Abstract:
Dimming ballasts and methods are presented for powering a plurality of fluorescent lamps in which at most one of the lamps is selectively dimmed while all the remaining lamps are turned on or off according to a dimming level setpoint to allow dimming to match a user&#39;s desired lighting level while maintaining high efficiency.

Description:
BACKGROUND OF THE DISCLOSURE 
       [0001]    Dimmable ballast systems provide varying levels of light output through a variety of means. For multi-lamp fixtures, conventional dimming ballast techniques include discrete dimming (so-called “step-dimming”) and continuous dimming. One example of discrete dimming is a multiple-lamp discrete ballast in which one or more lamps are shut off to provide a lower light output. This is sometimes implemented using external controls to turn off individual ballasts or fixtures until the selected light level is achieved. Discrete dimming approaches, however, only provide a finite number of predefined lighting levels and transitions between these discrete levels are often perceptible by users. Some continuous dimming designs operate multiple lamps in series with the power applied to the lamps being reduced for dimming. Series-connected dimming ballasts, however, suffer from inability to produce light when one or more lamps fail. Other proposed approaches include varying a DC bus amplitude via pulse width modulation (PWM) control to power a voltage or current fed inverter for driving one or more lamps, but this dimming control technique adds cost and may not provide the desired amount of dimming for certain applications. Also, continuous dimming techniques can cause early cathode failure by dimming a lamp if no separate cathode heating power is provided to keep the cathode operating within its normal temperature range. However, separate cathode heating contributes to inefficiency at dimming levels below a critical arc power level since the cathode heating power supply loss is in addition to the fact that the lamp light output is not linearly proportional to the lamp power (i.e. it may take 75% lamp power to provide 50% lamp lumens.) Thus, conventional continuous dimming techniques can lead to premature lamp degradation or failure through undesirable lamp cooling and/or extinguishment unless additional cost is incurred for cathode heating to prevent premature lamp degradation caused by the dimming operation. Continuous dimming ballasts, moreover, suffer from reduced power efficiency. Thus, there is a need for improved fluorescent lamp dimming apparatus and techniques for efficiently providing varying lighting levels to match a desired lighting level while maintaining high efficiency and without lamp stress or damage or increased cost, thereby allowing a user to selectively achieve energy savings by dimming lighting installations. 
       SUMMARY OF THE DISCLOSURE 
       [0002]    Multi-lamp dimming ballasts and control methods are disclosed by which one or more of the above-mentioned deficiencies can be mitigated or overcome in driving fluorescent lamps. 
         [0003]    Dimming ballast embodiments are presented for operating multiple lamps, which include a DC power source receiving AC input power and providing DC electrical power, as well as a DC-AC converter that provides an AC output to drive the lamps and a controller operative to control power applied to the lamps. The controller implements dimming operation according to a dimming level setpoint by selectively dimming at most one of the lamps while controlling all the remaining lamps to be substantially at 0% or 100% power. The ballast may further provide a cathode heating circuit to selectively heat one or more cathodes of the lamp being dimmed according to the setpoint dimming level. 
         [0004]    In one embodiment, the controller selectively dims only a predetermined lamp while controlling all the remaining lamps to be substantially full on or off, so as to economize on cathode heating apparatus and dimming circuitry. In other embodiments, the controller selects one of the lamps for dimming operation and selectively dims only the selected lamp while controlling all the remaining lamps to be substantially on or off, where the selection can be by an algorithm such as random selection or round-robin selection in various embodiments. Certain embodiments of the ballast may provide a separate inverter for controlling the dimmed lamp, and may include a dedicated inverter to power each lamp. Further embodiments provide dimming at multiple predetermined levels according to the dimming level setpoint, where the controller selectively dims the selected lamp slowly in concert with selectively turning one or more of the other lamps on or off in order to smoothly transition between predetermined levels. 
