Abstract:
An electronic dimming ballast comprises a filament turn-off circuit for controlling the magnitudes of filament voltages supplied to the filaments of a gas discharge lamp. Each of a plurality of filament windings is directly coupled to one of the filaments and is operable to supply a small AC filament voltage to the filaments. The plurality of filament windings and a control winding are loosely magnetically coupled to a resonant inductor of an output circuit of the ballast. A controllably conductive device is coupled across the control winding. When the controllably conductive device is conductive, the voltage across the control winding and the filament windings falls to zero volts. The controllably conductive device is driven with a pulse-width modulated (PWM) signal so as to control the magnitudes of the filament voltages. The filament voltages are provided to the filaments before striking the lamp, and when dimming the lamp near low end.

Description:
RELATED APPLICATIONS  
       [0001]     This application claims priority from commonly-assigned U.S. Provisional Patent Application Ser. No. 60/748,861, filed Dec. 9, 2005, entitled APPARATUS AND METHOD FOR CONTROLLING THE FILAMENT VOLTAGE IN AN ELECTRONIC DIMMING BALLAST, the entire disclosure of which is hereby incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to electronic ballasts and, more particularly, to electronic dimming ballasts for gas discharge lamps, such as fluorescent lamps.  
         [0004]     2. Description of the Related Art  
         [0005]     The typical fluorescent lamp is a sealed glass tube with a rare earth gas and has an electrode at each end for striking and maintaining an electric arc through the gas. The electrodes are typically constructed as filaments to which a filament voltage is applied to heat the electrodes, thereby improving their capability to emit electrons. This results in improved electric arc stability and longer lamp life.  
         [0006]     Typical prior art ballasts apply the filament voltages to the filaments prior to striking the arc, and maintain the filament voltages throughout the entire dimming range of the lamp. At low end, when light levels are lowest and, consequently, the electric arc is at its lowest level, the filament voltages are essential for maintaining a stable arc current. However, at high end, when light levels are highest, and the electric arc current is at its highest level, the electric arc current contributes to heating the filaments. Consequently, the filament voltages are not essential for proper operation of the lamp at high end, and may be dispensed with. At high end, the filament voltages do not provide any benefit in maintaining the electric arc, and result in excessive power consumption and unwanted heat.  
         [0007]     An example of a prior art electronic dimming ballast  100  for driving three fluorescent lamps L 1 , L 2 , L 3  in parallel is shown in  FIG. 1 . Electronic ballasts typically can be analyzed as comprising a front end  110  and a back end  120 . The front end  110  typically includes a rectifier  130  for generating a rectified voltage from an alternating-current (AC) mains line voltage, and a filter circuit, for example, a valley-fill circuit  140 , for filtering the rectified voltage to produce a direct-current (DC) bus voltage. The valley-fill circuit  140  is coupled to the rectifier  130  through a diode  142  and includes one or more energy storage devices that selectively charge and discharge so as to fill the valleys between successive rectified voltage peaks to produce a substantially DC bus voltage. The DC bus voltage is the greater of either the rectified voltage or the voltage across the energy storage devices in the valley-fill circuit  140 .  
         [0008]     The back end  120  typically includes an inverter  150  for converting the DC bus voltage to a high-frequency AC voltage and an output circuit  160  comprising a resonant tank circuit for coupling the high-frequency AC voltage to the lamp electrodes. A balancing circuit  170  is provided in series with the three lamps L 1 , L 2 , L 3  to balance the currents through the lamps and to prevent any lamp from shining brighter or dimmer than the other lamps. A control circuit  180  generates drive signals to control the operation of the inverter  150  so as to provide a desired load current to the lamps L 1 , L 2 , L 3 . A power supply  182  is connected across the outputs of the rectifier  130  to provide a DC supply voltage, V CC , which is used to power the control circuit  180 .  
