WO2010125487A1

WO2010125487A1 - Low frequency fluorescent lamp driving, and dimming thereof

Info

Publication number: WO2010125487A1
Application number: PCT/IB2010/051603
Authority: WO
Inventors: Schelte Heeringa
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2009-04-29
Filing date: 2010-04-14
Publication date: 2010-11-04

Abstract

A lamp driving circuit and method for driving a gas discharge lamp provided with supply electrodes, and an ignition electrode. A DC supply current is generated by a buck converter having a buck converter operating frequency. An inverter circuit comprising at least two switches in a bridge arrangement inverts the DC supply current into an alternating lamp supply current having an alternating frequency. An ignition circuit, configured to be connected to the ignition electrode, generates a lamp ignition pulse. A control circuit is coupled to the buck converter, the inverter circuit, and the ignition circuit for controlling the operation thereof. The buck converter is operated by pulse width modulation, PWM, switching the buck converter on and off with a PWM frequency, which is lower than the buck converter operating frequency. The alternating frequency of the inverter circuit is lower than the PWM frequency. The ignition circuit generates at least one lamp ignition pulse a predetermined time period after the buck converter is switched on in its PWM switching.

Description

Low frequency fluorescent lamp driving, and dimming thereof

FIELD OF THE INVENTION

The invention relates to the field of gas discharge lamp driving, and more specifically to a driving circuit and method for driving and dimming a fluorescent lamp.

BACKGROUND OF THE INVENTION

Commonly, electronic driving circuits, or ballasts, operate to energize gas discharge lamps (such as HID lamps and fluorescent lamps) at high frequencies, e.g. 20-100 kHz. Such high frequencies allow the use of relatively small circuit components. The lamp is ignited by applying a high voltage signal to the supply electrodes of the lamp by means of a resonant circuit. The lamp ignites when this high voltage reaches the lamp breakdown voltage.

Some gas discharge lamps have a metal plate or strip in the (e.g. glass) enclosure of the lamp or on the outside thereof, connected to a lamp electrode to decrease the breakdown voltage of the lamp. In some types of lamps, such an electrode is connected between the lamp terminals to enable flicker-free deep dimming.

At the high operating frequency, the lamp is a considerable source of RFI (Radio Frequency Interference) or EMI (ElectroMagnetic Interference) emission or radiation. Several RFI/EMI radiation suppression components are needed in the lamp driving circuit to comply with RFI/EMI regulations. In most cases a protective earth line connection is utilized to reach the required RFI/EMI radiation level.

However, in some (mainly consumer) applications, a protective earth connection is not available, while a deep dimming architecture is required, which causes more RFI/EMI radiation than other electronic driving circuits. These factors make RFI/EMI radiation suppression a complicated task which results in expensive measures. In general, RFI/EMI emission is lower as operating frequencies of the lamp decrease. Accordingly, from an RFI/EMI perspective the lamp operating frequency should be as low as possible. However, the existing lamp driving circuit topologies known for HID lamps are not suitable for low frequency operation because then the resonant ignition is no longer possible. Reference US 7,023,144 discloses a lamp driving circuit and method for operating a high pressure discharge, HID, lamp comprising a discharge vessel in which a pair of opposed electrodes are located and on which a trigger (ignition) electrode is located. Ignition of the HID lamp requires a high ignition voltage. The lamp driving circuit, comprising a buck converter and a full bridge inverter, supplies the lamp with rectangular alternating waves in the range of 60-1,000 Hz, alternated with rectangular alternating waves in the range of 5-50 Hz. The trigger electrode is driven by a starter circuit. An inductor connected in series with the lamp limits the current through the lamp.

In the lamp driving circuit and method according to US 7,138,769, a problem exists in that at the low frequencies of the rectangular alternating waves supplied to the HID lamp, although an RFI/EMI emission is relatively low, dimming is difficult when the RFI/EMI emission is to be kept at a minimum.

SUMMARY OF THE INVENTION It would be desirable to provide a lamp driving circuit and method producing a low RFI emission. It would further be desirable to provide a lamp driving circuit and method allowing deep dimming of fluorescent lamps. It would further be desirable to provide a lamp driving circuit and method allowing the use of low-cost components.