         [0005]    Methods are disclosed for powering fluorescent lamps, including receiving a dimming level setpoint value or signal indicating a desired dimming level for the dimming ballast, and selectively dimming at most one of the lamps while controlling all the remaining lamps to be substantially at 0% or 100% power at least partially according to the dimming level setpoint. Embodiments of the method may further include receiving the dimming level setpoint value or signal indicating a desired one of a plurality of predetermined discrete levels for the dimming ballast, as well as dimming at most one of the lamps slowly in concert with selectively turning one or more of the other lamps on or off so as to smoothly transition between predetermined levels. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    One or more exemplary embodiments are set forth in the following detailed description and the drawings, in which: 
           [0007]      FIG. 1  is a schematic diagram illustrating an exemplary four-lamp dimming ballast with a controller that selectively dims at most one lamp while controlling all the remaining lamps to be substantially fully on or off based on a dimming level setpoint; 
           [0008]      FIG. 2  is a schematic diagram illustrating further details of an embodiment of the dimming ballast of  FIG. 1  in which the controller selectively dims only a predetermined lamp while controlling all the remaining lamps to be substantially full on or off with a dedicated cathode heating circuit for the predetermined lamp; 
           [0009]      FIG. 3  is a schematic diagram illustrating further details of another dimming ballast embodiment in which the controller selects one of the lamps for dimming operation and selectively dims only the selected lamp while controlling all the remaining lamps to be substantially on or off, including individual dimmable inverters and cathode heating circuits for each lamp; 
           [0010]      FIGS. 4 and 5  provide a flow diagram illustrating an exemplary method for powering fluorescent lamps; and 
           [0011]      FIGS. 6-16  are simplified schematic diagrams illustrating operation of the ballast embodiment of  FIGS. 1 and 3  for dimming at various exemplary levels. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0012]    Referring now to the drawings, where like reference numerals are used to refer to like elements throughout, and wherein the various features are not necessarily drawn to scale,  FIG. 1  illustrates a lighting system  100  including an exemplary dimming ballast  102  with a DC power source including a rectifier  110  and a DC-DC converter  120  to receive AC power at an input  104  and to provide DC electrical power  122  to a DC-AC converter  140 . The DC-AC converter  140  converts the DC power  122  to provide an AC output  106  driving two or more lamps  108 . The rectifier  110  in the illustrated embodiment receives the input AC  104  and provides an intermediate DC  112  to the converter  120 , which is a switching type DC-DC converter  120  in one implementation, where the converter  120  can be a boost converter with a power factor correction (PFC) component  121  to also control the power factor of the ballast  102 . In other possible embodiments, the intermediate DC-DC converter can be omitted. 
         [0013]    The ballast  102  includes a controller  150  operatively coupled with the DC-AC converter  140  to control power applied to the lamps  108 , and may also provide control signals to a dimming circuit  142  of the DC-AC converter  140 , as well as to a cathode heating circuit  170  for selective heating of one or more lamp cathodes. The controller  150  can be any suitable types of hardware, software, or combinations thereof, and includes a dimming control component  152  and a heat control component  154 . Controller  150  receives a dimming level setpoint  160 , such as a signal or value and operates to selectively dim at most one of the lamps  108  while controlling all the remaining lamps  108  to be substantially at 0% or 100% power based at least in part on the dimming level setpoint  160 . The on/off control of the other lamps  108  need not be strictly 100% and 0% of rated power, respectively, wherein the on state can be within 2-3% of rated and the off state can be up to 2-3% of rated power to constitute substantially 100% and substantially 0% as used herein. 
         [0014]    By only dimming a single lamp at any given time, cathode heating only needs to be applied to the dimmed lamp, thereby reducing the amount of energy expended on non-lighting functions in the ballast  102 . Moreover, only one of the lamps  108  is in a lower efficiency dimmed mode of operation, thereby increasing the overall efficiency of the ballast  102  compared with conventional continuous dimming approaches. In this regard, linear fluorescent lamps  108  are most energy efficient when operating near their rated power, and as the power into the lamp is reduced (e.g., during dimming), the lumens drop off faster than watts, such that the user is provided with greatly reduced light levels for only slightly reduced power consumption. The disclosed ballast  102  thus facilitates reduction in user lighting energy consumption without significant ballast cost impact. Furthermore, the ballast  102  provides continuous dimming capabilities, and thus allows finer adjustment resolution than discrete step-dimming systems. 
