WO2007110800A1

WO2007110800A1 - Operating a gas discharge lamp

Info

Publication number: WO2007110800A1
Application number: PCT/IB2007/050915
Authority: WO
Inventors: Peter Lürkens; Carsten Deppe
Original assignee: Philips Intellectual Property & Standards Gmbh; Koninklijke Philips Electronics N.V.
Priority date: 2006-03-28
Filing date: 2007-03-16
Publication date: 2007-10-04

Abstract

The invention relates to a circuit for operating a gas discharge lamp 400. The circuit comprises an inverter 300 for generating a high frequency current for a gas discharge lamp 400. The inverter 300 includes a single power switch 312, in order to keep the costs of the circuit low. The generated high frequency current is further amplitude modulated, in order to reduce a burning back of the electrodes of the lamp 400 and to increase the stability of the arc of the gas discharge lamp 400, respectively. A controller 500 of the circuit controls the amplitude modulation of the high frequency current that is generated by the inverter 300. The invention relates equally to an apparatus 1 comprising such a circuit and to a method of operating such a circuit.

Description

OPERATING A GAS DISCHARGE LAMP

The invention relates to a circuit, an apparatus and a method for operating a gas discharge lamp.

Gas discharge lamps are well known from the state of the art. High Intensity Discharge (HID) gas discharge lamps, for instance, comprise a tube containing a gas or vapor. Further, two electrodes protrude into the tube. For operating the lamp, a suitable alternating current is supplied to these electrodes such that an arc is established and maintained between them. HID gas discharge lamps have the advantage that they are able to provide a high light emitting efficiency with a short arc. They are used for example as a light source in projectors, like video or PC projectors, but equally in general illumination systems, etc.

One special type of a HID lamp is the Ultra High Pressure (UHP) gas discharge lamp, which may employ for example mercury vapor and tungsten electrodes. UHP gas discharge lamps are used for instance for projection applications, in which the optical demands on the display require arc lengths in the order of lmm.

HID gas discharge lamps can be operated with an alternating current having a low frequency of 50 to 500 Hz. In order to ensure that the light output remains constant, a rectangular current form is used. That is, the absolute value of the current is always the same, only the sign changes.

Usually, at least four power transistors are used for generating the low frequency current. Power transistors are expensive components but due to the low frequency, circuits using only two power transistors would require very large and equally expensive electrolytic capacitors.

When using a higher frequency current with a low power, the arc attaches to the electrode in a punctual manner and burns in a so-called contracted mode. In this mode, the arc position is not stable, which results in a frequent change of position of the arc. If the HID gas discharge lamp is used in a projector, for instance, this can be seen as an annoying flickering in a projected image.

When using a higher frequency current with a high power, the arc attaches to the electrode in a planar manner and burns in a so-called diffuse mode. In this mode, the position of the arc changes only slowly so that a projection is not disturbed. However, a so-called "burning back" of the electrodes occurs. As a result, the gap between the electrodes is increased. A burning back of an electrode is a common reason for a significantly reduced lifetime of a HID gas discharge lamp.

The burning back can be reduced by applying an amplitude modulation to the high frequency current, while, at the same time, arc stability is maintained.

Patent application US 2003/0127995 Al, for example, proposes using a half bridge for providing an amplitude-modulated current having a carrier frequency of up to 500 kHz to a HID lamp for eliminating arc instabilities.

It is an object of the invention to provide a cost efficient power supply for a gas discharge lamp.

The object is reached with a circuit for operating a gas discharge lamp. The circuit comprises an inverter having a single power switch. The inverter is adapted to generate an amplitude-modulated high frequency current for a gas discharge lamp. The circuit further comprises a controller adapted to control an amplitude modulation of a high frequency current generated by the inverter.

The object is moreover achieved with an apparatus comprising such a circuit, for instance a projector.

The object is moreover achieved with a method of operating a gas discharge lamp. The method comprises generating a high frequency current by means of an inverter having a single power switch. The high frequency current is amplitude modulated. The method further comprises applying the amplitude-modulated high frequency current to the gas discharge lamp. On the one hand, an amplitude modulation of a high frequency lamp current is suited to prevent the problems arising with a high frequency lamp current having constant amplitude. When using an amplitude modulation, a tip- or tower-like elevation is formed on the surface of the electrode. The arc will fix to this elevation so that a variation of the arc position is no longer possible, even when using a low power lamp current. At the same time, the burning back is reduced to a value, which corresponds to an operation in the contracted mode, even when using a high power lamp current.

On the other hand, a high frequency lamp current can be provided even by an inverter employing only a single power switch without requiring large electrolytic capacitors.