To better address one or more of these concerns, in a first aspect of the invention a lamp driving circuit for driving a gas discharge lamp provided with supply electrodes, and an ignition electrode is provided. The lamp driving circuit comprises: a current supply for supplying a DC supply current; an inverter circuit coupled to the current supply for generating an alternating lamp supply current having an alternating frequency, the inverter circuit comprising at least two switches in a bridge arrangement; an ignition circuit configured to be connected to the ignition electrode for generating a lamp ignition pulse; and a control circuit coupled to the current supply, the inverter circuit, and the ignition circuit for controlling the operation thereof. The control circuit is configured to control the operation of the current supply by pulse width modulation, PWM, switching the current supply on and off with a PWM frequency. The control circuit is further configured to control the alternating frequency of the inverter circuit to be lower than the PWM frequency. The control circuit is further configured to control the ignition circuit to generate at least one lamp ignition pulse a predetermined time period after the control circuit switches the buck converter on in its PWM switching. In an embodiment, the current supply is a buck converter having a buck converter operating frequency for converting a DC supply voltage obtained by rectification from an AC mains supply voltage.

In a further aspect of the invention, a method of driving a gas discharge lamp provided with supply electrodes, and an ignition electrode is provided. The method comprises: providing a DC supply current; inverting the DC supply current into an alternating lamp supply current having a alternating frequency; supplying the alternating lamp supply current to the supply electrodes of the gas discharge lamp; switching the DC supply current on and off in pulse width modulation, PWM, mode with a PWM frequency, the alternating frequency being lower than the PWM frequency; and supplying at least one lamp ignition pulse to the ignition electrode a predetermined time period after the DC supply current is switched on in its PWM mode.

These and other aspects of the invention will be more readily appreciated as the same becomes better understood by reference to the following detailed description and considered in connection with the accompanying drawings in which like reference symbols designate like parts.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 schematically depicts a prior art lamp driving circuit comprising a half bridge converter.

Figure 2 schematically depicts a prior art lamp driving circuit comprising a full bridge converter driving a gas discharge lamp having an ignition electrode.

Figure 3 schematically depicts an embodiment of a lamp driving circuit of the present invention comprising a full bridge converter driving a gas discharge lamp having an ignition electrode.

Figure 4 schematically depicts an embodiment of the lamp driving circuit of Figure 3 in more detail.

Figure 5 schematically depicts another embodiment of a lamp driving circuit of the present invention comprising a half bridge converter driving a gas discharge lamp having an ignition electrode.

Figure 6 schematically depicts a further embodiment of a lamp driving circuit of the present invention comprising a full bridge converter driving a gas discharge lamp having an ignition electrode. Figure 7 schematically depicts an embodiment of a pulse generator for use in the lamp driving circuit of the present invention.

Figures 8a and 8b show graphs of currents in the lamp driving circuit of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

Figure 1 shows a prior art half bridge converter comprising a first switch 101 and a second switch 102 connected in series between a first supply voltage terminal 103 and a second supply voltage terminal 104. The first and second switch 101, 102 are shown in Figure 1 as electronic switches, in particular as MOSFET (Metal Oxide Semiconductor Field Effect Transistors) type transistors, but other types of transistors, such as bipolar transistor, or switchable semiconductor elements may be used instead. The same goes for the switches shown in the remaining Figures. A supply voltage applied at the first and second supply voltage terminals 103, 104 may be a DC voltage, obtained by rectifying an AC supply mains voltage, or by a battery and the like. The same goes for the supply voltage in the remaining Figures. Between a node 105 connecting the first switch 101 and the second switch 102, and the second supply voltage terminal 104, a resonant circuit comprising a series connection of an inductor 106 and a capacitor 107 is connected. At a node 108 connecting the inductor 106 and the capacitor 107, a first electrode 111 of a gas discharge lamp 110 is connected. A second electrode 112 of the gas discharge lamp 110 is connected to a node 113 connecting two capacitors 114 connected in series between the first and second supply voltage terminals 103, 104. The gas discharge lamp 110 has heating terminals 130, 131 to supply heating currents to the corresponding first and second electrodes 111, 112, respectively.

The first and second switches 101, 102 have control terminals 115, 116, respectively, connected to a level shifter and switch driving circuit 117, which is controlled by a control circuit 118 or the like, connected to the level shifter and switch driving circuit 117.