         [0015]      FIG. 2  illustrates an embodiment of the dimming ballast  102  in which the controller  150  selectively dims only a predetermined lamp  108   a  while controlling all the remaining lamps  108   b,    108   c,  and  108   d  to be substantially full on or off, with a dedicated cathode heating circuit  170  for the predetermined lamp  108   a.  This embodiment provides four inverters  146   a - 146   d  individually coupled to drive lamps  108   a - 108   d,  respectively, where the inverter  146   a  associated with the predetermined dimming lamp  108   a  includes dimming circuitry  142  operative to selectively reduce the output of the inverter  146   a,  and hence reduce the light output of the lamp  108   a  based at least in part on a 0-100% signal or value from the dimming control component  152  of the controller  150 . The remaining inverters  146   b - 146   d  are operated at or near 100% or 0% for on off control of the corresponding lamps  108   b - 108   d  according to on/off signals or values provided by the dimming control component  152 . 
         [0016]    The heat control component  154  of the controller  150  in this embodiment also provides a control signal or value to the cathode heating circuit  170  to selectively heat one or more cathodes of the dimming lamp  108   a  during all or a portion of the dimming operation to extend the life of the lamp  108   a.  The controller  150  may provide any suitable control signaling or messaging to the cathode heating circuit  170  to implement a heating function, which may but need not correlate with the 0-100% signal used to actuate the dimming circuit  142 , where the dimming control and heat control components  152  and  154  may implement different control algorithms based on the received dimming level setpoint  160 . The setpoint  160 , in this regard, may be an analog signal, such as a 0-10 v DC electrical signal set by a user whose voltage level represents the desired overall ballast light output amount, or may be a digital value communicated to the controller  150 , or may be any other suitable signal or value that indicating the desired light level. The controller  150  may be implemented as a processor-based system having a microprocessor, microcontroller, or other programmable or configurable processing or logic components, and the controller  150  and the components  152 ,  154  thereof can be implemented in software, firmware, or combinations of various hardware, software, firmware, etc., in a single control device  150  or in distributed fashion with one or more functions being implemented separately from others. 
         [0017]    In operation, the controller  150  receives the setpoint  160  and determines the on or off status of inverters  146   b - 146   d  based on the setpoint  160  to be at or below the desired light output value, and determines the amount of dimming for the inverter  146   a  to set the overall output of the ballast  102  to meet the setpoint amount. In this regard, for a given non-zero setpoint  160 , the controller  150  will provide the dimming control signals via component  152  so that all, some, or none of the inverters  146   b - 146   d  are on, and will control the dimming circuitry  142  so that the first inverter  146   a  powers the corresponding predetermined lamp  108   a  at 0-100% of its rated output. For example a dimming setpoint  160  having a value in the range of 75 to 100% light output, the controller  150  will dim the lamp  108   a  as needed to achieve that average light level while holding the other inverters  146   b - 146   d  on. For a desired setpoint of 75% light level, the inverter  146   a  is off (0%) with the other inverters  146   b - 146   d  on. For a setpoint between 50 and 75%, one of the three lamps  108   b - 108   d  is turned off, and the lamp  108   a  is dimmed to a level so that the average light level from the entire fixture is equal to the setpoint value. For the fully dim (0%) to 25% range of the setpoint  160 , the controller  150  turns the inverters  146   b - 146   d  off and operates the dimming circuit  142  to drive the lamp  108   a  between its full-bright and dimmest level. 
