . The AC voltage source  102  is configured to generate an AC voltage, for example but not limited to, a sinusoidal AC voltage. Alternatively or additionally, the AC voltage source  102  may be a ballast source associated with a gas discharge lamp fixture, and may thus be configured to supply voltage in the range of 600 VAC operating at 20 to 200 KHz, depending on the type of gas discharge lamp conventionally used. Of course, these are only examples of the types of voltage sources that may be utilized herein, and those skilled in the art will recognize that other voltage sources may be used without departing from the scope of embodiments described herein. Since the drive current required by a typical LED string is much less that may be generated by the AC voltage source  102 , embodiments may also include the current source circuitry  104  coupled to one or more voltage rails of the AC voltage source  102  and configured to generate a current from the AC voltage source  102 . In this example, the current source circuitry  104  may include a ballast capacitor Cb that is configured to generate a constant AC current and is coupled to the positive voltage rail of the AC voltage source  102  and in series with the LED string  112 , which is the load. The capacitance value of the ballast capacitor Cb may be selected based on the operating frequency of the AC voltage source  102 , and may be generally given by the equation Cb=I/2πfV, where I is the output current of the ballast capacitor Cb, V is the voltage of the AC voltage source  102 , and f is the frequency of the AC voltage source  102 . 
         [0025]    The rectifier circuitry  110  may be coupled to the current source circuitry  104  and configured to rectify and filter the AC current generated by the current source circuitry  104 . In some embodiments, and as shown in  FIG. 1 , the rectifier circuitry  110  may include full wave bridge circuitry (FWB) that includes four diodes arranged to rectify the AC current into a full wave rectified AC current. This arrangement is also known as a full wave rectifier, and may be referred to herein as either a full wave bridge, FWB or full wave rectifier. A filter capacitor Cf may be provided to filter the rectified AC current and generate a DC or quasi-DC current. The LED string  112  may be coupled to the rectifier circuitry  110 . In some embodiments, the LED string  112  may include a plurality of LED and/or other solid state light source devices configured to emit light. The LED string  112  may be driven by the DC current generated by the rectifier circuitry  110 . While the filter capacitor Cf may smooth the rectified DC current into a DC or quasi-DC signal, such a smoothed signal may still produce significant DC variations in relation to the peak-to-trough values of the AC current. Thus, to reduce or eliminate perceptible flicker due to the incomplete smoothing effect of the filter capacitor Cf, the capacitance value of Cf may be selected to have a large enough time constant, based on, for example but not limited to, the operating frequency of the AC voltage source  102  and required supply LED current. In  FIG. 1 , the ballast capacitor Cb may be much smaller than the filter capacitor Cf, for example, by orders of magnitude. The LED string  112  may be coupled to a ground  116 , which may include, for example, a system MAINS ground and/or common (earth) ground. Coupling the LED string  112  to the ground  116  may reduce or eliminate the LED string  112  from being in a “floating” state, which may reduce or eliminate electro-magnetic interference emanated by the LED string  112 . 
         [0026]    The solid state light source driver system  100  shown in  FIG. 1  may also be configured for pulse width modulated (PWM) dimming to provide dimming control over the LED string  112 . To that end, the solid state light source driver system  100  may, in some embodiments, include shunt circuitry  106  and dimming circuitry that includes a switch  108  and a PWM signal source  114 . In such embodiments, the shunt circuitry  106  may include two diodes D 1  and D 2  coupled to respective rails of the AC voltage source  102  and forward biased into the switch  108 . The shunt circuitry  106  is configured to shunt the AC voltage source  102  depending on the conduction state of the switch  108 , as will be described below. The switch  108  may be operably coupled to the shunt circuitry  106  and the FWB circuitry in the rectifier circuitry  110 . In operation, the PWM signal source  114  is configured to generate a PWM signal to control the conduction state of the switch  108 . When the PWM signal is ON (high), the switch  108  may close, thus creating a conduction path through the switch  108 . During the positive half wave of a signal from the AC voltage source  102 , current may flow through the diode D 1 , through the switch  108 , through a lower left diode of the FWB circuitry, and back to the AC voltage source  102 . During the negative half wave of the signal from the AC voltage source  102 , current may flow through the diode D 2 , through the switch  108 , through the upper left diode of FWB circuitry, and back to the AC voltage source  102 . Thus, when the switch  108  is conducting, the AC voltage source  102  may be shunted to interrupt current flow to the LED string  112 . 