In operation, the first and second switches 101, 102 are operated by the level shifter and switch driving circuit 117 at high frequencies of e.g. 20-100 kHz or higher. An ignition of the gas discharge lamp 110 is produced by a resonant voltage at the node 108 at a resonance frequency determined by the inductance of the inductor 106 and the capacitance of the capacitor 107. During the ignition phase of operation, the operating frequency of the level shifter and switch driving circuit 117 is sweeped across a frequency range along the resonance frequency. Once the lamp is ignited, an appropriate predetermined operating frequency of the level shifter and switch driving circuit 117 away from the resonance frequency is selected by the control circuit 118. The current in the gas discharge lamp 110 is then determined by the inductor 106 and the operating frequency. Since the operating frequency is high, the gas discharge lamp will be a major source of RFI/EMI radiation, which is undesirable.

Figure 2 shows a prior art full bridge converter comprising a first switch 201 and a second switch 202 connected in series between a first supply voltage terminal 205 and a second supply voltage terminal 206, and a third switch 203 and a fourth switch 204 connected in series between the first supply voltage terminal 205 and the second supply voltage terminal 206. Between a node 207 connecting the first switch 201 and the second switch 202, and a node 208 connecting the third switch 203 and the fourth switch 204, a series arrangement of an inductor 209 and a gas discharge lamp 210 is connected. The gas discharge lamp 210 has a first electrode 211 connected to the inductor 209, and a second electrode 212 connected to the node 208. The gas discharge lamp 210 is provided with an ignition electrode 213, which may be implemented as an electrically conducting (e.g. metal) structure (e.g. a plate or wire or strip or meshed structure) provided on, or embedded in an enclosure of the gas discharge lamp 210. The ignition electrode 213 is connected to a node 214 between capacitors 215, 216 connected in series between the electrodes 211, 212. The inductor 209 and the capacitors 215, 216 form a resonance circuit. The control terminals (gates) of the first, second, third and fourth switches

201, 202, 203, 204 are connected to a level shifter and switch driving circuit 217, which is controlled by a microcontroller 218 or the like, connected to the level shifter and switch driving circuit 217.

In operation, the first and fourth switches 201, 204 conduct simultaneously, alternating with the second and third switches 202, 203 conducting simultaneously, at high frequencies up to e.g. 100 kHz or higher. An ignition of the gas discharge lamp 210 is produced by a resonant voltage at the first electrode 211 of the gas discharge lamp 210 at a resonance frequency determined by the inductance of the inductor 209 and the capacitance of the capacitors 215, 216. The operating frequency may be fixed, where the resonance circuit resonates at the third harmonic frequency of the (square wave) full bridge voltage as supplied through the first and second supply voltage terminals 205 and 206. When the lamp is ignited, the inductor 209 and the operating frequency of the level shifter and switch driving circuit 217 selected by the control circuit 218 determine the current flowing in the gas discharge lamp 210. Since this operating frequency is high, the gas discharge lamp will be a major source of RFI/EMI radiation, which is undesirable.

Figure 3 shows a lamp driving circuit in an embodiment of the present invention comprising a first switch 301 and a second switch 302 connected in series between a first supply terminal 305 and a second supply terminal 306, and a third switch 303 and a fourth switch 304 connected in series between the first supply terminal 305 and the second supply terminal 306. A gas discharge lamp 310 has a first electrode 311 connected to a node 307 between the first switch 301 and the second switch 302, and a second electrode 312 connected to a node 308 between the third switch 303 and the fourth switch 304. The switches 301-304 form an inverter circuit. The gas discharge lamp 310 is provided with an ignition electrode 313 connected to a pulse generator 314. The gas discharge lamp 310 has heating terminals 330, 331 to supply heating currents to the corresponding first and second electrodes 311, 312, respectively.

A current supply or current source 320 connected to the first supply terminal 305 is adapted to supply current to the gas discharge lamp 310. The current source is controllable to be switched on and off in a pulse width modulation, PWM, mode with a PWM frequency. The current source 320 is also controllable to supply a current of varying average value, when it is switched on in PWM mode. A capacitor 322 connected between the first supply terminal 305 and the second supply terminal 306 is adapted to smooth the DC voltage across the gas discharge lamp 310 in operation.

The current source or current supply 320 may be implemented as a buck converter, but may also be implemented as a flyback converter, or as an arbitrary converter functioning as a current source through the use of a closed feedback loop.

