WO2008084349A1

WO2008084349A1 - Method and circuit arrangement for driving a discharge lamp

Info

Publication number: WO2008084349A1
Application number: PCT/IB2007/055148
Authority: WO
Inventors: Georg Greuel; Wolfgang S. Schiene; Marc M. A. Bleukx
Original assignee: Koninklijke Philips Electronics N.V.; Philips Intellectual Property & Standards Gmbh
Priority date: 2007-01-10
Filing date: 2007-12-17
Publication date: 2008-07-17

Abstract

A method and circuit arrangement for driving a gas discharge lamp by means of at least one half-bridge for commutating a supplied DC power to a bipolar lamp current is disclosed, wherein the half-bridge comprises two series-connected semiconductor switches (T1, T2) with intrinsic body diodes, like transistor switches and especially field effect 5 transistor (FET) switches like MOSFET (metal oxide semiconductor FET) switches. The switching on of one of the semiconductor switches (T1; T2) is inhibited until the body diode of the other semiconductor switch (T2; T1) is in an at least substantially non-conduct ing state, so that a destruction of the semiconductor switches due to a short circuiting of the DC power supply (1) via the conducting body diode (corresponding to its high recovery time) and 10 via the switched-on semiconductor switch is prevented.

Description

Method and circuit arrangement for driving a discharge lamp

FIELD OF THE INVENTION

The invention relates to a method and circuit arrangement for driving a gas discharge lamp (lamp driver) by means of at least one half-bridge for commutating a supplied DC power (i.e. DC voltage or current) to an especially bipolar or alternating lamp current, wherein the half-bridge comprises two series-connected semiconductor switches with internal or intrinsic body diodes, like transistor switches and especially field effect transistor (FET) switches like MOSFET (metal oxide semiconductor FET) switches.

BACKGROUND OF THE INVENTION Especially high efficiency discharge lamps like DBD (dielectric barrier discharge) lamps for emitting UV light, or high intensity (HID) or ultra high pressure (UHP) discharge lamps are usually operated with high power and high speed drive pulses (pulsed driving) in excess of e.g. about 16 kV/μs. Especially lamp drivers for such high power and high speed operation comprise MOSFET switches which are connected in a half-bridge or full-bridge driver circuit for commutating a supplied DC power to a bipolar lamp current.

US 2005/0269969 Al discloses a driver for a gas discharge lamp with a commutating stage having at least one chain of two series connected MOSFET switches, wherein the chain is connected between two input terminals, and wherein the gas discharge lamp to be driven is connected with the node between said two switches. This driver furthermore comprises a control unit for providing control signals to said two switches. During a first commutation interval, a lamp circuit current has only a first direction while during a second commutation interval said lamp circuit current has only an opposite direction. The control unit is designed to generate its control signals such that said two switches are always switched substantially simultaneously in counter-phase. The MOSFET switches are used in reverse conduction mode also in order to avoid body diode conduction.

SUMMARY OF THE INVENTION

Especially in connection with the above mentioned lamp drivers with high power and/or high speed drive pulses, it has revealed that during the ignition phase of the lamp in which the discharge gas is not yet conductive, the half-bridge is not yet resonant so that a zero voltage switching of the half-bridge cannot be achieved.

If the semiconductor switches of the half-bridge comprise internal or intrinsic body diodes, like especially in case of MOSFET switches, current can still be flowing through the body diode of the semiconductor switch of one side of the half-bridge, while the semiconductor switch of the other side of the half-bridge is going to switch on. Because of the usually high recovery times of the body diodes of the semiconductor switches in excess of about 1 μs, the switches can be destroyed by these currents.

One object underlying the invention is to provide a method and a circuit arrangement for driving a gas discharge lamp as mentioned in the introductory part above, by which semiconductor switches comprising an internal or intrinsic body diode (like especially transistor switches, FET switches or MOSFET switches) can be protected against destruction especially when high power and/or high speed drive pulses have to be generated.