         [0027]    When the PWM signal is OFF, the switch  108  may open, thus decoupling the shunt circuitry  106  and the switch  108  from the AC voltage source  102 . In that case, during a positive half wave of a signal from the AC voltage source  102 , current flows through the upper right diode of the full wave rectifier FWB, through the LED string  112 , through the lower left diode of the FWB and back to the AC voltage source  102 . During a negative half wave of the signal from the AC voltage source  102 , current flows through the lower right diode of the FWB, through the LED string  112 , through the upper left diode of the FWB and back to the AC voltage source  102 . Decoupling the shunt circuitry  106 , such that there no power loss on the elements in the shunt circuitry  106 , when power is delivered to the LED string  112 , may offer significant efficiency and power factor enhancements and may further operate to increase a signal to noise ratio of power delivered to the LED string  112 . 
         [0028]    In some embodiments, the filter capacitor Cf may have a capacitance value that enables the filter capacitor Cf to still deliver energy to the LED strings  112  when the AC voltage source  102  is shunted, but also to de-energize quickly enough to allow for adequate dimming control using the duty cycle of the PWM signal generated by the PWM signal source  114 . Thus, for example, the filter capacitor Cf may have a value that allows it to drain energy to the LED string  112  within a few percent of the ON time of the switch  108 . The PWM signal source  114  may be coupled to the ground  116 , which may include, for example, a system MAINS ground and/or common (earth) ground. Coupling the PWM signal source  114  to the ground  116  may reduce or eliminate the PWM signal source  114  from being in a “floating” state, which may reduce or eliminate harmonic noise in the switch  108  and shunt circuitry  106  and enable finer control over the LED string  112 . While the switch  108  is depicted as a generalized switching circuit, those skilled in the art will recognize that the switch  108  may include a FET switch, BJT switch or other electronic circuit capable of switching conduction states. As is known, the PWM signal generated by the PWM signal source  114  may have a controllable duty cycle to control the brightness and/or color of the LED string  112 . For example, assuming a 50% duty cycle, drive current is delivered to LED string  112  during the OFF time of the switch  108  and interrupted during the ON time of the switch  108 . To control the overall brightness in the LED string  112 , the duty cycle of the PWM signal may be adjusted. For example, the duty cycle may range from 0% (the switch  108  is always open) to 100% (the switch  108  is always closed) to control the overall brightness (luminosity) and/or color of the LED string  112 . 
         [0029]      FIG. 2  shows a solid state light source driver system  200  according to embodiments described herein. The solid state light source driver system  200  is configured to drive a plurality of LED strings  112 A,  112 B, . . . ,  112   n  from a single AC voltage source  102 , and includes a plurality of LED driver circuits  201 A,  201 B, . . . ,  201   n . An AC voltage source  102  is coupled to each of the LED driver circuits  201 A,  201 B, . . . ,  201   n , each of which, in whole or in part, may represent an LED channel, and the LED driver circuits  201 A,  201 B, . . . ,  201   n , each as a whole or in part thereof, may be referred to herein as a “channel”, and vice versa. Each of the LED driver circuits  201 A,  201 B, . . . ,  201   n  have a similar topology and operate in a similar manner as the circuit shown in  FIG. 1 , except as described below. Each LED driver circuit  201 A,  201 B, . . . ,  201   n  may include respective current source circuitry  104 A,  104 B, . . . ,  104   n , a respective switch  108 A,  108 B, . . . ,  108   n , respective PWM signal source circuitry  114 A,  114 B, . . . ,  114   n , respective rectifier circuitry  110 A,  110 B, . . . ,  110   n  and a respective LED string  112 A,  112 B, . . . ,  112   n . Here, the designation A, B, . . . , N in connection with reference numerals should be interpreted as a repetition of like components. The description and operation of these components are described above with reference to  FIG. 1 . 