         [0018]    In this manner, the ballast  102  can achieve continuous dimming at any value of the setpoint  160  by selectively dimming only the lamp  108   a  while individually controlling all the remaining lamps  108   b - 108   d  to be substantially at 0% or 100% power. Other embodiments are possible in which two or more of the lamps  108   b - 108   d  are driven by a shared inverter with on/off control. For example, a single inverter  146  could drive lamps  108   c  and  108   d  with on/off capability controlled by the dimming component  152 , with another on/off controlled inverter  146  driving the lamp  108   b  and the dimming-capable inverter  146   a  driving the predetermined lamp  108   a  with selective cathode heating being provided for the lamp  108   a  via the heat control component  154  and the heating circuitry  170 . In other possible implementations, the cathode heating circuit  170  can be operable to selectively heat one or more cathodes of more than one of the lamps  108 . Moreover, the controller  150  in the embodiment of  FIG. 2  may be configured to provide dimming at a plurality of predetermined discrete levels according to the dimming level setpoint  160  (e.g. discrete dimming) and the controller ( 150 ) selectively dims the predetermined lamp  108   a  slowly in concert with selectively turning one or more of the other lamps  108   b - 108   d  on or off so as to smoothly transition between predetermined levels. 
         [0019]      FIGS. 3-16  illustrate another dimming ballast embodiment  102  ( FIG. 3 ) in which the controller  150  selects one of the lamps  108  for dimming operation and selectively dims only the selected lamp  108  while controlling all the remaining lamps  108  to be substantially on or off. In this implementation, the DC-AC converter  140  includes four individually dimmable inverters  146   a - 146   d,  each having dimming circuitry  142  that receives a 0%-100% control signal or value from the dimming control component  152  of the controller  150 . Moreover, the cathode heating circuitry  170  in this embodiment provides cathode heating circuits for each lamp  108  that are separately controllable. The dimming control component  152  of the controller  150  selects one of the lamps  108  for dimming operation at any given time and selectively dims only the selected lamp  108  while controlling all the remaining lamps  108  to be substantially at 0% or 100% power at least partially according to the dimming level setpoint  160 . Any suitable selection algorithm or scheme can be employed, preferably to distribute the dimming operation time among the lamps  108   a - 108   d,  such as random selection or round-robin selection, for example. 
         [0020]      FIGS. 4 and 5  depict a flow diagram illustrating an exemplary method  200  for powering fluorescent lamps, which may be implemented by the controller  150  in the ballasts  102  illustrated and described herein, and  FIGS. 6-16  illustrate operation of the ballast  102  of  FIGS. 1 and 3  for dimming at various exemplary levels of the setpoint  160 . While the method  200  is illustrated and described below in the form of a series of acts or events, it will be appreciated that the various methods of the disclosure are not limited by the illustrated ordering of such acts or events. In this regard, except as specifically provided hereinafter, some acts or events may occur in different order and/or concurrently with other acts or events apart from those illustrated and described herein in accordance with the disclosure. It is further noted that not all illustrated steps may be required to implement a process or method in accordance with the present disclosure, and one or more such acts may be combined. The illustrated methods and other methods of the disclosure may be implemented in hardware, software, or combinations thereof, such as in the exemplary controller  150  above, in order to provide the selective dimming control concepts illustrated and described herein. 
         [0021]    The method  200  begins in  FIG. 4  with receipt at  202  of a dimming level setpoint value or signal (e.g., setpoint  160  above) indicating a desired dimming level for the dimming ballast  102 . Selective dimming is then performed at  250  of at most one of the lamps  108  while controlling all the remaining lamps  108  to be substantially at 0% or 100% power at least partially according to the dimming level setpoint  160  received at  202 . In the example of  FIG. 4 , a determination is made at  204  as to whether the setpoint dimming level is less than 100%. If not (NO at  204 ), all lamps are turned on at  206  (exemplary ballast condition shown in  FIG. 6 ), and the process  200  returns to receive another setpoint at  202 . If the dimming level is below 100% (YES at  204 ), a determination is made at  210  as to whether the dimming level is between 75% and 100%. If so, three lamps are turned on and 1 lamp is dimmed at  212  (exemplary ballast conditions shown in  FIGS. 7 and 8 ) and the process  200  returns to receive another setpoint at  202 . However, if the dimming level is not between 75% and 100% (NO at  210 ), a determination is made at  214  as to whether the dimming level equals 75%. If so (YES at  214 ), three lamps are turned on and the other lamp is turned off at  216  (ballast condition shown in  FIG. 9 ) and the process  200  returns to receive another setpoint at  202 . If not (NO at  214 ), a determination is made at  220  as to whether the dimming level is between 50% and 75%. If so, two lamps are turned on, 1 lamp is turned off, and one lamp is dimmed at  222  (exemplary ballast conditions shown in  FIGS. 10 and 11 ) and the process  200  returns to receive another setpoint at  202 . 