         [0009]      FIG. 2  shows a simplified schematic diagram of the back end  120  of a prior art dimming ballast for driving the lamps L 1 , L 2 , L 3  in parallel. As previously mentioned, the back end  120  includes the inverter  150  and the output circuit  160 . The inverter input terminals A, B are connected to the output of the valley-fill circuit  140 . The inverter  150  provides the high-frequency AC voltage for driving the lamps L 1 , L 2 , L 3  and includes series-connected first and second switching devices  252 ,  254 , for example, two field effect transistors (FETs). The control circuit  170  drives the FETs  252 ,  254  of the inverter using a complementary duty cycle switching mode of operation. This means that one, and only one, of the FETs  252 ,  254  is conducting at a given time. When the FET  252  is conducting, then the output of the inverter  150  is pulled upwardly toward the DC bus voltage. When the FET  254  is conducting, then the output of the inverter  150  is pulled downwardly toward circuit common.  
         [0010]     The output of the inverter  150  is connected to the output circuit  160  comprising a resonant inductor  262  and a resonant capacitor  264 . The output circuit  160  filters the output of the inverter  150  to supply an essentially sinusoidal voltage to the parallel-connected lamps L 1 , L 2 , L 3 . A DC blocking capacitor  266  prevents DC current from flowing through the lamps L 1 , L 2 , L 3 .  
         [0011]     Filament windings W 1 , W 2 , W 3 , W 4  are magnetically coupled to the resonant inductor  262  of the output circuit  160  and are directly coupled to the filaments of lamps L 1 , L 2 , L 3 . Because the lamps are being driven in parallel in  FIG. 2 , the windings W 1 , W 2 , W 3  are each provided to the filaments of different lamps and winding W 4  is provided to the filaments of all three lamps L 1 , L 2 , L 3 . The filament windings provide AC filament voltages, having magnitudes of approximately 3-5 V RMS , to the filaments to keep the filaments warm through the entire dimming range. The filaments especially need to be heated when the ballast is dimming the lamps to low end and during preheating of the filaments before striking the lamp. However, the prior art ballast  100  constantly provides the filament voltages to the filaments, which increases the power consumption of the ballast.  
         [0012]     Some prior art ballasts provide the filament voltages to the filaments of the lamps before striking the lamps, but then cuts off the filament voltages in order to reduce the power consumed by the ballast during normal operation. An example of such a ballast is described in greater detail in U.S. Pat. No. 5,973,455 to Mirskiy et al., issued Oct. 26, 1999, entitled ELECTRONIC BALLAST WITH FILAMENT CUT-OUT, the entire disclosure of which is incorporated herein by reference. The ballast includes an AC switch having a diode bridge defining two AC terminals and two DC terminals and having a transistor connected across the DC terminals. The primary winding of a filament transformer is connected across the AC terminals of the bridge. The transistor is coupled to a microprocessor for controlling the current through the primary winding of the filament transformer. The microprocessor is programmed to close the AC switch while the lamps are starting and to open the switch after the lamps are started, thereby cutting off the filament voltages from the lamps.  
         [0013]     However, in order to control the filament voltages, the ballast of Mirskiy et al. requires two magnetics: a first magnetic for coupling to the source of AC power and the second magnetic for coupling to the filaments. The requirement of two magnetics adds cost and requires control space in the ballast. Further, the ballast of Mirskiy et al. is only operable to turn off the filament voltage after the lamps have been struck and does not allow for control of the filament voltage throughout the dimming range of the ballast. Because of this, the ballast does not allow for a reduced power dissipation throughout the dimming range of the ballast.  
         [0014]     Thus, there exists a need for a ballast back end circuit that is operable to control the filament voltages provided to the filaments of the lamps that requires fewer parts, in particular, fewer magnetics. Also, there exists a need for a method of controlling the back end of a ballast in order to control the magnitude of the filament voltages provided to the filaments of the lamps throughout the dimming range of the ballast.  