The operation of the lamp driving circuit of Figure 3 is controlled by a control circuit (not shown). In operation, the first and fourth switches 301, 304 conduct simultaneously, alternating with the second and third switches 302, 303 conducting simultaneously. The frequency of operation of the inverter circuit is low, e.g. lower than 100 Hz, or lower than 10 Hz, or lower than 1 Hz, or lower than 0.1 Hz. The frequency of operation of the inverter circuit is lower than the PWM frequency. An ignition of the gas discharge lamp 310 is produced by the pulse generator 314 generating at least one voltage pulse at the ignition electrode 313 a predetermined time period after the current source 320 is switched on. The predetermined time period may be e.g. between 1 and 100 μs.

Figure 4 shows a lamp driving circuit of Figure 3 in an embodiment in more detail. The current source 320 is implemented as a buck converter comprising a series arrangement of a buck converter switch 401 and an inductor 402 between a supply voltage terminal 405 and the first supply terminal 305, and a diode 403 having its cathode connected to a node 404 and its anode connected to the second supply terminal 306. Control terminals (gates) of the first, second, third and fourth switches 301, 302, 303, 304 and the buck converter switch 401 are connected to a control circuit 410, either directly, or indirectly through a level shifter 411. The heating terminals 330, 331 of the gas discharge lamp 310 are connected to electrode heating circuits 340, 341, respectively, which are also connected to the control circuit 410 to be controlled thereby.

In operation, the buck converter may operate at an operating frequency of e.g. 100 kHz or higher in discontinuous (boundary condition) mode to facilitate control of the DC current supplied by the current source 320 implemented as the buck converter. In the discontinuous mode, the buck converter switch 401 is closed when the current in the diode 403 is (or has become again) zero, whereby the average current basically is controlled by controlling the turn-on time (duty cycle) of the buck converter switch 401. The discontinuous mode leads to low (the lowest of all modes) switching losses, and to small inductor size. There are no reverse recovery losses in the diode 403. A closed loop current control loop is unnecessary in the discontinuous mode. A further explanation of the operation of the buck converter is given below by reference to Figures 8a, 8b.

The buck converter is operated in a pulse width modulation, PWM, mode at a PWM frequency. During a pulse of the PWM mode, the buck converter is operated in the discontinuous mode at a buck converter frequency which is higher than the PWM frequency. At the start of a pulse in the PWM mode, an ignition voltage is supplied to the ignition electrode 313 by the pulse generator 314. The direction of current flow through the gas discharge lamp is reversed by causing the switches 301 and 304 to conduct in alternation with the causing the switches 302 and 303 to conduct at an inverter frequency which is lower than the PWM frequency. For example, the buck converter frequency may be 20-100 kHz, the PWM frequency may be 100 Hz (e.g. triggered by a 100 Hz ripple on a rectified 50 Hz AC mains supply voltage), and the inverter frequency may be 0.001 Hz. Each pair of switches 301, 304 and 302, 303 is closed when the buck converter current is zero. In operation, at the low operation frequency of the full bridge circuit, the heating circuit 340 or 341 at the first or second electrode 311, 312 acting as an anode can be switched off temporarily, since only the cathode needs to be heated to stably maintain a DC current in the gas discharge lamp 310. As from the start of a predetermined period of time preceding a current reversal in the gas discharge lamp 310, the first or second electrode 311, 312 acting as anode is heated (preheated) again by its corresponding heating circuit 340, 341, so that when the current reverses, this electrode has a predetermined temperature to act as a cathode. At the same time, the heating circuit 340, 341 of the electrode formerly acting as the cathode, then acting as anode, may be switched off. Switching off the heating of one of the first and the second electrodes 311, 312 of the gas discharge lamp 310 improves the efficiency of (operating) the gas discharge lamp 310, which may compensate a lower lamp efficiency at the lower operating frequencies according to the present invention.

Figure 5 shows a buck converter comprising a first switch 501 and a second switch 502 connected in series between a first supply voltage terminal 505 and a second supply voltage terminal 506. A half bridge converter comprises a third switch 503 and a fourth switch 504 connected in series between the first supply voltage terminal 505 and the second supply voltage terminal 506. Between a node 507 connecting the first switch 501 and the second switch 502, and a node 508 connecting the third switch 503 and the fourth switch 504, a series arrangement of an inductor 509, being part of the buck converter, and a gas discharge lamp 510 is connected. The gas discharge lamp 510 has a first electrode 511 connected to the inductor 509, and a second electrode 512 connected to the node 508. The gas discharge lamp 510 is provided with an ignition electrode 513 connected to a pulse generator 314. The gas discharge lamp 510 has heating terminals 530, 531 to supply heating currents to the corresponding first and second electrodes 511, 512, respectively. A capacitor 522 connected between the first electrode 511 and the second supply terminal 506 is adapted to smooth the voltage across the gas discharge lamp 510 in operation.

