METHOD AND DEVICE FOR DRIVING A GAS DISCHARGE LAMP

FIELD OF THE INVENTION

The present invention relates in general to a method and a device for driving a gas discharge lamp, using an alternating lamp current. The invention particularly relates to driving a High Intensity Discharge lamp (HID), i.e. a high-pressure lamp, such as, for instance, a high-pressure sodium lamp, a high-pressure mercury lamp, or a metal-halide lamp. The invention will be specifically explained hereinafter with reference to a HID lamp, but is not restricted thereto, as it can also be applied more generally to other types of gas discharge lamps.

BACKGROUND OF THE INVENTION

Gas discharge lamps are known in the art, and their elaborate explanation is therefore not necessary in this description. In general, a gas discharge lamp comprises two electrodes located in a closed vessel filled with an ionizable gas or vapor. The vessel is typically made of quartz or of a ceramic material, specifically poly crystalline alumina (PCA). The electrodes are arranged at a certain distance from each other, and an electric arc is maintained between these electrodes during operation.

In practice, the arc may not have a straight shape but is more or less curved (hence its name). This phenomenon is typically caused by magnetic fields and convection, and is strongest when the lamp is operated in a horizontal orientation, i.e. with the electrodes and the arc directed substantially horizontally. Curving of the arc is undesirable, especially in HID lamps in which the vessel is relatively small, because it may lead to increased erosion of the electrodes and the risk of the vessel becoming damaged by overheating and/or strong temperature gradients in the vessel material. It is therefore desirable to take arc-straightening measures in order to maintain a more gradual temperature profile in the vessel. The problem mentioned above is known, and arc-straightening measures have already been proposed. US-6, 130,508 discloses a method wherein the lamp is operated at a low-frequency commutating DC current, also indicated as low-frequency square-wave operation, and arc straightening is promoted by superposing a high-frequency ripple on the lamp current, with a suitable modulation depth, wherein the frequency of the high-frequency

ripple is selected to correspond to an acoustic resonance frequency. A driver circuit disclosed in the US document cited above has a structure of a first stage for generating a DC current with a HF ripple, a second stage implementing a commutator, and an igniter coupled to the output of the commutator.

OBJECT AND SUMMARY OF THE INVENTION

It is a general object of the present invention to improve the known driver. The invention has the particular object of providing a simpler circuit design with fewer components, thus saving costs. According to an important aspect of the present invention, the igniter is used to provide the high-frequency current component for arc-straightening purposes. The first stage of the driver can thus be designed in a much simpler way, while the igniter performs three functions. In a first mode, during start-up, the igniter provides the desired ignition voltage. In a second mode, after ignition, the igniter provides the take-over power. In a third mode, the igniter may be switched off; the first stage provides a DC voltage, and the power stage converts this voltage to an alternating current to drive the lamp. If desired, the igniter can operate in a third mode, in which it generates an arc-straightening high-frequency current component.

Further advantageous elaborations are defined in the dependent claims.

BRIEF DESCRIPTION OF THE DRAWING

These and other aspects, features and advantages of the present invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.

In the drawing,

Fig. 1 is a block diagram schematically illustrating an embodiment of a device according to the present invention.

DESCRIPTION OF EMBODIMENTS

Fig. 1 is a block diagram schematically illustrating an embodiment of a device, or driver, 1 for driving a HID lamp 2. The driver 1 comprises a lamp current-generating section 10, a high-frequency section 20, and a coupling transformer 40 coupling the output of the high-frequency section 20 to the output of the lamp current-generating section 10.

The lamp current-generating section 10 comprises a first stage 11 and a second stage or power stage 12. Although it is possible to power the device from a DC power source, it will typically be powered from a mains socket, which typically provides 23 OVAC @ 50 Hz (in Europe). The first stage performs the functions of power factor correction, rectifying, and conversion from AC to DC voltage. The power stage 12 converts the DC voltage from the first stage 11 into a commutating lamp current IL at a commutation frequency. The commutation frequency may be typically of the order of about 100 Hz. It is noted that such lamp current-generating sections 10 are known per se, so that a more elaborate description is not necessary here. It is further noted that different designs are possible for the power stage 12. In this embodiment, the half-bridge commutating forward (HBCF) design is described, which design is preferred in view of its relative simplicity and because it is also known per se.

The driver 1 has output terminals 3, 4 for connecting the lamp 2. The driver output terminals 3, 4 are connected to the power stage output terminals 13, 14, in series with a secondary winding 42 of the coupling transformer 40. A first capacitor 43 is coupled in parallel with the secondary transformer winding 42. The secondary inductance of the transformer 40 in parallel with the first capacitor 43 form a parallel resonant circuit having a resonance frequency f _RE s defined by f _RE s ⁼ 2π -JLC , in which L indicates the secondary inductance of the transformer 40, and C indicates the capacitance of the first capacitor 43. In a typical embodiment, the resonance frequency f _RE s may be of the order of about 70 kHz. It is noted that, due to tolerances, the resonance frequency f _RE s may differ from device to device.

The high-frequency section 20 comprises a waveform generator 21 of well- known half-bridge topology, comprising two power rails 22, 23, a first branch with two controllable switches 24, 25 coupled in series between the two power rails 22, 23, a second branch with two capacitors 26, 27 coupled in series between the two power rails 22, 23, and a diagonal branch coupled between a first node A between said two controllable switches 24, 25 and a second node B between said two capacitors 26, 27. The diagonal branch comprises a series arrangement of the primary winding 41 of said coupling transformer 40, an inductor 28 and a capacitor 29. The waveform generator 21 further comprises a switch controller 30, which may be implemented as a microcontroller, for driving the two controllable switches 24, 25, which may be implemented as FETs.

