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Electronic ballast with lossless snubber capacitor circuit - Universal Lighting Technologies, Inc. An improved high input voltage instant start electronic ballast uses a substantially lossless snubber circuit. EP00617301982-10-06Transistor inverter device.EP05769911994-01-05Control apparatus of fluorescent lamp. STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENTNot ApplicableREFERENCE TO SEQUENCE LISTING OR COMPUTER PROGRAM LISTING APPENDIXNot ApplicableBACKGROUND OF THE INVENTIONOne 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. BACKGROUND OF THE INVENTIONThis invention relates generally to gas turbine engines, and more specifically to bleed scoops used in gas turbine engine high-pressure compressors.At least some known gas turbine engines include a compressor, a combustor, and at least one turbine. Jay uses his years of racing experiece and combines it with some of the most breathtaking courses and trails anywhere ensuring a memorable racing experience for both beginning and advanced runners! Abby Broughton moved to Teton Valley at age ten and spent her days skiing, playing soccer, and playing in the Teton High School Pep band. Abby was a 4-time US National Rowing team member and achieved a bronze and silver medal at the 2009 and 2010 World Rowing Championships, as well as a gold in the 2010 World Cup. Char O'brien grew up in upstate New York and moved here to explore the wild west about 15 years ago. Currently Char teaches reading at the Alta school, Fitness classes at Dreamchasers, and is a facilitator for Parenting with Love and Logic. Chris Heilman, PhD, ATC, CSCS is a ski, runner, and outdoor enthusiast who empowers athletes and exercisers to achieve a higher level of performance, health, and well-being. The substantially lossless snubber circuit is incorporated into the ballast to reduce turn off losses and increase the efficiency of the ballast. The electronic ballast of claim 1 wherein an extended dead time is created between gating signals of said pair of transistors to allow said snubber capacitors to discharge.
The electronic ballast of claim 1 further comprising an open circuit voltage control circuit that controls a voltage across said output terminals when said gas discharge lamp is not connected between said output terminals.
The electronic ballast of claim 3 wherein said open circuit voltage control circuit includes a resistor connected in series with a tank capacitor of said ballast. The electronic ballast of claim 1 further comprising a resonant tank having a series connected tank inductor and tank capacitor and an output voltage sensing circuit that senses an output voltage of said ballast by sensing a voltage across a sampling capacitor connected in series with said tank capacitor. The electronic ballast of claim 1 further comprising a cable compensation circuit that limits variations in the output voltage of said electronic ballast due to a cable being connected to the output terminals of the ballast. The electronic ballast of claim 6 wherein said cable compensation circuit limits variations in said output voltage by altering gating signals provided to said transistors. The ballast of claim 8 wherein said snubber circuit comprises a snubber capacitor connected in parallel with each of said first transistor and said second transistor.
The ballast of claim 9 wherein an extended dead time is created between said gating signals of said transistors to allow said snubber capacitors to discharge. The electronic ballast of claim 8 further comprising an open circuit voltage control circuit that controls a voltage across said output terminals of said ballast when a gas discharge lamp is not connected between said output terminals.
The electronic ballast of claim 8 further comprising a resonant tank having a tank capacitor and a tank inductor.
The electronic ballast of claim 12 further comprising a sampling capacitor connected in series with said tank capacitor wherein an output voltage of said ballast is sampled across said sampling capacitor. The electronic ballast of claim 14 wherein an extended dead time is created between gating signals of said pair of transistors to allow said capacitors to discharge.
The electronic ballast of claim 14 further comprising an open circuit voltage control circuit that controls a voltage across output terminals of said ballast when a gas discharge lamp is not connected between said output terminals.
The electronic ballast of claim 14 further comprising a cable compensation circuit that limits variations in an output voltage due to cables being connected to outputs of the ballast. The electronic ballast of claim 14 further comprising a resonant tank having a series connected tank inductor and tank capacitor.
The electronic ballast of claim 18 further comprising a sampling capacitor connected in series with said tank capacitor wherein an output voltage of said ballast is sampled across said sampling capacitor.
The electronic ballast of claim 19 further comprising a resistor connected in series with said sampling capacitor wherein a voltage across said resistor is used to limit said output voltage of said electronic 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. The casing extends around the stator assembly, the stator assembly including a plurality of stator vanes, and a plurality of bleed scoops.