The object is solved according to claim 1 by a method for driving a gas discharge lamp by means of at least one half-bridge comprising two series connected semiconductor switches with body diodes, for commutating a supplied DC power to an especially bipolar or alternating lamp current, wherein the method comprises a first step of detecting an at least substantially non-conducting state of the body diode of one of the two semiconductor switches, and a second step of inhibiting a switching-on of the other of the two semiconductor switches, until the body diode of the one semiconductor switch is in an at least substantially non-conducting state such that after switching-on the other of the two semiconductor switches, no or only a minimum current can flow through the body diode of the one semiconductor switch.

By such a minimum (or zero) current it is prevented that the semiconductor switches are destructed.

A switched-on semiconductor switch is considered to be in a state in which its two switching terminals (which are controlled via a control terminal) are conducting or in a closed state, so that a current can flow from one switching terminal to the other switching terminal.

The object is solved according to claim 4 by a circuit arrangement for driving a gas discharge lamp by means of at least one half-bridge comprising two series connected semiconductor switches with body diodes, for commutating a supplied DC power to an especially bipolar or alternating lamp current, comprising a detector for detecting an at least substantially non-conducting state of the body diode of one of the two semiconductor switches, and inhibiting elements for inhibiting the supplying of a received switch-on signal to the other semiconductor switch until an at least substantially non-conducting state of the body diode of the one semiconductor switch has been detected.

One advantage of these solutions is, that a reliable method and a simple and low cost lamp driver with minimum power losses can be realized which especially can protect MOSFET or other semiconductor switches having an internal or intrinsic body diode, under almost all intermediate lamp conditions between initiating switching-on of the lamp and until the lamp is fully ignited and burning.

The subclaims disclose advantageous embodiments of invention. The embodiments according to claims 2 and 6 have the advantage that the conducting or non-conducting state of the body diodes can be detected very easy and reliable. The embodiments according to claims 3, 5, 6, 8 and 9 have the advantage that they can be realized by a minimum number of additional components in a very reliable manner.

The embodiment according to claim 7 has the advantage that the detector output signals have a common voltage reference.

Further details, features and advantages of the invention will become apparent from the following description of preferred and exemplary embodiments of the invention which are given with reference to the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 diagrammatically shows a block diagram of a circuit arrangement according to the invention.

DETAILED DESCRIPTION OF EMBODIMENTS Figure 1 shows a schematic functional block diagram of substantial components of a circuit arrangement for driving a discharge lamp (lamp driver).

It generally comprises a half-bridge section 20 at the left side of the broken line L in Figure 1 and a (gate) drive timing section 30 at the right side of the broken line L.

The half-bridge section 20 comprises two semiconductor switches Tl, T2 which are connected in series between a DC power supply 1 and a reference potential

(especially ground) M, namely a high side (or first) semiconductor switch Tl (connected with the DC power supply 1) and a low side (or second) semiconductor switch T2 (connected with the reference potential). The DC power supply 1 can be provided for generating almost any voltage level between about 10 V and about 1000 V, depending on the kind of lamp to be driven and the kind of semiconductor switches Tl, T2.

The semiconductor switches Tl, T2 are especially (power) MOSFET switches. More in detail, as indicated in Figure 1, the drain terminal of the high side

MOSFET switch Tl is connected with the DC power supply 1, and the source terminal of the low side MOSFET switch T2 is connected with the reference potential or ground M. The source terminal of the high side MOSFET switch Tl and the drain terminal of the low side MOSFET switch T2 are connected with each other to form a node P of the half-bridge to which a known lamp unit 11 is connected.

The lamp unit 11 substantially comprises the lamp to be driven, wherein usually a transformer is provided via which the lamp is connected with the node P of the half- bridge.

The half-bridge section 20 furthermore comprises a known communication interface 2 for supplying input signals I to a known control unit 3, and a high side gate driver 4, the output of which is fed to the control terminal, i.e. the gate terminal of the high side MOSFET switch Tl, and a low side gate driver 5, the output of which is fed to the control terminal, i.e. the gate terminal of the low side MOSFET switch T2.