It is noted that the power stage 12 in a HBCF topology also comprises two controllable switches and a switch controller which may be different from the controller 30, but these two controllers may also be integrated.

The lamp operates as follows. As long as the lamp is off, lamp current-generating section 10 generates an alternating voltage with a square-wave shape, having a relatively low frequency of the order of, for instance, 100 Hz. The voltage will be of the order of about 250 V, which is sufficient to allow a smooth transition from an ignition phase to an arc phase but is usually insufficient to ignite an arc. During the ignition phase, the high-frequency section 20 generates high- voltage pulses typically of the order of about 3.5 kV. To this end, the controller 30 alternately opens and closes the two controllable switches 24, 25 in counter phase, such that an alternating current is generated in the primary winding 41 of transformer 40, causing an alternating voltage to be generated across the secondary winding 42. More particularly, the controller 30 initially uses a relatively high frequency, of the order of about 45 kHz, and slowly lowers this frequency to a frequency of the order of about 20 kHz. In doing so, the switching frequency approaches a frequency O, which is one third of the resonance frequency £RES, i-e. about 23 kHz in this example. More generally, this frequency is typically of the order of about 25 kHz. When this frequency O is reached, said parallel circuit 42, 43 resonates on the third harmonic of the switching frequency, resulting in high resonance voltage pulses causing ignition. The function of the capacitor 29 in series with the primary winding 41 of transformer 40 is to block DC voltages and currents. The function of the inductor 28 in series with the primary winding 41 of transformer 40 is primarily to limit the current. When the lamp ignites, the controller 30 will turn off the two controllable switches 24, 25, so that no current flows in the primary winding 41 of transformer 40. As far as the lamp current is concerned, only power stage 12 is active; this will be indicated as "OFF mode" of the high-frequency section 20. It is noted that the controller 30 will typically be provided with means for detecting the ignition of the arc, as is known per se and will therefore not be illustrated for the sake of simplicity.

Besides being capable of operating in an ignition mode and in an OFF-mode, the high-frequency section 20 is capable of operating in an arc-straightening mode, in which a ripple current component for the lamp is generated. To this end, the controller 30 alternately opens and closes the two controllable switches 24, 25 in counter phase, at a higher

frequency, typically of the order of about 20-40 kHz, such that an alternating current is generated in the primary winding 41 of transformer 40, causing an alternating current to be generated in the secondary winding 42 and constituting the current ripple. This current ripple has an amplitude which is lower than the amplitude of the ignition pulses, for instance, an amplitude of the order of about 500 mA, which is primarily determined by the inductance of the inductor 28 in series with the primary winding 41 of transformer 40, and by the "resistance" of the burning lamp. The frequency is maintained constant, and the actual value of this fixed frequency can be set in the controller software. In an experimental setup, a value of 33.5 kHz proved to be satisfactory. In any case, the frequency is higher than the normal operating frequency of the power stage 12, and differs from the frequency of the ignition pulses. The frequency is preferably between O and f _RE s-

The controller 30 decides on the transition from the OFF-mode to the arc- straightening mode on the basis of the lamp voltage. To this end, the driver 1 comprises a lamp voltage sensor 50 receiving the lamp voltage as input and providing the controller 30 with a sensor output signal indicating the sensed lamp voltage. The controller 30 compares this received sensor signal with a threshold signal VTH and starts the arc-straightening mode as soon as the sensor signal exceeds the threshold signal VTH- It will be evident to a person skilled in the art that, dependent on the lamp type, a suitable threshold for the lamp voltage is about 80 V, which corresponds to the nominal voltage during the steady state. The arc straightening results in a slight decrease of the lamp voltage, so that the temperature also decreases.

In summary, the present invention provides a device 1 for driving a gas discharge lamp 2, the device comprising: a lamp current-generating section 10 for generating a constant lamp current commutating at a commutation frequency, the lamp current-generating section 10 having output terminals 13, 14; a high-frequency section 20 comprising a waveform generator 21 for generating an alternating current component; a coupling device 40 for adding the alternating current component generated by the high-frequency section 20 to the commutating lamp current generated by the lamp current-generating section.

In an ignition mode, the high-frequency section is used to generate ignition pulses at an ignition frequency which is higher than the commutation frequency.

In an arc-straightening mode, the high-frequency section is used to generate a current ripple at a ripple frequency which is higher than the commutation frequency and differs from the ignition frequency, the ripple having an amplitude which is lower than the amplitude of the ignition pulses. According to the invention, the high-frequency section is thus advantageously used to perform the ignition function as well as the arc-straightening function.

While the invention has been illustrated and described in detail, it will be evident to a person skilled in the art that it is not limited to the disclosed embodiments. Several variations and modifications are possible within the protective scope of the invention as defined in the appending claims.

For instance, in the Figure showing the preferred embodiment, the two power rails 22, 23 are connected to the output of the first stage 11 of the lamp current-generating section 10. Alternatively, the two power rails 22, 23 may be connected to a different source of substantially constant voltage. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention from a study of the drawing, the disclosure, and the appended claims. In the claims, use of the verb "comprise" and its conjugations does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single processor or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. A computer program may be stored/distributed on a suitable medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems. Any reference sign placed between parentheses shall not be construed as limiting the claim.

The present invention has been explained with reference to a block diagram, which illustrates functional blocks of the device according to the invention. It is to be understood that one or more of these functional blocks may be implemented in hardware, in which the function of such a functional block is performed by individual hardware components. However, it is also possible to implement one or more of these functional blocks in software, so that the function of such a functional block is performed by one or more program lines of a computer program or a programmable device such as a microprocessor, a microcontroller, a digital signal processor, etc.