Igniter circuit for an HID lamp

An igniter circuit (10) for an HID lamp (11) has a DC input (VBUS) for coupling to a source of DC voltage, and an output (23, 24) for coupling to the HID lamp. A resonant ignition circuit (12) operating at a controlled resonant frequency is coupled to the DC input for producing successive bursts of voltage having a frequency equal to the resonant frequency and having an amplitude that increases with time. The resonant ignition circuit (12) feeds the bursts of voltage across the output of the igniter until an HID lamp coupled thereto reaches breakdown.

FIELD OF THE INVENTION

This invention relates to igniters for high intensity discharge lamps.

BACKGROUND OF THE INVENTION

The function of a high-intensity discharge (HID) electronic ballast is to supply ignition to the lamp for starting and then operating the lamp, such as a metal halide lamp. A metal halide lamp is a gas discharge lamp in which metal halides are enclosed, for example, in a quartz envelope.

To initiate its operation, a metal halide lamp demands a high ignition voltage. Once the lamp is ignited, the voltages falls to low voltage of the order of 20 V and the lamp it is then maintained for a short time (typically between 1-2 minutes) in so-called “current mode” where the current is constant and the voltage rises until the lamp reaches nominal power, whereafter the ballast serves to stabilize the power.

Prior art igniter circuits are known where an uncontrolled oscillator frequency is swept from a frequency that is less than the resonant frequency such that when it reaches resonance the voltage reaches maximum value and the lamp strikes. However, during this operation the frequency continues to rise and the voltage therefore falls.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an igniter circuit for an HID lamp that employs a self-oscillating power supply for applying across the lamp a high ignition voltage that increases with time.

It is a further object to provide such an igniter circuit that is configured for coupling directly to an inverter having a half bridge topology for feeding low frequency current to the lamp after ignition.

These objects are realized in accordance with a first aspect of the invention by an igniter circuit for an HID lamp, the igniter circuit comprising:

a DC input for coupling to a source of DC voltage,

an output for coupling to the HID lamp, and

a resonant ignition circuit operating at a controlled resonant frequency coupled to said DC input for producing successive bursts of voltage having a frequency equal to the resonant frequency and having an amplitude that increases with time and for feeding said bursts of voltage across the output of the igniter until an HID lamp coupled thereto reaches breakdown.

According to a second aspect of the invention, there is provided a method for igniting a HID lamp, the method comprising:

using a resonant circuit connected across the lamp to generate successive bursts of voltage having a frequency equal to the resonant frequency and having an amplitude that increases with time; and

applying said bursts of voltage across the HID lamp until the lamp ignites, thereby loading the resonant circuit so that its Q factor falls sufficiently to stop the resonant circuit resonating.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

FIG. 1is a block diagram showing the functionality of an igniter circuit10according to the invention for an HID lamp11. The igniter circuit10is operated from a DC power source, VBUS, which is not itself part of the igniter circuit10and may be generated in manner well known to those skilled in the art. The DC power source, VBUS, is fed to a controlled self-oscillator12comprising a self oscillator13controlled by an ignition pulse control circuit14. The HID lamp11is coupled to the controlled self-oscillator12which constitutes an igniter circuit for igniting the lamp. The lamp is powered by a power supply15, which conveniently is coupled to the DC power source, VBUS, although it could be independent thereof. In order to disable operation of the igniter circuit after ignition of the HID lamp11, a power sensor16is responsively coupled to the DC power source, VBUS, so as to sense the current supplied thereby. Before the lamp11ignites the current sensed by the power sensor16is low; but once the lamp11ignites it draws power from the DC power source, VBUS. The power sensor16thus serves to indicate whether or not the HID lamp11has ignited. The ignition pulse control circuit14is responsively coupled to the power sensor16so as to cease operation when the power sensor16senses that the HID lamp11has ignited. For the sake of completeness, although not relevant to the igniter circuit per se, the power sensor16serves a second function in that the power supply15includes a power regulator (not shown) that is responsive to the power sensed by the power sensor16for stabilizing the nominal power to the lamp11.

FIG. 2is a schematic circuit diagram showing in detail a preferred embodiment of the igniter circuit10shown inFIG. 1.

Resonant Ignition Circuit

The oscillator13comprises a drive transformer T1having first, second and third windings20,21and22, respectively, which are connected in the correct polarity for positive feedback (oscillation). A first end of the first winding20is coupled to the source terminal of a first MOSFET M1whose gate terminal is coupled via a resistor R1to a second end of the first winding20. The drain terminal of the first MOSFET M1is coupled to VBUS, typically having a voltage of 400 VDC. A pair of zener diodes D1and D2is coupled back to back across the first winding20, their anodes being commonly connected and their respective cathodes being connected to opposite ends of the first winding20. The zener diodes D1, D2, limit the gate voltage fed to the MOSFET M1and thereby ensure that when the resonant voltage increases, it does not damage the gate of the MOSFET M1.

In complementary trimmer, the first end of the second winding21is coupled via a resistor R2to the gate terminal of a second MOSFET M2whose source terminal is coupled to a second end of the second winding21and constitutes the ground rail, GND. The drain terminal of the second MOSFET M2is coupled to the source terminal of the first MOSFET M1. A pair of zener diodes D3and D4is coupled back to back across the first winding20, their anodes being commonly connected and their respective cathodes being connected to opposite ends of the second winding21. The zener diodes D3and D4limit the gate voltage fed to the MOSFET M2and thereby ensure that when the resonant voltage increases, it does not damage the gate of the MOSFET M2. The first end of the second winding21is coupled to an ‘ON’ control output of an ON-OFF splitter25, its second end being coupled to GND. An ‘OFF’ control output of the ON-OFF splitter25is connected to the gate of the MOSFET M2. An input of the ON-OFF splitter25is connected to an output of the ignition pulse control circuit14, as will be described in more detail below. The ON-OFF splitter25serves to convey an ignition pulse conveyed by the ignition pulse control circuit14to the winding21of the drive transformer T1to enable the oscillation process; and to convey a disable signal to the gate of the MOSFET M2to prevent oscillation after the lamp11has ignited.

