Abstract:
An electronic ballast has a resonant tank circuit that includes a tank inductor and a tank capacitor connected in series. Lossless sampling of the output voltage of the electronic ballast is achieved by monitoring the voltage across a sampling capacitor placed in series with the tank capacitor. A resistive and capacitive filter is used to filter the monitored voltage such that it can be accurately received by a microcontroller. A resistor is connected in series with the sampling capacitor to produce an open circuit output voltage control signal that is used by the microcontroller to limit the open circuit output voltage. A cable compensation circuit is utilized to minimize variations in the open circuit voltage due to the connecting and disconnecting of a cable to the ballast output terminals.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
   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 
   Not Applicable 
   REFERENCE TO SEQUENCE LISTING OR COMPUTER PROGRAM LISTING APPENDIX 
   Not Applicable 
   BACKGROUND OF THE INVENTION 
   Electronic ballasts are used to provide power to gas discharge lamps such as fluorescent lights. These ballasts often sense the voltage applied to the lamp to monitor the functioning of the lamp and ballast. To sense the lamp voltage with a microcontroller, prior art ballasts such as set forth in U.S. Pat. No. 5,925,990, sample the lamp voltage through a resistor connected in series with a resonant capacitor. The voltage across the resistor represents the current in a resonant inductor, which is in turn proportional to the voltage across the resonant capacitor. However, in order to fully utilize the resolution of the A/D converter of the microcontroller, the voltage drop across the resistor needs to be relatively high. As a result, more than 2 watts of power are typically consumed at the output of the resonant inverter which increases the required input power to the ballast. Therefore, since efficiency is very important in modern lighting designs, an improved method and apparatus for sensing lamp voltage that consumes less power is needed. 
   In some prior art ballasts, the voltage on the lamp voltage sensing resistor is also used to control the open circuit voltage during striking when no lamp is connected. To accomplish this, the pulse width of one or both switches 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 cables, 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 significant. However, potentially damaging hard switching or capacitive mode switching is often observed with this type of prior art open circuit voltage controlled ballast. 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 ultimately 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 since it may result in damage. 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 connecting cables 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 sensing and controlling the open circuit voltage of a ballast is needed. 
   BRIEF SUMMARY OF THE INVENTION 
   A preferred embodiment of the present invention is directed toward an electronic ballast for producing an output voltage for igniting and powering gas discharge lamps connected between a pair of output terminals. The ballast includes a microcontroller for controlling the ballast. An inverter including a first and second transistor and a resonant tank having a tank inductor and a tank capacitor are also included. A sampling capacitor is connected in series with the tank capacitor and an output voltage of the electronic ballast is monitored across the sampling capacitor. A pair of snubber capacitors is provided to reduce turn off losses in the first and second transistors. One of the snubber capacitors is connected in parallel with the first transistor and the other of the snubber capacitors is connected in parallel with the second transistor. An extended dead time is created between gating signals of the first and second transistors to allow the snubber capacitors to discharge. An open circuit voltage control circuit is provided that controls the output voltage 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 the sampling capacitor. A voltage across the resistor is used to trim a gating signal for at least one of the first and the second transistors. A cable compensation circuit is provided that limits variations in the output voltage due to cables being connected to the output terminals of the ballast. 
   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. A lossless snubber circuit is used to reduce turn off losses in the inverter. A resonant tank having a tank capacitor is included. A sampling capacitor is connected in series with the tank capacitor. The output voltage of the ballast is monitored by monitoring a voltage across the sampling capacitor. A resistor and capacitor filter the voltage across the sampling capacitor such that it can be properly received by a microcontroller for analysis. An open circuit voltage control circuit controls a voltage across the output terminals of the electronic ballast 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 the sampling capacitor. The electronic ballast further includes a cable compensation circuit for compensating for changes in the output voltage due to a cable being connected to the output terminals. The cable compensation circuit compensates for changes in the output voltage by altering gating signals of at least one of the transistors and, thereby, limiting fluctuations in the output voltage. 