       SUMMARY OF THE INVENTION  
       [0015]     According to the present invention, an electronic dimming ballast for driving a gas-discharge lamp having a plurality of filaments includes an output circuit operable to receive a high-frequency AC voltage. The ballast further comprises a plurality of filament windings magnetically coupled to an inductor of the output circuit. Each filament winding is connectable to one of the filaments of the lamp and operable to supply a small AC filament voltage to one of the plurality of filaments. The ballast further comprises a control winding magnetically coupled to the inductor. A controllably conductive device having a control input is coupled such that the controllably conductive device is operable to control a voltage across the control winding. A control circuit is coupled to the control input of the controllably conductive device and is operable to render the controllably conductive device conductive and non-conductive. When the controllably conductive device is non-conductive, the plurality of AC filament voltages each have a first magnitude. When the controllably conductive device is conductive, the plurality of AC filament voltages each have a second magnitude. In a preferred embodiment of the present invention, the controllably conductive device comprises a semiconductor switch coupled across the control winding. In addition, the second magnitude is preferably less than the first magnitude and substantially zero volts. Further, the control circuit is operable to drive the control input of the controllably conductive device with a pulse-width modulated (PWM) signal to control the magnitudes of the filament voltages.  
         [0016]     According to another embodiment of the present invention, an electronic ballast for driving a gas discharge lamp having a plurality of filaments comprises an output circuit operable to receive a high-frequency AC voltage, a plurality of filament windings, a filament turn-off circuit, and a control circuit. Each of the plurality of filament windings is connectable to one of the plurality of filaments of the lamp and operable to supply a small AC filament voltage to one of the plurality of filaments. The control circuit is operable to drive the filament turn-off circuit with a pulse-width modulated signal having a variable duty cycle to control the magnitude of each of the plurality of AC filament voltages.  
         [0017]     In addition, the present invention provides a circuit for an electronic ballast for controlling a plurality of AC filament voltages provided to a plurality of filaments of a gas discharge lamp. The circuit comprises a plurality of filament windings, a control winding, a controllably conductive device, and a control circuit. The plurality of filament windings and the control winding are magnetically coupled to a resonant inductor of the ballast. Each of the plurality of filament windings is operable to be connected to, and to provide a filament voltage to, one of the plurality of filaments of the lamp. The controllably conductive device has a control input and is coupled such that the controllably conductive device is operable to control a voltage across the control winding. The control circuit is coupled to the control input of the controllably conductive device and is operable to render the controllably conductive device conductive and non-conductive. Accordingly, when the controllably conductive device is non-conductive, the plurality of AC filament voltages each have a nominal magnitude, and when the controllably conductive device is conductive, the plurality of AC filament voltages each have a magnitude substantially less than the nominal magnitude.  
         [0018]     The present invention further provides a method for controlling a plurality of AC filament voltages provided to a plurality of filaments of a gas discharge lamp in an electronic ballast comprising an output circuit including an inductor. The method comprises the steps of magnetically coupling a plurality of filament windings to the inductor, connecting each of the filament windings to one of the plurality of filaments of the lamp, providing each of the plurality of AC filament voltages to one of the plurality of filaments, magnetically coupling a control winding to the inductor, and controlling a voltage across the control winding to control a magnitude of each of the plurality of AC filament voltages. In a preferred embodiment, the step of controlling a voltage across the control winding comprises the steps of coupling a controllably conductive device having a control input across the control winding such that the controllably conductive device is operable to control the voltage across the control winding, and controlling the controllably conductive device such that when the controllably conductive device is non-conductive, each of the plurality of AC filament voltages has a first magnitude, and when the controllably conductive device is conductive, each of the plurality of AC filament voltages has a second magnitude.  
         [0019]     According to another aspect of the present invention, a method for controlling a plurality of AC filament voltages provided to a plurality of filaments of a gas discharge lamp in an electronic ballast comprising an output circuit including an inductor comprises the steps of connecting each of the filament windings to one of the plurality of filaments of the lamp, providing each of the plurality of AC filament voltages to one of the plurality of filaments, coupling a filament turn-off circuit comprising a controllably conductive device to the output circuit, and driving the controllably conductive device with a pulse-width modulated signal to control the magnitude of each of the plurality of AC filament voltages.  