The gate drive timing section 30 comprises a high side voltage detection unit 6, the input terminals of which receive the voltage between the switching terminals, i.e. the drain and the source terminal (i.e. the voltage over the body diode) of the high side MOSFET switch Tl, and a low side voltage detection unit 8, the input terminals of which receive the voltage between the switching terminals, i.e. the drain and the source terminal (i.e. the voltage over the body diode) of the low side MOSFET switch T2. The output signal of the high side voltage detection unit 6 is fed to the input of a fast high to low side transmitter unit 7 for referencing the output signal of the high side MOSFET voltage detection unit 6 to the same reference potential (preferably ground) as the output signal of the low side voltage detection unit 8.

The output signal of the high to low side transmitter unit 7 and the output signal of the low side voltage detection unit 8 (a related transmitter is not necessary at its output because all signals at the output of the low side voltage detection unit 8 are already referenced to the low side or ground) are fed to inhibiting elements 9, 10 for inhibiting the supplying of a received switch-on signal to one of the semiconductor switches Tl (T2) until an at least substantially non-conducting state of the body diode of the other semiconductor switch T2 (Tl) has been detected.

More in detail and according to Figure 1, the output signal of the high to low side transmitter unit 7 is fed to a first input of a second logical AND gate 10, and the output signal of the low side voltage detection unit 8 is fed to a first input of a first logical AND gate 9.

A second input of the first logical AND gate 9 receives a high side gate signal generated by the control unit 3. A second input of the second logical AND gate 10 receives a low side gate signal generated by the control unit 3. The output of the first logical AND gate 9 is fed to an input of the high side gate driver 4, and the output of the second logical AND gate 10 is fed to an input of the low side gate driver 5.

This circuit arrangement operates as follows:

Generally, the first (high side) and the second (low side) MOSFET switches Tl, T2 are alternatingly switched on and switched off, wherein when one of the switches is switched on, the other switch is switched off and vice versa, so that a bipolar or alternating lamp current is generated from a DC current or DC voltage, supplied by the DC power supply 1.

Generally, a switched-on condition is considered as a conducting condition, in which the semiconductor switch is closed and a current flows between the drain and the source terminals of the related semiconductor (MOSFET) switch Tl; T2. A switched-off condition is considered as a non-conducting condition, in which the semiconductor switch is open and no considerable current flows between the drain and the source terminal of the related semiconductor (MOSFET) switch Tl; T2. Generally, the switching of the first MOSFET switch Tl is initiated by the high side gate signal, and the switching of the second MOSFET switch T2 is initiated by the low side gate signal.

In a standard configuration of a half-bridge, the high side gate signal and the low side gate signal would be directly fed to the gate terminal of the related MOSFET switch. However, it has to be considered that a power MOSFET switch usually comprises an internal or intrinsic body diode which is comparatively slow and has a comparatively long recovery time e.g. in the range of about 1 μs or more. If for example the second MOSFET switch T2 would have to be switched on by the low side gate signal, while current is still flowing through the body diode of the first MOSFET switch Tl although it has been switched off by the high side gate signal, the DC power supply 1 would be momentarily short-circuited through the body diode of the first MOSFET switch Tl and the conducting second MOSFET switch T2. This in turn would lead to a destruction of the second MOSFET switch T2 and possibly of the first MOSFET switch Tl as well. In order to avoid this, the circuit arrangement according to the invention is provided with the gate drive timing section 30 to which the high side gate signals and the low side gate signals are fed and which does not allow or inhibit switching-on of one of the MOSFET switches Tl (T2) until the body diode of the other (opposite) MOSFET switch T2 (Tl) is detected off or at least substantially non-conducting, i.e. non-conducting to such an extent that - after switching on the one switch - no or only a minimum current can flow through the body diode of the other switch, by which current none of the MOSFET switches can be destructed.

The gate drive timing section 30 controls the switching on and switching off of the MOSFET switches Tl, T2 in such a way that the recovery time of the body diodes is considered and the DC power supply 1 is not short-circuited through the body diode of one of the MOSFET switches Tl, T2.

More in details, the recovery time of the body diodes and thus a conducting or non-conducting state of the body diodes is considered by detecting and evaluating the voltage level between the drain and the source terminal of the related MOSFET switch, because as long as the body diode has not recovered from a conducting state, this voltage has a minimum level, in comparison to a high level after the expiration of the recovery time, at which the body diode has reached the non-conducting state.