   Yet another embodiment of the present invention is directed toward an electronic ballast having a pair of output terminals for providing power to gas discharge lamps. The ballast includes a microcontroller for controlling the ballast and a half-bridge inverter circuit having a pair of series connected transistors. A resonant tank circuit is connected between the series connected transistors. The resonant tank circuit includes a resonant inductor connected in series with a resonant capacitor. A sampling capacitor is connected in series with the resonant capacitor such that a voltage across the sampling capacitor is proportional to a voltage across the output terminals. The microcontroller monitors the voltage across the sampling capacitor. A resistive and capacitive filter filters the sampled voltage across the sampling capacitor before the sampled voltage is provided to the microcontroller. A resistor is connected in series with the sampling capacitor such that a voltage across the resistor is used to control an open circuit output voltage of the electronic ballast. An open circuit voltage control circuit produces a transistor gating signal based upon the voltage across the resistor that is used by gating logic to control the output voltage. A cable compensation circuit limits variations in the output voltage due to cables being connected to outputs of the ballast. The cable compensation circuit also limits variations in the output voltage by providing signals to the microcontroller that are used to alter the turn-on times of at least one of the pair of transistors. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       FIG. 1  is a schematic diagram of a lossless lamp voltage sampling circuit for an A/D converter of a microcontroller constructed in accordance with a preferred embodiment of the present invention; 
       FIG. 2  is a schematic diagram of a lossless lamp voltage sampling circuit having open circuit voltage control circuitry constructed in accordance with an embodiment the present invention; and 
       FIG. 3  is a schematic of a lossless lamp voltage sensing circuit including a cable compensation circuit constructed in accordance with a preferred embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   A preferred embodiment of the present invention is directed toward an instant start electronic ballast for a gas discharge lamp that overcomes the aforementioned deficiencies of the prior art. Referring now to  FIG. 1 , an electronic ballast  2  that utilizes lossless sampling of the lamp voltage  4  in accordance with a preferred embodiment of the present invention is shown. The electronic ballast  2  includes a bulk DC voltage source  6  that provides power to the inverter circuit transistors  8  and  10  each of which is connected in parallel with a respective snubber capacitor  17  and  18 . In the normal case, the capacitors  17  and  18  reduce the turn-off losses associated with the switches  8  and  10 . However, all of the energy stored in the capacitors  17  and  18  when the switches  8  and  10  are turned off will be dissipated on the switches  8  and  10  at the turn-on. Thus, in a preferred embodiment such as an IHRV ballast, an extended dead time that allows the capacitors  17  and  18  to discharge is created between the gating signals of the transistors  8  and  10  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  17  and  18  during the extended dead time. 
   The series resonant tank is comprised of a resonant tank inductor  12  and a resonant tank capacitor  14 . Prior art circuits use a resistor connected in series with the resonant capacitor  14  to sense the lamp voltage  4  and control the open circuit voltage. However, in a preferred embodiment of the present invention for an Instant-Start, High Range Voltage (IHRV) ballast and/or sign ballast, the lamp voltage  4  is sensed by a sampling capacitor  16  that is connected in series with the resonant capacitor  14  as shown in  FIG. 1 . Using the principle of voltage division with capacitors, when the sampling capacitor  16  is much smaller than the resonant capacitor  14 , the voltage drop on the sampling capacitor  16  is very large compared to resonant capacitor  14 . This is beneficial in that it is relatively easy to find a film capacitor  16  that satisfies these criteria, has a small package size and is inexpensive. Moreover, the creation of large voltage drop across capacitor  16  provides a wide voltage range for the voltage sampling such that the full range of the A/D converter of the microcontroller  11  can be utilized. Most preferably, the capacitor  16  is a 330 nF 60V or a 680 nF 60V capacitor. A sampling circuit comprised of capacitors  20  and  22  and resistors  24  and  26  is used to condition and filter the sampled voltage on capacitor  16  for processing by the microcontroller. The sampling circuit of  FIG. 1  provides a low output impedance, strong signal with an excellent signal to noise ratio and a quick response time to an AID converter input of an associated microcontroller  11 . More importantly, since the sampling is accomplished across the capacitor  16 , the sampling process is substantially lossless and does not significant decrease the efficiency of the ballast  2 . Thus, the circuit of  FIG. 1  uses lossless capacitor based voltage division to improve the efficiency of the ballast  2  without sacrificing performance. 