         [0020]     Other features and advantages of the present invention will become apparent from the following description of the invention that refers to the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]      FIG. 1  is a simplified block diagram of a prior art dimming ballast;  
         [0022]      FIG. 2  is a simplified schematic diagram of the back end of the prior art dimming ballast of  FIG. 1  for driving multiple lamps in parallel;  
         [0023]      FIG. 3  is a simplified block diagram of a ballast according to the present invention;  
         [0024]      FIG. 4  is a simplified schematic diagram of a ballast back end comprising a filament turn-off circuit according to a first embodiment of the present invention;  
         [0025]      FIG. 5A  is a top view of a bobbin of the ballast back end of  FIG. 4  with a ferrite core installed;  
         [0026]      FIG. 5B  is a top view of the bobbin of  FIG. 5A  without the ferrite core installed;  
         [0027]      FIG. 5C  is a perspective view of the bobbin of  FIG. 5A  without the ferrite core installed;  
         [0028]      FIG. 5D  is a plot of the magnitude of the filament voltage versus the dimming level of the ballast demonstrating a control scheme for linearly controlling the filament turn-off circuit of  FIG. 4 ;  
         [0029]      FIG. 5E  is a plot of the magnitude of the filament voltage versus the dimming level of the ballast demonstrating a simple control scheme for controlling the filament turn-off circuit of  FIG. 4 ;  
         [0030]      FIG. 6  is a simplified schematic diagram of a filament turn-off circuit according to a second embodiment of the present invention;  
         [0031]      FIG. 7  is a simplified plot of various voltage waveforms of the filament turn-off circuit of  FIG. 6 ; and  
         [0032]      FIG. 8  is a simplified schematic diagram a ballast back end comprising a filament turn-off circuit according to a third embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0033]     The foregoing summary, as well as the following detailed description of the preferred embodiments, is better understood when read in conjunction with the appended drawings. For the purposes of illustrating the invention, there is shown in the drawings an embodiment that is presently preferred, in which like numerals represent similar parts throughout the several views of the drawings, it being understood, however, that the invention is not limited to the specific methods and instrumentalities disclosed.  
         [0034]     Turning first to  FIG. 3 , there is shown a simplified block diagram of an electronic dimming ballast  300  according to the present invention. The ballast  300  includes many similar blocks as the prior art ballast  100  of  FIG. 1 , which have the same function as described previously. However, those components of the ballast  300  that differ from the prior art ballast  100  will be described in greater detail below.  
         [0035]     The ballast  300  comprises a back end  320  that includes an output stage  360  according to the present invention. A control circuit  380  provides a control signal to a filament turn-off circuit  390  to control when the filament voltages are provided to the lamps L 1 , L 2 , L 3  and to control the magnitude of the filament voltages. The filament turn-off circuit  390  accordingly controls the output circuit  360  in response to the control signal from the control circuit  380 . The control circuit  380  may comprise an analog circuit or any suitable processing device, such as a programmable logic device (PLD), a microcontroller, a microprocessor, or an application specific integrated circuit (ASIC).  
         [0036]     Referring to  FIG. 4 , there is shown a simplified schematic diagram of the back end  320  of the ballast  300  according to a first embodiment of the present invention. The output circuit  360  includes a resonant inductor  462 , a resonant capacitor  464 , and a DC blocking capacitor  466 . The lamps L 1 , L 2 , L 3  and the balancing circuit  170  are coupled across the resonant capacitor  464 . The filament windings W 1 , W 2 , W 3 , W 4  are magnetically coupled to the resonant inductor  462  and directly coupled to the lamps L 1 , L 2 , L 3  to provide the filament voltages to the lamps (in the same manner as shown in  FIG. 2 ). A control winding W 5  is also magnetically coupled to the resonant inductor  462 .  
         [0037]     Note that all windings W 1 , W 2 , W 3 , W 4 , W 5  are loosely coupled to the resonant inductor  462 , such that if any of the windings are electrically shorted, the inductance of the resonant inductor is not greatly affected. For example, if the nominal inductance of the resonant inductor  462  is 470 μH, the inductance preferably shifts no more than approximately 30 μH—to 440 μH—when the control winding W 5  is shorted. This approximately 6.4% change in inductance does not significantly alter the inductance of the resonant inductor  462  or the operation of the output circuit  360 .  