It is thus an advantage of the invention that it allows providing a power supply for a gas discharge lamp which is particularly cost efficient, which can be implemented with small dimensions and which has nevertheless a high performance.

The single power switch of the inverter can be for example a power transistor. The inverter comprising a single power switch can be for instance, though not exclusively, a class E converter. A class E converter has the advantage that it is highly efficient.

The gas discharge lamp can be for example an HID gas discharge lamp, like a UHP gas discharge lamp.

It is to be understood that the generated high frequency lamp current can be of any frequency exceeding 1 kHz. Advantageously, though, the generated high frequency current has a frequency above the acoustical resonance of the employed gas discharge lamp. Otherwise, acoustic or mechanical oscillations in the gas discharge lamp may result in an instable arc. As a result, the quality of light could be insufficient at least for projection applications. In particularly severe cases, the gas discharge lamp could even be destroyed. In the case of a HID gas discharge lamp, the inverter is advantageously adapted to generate a high frequency current having a frequency of at least 150 kHz. In the case of a UHP gas discharge lamp, the inverter is advantageously adapted to generate a high frequency current having a frequency of at least 3 MHz.

The amplitude modulation may have a low frequency of less than 500 Hz, for example a frequency in a range of 40 Hz to 300 Hz. The extent of the amplitude modulation may further lie in a range of 5-50% of the basic amplitude of the high frequency current.

The amplitude modulation can be achieved in various ways.

If the controller is linked to the inverter, the controller may be adapted for example to control a switching of the single power switch to obtain a desired amplitude modulation.

When the switching of the single power switch is adjusted for generating the amplitude modulation, though, the inverter might occasionally have to be operated at a less favorable operating point, which might reduce the efficiency of the inverter.

Alternatively, the controller could thus be adapted for example to control an input voltage to the inverter to obtain a desired amplitude modulation.

In case the circuit comprises a pre-conditioner adapted to provide an input voltage to the inverter, the controller may be linked to this end to the pre- conditioner. The controller may then be adapted to control the operation of the pre- conditioner to obtain a desired amplitude modulation. A pre-conditioner may be needed anyhow, if the inverter requires a particular input voltage, or in order to fulfill mains harmonics regulations. If the inverter is realized with a class E converter, for example, an input voltage of about 100 V is best for supporting the power and frequency range that is needed for a UHP gas discharge lamp. Further, those Metal Oxide Semiconductor Field Effect Transistors (MOSFETs), which are optimally suited as power switches for a power class of 100-200 W, generally require an input voltage of approximately 80- 120V, which lies below the mains voltage.

Depending on the available power source and the voltage requirements of the inverter, the pre-conditioner can be for example a step-down converter, like a buck converter, a flyback converter, a forward converter or a single-ended primary inductance converter (SEPIC), etc., or a step-up converter.

The amplitude modulation can further be realized for instance by a source or emitter modulation, in the manner of the so-called "cathode" modulation. Source and emitter modulation is widely used in the field of radio transmitters and known as a so- called cathode modulation from the tube technology. In general, the term "cathode" stands for the reference terminal of the amplifying input of a triode component, like the cathode of a tube triode, the emitter of a transistor or the source of a MOSFET, etc. Instead of connecting the source-terminal directly to ground, it is connected to a variable voltage source. The high frequency current is shortcut via a capacitor to ground. The modulation is achieved in that the low frequency alternating voltage generates a corresponding current modulation in the transistor, which results in turn in a modulation of the high frequency signal.

In general, the invention can be realized in a particularly simple and cost efficient manner, if the control of a component that is present anyhow for some other function is modified for obtaining the amplitude modulation.

In some cases, the amplitude modulation may also be synchronized to the operation of some other component. Such another component can be any component that is operated with a known frequency and that belongs to an apparatus comprising the proposed circuit. For example, if the proposed circuit is provided for a projector comprising a display, the controller may be adapted to synchronize the amplitude modulation to an operating frequency of this display.

In sequentially operating image generators, like Digital Light Processing (DLP™) or Digital Micro Mirror Device (DMD) displays using a rotating color wheel, the image quality can be improved, if the temporal course of the lamp current is synchronized with separate phases of the image generation. As a result, the input lamp current may have the same course for each presented frame in the same temporal sequence. Thus, errors in the representation of color grades and brightness grades can be avoided. A synchronization might even enable an optimization, for instance such that in dark parts of an image, a higher number of grades is achieved than in image parts with medium or higher brightness. This is of advantage for typical feature film material.

The invention is particularly suited for use with projectors, in particular 3 panel LCD projectors, and with lamps for general illumination, but it can be employed with various other systems as well.

