Abstract:
An improved ignition circuit for a high intensity discharge (HID) lamp is disclosed. The ignition voltage is provided across a single capacitor, and at the end of the ignition phase, a second capacitor is switched into the circuit to divide the voltage across two capacitors, and provide a steady state square wave current and voltage.

Description:
TECHNICAL FIELD 
     This invention relates to a method and apparatus for igniting a High Intensity Discharge (HID) lamp. The invention has particular application in high volume commercial HID devices, where cost is an important consideration. 
     BACKGROUND OF THE INVENTION 
     HID lamps typically use a gas sealed within a glass container which conducts electricity and emits light at a particular wavelength. The wavelength is a function of the type of gas used. 
     In order to start, or ignite, an HID lamp, there are generally four phases the ballast must account for. The first phase is the breakover phase, in which a relatively high voltage pulse (e.g., 3 kilovolts) is applied between two electrodes of the HID lamp in order to free electrons from the gas molecules and start the conduction process. Typically the ballast must supply the high voltage pulse for a duration of approximately 10 microseconds. After the 10 microsecond 3 kilovolt pulse, a takeover state is entered. Depending on the lamp and ballast conditions, the takeover state can last on the order of hundreds of microseconds, during which the ballast must be capable of supplying approximately 280 to 300 volts to the lamp. This continues the process of bringing the gas towards a steady state of conduction. 
     After takeover, the HID lamp enters the run up phase. At the beginning of the run up phase, the temperature, internal pressure, and voltage within the lamp are relatively low. During the run up phase, the voltage ramps up from approximately 20 volts to approximately 90 volts over the course of a minute or even more. After that minute, the lamp enters its fourth and final stage, which is the steady state operating phase. During steady state, the lamp emits light at its normal temperature and pressure for which it was designed. 
     During steady state, the lamp is operating based upon a current signal which must oscillate. More specifically, because of the physics of such devices, they cannot operate on DC but must instead be operated based upon preferably a low frequency square wave signal which oscillates between a positive and negative current. Thus, the steady state may be, for example, a square wave current at 100 Hz that results in a lamp voltage which oscillates between plus and minus 90 volts. 
     The above four phases require that the circuitry to drive the lamp deliver a prescribed signal. More specifically, the drive circuitry must deliver the breakover ignition pulse of approximately 10 microseconds, followed by the takeover voltage of approximately 280 to 300 volts for on the order of hundreds of microseconds, and then the run up and steady state voltages. FIG. 1 is an exemplary prior art arrangement for delivering the above-prescribed signal. At ignition, a signal of approximately 400 volts is placed across capacitor  150 . The 400 volts is conveyed through device  130  and inductor  134 , and causes a signal of approximately 300 volts to appear across capacitor  132 . 
     Capacitor  132  causes igniter  105  to generate a pulse of approximately 3 kilovolts for approximately 10 microseconds, after which the igniter  105  appears essentially as a short circuit. The igniter is typically triggered by the voltage across capacitor  132  to generate the 3 kilovolt pulse. After the initial pulse, and when the igniter acts as an effective short circuit, the voltage of approximately 280 to 300 volts from capacitor  132  is delivered from capacitor  132  through igniter  105  to the HID lamp  108 . These 280-300 volts are maintained for on the order of hundreds of microseconds, until the takeover phase is complete. Immediately after the takeover phase, controller  110  begins the run up and steady state process. During steady state, controller  110  controls the gate voltages of  136  through  139  such that the oscillating square wave described above is delivered to the lamp  108 . 
     A problem with the arrangement of FIG. 1 is that the cost is relatively expensive due to the number of components. More specifically, because it is required to generate a square wave which varies its polarity periodically, four transistors are required within commutator  120 . The four transistors act in conjunction with the control voltages applied to their gates by controller  110  in order to generate the required square wave. 
     FIG. 2 shows an alternative prior art embodiment for delivering the prescribed four phases of signal to an HID lamp. The arrangement of FIG. 2 utilizes two capacitors  220  and  222  in series as a voltage divider. HID lamp  108  is connected between igniter  105  and point  208 . The system need not use four different transistors to create the square wave utilized during steady state. Instead, only two transistors  224  and  226  are needed. During steady state, lamp electrode  210  is connected to point  208 , and transistors  224  and  226  can be operated at high frequency and at varying duty ratios. Thus, by operating controller  218  in a fashion such that transistors  224  and  226  are alternatively switched on and off with the proper durations, the required steady state current and voltage waveforms can be delivered. Since one of the HID lamp electrodes is connected to the middle of the divider formed by capacitors  220  and  222 , only two transistors  224  and  226  are needed to generate a square wave with changing polarity. 
     The problem with the arrangement of FIG. 2 occurs during ignition. More specifically, in order to supply the approximately 280 volts needed to be present across the lamp  108  during takeover phase, 560 volts must be present between points  214  and  216 . This increased voltage, which is used only during the initial ignition process, creates relatively high stress on the circuit components. This either causes failures or, in the case of quality components that can withstand the stress, drives up the cost to nearly the point of using the four devices in FIG.  1 . 
     In view of the above, there exists a need in the art for an ignition circuit for an HID lamp which can utilize only a small number of switching devices but yet can operate without the relatively high voltages required in arrangements such as that in FIG.  2 . 