         [0038]     Preferably, the resonant inductor  462 , the filament windings W 1 , W 2 , W 3 , W 4 , and the control winding W 5  are wound on a single bobbin  560 .  FIG. 5A  is a top view of the bobbin  560  with a ferrite core  562  installed.  FIG. 5B  is a top view and  FIG. 5C  is a perspective view of the bobbin  560  without the ferrite core  562  installed. The bobbin  560  comprises a first bay  564  around which the wire (not shown) of the resonant inductor  462  is wound. The windings W 1 , W 2 , W 3 , W 4 , W 5  (not shown in  FIGS. 5A-5C ) are all wound in a second bay  566 . The bobbin  560  comprises a spacing  568  between the first bay  564  and the second bay  566 . The spacing  568  allows the windings W 1 , W 2 , W 3 , W 4 , WS to be loosely magnetically coupled to the resonant inductor  462 .  
         [0039]     Referring back to  FIG. 4 , the filament voltage turn-off circuit  390  is coupled across the control winding WS and includes a controllably conductive device, for example, a FET  492  in a full-wave rectifier bridge  494 , which comprises four diodes. Alternatively, the filament voltage turn-off circuit may be a relay or any type of bidirectional semiconductor switch, such as two FETs in anti-series connection. Also alternatively, the controllably conductive device may be a bipolar junction transistor (BJT), an insulated gate bipolar transistor (IGBT), or some such similar controllable switching device. The FET  492  has a control input that is coupled to the control circuit  380  and is utilized to render the FET conductive or non-conductive. When the FET  492  is non-conductive, current is not able to flow through the control winding WS. This allows the filament windings W 1 , W 2 , W 3 , W 4  to operate normally and to provide the filament voltages to the filaments of the lamps L 1 , L 2 , L 3  in the same manner as the prior art ballast  100 . However, when the FET  492  is conductive, the filament voltage turn-off circuit  390  essentially electrically shorts out the control winding W 5 , i.e., the voltage across the control winding WS is substantially zero volts. This in turn collapses the filament voltages across windings W 1 , W 2 , W 3 , W 4  to substantially low voltages, e.g., preferably substantially zero volts. Since the windings are loosely coupled to the resonant inductor  462 , this operation does not significantly affect the inductance of the resonant inductor  462  and the operation of the ballast  300 .  
         [0040]     As previously mentioned, the filaments of the lamps L 1 , L 2 , L 3  need to be heated prior to striking the lamps and when dimming to a low light intensity. To strike the lamps L 1 , L 2 , L 3 , the control circuit  380  first preheats the filaments of the lamps by driving the FETs  252 ,  254  of the inverter  150  at a high frequency (e.g., approximately 100 kHz). This causes a large voltage to develop across the resonant inductor  462 , while a smaller voltage, which is not great enough to strike the lamps L 1 , L 2 , L 3 , develops across the resonant capacitor  494 . At this time, the control circuit  380  drives the FET  492  to be non-conductive, such that the filament voltages are provided to the filaments of the lamps L 1 , L 2 , L 3 .  
         [0041]     After a predetermined period of time, the control circuit  380  reduces the operating frequency of the FETs  252 ,  254  to close to the resonant frequency of the output circuit  360  (e.g., 70 kHz), which increases the voltage across the resonant capacitor  464  to strike the lamps L 1 , L 2 , L 3 . Since a voltage is still produced across the resonant inductor  462 , the filament voltages will continue to be provided to the lamps. After the lamps L 1 , L 2 , L 3  are operating normally, the control circuit  380  is operable to cause the FET  492  to conduct, which removes (or reduces) the filament voltages from the filaments of the lamps.  