         [0030]    Each LED driver circuit  201 A,  201 B, . . . ,  201   n  may also include respective shunt circuitry  206 A,  206 B, . . . ,  206   n . Each respective shunt circuitry  106 A,  106 B, . . . ,  106   n  may include three diodes D 1 , D 2  and D 3 , where the diodes D 1  and D 3  are coupled to the negative rail of the AC voltage source  102  and forward biased into the respective switch  108 , and the diode D 2  is coupled to the positive rail of the AC voltage source  102  and forward biased into the respective switch  108 . The shunt circuitry  206 A,  206 B, . . . ,  206   n  is configured to independently shunt the AC voltage source  102  depending on the conduction state of the respective switch  108 A,  108 B, . . . ,  108   n , as will be described below. Embodiments may also include a return diode (Dc)  218  that is shared by each of the driver circuits  201 A,  201 B, . . . ,  201   n  and coupled to each respective shunt circuitry  206 A,  206 B, . . . ,  206   n  and switch  108 A,  108 B, . . . ,  108   n . Each switch  108 A,  108 B, . . . ,  108   n  may be operably coupled to respective shunt circuitry  106 A,  106 B, . . . ,  106   n  and the return diode  218 . 
         [0031]    In operation, each respective PWM signal source circuitry  114 A,  114 B, . . . ,  114   n  is configured to generate a PWM signal to control the conduction state of a respective switch  108 A,  108 B, . . .  108   n . Using the driver circuit  201 A as an example, when the PWM signal is ON (high), the switch  108 A may conduct, thus closing the switch  108 A. During the positive half wave of a signal from the AC voltage source  102 , current may flow through the diode D 2 , through the switch  108 A, through the return diode  218 , and back to the AC voltage source  102 . During the negative half wave of a signal from the AC source  102 , current may flow through the diode D 3 , through the switch  108 A, through the diode D 1 , and back to the AC voltage source  102 . Thus, when the switch  108 A is conducting, the AC voltage source  102  may be shunted to interrupt current flow to the LED string  112 A. When the PWM signal is OFF (low), the switch  108 A may open, thus decoupling the shunt circuitry  206 A from the AC voltage source  102 . In that case, current flows through the rectifier circuitry  110 A to power the LED string  112 A, as described above in regards to  FIG. 1 . Decoupling the shunt circuitry  206 A, such that there is no power loss on the elements in the shunt circuitry  206 A when power is delivered to the LED string  112 A, may offer significant power factor enhancements and may further operate to increase a signal to noise ratio of power delivered to the LED string  112 A. Each of the other driver circuits  201 B, . . . ,  201   n  may, and in some embodiments do, operate in a similar manner. 
         [0032]    Each LED string  112 A,  112 B, . . . ,  112   n  may include one or more individual LED devices. Each string may be arranged by color, for example but not limited to a red, green, blue (RGB) topology in which the LED string  112 A may include one or more red LEDs, the LED string  112 B may include one or more green LEDs, and the LED string  112   n  may include one or more blue LEDs. Of course, this is only an example and other color arrangements are equally contemplated herein, for example, RGW (red, green, white), RGBY (red, green, blue, yellow), infrared, etc., without departing from the scope of the embodiments described herein. By controlling the brightness in each LED string  112 A,  112 B, . . . ,  112   n , the overall brightness and/or perceived color of the collection of the LED strings  112 A,  112 B, . . . ,  112   n  may be controlled. Thus, in such embodiments, each PWM signal source  114 A,  114 B, . . . ,  114   n  may be independently controlled with its own duty cycle to independently control each LED string  112 A,  112 B, . . . ,  112   n . To that end, the return diode  218  may operate to reduce or eliminate crosstalk between each driver circuit  201 A,  201 B, . . . ,  201   n , i.e., reduce or eliminate the effect of varying current between LED strings  112 A,  112 B, . . . ,  112   n.    