A compressor in accordance with claim 1 wherein said stator assembly comprises an outer band aft radius, said bleed scoop aft side extending through said outer band aft radius. A compressor in accordance with claim 1 wherein said bleed scoop defines a cross-sectional area greater than about 0.08 square inches.
A compressor in accordance with claim 1 wherein said bleed scoop defines a non-elliptical cross-sectional area. A compressor in accordance with claim 1 wherein said bleed scoop defines a generally triangular cross-section. A compressor in accordance with claim 1 wherein under operating conditions the gas flowpath defines a main pressure and a main flowrate, said bleed plenum defines a plenum pressure and a plenum flowrate, said bleed scoop operates with a main pressure to plenum pressure ratio of about 1.21 at a plenum flowrate to main flowrate of about 2%. A compressor in accordance with claim 1 wherein said bleed scoop comprises a contoured outer surface to facilitate reducing a pressure drop between said gas flowpath and said bleed plenum.
A gas turbine engine comprising a compressor comprising a casing and a stator assembly, said casing extending around said stator assembly, said stator assembly comprising a plurality of stator vanes, and a plurality of bleed scoops, adjacent stator vanes defining a static high-pressure region, said bleed scoops formed at least partially within the high pressure region, each said compressor bleed scoop cross-sectional area is greater than about 0.08 square inches. A gas turbine engine in accordance with claim 8 wherein each said compressor stator vane comprises a pressure sidewall, each said compressor bleed scoop comprises an adjacent side, an aft side, and a contoured side, said adjacent side adjacent said stator vane pressure sidewall. A gas turbine engine in accordance with claim 8 wherein said compressor stator assembly further comprises an outer band aft radius, said compressor bleed scoop aft side extends through said outer band aft radius. A gas turbine engine in accordance with claim 8 wherein each said compressor bleed scoop defines a non-elliptical cross-sectional area. A gas turbine engine in accordance with claim 11 wherein each said compressor bleed scoop defines a substantially triangular cross-sectional area. A gas turbine engine in accordance with claim 8 wherein said compressor stator assembly further comprises a bleed plenum, each said compressor bleed scoop comprises a contoured outer surface to facilitate reducing a pressure drop between said gas flow path and a bleed plenum.
We offer personalized training and camps headed by coach and motivator, Lisa Smith-Batchen. 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 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.
Adjacent stator vanes define a static high-pressure region, the bleed scoops formed at least partially within the high pressure region. The mixture is then ignited for generating hot combustion gases, and the combustion gases are channeled to the turbine which extracts energy from the combustion gases for powering the compressor, as well as producing useful work to propel an aircraft in flight or to power a load, such as an electrical generator.The compressor includes a rotor assembly and a stator assembly. After graduating from college with an art degree she moved to Seattle to further pursue her rowing. Mountain biking , hiking, swimming, skiing, camping, eating and cooking, pretty much anything with friends and family makes her feel rich and fills her soul!! These experiences include rehabilitating injured athletes as a board-certified athletic trainer, personal training and wellness coaching as a board-certified strength coach and a PhD in Sport and Exercise Psychology, coaching mental skills to athletes of all abilities, lecturing at a university, and instructing a variety of fitness classes (water aerobics, spinning, ski conditioning, weight lifting, cardio core, flexibility for fitness).
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. 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.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. The rotor assembly includes a plurality of rotor blades extending radially outward from a disk. After six years of elite rowing, she has recently moved back to the Valley to be closer to the mountains and is excited to be on the Dreamchasers team!
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. To accomplish this, the pulse width of one switch of the half bridge is typically controlled. More specifically, each rotor blade extends radially between a platform adjacent the disk, to a tip. Controlling the pulse width controls the open circuit voltage indirectly by using inductor current to control the voltage on the capacitor. A gas flowpath through the rotor assembly is bound radially inward by the rotor blade platforms, and radially outward by a plurality of shrouds.The stator assembly includes a plurality of stator vanes that form nozzles that direct the compressed gas entering the compressor to the rotor blades. 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. The outer band retains the stator vanes and mounts the stator assembly within the compressor casing.
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. The stator assembly also forms a bleed plenum between the outer band and the compressor casing.Within at least some known gas turbine engines, a portion of high-pressure air is extracted or bled from the compressor and directed to a clearance control system.