         [0042]     Further, the control circuit  380  is operable to drive the FET  492  with a pulse-width modulated (PWM) signal in order to obtain different magnitudes of the filament voltages on the filament windings W 1 , W 2 , W 3 , W 4 . This allows the control circuit  380  to reduce magnitude of the filament voltages—and the power consumption of the ballast—without completely removing the filament voltages from the filaments of the lamps. For example, when dimming a lamp to the midpoint of the dimming range, some heating of the filaments is required. However, at this point, it may not be necessary to provide the maximum filament voltage to the filaments, so a filament voltage having a magnitude less than the maximum filament voltage may be provided to the filaments.  
         [0043]     The magnitude of a filament voltage is dependent on the duty cycle of the PWM signal, e.g., inversely proportional to the duty cycle. The control circuit  380  is operable to control the duty cycle of the PWM signal in order to vary the magnitude of the filament voltage between the maximum filament voltage (typically about 3-5 V RMS ) and zero volts. The frequency of the PWM signal is preferably about 25 kHz, which is above the audible frequency range. However, the frequency of the PWM signal is not limited to 25 kHz, but may range up to or greater than the operating frequency of the back end  320  of the ballast  300 .  
         [0044]     Accordingly, the magnitudes of the filament voltages can be controlled throughout the dimming range of the ballast  300 .  FIG. 5D  shows a plot of the magnitude of the filament voltage versus the dimming level of the ballast, which demonstrates a possible control scheme for controlling the filament voltage. The magnitude of the filament voltage is held constant at five volts when the dimming level is below a first threshold TH 1  (e.g., 30% in  FIG. 5D ) and is held constant at zero when the dimming level is above a second threshold TH 2  (e.g., 80% in  FIG. 5D ). Between the first and second thresholds, the magnitude of the filament voltage is linearly changed from approximately five volts to approximately zero volts. However, the present invention is not limited to using a linear function. Alternatively, a piece-wise step function or a complex curve may be used to decrease the magnitude of the filament voltage as the dimming level increases.  
         [0045]      FIG. 5E  shows a plot of the magnitude of the filament voltage versus the dimming level of the ballast showing a simple control scheme of the filament voltage. The filament voltage is simply turned off near the high end of the dimming range of the ballast. When the dimming level is below a threshold TH 3  (e.g., 80% in  FIG. 5E ), the filament voltages are held constant at an on-magnitude of approximately five volts RMS, and when the dimming level is above the threshold, the filament voltages are held constant at an off-magnitude of approximately zero volts. When the dimming level is changed such that the dimming level crosses the threshold, the magnitude of the filament voltages is stepped from the on-magnitude to the off-magnitude, or vice versa. Preferably, the filament voltages are “faded”, i.e., continuously varied over a period of time from the on-magnitude to the off-magnitude (and vice versa), to avoid a step response of the lamp current through the lamps, which can cause a visible flickering of the lamps. The fading occurs over an appropriate amount of time that allows a control loop of the control circuit to properly regulate the current to the lighting load without causing a visible flickering. For example, if the control loop has a response time of 2 msec, the fading preferably occurs over a time period of about 500 msec.  
         [0046]      FIG. 6  shows a simplified schematic diagram of a filament turn-off circuit  690  according to a second embodiment of the present invention. Once again, the filament turn-off circuit  690  is coupled across the additional winding W 5  of the output circuit  360  and is operable to control the voltage across the control winding to substantially zero volts. The filament turn-off circuit  690  comprises a FET  692  in a rectifier bridge  694 . A saw-tooth waveform generator  695  produces a triangle wave V TR1  at the frequency of the PWM signal, i.e., preferably 25 kHz, as shown in  FIG. 7 ( a ). For this embodiment, the control circuit  380  is operable to provide a DC control voltage V DC , shown in  FIG. 7 ( a ), to the filament turn-off circuit  690 . The triangle wave V TR1  is provided to the negative input of a comparator  696  and the DC control voltage V DC  is provided to the positive input. When the triangle wave V TR1  is less than the DC control voltage V DC , the output of the comparator  696  will be pulled “high”, i.e. to approximately the magnitude of the DC supply voltage V CC  of the power supply  182 . When the triangle wave V TR1  is greater than the DC control voltage V DC , the output of the comparator  696  will be pulled “low”, i.e., to approximately zero volts. Thus, the comparator  696  generates a PWM signal V PWM , shown in  FIG. 7 ( b ), which has a duty cycle that is dependent on the magnitude of the DC control voltage V DC .  
         [0047]     Accordingly, the comparator  696  is operable to drive the FET  692  with the PWM signal V PWM  in response to the DC control voltage V DC . However, the frequency of the PWM signal (e.g., 25 kHz) and the frequency of the current that flows through the FET  692  when the FET is conductive (e.g., 70 kHz during normal operation of the ballast  300 ) are typically not the same. Therefore, when the PWM signal transitions from high to low, the current through the FET  692  is most likely not near zero amps. It is not desirable to cause the FET  692  to stop conducting when current through the FET has a substantially large magnitude, since this can cause large voltage spikes across the control winding W 5  and damage the FET  692  and the filaments of the lamps L 1 , L 2 , L 3 .  
         [0048]     Thus, the filament turn-off circuit  690  comprises additional circuitry to cause the FET  692  to stop conducting when the current through the FET is substantially zero amps. A resistor  697  is coupled in series with the FET  692  in the rectifier bridge  694 . A zero-cross detect circuit  698  is coupled to the resistor  697  and is operable to determine when the voltage across the resistor  697  is substantially zero volts, i.e., when the current through the FET  692  is substantially zero amps. The zero-cross detect circuit  698  provides a zero-cross signal, V ZC , shown in  FIG. 7 ( c ), which has negative pulses that correspond to the zero-crossings of the current through the FET  692 .  
         [0049]     The output of the comparator  696 , i.e., the PWM signal V PWM , is provided to the active-high data input D and the active-low reset input RST of a flip-flop  699 . The zero-cross signal V ZC  is provided to the active-low clock input CLK of the flip-flop  699 . A FET drive signal V DRIVE , shown in  FIG. 6 ( d ), is produced at the negative output  Q  of the flip-flop  699  and is coupled to the gate of the FET  692 . When the reset input RST is low, the flip-flop  699  will provide a high voltage at the negative output  Q . For the flip-flop  699  to drive the negative output  Q  low, both the data input D and the reset input RST must be high when the clock input CLK receives a high-to-low transition. Thus, after the PWM signal VPWM transitions from low to high, the flip-flop  699  “holds” the negative output  Q  high until a negative pulse occurs on the zero-cross waveform V ZC . When a negative pulse occurs on the zero-cross waveform V ZC , the flip-flop  699  drives the negative output  Q  low. Hence, the FET drive signal V DRIVE  does not transition from high to low, i.e., does not cause the FET to stop conducting, until the current through the FET  692  is substantially zero amps.  
         [0050]      FIG. 8  shows a simplified schematic diagram of a back end  820  according to a third embodiment of the present invention. An output circuit  860  includes a tapped winding W 6 , which is coupled to a filament voltage turn-off circuit  890 . The filament voltage turn-off circuit  890  comprises a FET  892  having a drain terminal coupled to circuit common and the tap of the tapped winding W 6  and a source terminal coupled a first end of the tapped winding through a first diode  894 A and to a second end of the tapped winding through a second diode  894 B. The control input of the FET  892  is coupled to the control circuit  380 . When the FET  892  is non-conductive, the filament windings W 1 , W 2 , W 3 , W 4  operate normally and provide the filament voltages to the filaments of the lamps L 1 , L 2 , L 3 . When the FET  892  is conductive, a current flows through the first end of the tapped winding and the first diode  894 A during the positive half-cycles, and through the second end of the tapped winding and a second diode  894 B during the negative half-cycles. The total resulting voltage across the tapped winding, i.e., from the first end to the second end, is substantially zero volts. Accordingly, when the FET  892  is conductive, the filament voltages across the windings W 1 , W 2 , W 3 , W 4  are substantially zero volts.  
         [0051]     Although the present invention has been described in relation to particular embodiments thereof, many other variations and modifications and other uses will become apparent to those skilled in the art. It is preferred, therefore, that the present invention be limited not by the specific disclosure herein, but only by the appended claims.