         [0033]    In embodiments as shown in  FIG. 2 , the PWM signal source circuitry  114 B may be coupled to a ground  116 , which may include, for example, a system MAINS ground and/or common (earth) ground. Coupling the PWM signal source circuitry  114 B to the ground  116  may reduce or eliminate the PWM signal source circuitry  114 B from being in a “floating” state, which may reduce or eliminate harmonic noise in the respective switch  108 B and the respective shunt circuitry  206 B and enable finer control over the LED string  112 B. However, in such embodiments, each LED string  112 A,  112 B, . . . ,  112   n  may not be coupled to a ground (due to potential shorting issues), and thus, the LED strings  112 A,  112 B, . . . ,  112   n  may be in a floating condition which could introduce noise and/or other non-controllable factors into the solid state light source driving system  200 . 
         [0034]      FIG. 3  shows a solid state light source driver system  300  according to embodiments described herein, which are configured to drive a plurality of LED strings  112 A,  112 B, . . . ,  112   n  from a single AC voltage source, similar to the embodiment of  FIG. 2 . Here, a plurality of LED driver circuits  301 A,  301 B, . . . ,  301   n  are each coupled to an AC voltage source  102 . Each of the LED driver circuits  301 A,  301 B, . . . ,  301   n  have a similar topology and operate in a similar manner as the system  100  shown in  FIG. 1 , except as described below. Each LED driver circuit  301 A,  301 B, . . . ,  301   n  may include respective current source circuitry  104 A,  104 B, . . . ,  104   n , a respective switch  108 A,  108 B, . . . ,  108   n , respective PWM signal source circuitry  114 A,  114 B, . . . ,  114   n , respective shunt circuitry  206 A,  206 B, . . . ,  206   n , and respective LED strings  112 A,  112 B, . . . ,  112   n . Here, the designation A, B, . . . , N in connection with reference numerals should be interpreted as a repetition of like components. The description and operation of these components are described above with reference to  FIGS. 1 and 2 . 
         [0035]    Embodiments may also include first and second return diodes (Dc and Dc 1 )  218  and  320  that are shared by each of the LED driver circuits  301 A,  301 B, . . . ,  301   n . The first return diode  218  may be coupled to each respective shunt circuitry  206 A,  206 B, . . . ,  206   n  and each respective switch  108 A,  108 B, . . . ,  108   n . The second return diode  320  may be coupled to each respective LED string  112 A,  112 B, . . . ,  112   n  and each respective rectifier circuitry  310 A,  310 B, . . . ,  310   n . Each switch  108 A,  108 B, . . . ,  108   n  may be operably coupled to the respective shunt circuitry  206 A,  206 B, . . . ,  206   n  and the first return diode  218 . The rectifier circuitry  310 A,  310 B, . . . ,  310   n  may include three diodes D 4 , D 5  and D 6  instead of the FWB topology that comprises four diodes as shown in  FIGS. 1 and 2 . 
         [0036]    In operation, each respective PWM signal source circuitry  114 A,  114 B, . . . ,  114   n  is configured to generate a PWM signal to control the conduction state of a respective switch  108 A,  108 B, . . .  108   n . Using the LED driver circuit  301 A as an example, when the PWM signal is ON (high), the switch  108 A may close, creating a conduction path through the switch  108 A. During the positive half wave of a signal from the AC voltage source  102 , current may flow through the diode D 2 , through the switch  108 A, through the first return diode  218 , and back to the AC voltage source  102 . During the negative half wave of a signal from the AC voltage source  102 , current may flow through the diode D 3 , through the switch  108 A, through the diode D 1 , and back to the AC voltage source  102 . Thus, when the switch  108 A is conducting, the AC voltage source  102  may be shunted to interrupt current flow to the LED string  112 A. When the PWM signal is OFF (low), the switch  108 A may open, thus decoupling the shunt circuitry  106 A from the AC voltage source  102 . In that case, during the positive half wave of a signal from the AC voltage source  102 , current may flow through the diode D 5 , through the LED string  112 A, through the second return diode  320 , and back to the AC voltage source  102 . During the negative half wave of a signal from the AC voltage source  102 , current may flow through the diode D 6 , through the LED string  112 A, through the diode D 4 , and back to the AC voltage source  102 . As with previously described embodiments, decoupling the shunt circuitry  206 A, such that there is no power loss on the elements in the shunt circuitry  206 A, when power is delivered to the LED string  112 A, may offer significant power factor enhancements and may further operate to increase a signal to noise ratio of power delivered to the LED string  112 A. Each of the other LED driver circuits  301 B, . . . ,  301   n  may operate in a similar manner. 