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter. Fig. 1 is a schematic block diagram of an apparatus according to an embodiment of the invention.

Fig. 2 is a simplified circuit diagram illustrating details of a first exemplary implementation of the apparatus of Figure 1.

Fig. 3 is a simplified circuit diagram illustrating details of a second exemplary implementation of the apparatus of Figure 1.

Fig. 4 is a simplified circuit diagram illustrating details of a third exemplary implementation of the apparatus of Figure 1.

Fig. 5 is a simplified circuit diagram illustrating details of a fourth exemplary implementation of the apparatus of Figure 1.

Fig. 6 is a simplified circuit diagram illustrating details of a fifth exemplary implementation of the apparatus of Figure 1.

Fig. 7 is a flow chart illustrating an operation of the apparatus of Figure

1.

Fig. 8 is a diagram presenting an amplitude modulated high frequency current that may be provided to a UHP gas discharge lamp of the apparatus of Figure 1.

Fig. 9 is a diagram presenting an alternative amplitude modulated high frequency current that may be provided to an UHP gas discharge lamp of the apparatus of Figure 1.

In the figures, state-of-the-art demagnetizing means for the presented transformers are omitted for better readability.

Figure 1 is a schematic block diagram of an exemplary apparatus in which the invention can be implemented.

The apparatus is an LCD projector 1 that is connected to an AC voltage source 2, for instance a mains connection.

Within the projector 1, the voltage source 2 is connected via a rectifier 100, a pre-conditioner 200 and an inverter 300 to a UHP gas discharge lamp 400. In addition, a controller 500 is connected to the pre-conditioner 200 and/or to the inverter 300. The lamp 400 is used as a light source for a 3 panel LCD display 600 of the projector 1.

The voltage source 2 provides an AC voltage of 230 V. The rectifier 100 converts this AC voltage into a DC voltage. The pre-conditioner 200 is a suitable step- down DC to DC converter and used for a power factor correction. That is, it converts the DC voltage provided by the rectifier 100 into a DC voltage which is suited as an input voltage for the employed inverter 300, for instance to a voltage of about 100 V. The inverter 300 generates a high frequency current of e.g. 13.56 MHz for operating the UHP gas discharge lamp 400. The inverter 300 uses a single power switch for realizing this function. The light provided by the UHP gas discharge lamp is used for projecting an image generated by the display 600 onto some screen (not shown).

The controller 500 may be used for controlling the regular operation of both the pre-conditioner 200 and the inverter 300. In addition, it continuously adjusts one of these components to ensure that the high frequency current output by the inverter 300 is amplitude modulated by way of example by around 10 % with a frequency of about 100 Hz. The modulating frequency may be synchronized for example with the operating frequency of the display 600, which usually lies in a range of 50 Hz to 200 Hz.

Figures 2 to 6 are simplified circuit diagrams of five exemplary implementations of the components 100, 200 and 300 of Figure 1. In each of these implementations, the controller 500 adjusts the pre-conditioner 200 for causing the amplitude modulation.

In the implementation presented in Figure 2, the pre-conditioner 200 is a flyback pre-conditioner.

The rectifier 100 comprises four diodes 111 - 114. A first terminal of the AC voltage source 2 is connected via the first diode 111 to an input of the flyback pre- conditioner 200. A second terminal of the AC voltage source 2 is connected via the second diode 112 to the input of the flyback pre-conditioner 200. A grounding of the projector 1 is connected via the third diode 113 to the first terminal of the AC voltage source 2. The grounding of the projector 1 is further connected via the fourth diode 114 to the second terminal of the AC voltage source 2. Within the flyback pre-conditioner 200, the input of the flyback pre- conditioner 200 is connected via a primary winding of a transformer 211 and a transistor

212 to ground. A first terminal of the secondary winding of transformer 213, which has an opposite winding direction compared to the primary winding, is connected via a diode

213 and a capacitor 214 to a second terminal of the secondary winding of the transformer 213. The voltage across capacitor 214 is applied as a supply voltage to the input of the inverter 300.

The inverter 300 is a class E converter. The supply voltage provided to the class E converter 300 is applied to a series connection of a choke 311 and a transistor 312. A capacitor 313 is arranged in parallel to transistor 312. Capacitor 313 maybe considered to be realized partially or completely by the output capacity of the transistor 312. In addition, a series connection of a further capacitor 314, an inductance 315 and a capacitor 316 is arranged in parallel to transistor 312. Capacitors 314 and 316 and inductance 315 form a resonating filter, with matching of the lamp impedance to the requirements of the converter. The UHP gas discharge lamp 400 is switched in parallel to capacitor 316.