The first and second switches 501, 502 have control terminals (gates) 515, 516, respectively, connected to a level shifter and switch driving circuit 517, which is controlled by a control circuit 518 or the like, connected to the level shifter and switch driving circuit 517. The control terminals (gates) of the third and fourth switches 503, 504 are directly or indirectly, through a level shifter 519, connected to the control circuit 518.

In operation, the level shifter and switch driving circuit 517 operates one of the first and the second switch 501, 502 at a high frequency of e.g. 100 kHz, while the other one of the first and the second switch 502, 501 acts as a diode. In this way, the first and the second switches 501, 502 and the inductor 509 implement a buck converter.

In operation, the third and fourth switches 503, 504 are controlled by the control circuit 518 to conduct in alternation at a low frequency, e.g. lower than 100 Hz, or lower than 10 Hz, or lower than 1 Hz, or lower than 0.1 Hz, or even lower. An ignition of the gas discharge lamp 510 is produced by the pulse generator 314 generating at least one voltage pulse at the ignition electrode 513. The operation of the buck converter of Figure 5 is designed similar to that of the buck converter of Figure 4, and as further explained with reference to Figures 8a and 8b.

The lamp driving circuit of Figure 5 is more economical than the lamp driving circuit of Figure 4, since the buck converter switch 401 and the diode 403 present in the lamp driving circuit of Figure 4 are absent in the lamp driving circuit of Figure 5.

Again returning to Figure 4, the full bridge circuit comprising first, second, third and fourth switches 301, 302, 303 and 304, can reverse the lamp polarity when the buck converter comprising buck converter switch 401, inductor 402 and diode 403, is switched off (i.e. when the current outputted by the buck converter is zero). This means that the switching losses are very low, and that slow switching devices can be used, such as bipolar transistors or MOSFETs with slow gate drivers.

The maximum voltage across the first, second, third and fourth switches 301, 302, 303 and 304 is equal to the lamp voltage, which may be e.g. about 80-100 V. Therefore, switches with a lower voltage rating than the voltage on the supply voltage terminal 405 can be applied. Usually the buck converter input voltage is generated in a rectifier circuit (which may include a power factor correction circuit) with an output voltage of e.g. about 380- 420 V.

Having the lower voltage and switching speed requirements on the switches will result in lower costs of the lamp driving circuit when compared to the solution explained originally with reference to Figures 3 or 4.

Figure 6 shows an alternative embodiment of the lamp driving circuit of Figure 4. In the embodiment of Figure 6, the second and the fourth MOSFET switches 302, 304 of Figure 4 have been replaced with bipolar transistor switches 602, 604 having a control terminal (base) connected to the control circuit 410.

Figure 6 further illustrates slow level shifting circuits connected to the control terminals (gates) of the first switch 301 and the third switch 303. Each level shifting circuit comprises a parallel arrangement of a first resistor 605 and a zener diode 606, connected between the first supply terminal 305 and the control terminal (gate) of the first or the third switch 301, 303. A second resistor 607 is connected between the control terminal (gate) of the first or the third switch 301, 303 and the control circuit 410.

Figure 7 shows an embodiment of a pulse generator 314 (Figures 3-6) comprising a first pulse generator switch 701 connected in series with a second pulse generator switch 702. A terminal 703 of the pulse generator 314 is configured to be connected to the first supply (voltage) terminal 305, 405 of the lamp driving circuit. A terminal 704 is configured to be connected to the ignition electrode 313, 513 of the gas discharge lamp 310, 510. A terminal 705 is configured to be connected to the second supply voltage terminal 306 of the lamp driving circuit. In Figure 7, any current limiting circuit arrangements or other protective circuit arrangements have been omitted for clarity.

In the embodiment shown, a high dV/dt can be obtained by closing the first pulse generator switch 701 while opening the second pulse generator switch 702, or operating the first and second pulse generator switches in the opposite way.

Figure 8a shows a graph of a DC supply current I_s generated by the buck converter shown in Figures 4, 5 and 6. At a time tl or t3, the buck converter switch 401

(Figures 4, 6), or 501/502 (Figure 5) is closed, which causes the current I_s to increase from zero to a peak value, as essentially determined by the inductor 402, 509, respectively. At a subsequent time t2 or t4, the buck converter switch 401 or 501/502, respectively, is opened, which causes the current Is to decrease from its peak value to zero. Between times t2 and t3, and between times t4 and t5, the current Is flows through the diode 403 (Figures 4, 6) or 502/501 (Figure 5) and the inductor 402, 509, respectively. Between times tl and t5, an average current value I_AV of the DC supply current I_s is generated.