A first end of the third winding22of the oscillator drive transformer T1is connected to a first capacitor C1connected in series with a first end of a resistor R3, whose second end is coupled to the common junction of a split winding of a transformer T2, comprising windings L1, L2. The coils L1and L2are wound such that a first end of the coil L1is connected to a second end23of the coil L2, whose first end is connected to a first end of the HID lamp11. A second end of the third winding22is connected to the common junction of the two MOSFETs M1and M2, i.e. to the source of M1and to the drain of M2.

The DC power source, VBUS, comprises pair of large series connected electrolytic capacitors C2and C4connected between VBUS and GND, their common junction24being connected to a second end of the HID lamp11and to the second end of the coil L1via a capacitor C5. The capacitors C2and C4serve as storage capacitors for storing DC voltage for powering the controlled self-oscillator12and the power supply15. The power supply15operates as a low frequency square wave current source controlled power shown as26inFIG. 2that is connected to the common junction of the coil L1and the capacitor C5. The low frequency square wave current source is produced in known manner by an inverter (not shown). Preferably, the inverter is a half-bridge topology of which the capacitors C2and C4are integral components. The junction of the capacitors C2and C4and the first end of the coil L2constitute output terminals of the igniter circuit10across which the HID lamp11is coupled.

Having described the topology of the resonant ignition circuit12, its operation will now be described.

C4and C5have very low impedance at the resonant frequency and so practically behave as short circuits. The Q factor is determined by the values of R1, R2, R3. The resistors R1and R2together with the input capacitances of the gates of the two MOSFETs M1and M2create a phase shift which causes a reduction in the resonant voltage fed to the lamp.

The Q factor determines the maximum peak voltage that may be fed to the HID lamp11before breakdown, which may be several kilovolts, whereafter the voltage fed to the lamp falls to a low voltage, typically in the order of 20V and is maintained at constant current until it reaches the nominal power of the lamp.

A train of ignition pulses shown graphically inFIG. 3at the resonant frequency f0is fed to the junction between the source of M1and the drain of M2through the resonant circuit constituted by C1and L1, so that the resonant circuit resonates with increasing amplitude for the duration of each ignition pulse as shown graphically inFIG. 4due to the positive feedback produced by the windings of the drive transformer, T1. At the end of each ignition pulse, the amplitude of the resonant lamp voltage decreases until it reaches substantially zero until the arrival of the next ignition pulse, when the cycle is repeated. As noted, C4has low impedance at the resonant frequency and acts as a short circuit.

When the lamp11starts to conduct, the lamp acts as a low impedance, and the current through the lamp fed by the low frequency current source26(corresponding to the power supply15shown inFIG. 1) flows through L1and L2which together operate as a choke, which filters some of the high frequency ripple. C5acts as a first filter for removing the high frequency ripple superimposed on the low frequency current. C2and C4whose mid-point voltage is equal to half VBUS form part of a half bridge inverter that serves to supply low frequency current to the lamp11after ignition; and are thus integral components of the power supply shown as15inFIG. 1and of the low frequency current source shown as26inFIG. 2.

Before lamp breakdown, the transformer T2serves as the lamp igniter; and after breakdown when the lamp starts to conduct in the current mode, it serves as a choke for removing the high frequency ripple.

The object is to generate a high voltage waveform with increasing amplitude that is applied to the lamp as shown graphically inFIG. 4. When the lamp voltage reaches a certain voltage (1 kV-4 kV depending on lamp temperature), the lamp ignites. When this happens, the lamp impedance falls to a low value and loads the resonant circuit so that its Q factor falls significantly and it stops resonating. The self-oscillation circuit stops the oscillator coil T1from oscillating.

Ignition Pulse Control Circuit

As noted above, the oscillator13stops oscillating when the HID lamp11ignites owing to the fact that the low lamp impedance after ignition loads the resonant circuit causing a marked reduction in its Q factor. However, rather than rely on this alone, it is considered preferable to disable the ignition circuit once the lamp has ignited, this being achieved by the igniter pulse control circuit14. The igniter pulse control circuit14comprises a comparator27having a positive input to which a reference voltage signal PREF is fed and having a negative input coupled to the power sensor16so as to receive a voltage signal PIN that is proportional to the power across the HID lamp11. Ignition pulses shown graphically inFIG. 3having a duty cycle determined by TONand TOFFare fed to one input of a 2-input AND-gate28while the logic signal at the output of the comparator is fed to the second input of the AND-gate28. Before the lamp starts conducting, PIN is low and the comparator output is logic HIGH; the AND-gate28therefore conveys the ignition pulses to the ON-OFF splitter25. When the lamp ignites, PIN is larger than PREF and the output of the comparator27goes to LOW, whereupon the AND-gate28stops feeding the ignition pulses to the ON-OFF splitter25.

The oscillator13is self-controlled to operate at the resonant frequency as determined by C1and L1such that although differences in the values of C1and L1, as may occur in mass production owing to component tolerances will give rise to different resonant frequencies, the oscillator13will always operate at resonant frequency.

Moreover, the resonant frequency at which the oscillator13resonates is also a function of the parasitic capacitance of the wires connecting the HID lamp11to the resonant ignition circuit12, being a function of their length. Therefore, the oscillator13resonates at resonant frequency regardless of the length of the wires connecting the HID lamp11to the resonant ignition circuit12.