In the implementation of Figure 3, the pre-conditioner 200 is a buck pre- conditioner.

The rectifier 100 is implemented and arranged in the same way as in the implementation presented in Figure 2. Thus, corresponding components have been provided with corresponding reference numbers.

Within the buck pre-conditioner 200, the input ofthe buck pre-conditioner 200 is connected via a transistor 221 , a smoothing inductor 222 and a capacitor 223 to ground. The grounding terminal is further connected via a diode 224 to the junction between transistor 221 and smoothing inductor 222. The voltage across capacitor 223 is applied as a supply voltage to the input ofthe inverter 300.

The inverter 300 is implemented and arranged in the same way as in the implementation presented in Figure 2. Thus, corresponding components have been provided with corresponding reference numbers.

In the implementation of Figure 4, the pre-conditioner 200 is a forward- converter pre-conditioner. The rectifier 100 is implemented and arranged in the same way as in the implementation presented in Figure 2. Thus, corresponding components have been provided with corresponding reference numbers.

Within the forward-converter pre-conditioner 200, the input of the forward-converter pre-conditioner 200 is connected via a primary winding of a transformer 231 and a transistor 232 to ground. The sequence order of the transistor and the transformer is irrelevant for the principle of the circuit. A first terminal of the secondary winding of transformer 231, which has the same winding direction as the primary winding, is connected via a diode 233, a smoothing inductor 234 and a capacitor 235 to a second terminal of the secondary winding of transformer 231. The link between capacitor 235 and the secondary winding of transformer 231 is further connected via a further diode 236 to the link between diode 233 and smoothing inductor 234. The voltage across capacitor 235 is applied as a supply voltage to the input of the inverter 300.

In general, the smoothing inductor of a buck pre-conditioner or of a forward-converter pre-conditioner could also be combined with the choke of the class E converter 300. This option is illustrated for both cases in Figures 5 and 6.

The implementation of Figure 5 corresponds basically to the implementation presented in Figure 3. The buck pre-conditioner 200 of Figure 5, however, comprises only a transistor 241 and a diode 244. The first terminal of transistor 241 is connected to the input of the pre-conditioner 200, and diode 244 is connected between ground and the second terminal of transistor 241. The voltage across the diode 244 is applied to the input of the class E converter 300. The function of smoothing inductor 222 of the circuit of Figure 3 is realized by the choke 311 of the class E converter 300.

The implementation of Figure 6 corresponds basically to the implementation presented in Figure 4. The input of the forward-converter pre- conditioner 200 is connected again via a primary winding of a transformer 251 and a transistor 252 to ground. In this case, however, a first terminal and a second terminal of the secondary winding of transformer 251 are connected to each other via a respective diode 253 and 256. The voltage across diode 256 is applied as a supply voltage to the input of the class E converter 300. The function of smoothing inductor 234 of the circuit of Figure 4 is realized by choke 311 of the class E converter 300.

The operation of the projector 1 with either of the presented implementations is illustrated in the flow chart of Figure 7. Each of the rectifier 100, the employed pre-conditioner 200 and the class E converter 300 may operate in a conventional manner. Only the control of the conditioner 200 that is applied by the controller 500 is adapted in accordance with the invention.

The AC voltage provided by the AC voltage source 2 is rectified by the rectifier 100 to obtain a DC voltage (step 701).

The controller 500 controls the gate voltage of the transistor 212, 221, 232, 241, 252 of the employed pre-conditioner 200 such that a basic DC voltage of 80- 120 V is obtained. The controller 500 further varies the gate voltage of the transistor 212, 221, 232, 241, 252 of the employed pre-conditioner 200 such that the amplitude of the adjusted voltage varies by +/- 10 % (step 702)

The class E converter 300 converts the amplitude modulated supply voltage into an amplitude modulated very high frequency current of 13.56 MHz. This frequency is well above the acoustic resonance of a UHP gas discharge lamp 400 and practically all other HID lamps. The controller 500 controls the gate voltage of the transistor 312 of the class E converter 300 to this end such that the desired frequency is achieved. Due to the amplitude modulated supply voltage, also the generated very high frequency current comprises automatically an amplitude modulation of at least +/- 10 % as well (step 703).

The generated amplitude modulated very high frequency current is then applied to the UHP gas discharge lamp 400, which operates in a stable manner (step 704).