The mode of operating the buck converter switch 401, or 501/502 such that the current Is varies between zero and a peak value, is defined as a discontinuous mode of the buck converter. It is noted that in the discontinuous mode, at the time t3 when the current IS has decreased to zero, a renewed closing of the buck converter switch 401, or 501/502 may be delayed by introducing a delay time in which the current Is remains zero.

The average current value I_AV may be increased by increasing the time period tl-t2 or t3-t4, thereby also increasing the time period t2-t3 or t4-t5, respectively. The average current value I_AV may be decreased by introducing said delay time, and/or by decreasing the time period tl-t2 or t3-t4, thereby also decreasing the time period t2-t3 or t4-t5, respectively.

Figure 8b illustrates a pulse width modulation, PWM, operation of the buck converter, thereby switching the buck converter on and off during predetermined time periods. This is one way of dimming a gas discharge lamp according to the present invention. Separately, or in combination, another way of dimming a gas discharge lamp according to the present invention is to vary the closing time of the buck converter switch when the buck converter is switched on, thereby determining an average current value when the buck converter is switched on. Combining the PWM and the closing time control of the buck converter switch allows for a deep dimming of the gas discharge lamp. Between t6 and t7, the buck converter is switched on in discontinuous mode to generate a current Is having an average current value I_AVI - Between each triangular current peak, a delay time in which the current Is is zero, is inserted. Between t7 and t8, the buck converter is switched off. Between t8 and t9, which time period is longer than the time period between t6 and t7, the buck converter is switched on again in discontinuous mode to generate a current Is having an average current value I_AV2 which is lower that I_AVI - Between t8 and t9, a delay time between the triangular current peaks is zero.

In the timing of the operation of the buck converter, an ignition pulse is provided to the gas discharge lamp a predetermined time period after time t6 and time t8. As explained in detail above, in an embodiment the present invention provides a lamp driving circuit and method for driving a gas discharge lamp provided with supply electrodes, and an ignition electrode. A DC supply current is generated by a buck converter having a buck converter operating frequency. An inverter circuit comprising at least two switches in a bridge arrangement inverts the DC supply current into an alternating lamp supply current having an alternating frequency. An ignition circuit, configured to be connected to the ignition electrode, generates a lamp ignition pulse. A control circuit is coupled to the buck converter, the inverter circuit, and the ignition circuit for controlling the operation thereof. The buck converter is operated by pulse width modulation, PWM, switching the buck converter on and off with a PWM frequency, which is lower than the buck converter operating frequency. The alternating frequency of the inverter circuit is lower than the PWM frequency. The ignition circuit generates at least one lamp ignition pulse when the buck converter is switched on in its PWM switching.

As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention, which can be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention in virtually any appropriately detailed structure. Further, the terms and phrases used herein are not intended to be limiting, but rather, to provide an understandable description of the invention.

The terms "a" or "an", as used herein, are defined as one or more than one. The term plurality, as used herein, is defined as two or more than two. The term another, as used herein, is defined as at least a second or more. The terms including and/or having, as used herein, are defined as comprising (i.e., open language, not excluding other elements or steps). Any reference signs in the claims should not be construed as limiting the scope of the claims or the invention.

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 single processor, controller, microcontroller or other unit, or a multiplicity of said units, may fulfil the functions of several items recited in the claims.

Claims

CLAIMS:

1. A lamp driving circuit for driving a gas discharge lamp provided with supply electrodes, and an ignition electrode, the lamp driving circuit comprising: a current supply for supplying a DC supply current; an inverter circuit coupled to the current supply for generating an alternating lamp supply current having an alternating frequency, the inverter circuit comprising at least two switches in a bridge arrangement; an ignition circuit configured to be connected to the ignition electrode for generating a lamp ignition pulse; and a control circuit coupled to the current supply, the inverter circuit, and the ignition circuit for controlling the operation thereof, wherein the control circuit is configured to control the operation of the current supply by pulse width modulation, PWM, switching the current supply on and off with a PWM frequency, wherein the control circuit is configured to control the alternating frequency of the inverter circuit to be lower than the PWM frequency, and wherein the control circuit is configured to control the ignition circuit to generate at least one lamp ignition pulse a predetermined time period after the control circuit switches the current supply on in its PWM switching.

