CIRCUIT ARRANGEMENT

Abstract: An improved circuit arrangement for igniting and operating 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: Circuit arrangement This invention relates to a method and a circuit for igniting and operating a High Intensity Discharge (HID) lamp. The invention has particular application in high volume commercial HID devices, where cost is an important consideration. HID lamps typically use a gas and/or vapour sealed within a container or discharge vessel which during stable operation of the lamp conducts electricity and emits light at a particular wavelength. The wavelength depends on the type of vapour or vapours used. HID lamps commonly have a negative voltage-current characteristic. Thus these lamps need a ballast circuit during steady or stable lamp operation to stabilize the current through the lamp. To start operation of a HID lamp a high voltage pulse is required which is generally supplied by an igniter or igniter circuit. The igniter can form part of the ballast, in particular in modern electronic ballast circuits. Typically a HID lamp has two electrodes between which a discharge takes place during operation of the lamp. In order to start, or ignite, an HID lamp, there are generally four phases the ballast in combination with the igniter 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 and/or vapour molecules and start the conduction process. Typically the ballast and igniter 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 and/or vapour towards a steady state of conduction resulting in a stable discharge. 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 the relative low value, e.g. approximately 20 volts to a value corresponding to steady lamp operation, the so-called lamp voltage, e.g. 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. In general stable operation of HID-lamps is on AC, thus during steady state, the lamp is operating based upon an oscillating current signal. In modern electronic ballasts it is preferable to operate the HID-lamp upon 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 described four phases require that the circuit to drive the lamp that is to ignite and to operate 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 circuit arrangement for delivering the above-prescribed signal. Typically modern electronic ballasts or switch mode power controls for HID-lamp are provided with a pre-conditioner which transforms the input voltage, e.g. the voltage from the public supply source, into a relative high DC voltage e.g. 400 volts. The thus formed relative high DC voltage forms the source for the lamp igniting and lamp operating circuit part of the ballast circuit arrangement. At ignition, a signal of approximately 400 volts is placed across terminals 102, 104. 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 switches 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 switches e.g. 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 210 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 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 210 during the 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 an increased failure rate or, in the case of the use of quality components for the arrangement 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. _ The above and other problems of the prior art are overcome in accordance with the teachings of the present invention which relates to a circuit for igniting and operating a HID lamp. A two-transistor circuit is utilized which is sufficient to provide the steady state square wave voltage providing the lamp voltage. 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 into the circuit, thereby forming a voltage divider and permitting the pulsed steady state voltage to be delivered to the lamp. 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 ignition of HID device. The circuit arrangements shown in Figs. 1 and 2 have been described herebefore. 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. 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 short-cut to terminal 316, and thereby places 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 capacitor 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 HID lamp 314 for its stable 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 terminals 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 stable operate the HID lamp. By selecting capacitors 350 and 352 to have typical values on the order of 47 microfarads, the period of time it takes to charge capacitor 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 otherwise could cause damage to the lamp. If the lamp were to extinguish during operation while the power is still applied, then capacitors 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 through control 310 in order to avoid massive currents destroying switch 320. Such a discharge can be accomplished by control 310 switching an additional resistor into the path between capacitor 352 and ground in order to limit the current discharge path, as provisional indicated in Fig. 3 by resistor 321. Preferably such discharge resistor (321) is positioned in the branch of the switch 320, as this implies that the resistor is switched out of the circuit by the switch 320 during stable lamp operation. Alternatively, the conteoUer 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, conteoUer 310 may then initiate the ignition sequence again by closing switch 320 and thus, placing 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.

Claims: CLAIMS: 1. A circuit arrangement suitable for igniting and operating a high intensity discharge (HID) lamp (314) comprising: a voltage divider (350, 352) for delivering a drive voltage; and means for switching (320) a portion of the voltage divider into said circuit after a predetermined phase of ignition of said HID lamp (314). 2. The circuit arrangement of claim 1 wherein the predetermined phase is a takeover phase. 3. The circuit arrangement of claim 2 wherein said voltage divider comprises at least two capacitors (350, 352). 4. The circuit arrangement of claim 3 wherein said means for switching (320) further comprises a device for momtoring a voltage or current (310) delivered to said HID lamp (314) and for switching said portion of said voltage divider (350, 352) when a predetermined change in said voltage or current occurs. 5. A method of delivering electrical power to a High Intensity Discharge (HID) lamp (314) , the method comprising the steps of: delivering an initial signal sufficient to ignite the HID lamp by causing a breakover phase to occur; maintaining said signal long enough to provide for a takeover phase; and dividing said signal after said takeover phase of said HID lamp (314), and continuing to provide two steady state signals resulting from said division to operate said lamp (314) in a steady state phase. 6. The method of claim 5 further comprising the step of measuring the current delivered to the HID lamp (314) in order to determine when to divide the initial signal. 7. The method of claim 6 further comprising the steps of: monitoring electrical signals present across said HID lamp (314) during said steady state; determining, in response to said step of monitoring, whether said HID lamp (314) has become extinguished during steady state operation thereof; and if so, causing a gradual increase in one of said two steady state signals to increase it to a value equal to that of said initial signal. 8. The method of claim 7 further comprising the step of igniting said lamp after said gradual increase to said equal value is complete. 9. The method of claim 8 wherein said gradual increase is affected by switching a discharge resistor (321) in series with a capacitor (320). 10. The method of claim 6 wherein each said two steady state signals are delivered to a separate one of two switching devices (354, 356), and wherein said two switching devices (354, 356) are alternatively switched on and off. 11. A circuit arrangement suitable for driving a High Intensity Discharge (HID) lamp (314) comprising: a first capacitor (350) connected between a first terminal (318) and a first point (328), and a second capacitor (352) connected between said first point and a second terminal (316), an igniter (312), connected between said first point and a second point (330); a first switch (354) connected between the first terminal and second point, and a second switch (356) connected between the second terminal and second point; a third switch (320) connected between the first point and second terminal; and a controller (310) for igniting the HID lamp (314) with the third switch (320) closed (conducting), and for switching the third switch (320) open when the HID lamp (314) enters a run up operation phase. 12. The circuit arrangement of claim 11 wherein the controller (310) also alternatively switches the first and second switches (354, 356) on and off during steady state operation in order to generate a substantially square wave from an input voltage. 13. The circuit arrangement of claim 11 wherein the controller (310) includes means for ascertaining if the HID lamp becomes extinguished during operation, and if so, for causing a gradual discharge of said second capacitor (352) . 14. The circuit arrangement of claim 13 wherein said gradual discharge is caused by driving a gate of a teansistor (320) in a manner sufficient to limit current through said transistor in a prescribed manner. 15. A circuit arrangement for driving an HID lamp (314) comprising; means for delivering power from a power source to said HID lamp (314) from a power supply; means for momtoring a characteristic of said HID lamp (314) in response delivering said power, and means for switching a device (352) into said circuit so as to implement a voltage divider (350, 352). 16. The circuit arrangement of claim 15 wherein the monitored characteristic is electrical current. 17. The circuit arrangement of claim 16 further comprising a monitor for monitoring said HID lamp (314) to determine if said HID lamp has extinguished during operation, and if so, gradually discharging a capacitor (352).

Inventors: SHEN ERIC B, SHEN, ERIC, B.
Applicants: KONINKL PHILIPS ELECTRONICS NV, KONINKLIJKE PHILIPS ELECTRONICS N.V.
Application year: 2001
Main classification: H05B41
IPC codes: H05B41