     SUMMARY OF THE INVENTION 
     The above and other problems of the prior art are overcome in accordance with the teachings of the present invention which relates to an ignition circuit for HID lamps. A two-transistor circuit is utilized which is sufficient to provide the steady state square wave voltage of approximately 90 volts. During the ignition process, a switch is utilized to switch one of two capacitors forming a voltage divider out of the ignition circuit. This results in the entire input voltage being applied to one capacitor, thereby delivering a sufficient voltage for ignition. After the ignition period, the second capacitor is switched back into the circuit, thereby forming a voltage divider and permitting the pulsed steady state voltage to be delivered to the lamp. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 depicts an exemplary prior art arrangement for use in igniting an HID device; 
     FIG. 2 depicts an alternative prior art arrangement for use in igniting an HID device; and 
     FIG. 3 depicts an exemplary embodiment of the present invention for use in the ignition of an HID device. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The arrangement of FIG. 3 includes two transistors  354  and  356 , supply capacitors  350  and  352 , capacitor  360 , and control circuitry  310 . As is conventional, an igniter  312  is connected to the HID lamp  314 . 
     Although not shown for purposes of simplicity, the gates of transistors  354  and  356  are connected to controller  310 , so that controller  310  may switch devices  354  and  356  on and off at the appropriate times as described hereafter. A switch  320  is placed across capacitor  352 . The switch may be implemented in the form of a solid-state device such as a MOSFET or any other conveniently available switch. The switch may also be connected to controller  310  in order to facilitate control thereof. A resistor  322  is connected in series with the switch  320 . Typical values of capacitors  350  and  352  range from 22 to 68 microfarads. 
     In operation, an initial bus voltage of approximately 400 volts is applied across terminals  318  and  316 , and the switch  320  is kept closed by controller  310 . This causes point  328  to be connected to terminal  316 , and thereby places approximately the entire 400 volts across capacitor  350 . Transistors  354  and  356  are operated at high frequency and at the proper duty ratios such that 280 to 300 volts are delivered across capacitor  360 , triggering the igniter  312 . 
     After the breakover and takeover periods, the system enters the run up phase and it is necessary to prepare the system for AC operation. Controller  310  monitors the status of the system, and after the lamp has entered the run up phase, places switch  320  into the open position. This may be accomplished for example, by removing an appropriate gate voltage from a MOSFET or similar device. 
     By opening such a switch, the 400 volts across terminals  316  and  318  is now divided between capacitors  350  and  352 . The circuit thus is in the arrangement shown in FIG. 2 for delivering steady state square wave current and voltage to the lamp  314  for operation. Thus, by utilizing two capacitors in series and switching one of the them out of the circuit for the breakover and takeover periods, the benefits of a reduced number of components are achieved without the expense of having to use high stress components to withstand increased voltages between terminals  316  and  318 . 
     The transition from the ignition phase to steady state phase must be timed correctly. More specifically, we refer to the steady state phase as AC operation, since the signal driving the HID lamp is alternating polarity square wave. It is important that the controller sense the end of takeover and immediately open switch  320  in order to place capacitor  352  back into the circuit. There are several manners in which this can be accomplished. One is to have the controller monitor the impedance measured across the HID lamp  314 . A drop in impedance occurs at the end of takeover, since the gas becomes more conductive. 
     Another technique is for the controller to measure the current being delivered to the HID lamp, since the end of takeover phase is marked by a sudden increase in the current being delivered to the lamp due to the lower impedance of the lamp. 
     Regardless of how the end of takeover is sensed, the controller  310  opens switch  320  upon sensing the end of takeover, and capacitor  352  then begins to charge naturally as capacitor  350  discharges. The total voltage between points  318  and  316  nonetheless remains substantially constant. When the voltage across each of capacitors  350  and  352  reaches approximately 200 volts, the controller  310  begins switching transistors  354  and  356  on and off appropriately to generate the steady state AC pulse signal required to drive the HID device. 
     By selecting capacitors  350  and  352  to have typical values on the order of 47 microfarads, the period of time it takes to charge  352  can be kept to under 100 milliseconds. This timing is important since by keeping the charge time small, the HID lamp  314  is not operating in a DC mode for an extended length of time, which could cause damage to the lamp. 
     If the lamp were to extinguish during operation while the power is still applied, then capacitor  352  should be discharged before re-ignition. The controller  310  will sense the extinguishing of the lamp and discharge capacitor  352  through switch  320  prior to the ignition. This discharge should be limited by the controller  310  in order to avoid massive currents destroying switch  320 . 
     Such a discharge is accomplished by controller  310  switching an additional resistor  322  into the path between capacitor  352  and ground in order to limit the current in the capacitor discharge path. Alternatively, the controller can properly drive switch  320  in order to limit the current permitted therethrough by regulating the gate voltage in a conventional fashion to provide for the correct current in accordance with the device characteristics. Once capacitor  352  is discharged, controller  310  may then initiate the ignition sequence again by closing switch  320  and thus place the appropriate voltage across capacitor  350 . 
     While the above describes the preferred embodiment of the invention, it is understood by those of skill in the art that various modifications and variations may be utilized. For example, separate power supplies may be utilized during the ignition phase and later during the steady state phase. The switch  320  may be implemented using a variety of switching devices. Such modifications are intended to be covered by the following claims.