Furthermore, the use of a large resonant capacitor makes the resonant tank difficult to design. More specifically, a bleed scoop in the stator assembly outer band channels the extracted air to a bleed plenum for the clearance control system. However, the effectiveness of the clearance control system may be reduced because the pressure of the extracted air drops as it passes through the bleed scope. Conduction losses and hard switching are undesirable in that they may cause the ballast to fail. Further, the bleed scoop can provide compressed air with both static and dynamic pressure components.BRIEF SUMMARY OF THE INVENTIONIn an exemplary embodiment, a method for operating a gas turbine engine includes a compressor including stator vanes, a stator assembly, a compressor casing, and a bleed plenum. 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 method includes extracting pressurized air from the compressor into at least one bleed scoop formed within the high static pressure regions defined by the adjacent stator vanes and directing the airflow from the bleed scoops into a bleed plenum.In another aspect, a compressor for a gas turbine engine, the compressor comprises a casing and a stator assembly. The stator assembly comprises a plurality of stator vanes, adjacent stator vanes defining a static high-pressure region between the vanes.
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. The stator assembly also defines at least one bleed scoop extending at least partially through the static high-pressure region adjacent the stator vanes and in flow communication with the gas flowpath and a bleed plenum.In another aspect, a gas turbine engine includes a compressor including a casing and a stator assembly. 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.
Adjacent stator vanes define a static high-pressure region, the bleed scoops formed at least partially within the high-pressure region.BRIEF DESCRIPTION OF THE DRAWINGSFIG. However, these circuits are often complex and decrease the efficiency, while increasing the cost, of the ballast.
2 is a schematic illustration of a compressor that may be used with a gas turbine engine shown in FIG.
The snubber circuit includes a pair of series connected snubber capacitors connected in parallel with the pair of transistors. 4 is a plan view of an exemplary stator assembly that may be used with the gas turbine engine shown in FIG.
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. 1 is a schematic illustration of a gas turbine engine 10 including a fan assembly 12, a compressor 14, and a combustor 16. 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.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.
In one embodiment, engine 10 is a CFM 56 engine commercially available from General Electric Company, Cincinnati, Ohio.In operation, air flows through fan assembly 12 and compressed air is supplied to compressor 14.
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. Airflow from combustor 16 drives turbines 18 and 20, and turbine 20 drives fan assembly 12.FIG. The snubber circuit includes a snubber capacitor connected in parallel with each of the first transistor and the second transistor. 2 is a schematic illustration of a compressor 40 that may be used in a gas turbine engine, such as gas turbine engine 10 (shown in FIG. 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. Compressor 40 includes a stator assembly 48 and a compressor casing 50 extending around stator assembly 48. A sampling capacitor connected in series with the tank capacitor wherein an output voltage of the ballast is sampled across the sampling capacitor.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.
Stator assembly 48 includes a plurality of stator vanes 52 within a gas flowpath 54, a bleed plenum 56 and an outer band 60. 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. In the exemplary embodiment, bleed plenum 56 is radially outward from a plurality of 4th stage stator vanes 52.FIG. 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. 3 a plan view of a known stator assembly 70 including a bleed scoop 78 defined within an outer band 72.
The electronic ballast has a resonant tank having a series connected tank inductor and tank capacitor. More specifically, stator assembly 70 is a known 4th stage stator assembly used in a CFM-56 engine.
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. Stator assembly 70 includes an outer band 72, which includes a raised aft radius 74, a raised forward radius 76, and an area 62 between vanes 52.
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. During operating conditions, a static high-pressure region 64 is formed in area 62 between adjacent vanes 52, adjacent outer band 72. 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 DRAWINGSFIG.
Bleed scoop 78 is located in outer band 72 and is generally equidistant between adjacent stator vanes 52 within area 62. 1 is a schematic diagram of a lossless snubber circuit constructed in accordance with a preferred embodiment of the present invention;FIG.
At least one known bleed scoop 78 has an elongated oval cross-sectional area 80 and includes a cross-sectional area of approximately 0.06 square inches through outer band 72. 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;FIG. Oval cross-sectional area 80 includes a long axis 81 that is oriented circumferentially from pressure sidewall 68 to suction sidewall 66. 3 is a schematic diagram of a hybrid sampling circuit having a lossless snubber circuit constructed in accordance with the present invention; andFIG. 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 INVENTIONA 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. 4 is a plan view of an exemplary stator assembly 90 that may be used with the gas turbine engine 10 shown in FIG. 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.
More specifically, stator assembly 90 is an exemplary embodiment of a 4th stage stator assembly. 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. Stator assembly 90 includes stator vane 52 with suction sidewall 66, pressure sidewall 68 and area 62 between suction sidewall 66 and pressure sidewall 68. This increased gate charge increases the turn-off losses for a high voltage inverter.A simplified schematic of an electronic ballast 2 constructed in accordance with a preferred embodiment of the present invention is shown in FIG.
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. During operating conditions, static high-pressure region 64 is located adjacent pressure sidewall 68 and raised aft radius 96 within area 62.


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.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. Further, bleed scoop 94 is contoured to conform to and substantially encompass high-pressure region 64, extending both circumferentially and radially.
In one embodiment, bleed scoop 94 is formed in outer band 92 and raised aft radius 96, extending radially through outer band 92 and raised aft radius 96. 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. Bleed scoop 94 includes a cross-sectional area 110 of greater than approximately 0.08 square inches. In the exemplary embodiment, bleed scoop 94 presents a generally triangular cross-sectional area 110, including 0.09 square inches.
In the normal case, the capacitors 16 and 18 reduce the turn-off losses associated with the switches 6 and 8. Bleed scoop adjacent side 98 is adjacent stator vane pressure sidewall 68, within a first offset distance 114 of less than 0.06 inches from pressure sidewall 68 to adjacent side 98.
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. Also in the exemplary embodiment, bleed scoop 94 is a second offset distance 116 of approximately 0.1 inches from suction sidewall 66 of adjacent stator vane 86. 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. Contoured side 102 corresponds to the predetermined pressures contours of high-pressure region 64. Thus, at the turn-on, there are no switching losses in the transistors 6 and 8 of a preferred embodiment of the present invention.
Bleed scoop 94 facilitates pressurized air flow from area 62 to bleed plenum 56.In use, compressor 40 supplies compressed air to combustor 16 (shown in FIG.
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.
A portion of the compressed air, generally less than 5%, is extracted or bled off by bleed scoop 94 to bleed plenum 56 (shown in 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.
Field studies, computer modeling or combinations of methods are used to determine the pressure contour of high-pressure region 64 between stator vanes 52 adjacent outer band 60. The length of the dead time is adjusted by controlling the gating signals provided from the microcontroller 11 to the transistors 6 and 8.
Forming bleed scoop 94 in high-pressure region 64 provides higher pressure compressed air to bleed plenum 56. 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.
With bleed scoop 94 in high-pressure region 64, pressurized air is extracted from compressor 40 into bleed scoop 94.
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. Further, contouring bleed scoop cross-sectional area 110 to the shape of high-pressure region 64 reduces the pressure drop at bleed scoop 94. Thus, if there is only one capacitor, then the dissipated energy is concentrated in only one FET or transistor. More specifically, during engine operation with bleed scoop 94, at a 2% bleed rate (the ratio of airflow through bleed scoop 94 to airflow through compressor 40), the ratio of gas flowpath pressure to bleed plenum pressure is approximately 1.21, a significant improvement. 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.Referring now to FIG. 3), at a 2% bleed rate, the ratio of gas flowpath pressure to bleed plenum pressure is approximately 1.24.
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.
Bleed scoop 94 facilitates more efficient extraction of compressed air, with reduced pressure loss. The electronic ballast 30 includes a bulk DC voltage source 32 that provides power to the inverter circuit transistors 46 and 48.
Bleed scoop 94 facilitates a higher bleed plenum 56 pressure at a given bleed rate, enabling more efficient operation clearance control system operation.The above-described bleed scoop is cost-effective and highly reliable.
The series resonant tank is comprised of a resonant tank inductor 40 and a resonant tank capacitor 42. The bleed scoop is located in the high-pressure regions adjacent compressor stator vanes and contoured to provide a larger cross-section in the high-pressure regions. 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. Reducing the pressure drop in the extracted air as it passes through the bleed scoop to the bleed plenum facilitates higher pressure in the bleed plenum at a given bleed rate, and improved engine efficiency. As a result, the pressure drop through the bleed scoop is reduced, facilitating improved turbine operations.While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims. 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. A sampling circuit comprised of capacitors 50 and 52 and resistors 54 and 56 is used to sample the voltage on capacitor 44. 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.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. 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.The sampling circuit described above with respect to FIG. 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. 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. Therefore preferred embodiments of the present invention include a cable compensation circuit.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. 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 true open circuit voltage is sensed in this way to change the current threshold required to turn-off the switch 46.
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.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.



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