2. The lamp driving circuit of claim 1, wherein the control circuit is configured to control the alternating frequency of the inverter circuit to be lower than 1 Hz, or lower than 0.1 Hz, or lower than 0.01 Hz.

3. The lamp driving circuit of claim 1 or 2, wherein the control circuit is configured to control the PWM frequency of the current supply to be at least equal to a frequency of the AC mains supply, or at least equal to twice a frequency of the AC mains supply.

4. The lamp driving circuit of any of the preceding claims, wherein the current supply is a buck converter having a buck converter operating frequency for converting a DC supply voltage obtained by rectifying an AC mains supply voltage.

5. The lamp driving circuit of claim 4, wherein the PWM frequency is lower than the buck converter operating frequency.

6. The lamp driving circuit of claim 4 or 5, wherein the control circuit is configured to control the buck converter to operate in a discontinuous mode.

7. The lamp driving circuit of any of claims 4-6, wherein the control circuit is configured to control the switches of the inverter circuit to switch when the current supplied by the buck converter is zero.

8. The lamp driving circuit of any of claims 4-7, wherein the buck converter comprises a series arrangement of a buck converter switch and a diode connected between DC supply terminals, the buck converter switch being coupled to the control circuit for controlling the operation of the buck converter switch, wherein the inverter circuit comprises a series arrangement of a third switch and a fourth switch connected between the DC supply terminals, wherein a series arrangement of a buck converter inductor and a smoothing capacitor is connected in parallel to the diode, a common node of the buck converter inductor and the smoothing capacitor, and a common node of the third and the fourth switch being configured to be connected to the gas discharge lamp, and wherein: the control circuit is configured to control, in alternation and at a high frequency, the buck converter switch to conduct while the diode does not conduct, whereas the diode conducts while the buck converter switch does not conduct, and the control circuit is configured to control, in alternation and at a low frequency, the third switch to conduct while the fourth switch does not conduct, and the fourth switch to conduct while the third switch does not conduct.

9. The lamp driving circuit of any of claims 4-7, wherein the buck converter comprises a series arrangement of a first switch and a second switch connected between DC supply terminals and being coupled to the control circuit for controlling the operation of the first switch and the second switch, wherein the inverter circuit comprises a series arrangement of a third switch and a fourth switch connected between the DC supply terminals, wherein a series arrangement of a buck converter inductor and a smoothing capacitor is connected in parallel to the second switch, a common node of the buck converter inductor and the smoothing capacitor, and a common node of the third and the fourth switch being configured to be connected to the gas discharge lamp, and wherein: the control circuit is configured to control, in alternation and at a high frequency, the first switch to conduct while the second switch does not conduct, and the second switch to conduct while the first switch does not conduct, and the control circuit is configured to control, in alternation and at a low frequency, the third switch to conduct while the fourth switch does not conduct, and the fourth switch to conduct while the third switch does not conduct.

10. The lamp driving circuit of any of the preceding claims, wherein the lamp driving circuit, for each heating electrode of the gas discharge lamp, comprises a heating circuit for supplying the heating electrode, the control circuit being coupled to the heating circuit for controlling the operation thereof, and wherein the heating circuit is configured to switch off the heating of the corresponding heating electrode for a predetermined time period when the lamp supply current direction is such that the heating electrode is at a lamp anode side.

11. An assembly of a lamp driving circuit of any of the preceding claims, and a gas discharge lamp.

12. The assembly of claim 11, wherein the gas discharge lamp is a fluorescent lamp.

13. A method of driving a gas discharge lamp provided with supply electrodes, and an ignition electrode, the method comprising: providing a DC supply current; inverting the DC supply current into an alternating lamp supply current having a alternating frequency; supplying the alternating lamp supply current to the supply electrodes of the gas discharge lamp; switching the DC supply current on and off in pulse width modulation, PWM, mode with a PWM frequency, the alternating frequency being lower than the PWM frequency, and supplying at least one lamp ignition pulse to the ignition electrode a predetermined time period after the DC supply current is switched on in its PWM mode.

14. The method of claim 13, wherein the DC supply current is generated by a buck converter having a buck converter operating frequency.

15. The method of claim 14, wherein the PWM frequency is lower than the buck converter operating frequency.