   The sampling capacitor  16  used in the ballast of  FIG. 1  can also be connected in series with a low value resistor  30 , which can be used to control the open circuit voltage  4 , as shown in  FIG. 2 . The hybrid sampling circuit shown in  FIG. 2  samples a large amplitude version of the lamp voltage  4  across capacitor  30 . The sampled signal is smoothed by RC filters constructed from capacitors  20  and  22  and resistors  24  and  26  and then fed to the A/D converter of the microcontroller. The response time of the sampled lamp voltage  4  is not fast, but it is almost entirely lossless. For open circuit voltage control, the amplitude of the voltage across resistor  30  is large enough at lamp striking to turn on transistor  36  and activate the gating logic  7  to trim the pulse width of the gating signal generated by the half-bridge driver  9  for the upper switch  8  of the half bridge. The exact value at which such gate signal trimming will occur is determined by the value of the resistors  32  and  34  which control the turning off and on of transistor  36 . Most preferably, resistors  32  and  34  both have a value of 1 kΩ. Trimming the pulse width of the gating signal of the upper switch  8  limits the open circuit voltage  4 . However, during steady state operation, the voltage on the resistor  30  is very small, out of phase with the voltage on capacitor  16 , and still proportional to the lamp voltage  4 . Hence, lamp voltage sensing is not affected by the resistor  30  when the ballast  2  is operating in the steady state. 
   The sampling circuit described above with respect to  FIG. 2  can be used independently with or without a cable compensation circuit. Since the voltage on resistor  30  is in phase with the current of the upper switch  8 , it is convenient to use the voltage on resistor  30  to control the open circuit voltage  4  when no lamp is connected and to trim the pulse width of the upper switch  8  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  14 , the parameters of the resonant tank constructed from inductor  12  and capacitor  14  changes. As a result, the open circuit voltage  34  decreases when a cable is connected to the ballast  2 . When the value of the resonant capacitor  14  is small, the decrease in the open circuit voltage  34  is significant and the ballast will not strike the lamp. The open circuit voltage  4  can be set high to start a lamp with a long cable. However, in applications where no cable is attached, the open circuit voltage  4  will then be too high, which may cause the ballast to fail the through-lamp leakage test, or damage the sampling film capacitor  16 . Increasing the capacitance of the resonant capacitor  14  helps to decrease the variation of the open circuit voltage but increases the conduction losses due to the circulation currents in the resonant capacitor  14 . Furthermore, larger capacitor values lead to saturation of the resonant inductor  12 . Therefore, the most preferred embodiments of the present invention include a cable compensation circuits. 
   Capacitor sampling provides a strong sample signal with low output impedance and quick response. A cable compensation circuit is created by adding zener diode  44 , resistors  42  and  46 , and capacitor  40  as shown in  FIG. 3 . The open circuit voltage  4  as sampled by capacitor  16  rises very rapidly at node  48 . When the open circuit voltage  4  becomes too high, the zener diode  44  starts to conduct and feeds current to the base of transistor  36  such that the conductive threshold for the transistor  36  is decreased. Thus, the transistor  36  starts to turn-on earlier when the voltage on resistor  30  is lower. The pulse width of the gating signal of the upper switch  8  then becomes narrower thereby reducing the open circuit voltage  4 . So, the true open circuit voltage  4  is sensed in this way to change the current threshold required to turn-off the switch  8 . In an exemplary circuit constructed as described above, the open circuit voltage  4  varies from 1.9 kv to 2.6 kv without the cable compensation circuit of  FIG. 3  when 18 feet of cable is connected to or removed from the circuit.. However, with the cable compensation circuit of  FIG. 3 , the variation in the open circuit voltage  4  is within approximately 100V. Thus, an electronic ballast having a lossless voltage sampling and control circuit and a cable compensation circuit in accordance with the preferred embodiment of the present invention as shown in  FIG. 3  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 Circuit for Sampling of Lamp Voltage, 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.