         [0022]    If not (NO at  220 ), a determination is made at  224  as to whether the dimming level equals 50%. If so (YES at  224 ), two lamps are turned on and two lamps are turned off at  226  ( FIG. 12 ) and the process  200  returns to receive another setpoint at  202 . If the level is not equal to 50% (NO at  224 ), a determination is made at  230  as to whether the dimming level is between 25% and 50%, and if so, one lamp is turned on, two lamps are turned off, and 1 lamp is dimmed at  232  and the process  200  returns to receive another setpoint at  202 . If the dimming level is not between 25% and 50% (NO at  230 ), a determination is made at  234  as to whether the dimming level is equal to 25%. If so (YES at  234 ), one lamp is turned on and three lamps are turned off at  236  ( FIG. 13 ) and the process  200  returns to receive another setpoint at  202 . If the dimming level does not equal 25% (No at  234 ), the process  200  continues to  FIG. 5  with a determination being made at  240  as to whether the dimming level is between 0% and 25%. If so, three of the lamps are turned off and 1 lamp is dimmed at  242  (exemplary ballast conditions shown in  FIGS. 14 and 15 ) and the process  200  returns to receive another setpoint at  202  in  FIG. 4 . If not (NO at  240  in  FIG. 5 ), the dimming level is determined to be 0% at  244  and all lamps are turned off at  246  ( FIG. 16 ), after which the process  200  returns to  202  in  FIG. 4  to receive another setpoint  160 . 
         [0023]    Other embodiments of the method  200  are possible in which cathode heating is selectively provided to one or more cathodes of the lamp  108  being dimmed. In certain embodiments, moreover, receiving the dimming level setpoint value or signal at  202  may include receiving the dimming level setpoint value or signal  160  indicating a desired one of a plurality of predetermined discrete levels for the dimming ballast  102 . In this embodiment, the selective dimming at  250  may include selectively dimming at most one of the lamps  108  slowly in concert with selectively turning one or more of the other lamps  108  on or off so as to smoothly transition between predetermined levels. 
         [0024]    The exemplary ballasts  102  and method  200  facilitates maintenance of high fixture efficiency while not causing abrupt light level changes associated with conventional continuous and discrete dimming techniques. Various embodiments, moreover, provide for selective application of power to heat the cathodes of the dimmed lamps  108  in order to allow the dimmed lamp to operate to its rated life. The embodiments of  FIG. 3 , moreover, allows the controller  150  to vary which lamp which is dimmed for different light levels in order to even out any possible system effects on lamp life. These techniques, individually or in combination, provide for reduction in energy consumed by the ballast  102  compared to conventional dimming ballasts, and may further mitigate or avoid quick transients in lighting level when the dimming setpoint value is changed. 
         [0025]    The above examples are merely illustrative of several possible embodiments of various aspects of the present disclosure, wherein equivalent alterations and/or modifications will occur to others skilled in the art upon reading and understanding this specification and the annexed drawings. In particular regard to the various functions performed by the above described components (assemblies, devices, systems, circuits, and the like), the terms (including a reference to a “means”) used to describe such components are intended to correspond, unless otherwise indicated, to any component, such as hardware, software, or combinations thereof, which performs the specified function of the described component (i.e., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the illustrated implementations of the disclosure. Although a particular feature of the disclosure may have been illustrated and/or described with respect to only one of several implementations, such feature may be combined with one or more other features of the other implementations as may be desired and advantageous for any given or particular application. Furthermore, references to singular components or items are intended, unless otherwise specified, to encompass two or more such components or items. Also, to the extent that the terms “including”, “includes”, “having”, “has”, “with”, or variants thereof are used in the detailed description and/or in the claims, such terms are intended to be inclusive in a manner similar to the term “comprising”. The invention has been described with reference to the preferred embodiments. However, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations.