Abstract:
An improved high input voltage instant start electronic ballast uses a substantially lossless snubber circuit. The substantially lossless snubber circuit is incorporated into the ballast to reduce turn off losses and increase the efficiency of the ballast. The snubber circuit includes two capacitors connected in parallel with respect to the two switching transistors or FETS in the inverter of the ballast. A series-resonant lamp voltage sensing circuit is also provided that uses a voltage dividing capacitor to accomplish lossless monitoring of the open circuit voltage of the ballast. A cable compensation circuit minimizes variations in the open circuit voltage due to the connecting and disconnecting of a cable to the ballast by limiting the turn on times of the transistors during high voltage conditions.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS  
       [0001]     This application is a Non-Provisional Utility application which claims benefit of co-pending U.S. Patent Application Ser. No. 60/526,638 filed Dec. 3, 2003, entitled “High Input Voltage Microcontroller Based Instant Start Ballast” which is hereby incorporated by reference. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0002]     Not Applicable  
       REFERENCE TO SEQUENCE LISTING OR COMPUTER PROGRAM LISTING APPENDIX  
       [0003]     Not Applicable  
       BACKGROUND OF THE INVENTION  
       [0004]     One problem with prior art electronic ballasts is that the open circuit voltage of an instant-start ballast needs to be controlled when there is not a lamp coupled to the ballast. Unfortunately, prior art methods of providing this open circuit voltage control cause substantial variations in the open circuit voltage when used in conjunction with different lengths of cable, or require a high value resonant capacitor which results in a high circulating current. A high circulating current is undesirable in that it increases the conduction losses in the ballast and may result in damaging capacitive mode switching occurring during the striking transients. Therefore, an improved method and apparatus for controlling the open circuit voltage of a high input voltage electronic ballast without increasing the switching losses or creating high value circulating currents is needed.  
         [0005]     In some prior art ballasts, the voltage on the lamp voltage sensing resistor is used to control the open circuit voltage during striking when no lamp is connected. To accomplish this, the pulse width of one switch of the half bridge is typically controlled. Controlling the pulse width controls the open circuit voltage indirectly by using inductor current to control the voltage on the capacitor. As a result, large open circuit voltage variations often result when external connections to the fixture, such as a connecting cable, add extra capacitance. In ballast implementations that can afford to use a large resonant capacitor and a small inductor, the open circuit voltage variation problem is generally not too significant. However, potentially damaging hard switching or capacitive mode switching is often observed in these high capacitance types of prior art open circuit voltage controlled ballasts. Furthermore, the use of a large resonant capacitor makes the resonant tank difficult to design. As a result, these types of ballasts suffer from more conduction losses and/or hard switching during the striking of the lamp than do typical ballasts. Conduction losses and hard switching are undesirable in that they may cause the ballast to fail. A large resonant capacitor, with a striking voltage of two lamps across it, stores a substantial amount of energy. When the striking attempt occurs when there is no load, the striking energy is transferred to the resonant inductor and can saturate the inductor. The result is undesirable hard switching occurring during the striking. Even though a MOSFET can survive the high stress transients in ballasts with a 460V bulk voltage, hard switching is undesirable and should be avoided if possible. Furthermore, for some types of ballasts, it is critically important to avoid hard switching due to their particular susceptibility to damage from transients. Thus, in many of the prior art ballasts, the resonant capacitor value is minimized and a cable compensation circuit is utilized to control the open circuit voltage such that it is constant with various lengths of connected cable attached having varying amounts of capacitance. However, these circuits are often complex and decrease the efficiency, while increasing the cost, of the ballast. Therefore, an improved method and apparatus for controlling the open circuit voltage of a ballast and compensating for any attached cables is needed.  
         [0006]     Therefore what is needed is a new and improved electronic ballast that overcomes the above mentioned deficiencies of the prior art.  
       BRIEF SUMMARY OF THE INVENTION  
       [0007]     A preferred embodiment of the present invention is directed toward an electronic ballast for producing an output voltage on a pair of output terminals for igniting and powering a gas discharge lamp connected between the output terminals. The ballast includes an inverter having a pair of transistors. A snubber circuit reduces turn off losses in the transistors. The snubber circuit includes a pair of series connected snubber capacitors connected in parallel with the pair of transistors. An extended dead time is created between gating signals of the pair of transistors to allow the snubber capacitors to discharge. The electronic ballast includes a resonant tank having a series connected tank inductor and tank capacitor and an output voltage sensing circuit that senses an output voltage of the ballast by sensing a voltage across a sampling capacitor connected in series with the tank capacitor. An open circuit voltage control circuit is also preferably included that controls a voltage across the output terminals when a gas discharge lamp is not connected between the output terminals. The open circuit voltage control circuit includes a resistor connected in series with a tank capacitor of the ballast. A cable compensation circuit is also preferably included that limits variations in the output voltage of the electronic ballast due to a cable being connected to the output terminals of the ballast. The cable compensation circuit limits variations in the output voltage by altering the gating signals provided to the transistors.  
         [0008]     Another embodiment of the present invention is directed toward an electronic ballast for providing an output voltage on a pair of output terminals for use in powering a gas discharge lamp. The ballast includes an inverter circuit having a first transistor and a second transistor and a resonant tank having a tank capacitor and a tank inductor. A substantially lossless snubber circuit reduces turn-off losses in the first and second transistors of the inverter. The snubber circuit includes a snubber capacitor connected in parallel with each of the first transistor and the second transistor. A microcontroller provides gating signals to the transistors such that an extended dead time is created between the gating signals of the transistors to allow the snubber capacitors to discharge. An open circuit voltage control circuit controls a voltage across the output terminals of the ballast when a gas discharge lamp is not connected between the output terminals. A sampling capacitor connected in series with the tank capacitor wherein an output voltage of the ballast is sampled across the sampling capacitor.  
         [0009]     Yet another embodiment of the present invention is directed toward an electronic ballast having a half-bridge inverter circuit that includes a pair of transistors and a pair of capacitors. Each capacitor is connected in parallel with one of the transistors. A microcontroller generates transistor switching control signals that cause the transistors to switch on and off at a rate that allows the capacitors to reduce turn off losses in the transistors. This is preferably accomplished by creating an extended dead time between the gating signals of the pair of transistors that allows the capacitors to discharge. The electronic ballast has a resonant tank having a series connected tank inductor and tank capacitor. A sampling capacitor is connected in series with the tank capacitor wherein an output voltage of the ballast is sampled across the sampling capacitor. An open circuit voltage control circuit is also included that controls a voltage across the output terminals of the ballast when a gas discharge lamp is not connected between the output terminals. The open circuit voltage control circuit has a resistor connected in series with the sampling capacitor and the voltage across the resistor is used to limit the output voltage of the electronic ballast. A cable compensation circuit is also preferably provided to limit variations in the output voltage due to cables being connected to outputs of the ballast.  
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
       [0010]      FIG. 1  is a schematic diagram of a lossless snubber circuit constructed in accordance with a preferred embodiment of the present invention;  
         [0011]      FIG. 2  is a schematic diagram of a lossless lamp voltage sampling circuit having a lossless snubber circuit constructed in accordance with a preferred embodiment of the present invention;  
         [0012]      FIG. 3  is a schematic diagram of a hybrid sampling circuit having a lossless snubber circuit constructed in accordance with the present invention; and  
         [0013]      FIG. 4  is a schematic diagram of a cable compensation circuit having a lossless snubber circuit constructed in accordance with a preferred embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0014]     A preferred embodiment of the present invention is directed toward an instant start electronic ballast for a gas discharge lamp having a substantially lossless snubber circuit for reducing the turn off loses of the transistors in the inverter of the ballast. For voltage-fed, series-resonant, half-bridge inverters, the turning-on of the FETS or transistors involves zero voltage switching, but the turning-off of the FETS and transistors involves hard switching. For most ballasts, the turning-off current is small so that losses associated with the turning-off are not significant. However, for an Instant Start—High Range Voltage ballast, the current at turn-off is near its peak and, thus, the turn-off losses are relatively high. Furthermore, due to the larger die size of the high voltage FETS, more gate charge has to be removed from the gates before they can be turned-off. This increased gate charge increases the turn-off losses for a high voltage inverter.  
         [0015]     A simplified schematic of an electronic ballast  2  constructed in accordance with a preferred embodiment of the present invention is shown in  FIG. 1 . The electronic ballast consists of a bulk DC voltage stage  4  that provides a relatively constant DC voltage to the inverting transistors  6  and  8 . In a typical fluorescent light ballast, the bulk DC voltage stage  4  includes a bridge rectifier that converts a standard AC supply voltage into a DC voltage. However, it will be readily appreciated by those skilled in the art that a variety of power sources may be utilized to provide a DC voltage.  
         [0016]     A resonant tank comprised of an inductor  10  and capacitor  12  is used to filter the output of the inverting transistors  6  and  8  and provide the filter power signals to the lamp  14  which is represented by a resistor  14  in  FIG. 1 . To reduce the turn-off losses associated with the transistors  6  and  8 , two snubber capacitors  16  and  18  are connected in parallel with the transistors  6  and  8  of a preferred ballast  2  of the present invention as shown in  FIG. 1 . In the normal case, the capacitors  16  and  18  reduce the turn-off losses associated with the switches  6  and  8 . However, all of the energy stored in the capacitors  16  and  18  when the switches  6  and  8  are turned off, will be dissipated in the switches  6  and  8  at the turn-on. Thus, in a preferred embodiment such as an IHRV ballast, an extended dead time that allows the capacitors  16  and  18  to discharge is created between the gating signals of the transistors  6  and  8  to deal with this problem. Since the load current flowing through the lamp  14  is highly inductive during this extended dead time, the load  14  current will discharge the snubber capacitors  16  and  18  during the extended dead time. Thus, at the turn-on, there are no switching losses in the transistors  6  and  8  of a preferred embodiment of the present invention. Furthermore, at turn-off, the switching losses are also completely removed through the use of capacitors  16  and  18 . As a result, there are substantially no switching losses in the inverter of the ballast and the use of a 770V half bridge inverter becomes economically feasible. However, in order to use the lossless snubber capacitors  16  and  18  of  FIG. 1 , the amplitude of the load  14  current should be high at the turn-off and the dead time should be large enough to allow the snubber capacitors  16  and  18  to be discharged. The length of the dead time is adjusted by controlling the gating signals provided from the microcontroller  11  to the transistors  6  and  8 . As will be appreciated by those skilled in the art, the time required for the snubber capacitors  16  and  18  to discharge will depend upon the capacitance of the particular capacitors  16  and  18  and the amount of charge stored on the capacitors  16  and  18 . Alternatively, only one capacitor can be used instead of the two capacitors  16  and  18 . However, the use of a single capacitor may be disadvantageous in that, whenever there is not enough load current to discharge the capacitor, the energy stored in the capacitor will be dissipated in the transistor or FET connected in parallel with it. Thus, if there is only one capacitor, then the dissipated energy is concentrated in only one FET or transistor. With two capacitors  16  and  18 , as shown in  FIG. 1 , the voltage stress is substantially equally distributed across both switches  6  and  8  and, thus, the reliability and robustness of the ballast  2  is increased.  
         [0017]     Referring now to  FIG. 2 , an electronic ballast  30  with a series resonant tank that utilizes lossless sampling of the lamp voltage  34  in conjunction with the lossless snubber capacitors  36  and  38  of a preferred embodiment of the present invention is shown. The electronic ballast  30  includes a bulk DC voltage source  32  that provides power to the inverter circuit transistors  46  and  48 . The series resonant tank is comprised of a resonant tank inductor  40  and a resonant tank capacitor  42 . Prior art circuits use a resistor connected in series with the resonant capacitor  42  to sense the lamp voltage  32  and control the open circuit voltage. However, in the newly developed circuit of a preferred embodiment of the present invention for an IHRV ballast and/or sign ballast, the lamp voltage  34  is sensed by a sampling capacitor  44  connected in series with the resonant capacitor  42  as shown in  FIG. 2 . Using the principle of voltage division with capacitors, when the sampling capacitor  44  is much bigger than the resonant capacitor  42 , the voltage drop on the sampling capacitor  44  is very small and vice versa. This is beneficial in that it is relatively easy to find a film capacitor  44  that has a small package size and is relatively inexpensive. Most preferably, the capacitor&#39;s  44  values are 330 nF 60V or 680 nF 60V. A sampling circuit comprised of capacitors  50  and  52  and resistors  54  and  56  is used to sample the voltage on capacitor  44 . The sampling circuit of  FIG. 2  provides a low output impedance, a strong signal with excellent signal to noise ratio and a quick response time to an A/D converter input of an associated microcontroller. Thus, the circuit of  FIG. 2  uses lossless snubber capacitors  46  and  48  and capacitor  44  based voltage division to improve the efficiency of the ballast  30  without sacrificing performance.  
         [0018]     The sampling capacitor  44  used in the ballast of  FIG. 2  can also be connected in series with a low value resistor  60 , which can be used to control the open circuit voltage  34  as shown in  FIG. 3 . The hybrid sampling circuit shown in  FIG. 3  samples a large amplitude version of the lamp voltage  34  across a capacitor  44 . The sampled signal is smoothed by RC filters formed by capacitors  50  and  52  and resistors  54  and  56  and then fed to the A/D converter of the microcontroller. The response of the lamp voltage is not fast in the circuit of  FIG. 3 , but it is almost entirely lossless. For open circuit voltage control, the amplitude of the voltage across resistor  60  is large enough at the striking to turn on transistor  66  to trim the pulse width of the gating signal of the upper switch  46  of the half bridge. Trimming the pulse width of the gating signal of the upper switch  46  controls the open circuit voltage. However, during steady state operation, the voltage on the resistor  60  is very small, out of phase with the voltage on capacitor  44 , and still proportional to the lamp voltage  34 . Hence, the lamp voltage  34  sensing is not affected by the resistor  60  during steady state operation.  
         [0019]     The sampling circuit described above with respect to  FIG. 3  can be used independently without a cable compensation circuit. Since the voltage on resistor  60  is in phase with the current of the upper switch  46 , it is convenient to use it to control the open circuit voltage when no lamp is connected and to trim the pulse width of the upper switch  46  of the half-bridge as discussed above. However, when a long cable is connected and the capacitance of the cable is essentially in parallel with the resonant capacitor  42 , the parameters of the resonant tank circuit constructed from inductor  40  and capacitor  42  are changed. As the result, the open circuit voltage  34  decreases when a cable is connected to the output terminals of the electronic ballast. When the value of the resonant capacitor  34  is small, the decrease in the open circuit voltage  34  is significant and the ballast will not strike the lamp. The open circuit voltage can be set high to start a lamp with a long cable. However, in applications where no cable is attached, the open circuit voltage is then too high, which may cause the ballast to fail the through-lamp leakage test, or damage the film capacitor  44 . Increasing the capacitance of the resonant capacitor  42  helps to decrease the variation of the open circuit voltage but increases the conduction losses due to the circulation currents in the resonant capacitors. Furthermore, larger capacitor values lead to saturation of the resonant inductor  40 . Therefore preferred embodiments of the present invention include a cable compensation circuit.  
         [0020]     Capacitor sampling provides a strong sample signal with low output impedance and quick response. A cable compensation circuit is created by adding zener diode  70 , resistors  72  and  76 , and capacitor  74  to the circuit of  FIG. 3  as set forth in  FIG. 4 . The open circuit voltage  34  as sampled by capacitor  44  rises very rapidly at node  68 . When the open circuit voltage  34  becomes too high, the zener diode  52  starts to conduct and feeds current to the base of transistor  66  such that the conductive threshold for the transistor  66  is decreased. Thus, the transistor  66  starts to turn-on earlier when the voltage on resistor  60  is lower. The pulse width of the gating signal of the upper switch  46  then becomes narrower. The true open circuit voltage is sensed in this way to change the current threshold required to turn-off the switch  46 . In an exemplary circuit constructed as described above, the open circuit voltage varies from 1.9 kV to 2.6 kV without the cable compensation circuit when 18 feet of cable is connected to or removed from the circuit. However, with the cable compensation circuit of  FIG. 4 , the variation in the open circuit voltage is within approximately 100V. Thus, an electronic ballast having lossless snubber capacitors and a cable compensation circuit in accordance with the embodiment of the present invention shown in  FIG. 4  represents a substantial improvement upon the prior art.  
         [0021]     Thus, although there have been described particular embodiments of the present invention of a new and useful Lossless Snubber Capacitor Circuit, it is not intended that such references be construed as limitations upon the scope of this invention except as set forth in the following claims.