         [0037]    As with the previous described embodiments, each LED string  112 A,  112 B, . . . ,  112   n  may include one or more individual LED devices. Each LED string  112 A,  112 B, . . . ,  112   n  may be arranged by color, for example a red, green, blue (RGB) topology in which the LED string  112 A may include one or more red LEDs, the LED string  112 B may include one or more green LEDs, and the LED string  112   n  may include one or more blue LEDs. Of course, this is only an example, and other color arrangements are equally contemplated herein, for example, RGW (red, green, white), RGBY (red, green, blue, yellow), infrared, etc., without departing from the scope of embodiments described herein. By controlling the brightness in each LED string  112 A,  112 B, . . . ,  112   n , the overall brightness and/or perceived color of the collection of LED strings  112 A,  112 B, . . . ,  112   n  may be controlled. Thus, in such embodiments, each PWM signal source circuitry  114 A,  114 B, . . . ,  114   n  may be independently controlled with its own duty cycle to independently control each LED string  112 A,  112 B, . . . ,  112   n . To that end, the first and second return diodes  218  and  320  may operate to reduce or eliminate crosstalk between each LED driver circuit  301 A,  301 B, . . . ,  301   n , i.e., reduce or eliminate the effect of varying current between the LED strings  112 A,  112 B, . . . ,  112   n.    
         [0038]    Advantageously, in such embodiments, elimination of one of the diodes in each of the respective rectifier circuitry  310 A,  310 B, . . . ,  310   n  may enable the rectifier circuitry  310 A,  310 B, . . . ,  310   n  and the LED string  112 A,  112 B, . . . ,  112   n  in each LED driver circuit  301 A,  301 B, . . . ,  301   n  to be coupled to a ground  116 . Such an arrangement may reduce or eliminate noise and/or reduce electro-magnetic interference emanated by the LED string  112 A,  112 B, . . . ,  112   n  and other non-controllable factors into the system  300 . However, in this arrangement, the PWM signal source circuitry  114 A,  114 B, . . . ,  114   n  may not be coupled to a ground due to potential shorting issues, and thus, the PWM signal source circuitry  114 A,  114 B, . . . ,  114   n  may be in a floating condition, which could introduce noise and/or other non-controllable factors into the system  300 . 
         [0039]      FIG. 4  shows a solid state light source driver system  400  according to embodiments described herein. The driver system  400  is configured to drive a plurality of solid state lights source strings, here LED strings  112 A,  112 B, . . . ,  112   n , from a single AC voltage source, similar to the embodiments shown in  FIGS. 2 and 3 . The driver system  400  includes a plurality of LED driver circuits  401 A,  401 B, . . . ,  401   n  and an AC voltage source  102  coupled to each of the LED driver circuits  401 A,  401 B, . . . ,  401   n . Each of the LED driver circuits  401 A,  401 B, . . . ,  401   n  have a similar topology and operate in a similar manner as other LED driver circuits described throughout the specification. Each LED driver circuit  401 A,  401 B, . . . ,  401   n  may include respective current source circuitry  104 A,  104 B, . . . ,  104   n , a respective switch  108 A,  108 B, . . . ,  108   n , respective PWM signal source circuitry  114 A,  114 B, . . . ,  114   n , respective shunt circuitry  106 A,  106 B, . . . ,  106   n , and respective LED strings  112 A,  112 B, . . . ,  112   n . Here, the designation A, B, . . . , N in connection with reference numerals should be interpreted as a repetition of like components. The description and operation of these components are described above with reference to  FIGS. 1-3 . 