It is to be noted that the amplitude modulation could equally be achieved with a fixed DC voltage applied to the class E converter 300. In this case, the controller 500 controls the gate voltage of the transistor 212, 221, 232, 241, 252 of the employed pre-conditioner 200 only such that a basic DC voltage of 80-120 V is obtained. In addition, the controller 500 varies the gate voltage of the transistor 312 of the inverter class E converter 300 such that a very high frequency current is generated and that the amplitude of the very high frequency current varies by +/- 10 %.

Figure 8 presents an exemplary lamp voltage and an exemplary lamp current that may be applied to the UHP gas discharge lamp 400 of the projector 1 of Figure 1.

The lamp voltage is depicted in an upper diagram over time. It is a very high frequency voltage, with an almost constant amplitude.

The associated lamp current is depicted in a lower diagram over time. It is a very high frequency current with a sinusoidal amplitude modulation.

It has to be noted that for illustration purposes, the time scale was selected such, that the amplitude modulation becomes visible. Thus, the detailed waveforms of the very high frequency of voltage and current can no longer be distinguished in the same diagram.

Figure 9 presents an alternative exemplary lamp current over time that is applied to the UHP gas discharge lamp 400. The lamp current is again a very high frequency current. In this case, however, a square-wave amplitude modulation has been employed.

Again, the very high frequency component of the current can no longer be distinguished in relation to the modulation frequency.

With the presented amplitude modulation, the electrode burn back of the UHP gas discharge lamp 400 is strongly reduced compared to the behavior under unmodulated high frequency conditions, and a lamp lifetime is achieved that is similar to the lifetime that is achieved with a low frequency operation. At the same time, the electrodes exhibit the formation of tip structures that serve as a preferred arc attachment location. Thereby, the tendency of arc jumping and the associated flicker effects in the image projection is suppressed. In addition, the use of an inverter 300 employing a single power transistor 312 ensures that the production of the projector 1 is cost efficient.

It is understood that the described embodiments of the invention represent only some of a great variety of possible embodiments of the invention. For example, the amplitude modulation could be achieved by amplitude modulating the supply voltage that is provided by a voltage source even before it is input to a pre- conditioner. Moreover, reference signs in the claims are not intended to limit the scope of the claims but only to facilitate an easy understanding of the claims. It is further understood that the term "comprising" in the claims does not exclude other elements or steps, and that the terms "a" or "an" in the claims does not exclude a plurality.

Claims

CLAIMS:

1. Circuit for operating a gas discharge lamp (400), said circuit comprising an inverter (300) having a single power switch (312), said inverter (300) being adapted to generate an amplitude modulated high frequency current for a gas discharge lamp (400); and a controller (500) adapted to control an amplitude modulation of a high frequency current generated by said inverter (300).

2. Circuit according to claim 1, wherein said gas discharge lamp (400) is a high intensity discharge lamp, and wherein said inverter (300) is adapted to generate a high frequency current having a frequency of at least 150 kHz.

3. Circuit according to claim 1, wherein said gas discharge lamp (400) is an ultra high pressure lamp, and wherein said inverter (300) is adapted to generate a high frequency current having a frequency of at least 3 MHz.

4. Circuit according to claim 1, wherein said controller (500) is adapted to cause an amplitude modulation, which has a frequency of less than 500 Hz.

5. Circuit according to claim 1, wherein said controller (500) is linked to said inverter (300), and wherein said controller (500) is adapted to control a switching of said single power switch (312) to obtain a desired amplitude modulation.

6. Circuit according to claim 1, wherein said controller (500) is adapted to control an input voltage to said inverter (300) to obtain a desired amplitude modulation.

7. Circuit according to claim 1, further comprising a pre-conditioner (200) adapted to provide an input voltage to said inverter (300), wherein said controller (500) is linked to said pre-conditioner (200), and wherein said controller (500) is adapted to control an operation of said pre-conditioner (200) to obtain a desired amplitude modulation.

8. Circuit according to claim 1, wherein said pre-conditioner (200) is adapted to perform a step-down conversion.

9. Circuit according to claim 1, wherein said pre-conditioner (200) is one of a flyback converter (211-214), a buck converter (221-224; 241-244), a forward converter (231-236; 251-256), and a single-ended primary inductance converter.

10. Apparatus (1) comprising a circuit (300,500) according to claim 1.

11. Apparatus (1) according to claim 10, further comprising another component (600), which is operated at a known frequency, wherein said controller (500) is adapted to synchronize said amplitude modulation to an operating frequency of said other component (600).

12. Method of operating a gas discharge lamp (400), said method comprising: generating a high frequency current by means of an inverter (300) having a single power switch (312), wherein said generated high frequency current is amplitude modulated; and applying said amplitude modulated high frequency current to said gas discharge lamp (400).