         [0040]    Each LED driver circuit  401 A,  401 B, . . . ,  401   n  in this embodiment may also include respective isolation circuitry  403 A,  403 B, . . . ,  403   n  coupled to the negative voltage rail of the AC voltage source  102 . In some embodiments, the isolation circuitry  403 A,  403 B, . . . ,  403   n  may include a capacitor Cb 2 . The capacitance value of the capacitor Cb 2  may be the same or approximately the same as the ballast capacitor Cb 1  (element  104  in  FIG. 1 ) to reduce or eliminate uneven loading of the AC voltage source  102 . The isolation circuitry  403 A,  403 B, . . . ,  403   n  is configured to isolate each LED channel from other LED channels. Thus, advantageously, the isolation circuitry  403 A,  403 B, . . . ,  403   n  may reduce or eliminate crosstalk between the channels to enable more precise control over each channel. Also advantageously, the isolation circuitry  403 A,  403 B, . . . ,  403   n  enables each LED driver circuit  401 A,  401 B, . . . ,  401   n  to be coupled to a ground  116 , thus eliminating a floating condition in any of the LED driver circuit  401 A,  401 B, . . . ,  401   n . In other words, the isolation circuitry  403 A,  403 B, . . . ,  403   n  may enable both the PWM signal source circuitry  114 A,  114 B, . . . ,  114   n  and the LED strings  112 A,  112 B, . . . ,  112   n  to be coupled to the ground  116 . 
         [0041]    As with the embodiments described previously, each LED string  112 A,  112 B, . . . ,  112   n  may include one or more individual LED devices. Each string may be arranged by color, for example a red, green, blue (RGB) topology in which the LED string  112 A may include one or more red LEDs, the LED string  112 B may include one or more green LEDs, and the LED string  112   n  may include one or more blue LEDs. Of course, this is only an example and other color arrangements are equally contemplated herein, for example, RGW (red, green, white), RGBY (red, green, blue, yellow), infrared, etc., without departing from the scope of embodiments described herein. By controlling the brightness in each LED string  112 A,  112 B, . . . ,  112   n , the overall brightness and/or perceived color of the collection of the LED strings  112 A,  112 B, . . . ,  112   n  may be controlled. Thus, in such embodiments, each PWM signal source circuitry  114 A,  114 B, . . . ,  114   n  may be independently controlled with its own duty cycle to independently control each LED string  112 A,  112 B, . . . ,  112   n . To that end, the respective ballast capacitor Cb 1  in each respective current source circuitry  104 A,  104 B, . . . ,  104   n , and the respective isolation capacitor Cb 2  in each respective isolation circuitry  403 A,  403 B, . . . ,  403   n , may operate to reduce or eliminate crosstalk between each LED driver circuit  401 A,  401 B, . . . ,  401   n , i.e., reduce or eliminate the effect of varying current between LED strings  112 A,  112 B, . . . ,  112   n.    
         [0042]      FIG. 5  shows a solid state light source driver system  500  according to embodiments described herein. The driver system  500  shown in  FIG. 5  is configured to drive a plurality of solid state light sources, here LED strings, from a single AC voltage source, similar to the embodiments of  FIGS. 2 ,  3  and  4 . The driver system  500  includes a plurality of LED driver circuits  501 A,  501 B, . . . ,  501   n  and an AC voltage source  102  coupled to each of the LED driver circuits  501 A,  501 B, . . . ,  501   n . Each of the LED driver circuits  501 A,  501 B, . . . ,  501   n  have a similar topology and operate in a similar manner as those described throughout. Each LED driver circuit  501 A,  501 B, . . . ,  501   n  may include respective current source circuitry  104 A,  104 B, . . . ,  104   n , a respective switch  108 A,  108 B, . . . ,  108   n , respective PWM signal source circuitry  114 A,  114 B, . . . ,  114   n , respective shunt circuitry  106 A,  106 B, . . . ,  106   n , respective rectifier circuitry  110 A,  110 B, . . . ,  110   n  and respective LED strings  112 A,  112 B, . . . ,  112   n . Here, the designation A, B, . . . , N in connection with reference numerals should be interpreted as a repetition of like components. The description and operation of these components are described above with reference to  FIGS. 1-4 . 
         [0043]    The driver system  500  may also include an isolation transformer  503  coupled between the AC voltage source  102  and each of the LED driver circuits  501 A,  501 B, . . . ,  501   n . The isolation transformer  503  may be configured to supply each LED driver circuit  501 A,  501 B, . . . ,  501   n  with an AC voltage and to isolate each LED driver circuit  501 A,  501 B, . . . ,  501   n  from other driver circuits. The isolation transformer  503  may be, and in some embodiments is, a known isolation transformers of any type; such transformers are generally configured with a primary winding and a plurality of isolated secondary windings. The turn ration between the primary and secondary side may determine the voltage delivered by the isolation transformer  503 . Thus, advantageously, the isolation transformer  503  may reduce or eliminate crosstalk between the channels to enable more precise control over each channel. Also advantageously, the isolation transformer  503  may enable each LED driver circuit  501 A,  501 B, . . . ,  501   n  to be coupled to a ground  116 , thus eliminating a floating condition in any of the LED driver circuits  501 A,  501 B, . . . ,  501   n . In other words, the isolation transformer  503  may enable both the PWM signal source circuitry  114 A,  114 B, . . . ,  114   n  and the LED strings  112 A,  112 B, . . .  112   n  to be coupled to the ground  116 . 
         [0044]    As with other embodiments, each LED string  112 A,  112 B, . . . ,  112   n  may include one or more individual LED devices. Each string may be arranged by color, for example a red, green, blue (RGB) topology in which the LED string  112 A may include one or more red LEDs, the LED string  112 B may include one or more green LEDs, and the LED string  112   n  may include one or more blue LEDs. Of course, this is only an example and other color arrangements are equally contemplated herein, for example, RGW (red, green, white), RGBY (red, green, blue, yellow), infrared, etc., without departing from the scope of embodiments described herein. By controlling the brightness in each LED string  112 A,  112 B, . . . ,  112   n , the overall brightness and/or perceived color of the collection of LED strings  112 A,  112 B, . . . ,  112   n  may be controlled. Thus, in such embodiments, each PWM signal source circuitry  114 A,  114 B, . . . ,  114   n  may be independently controlled with its own duty cycle to independently control each LED string  112 A,  112 B, . . . ,  112   n.    
         [0045]    In any of the embodiments described herein, a feedback controller (not shown in any of  FIGS. 1-5 ) may be utilized to provide feedback current control over the LED strings  112  and/or  112 A,  112 B, . . . ,  112   n . For example, each LED driver circuit may include a feedback sense resistor coupled to the LED strings to generate a current feedback signal to a feedback controller. Alternatively, a photodetector may be disposed near the LED strings to receive light and generate a feedback signal proportional to the light of the LED strings. A feedback controller may be utilized to compare the feedback signal to user-defined and/or preset values to generate control signals to control the duty cycle of the PWM signal generated by the PWM signal source circuitry. Known feedback controllers, in accordance with the teachings of the present disclosure, may be used to control the duty cycle of power delivered to each LED string. 
         [0046]    As used in any embodiment herein, “circuit” or “circuitry” may comprise, for example, singly or in any combination, hardwired circuitry, programmable circuitry, state machine circuitry, and/or firmware that stores instructions executed by programmable circuitry. In at least one embodiment, the circuits and/or circuitry described herein may collectively or individually comprise one or more integrated circuits. An “integrated circuit” may include a digital, analog or mixed-signal semiconductor device and/or microelectronic device, such as, for example, but not limited to, a semiconductor integrated circuit chip. 
         [0047]    Unless otherwise stated, use of the word “substantially” may be construed to include a precise relationship, condition, arrangement, orientation, and/or other characteristic, and deviations thereof as understood by one of ordinary skill in the art, to the extent that such deviations do not materially affect the disclosed methods and systems. 
         [0048]    Throughout the entirety of the present disclosure, use of the articles “a” or “an” to modify a noun may be understood to be used for convenience and to include one, or more than one, of the modified noun, unless otherwise specifically stated. 
         [0049]    Elements, components, modules, and/or parts thereof that are described and/or otherwise portrayed through the figures to communicate with, be associated with, and/or be based on, something else, may be understood to so communicate, be associated with, and or be based on in a direct and/or indirect manner, unless otherwise stipulated herein. 
         [0050]    Although the methods and systems have been described relative to a specific embodiment thereof, they are not so limited. Obviously many modifications and variations may become apparent in light of the above teachings. Many additional changes in the details, materials, and arrangement of parts, herein described and illustrated, may be made by those skilled in the art.