Consequently, a high side gate signal which is generated by the control unit 3 is inhibited and only fed to the high side gate driver 4 if the voltage over the second (low side) MOSFET switch T2 has reached or exceeds a predetermined voltage level or threshold. The same applies vice versa in case that a low side gate signal is generated by the control unit 3, which signal is inhibited and only fed to the low side gate driver 5 if the voltage over the first (high side) MOSFET switch Tl has reached or exceeds a predetermined voltage level or threshold. The voltage levels or thresholds are selected such high that it is ensured that the body diode of the related first or second MOSFET switch Tl, T2 is non conducting at least to such an extent that there is no considerable risk that any one of the MOSFET switches Tl, T2 is destructed by currents flowing from the DC power supply 1 through the related body diode to the reference potential or ground M after the other of the MOSFET switches Tl, T2 has been switched on.

The high side and the low side MOSFET voltage detection units 6; 8 are provided for detecting and comparing the voltage level over the first and the second MOSFET switches Tl; T2, respectively, (i.e. over the switching terminals, i.e. the body diodes) with the predetermined voltage level or threshold and to generate a logical "high" output signal (release signal) to the related second and first AND gate 10; 9, respectively, only if the detected voltage level is at least equal to or higher than the predetermined voltage level, so that now the high side or low side gate signal, respectively, can be supplied via the related AND gate 9; 10 to the high or low side gate driver 4; 5, respectively, in order to close or switch on the related MOSFET switch Tl; T2.

The predetermined voltage level or threshold voltage is preferably set to a value of at least 2 Volt, but can be selected to any value between about 2 Volt and about 1000 Volt, depending on the type of MOSFET switch Tl; T2 and body diode. The total reaction speed of the gate drive timing part 30 must be fast enough to not inhibit normal operation when the body diodes are not carrying current and the operated lamp is in its normal running condition after the ignition phase. By using high-speed CMOS components and a fast digital isolator for the high to low transmitter unit 7, a total reaction speed of about 100 ns can be obtained. However, other components such as CMOS or TTL, discrete components, capacitive couplers, transformers or optocouplers can also be used as well, depending on the operating frequency of the half-bridge to be protected by the gate drive timing part 30.

The method and circuit arrangement according to the invention can be used advantageously for bipolar DBD UV lamp drivers, especially for high power drivers (e.g. about 2 kW) for DBD lamps, in particular DBD lamps equipped with an UVC emitting phosphor used in the field of fluid treatment or purification, or for other lamp applications which use a half-bridge topology and encounter hard-switching on a body diode of a FET switch, especially a MOSFET switch during certain lamp conditions.

In comparison to known other solutions using extra series and hyperfast parallel diodes, the method and circuit arrangement according to the invention is much more efficient regarding power, cost and space, and has a higher reliability.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive, and the invention is not limited to the disclosed embodiments. Modifications to embodiments of the invention described in the foregoing are possible without departing from the scope of the invention as defined by the accompanying claims. Expressions such as "including", "comprising", "incorporating", "consisting of, "have", "is" used to describe and claim the present invention are intended to be construed in a non-exclusive manner, namely allowing for items, components or elements not explicitly described also to be present.

Numerals included within parentheses in the accompanying claims are intended to assist understanding of the claims and should not be construed in any way to limit the subject matter claimed by these claims.1. Method for driving a gas discharge lamp by means of at least one half-bridge comprising two series connected semiconductor switches with body diodes, for commutating a supplied DC power to a lamp current, wherein the method comprises a first step of detecting an at least substantially non-conducting state of the body diode of one of the two semiconductor switches, and a second step of inhibiting a switching-on of the other of the two semiconductor switches, until the body diode of the one semiconductor switch is in an at least substantially non-conducting state such that after switching-on the other of the two semiconductor switches, no or only a minimum current can flow through the body diode of the one semiconductor switch.

Claims

CLAIMS:

1. Method for driving a gas discharge lamp by means of at least one half-bridge comprising two series connected semiconductor switches with body diodes, for commutating a supplied DC power to a lamp current, wherein the method comprises a first step of detecting an at least substantially non-conducting state of the body diode of one of the two semiconductor switches, and a second step of inhibiting a switching-on of the other of the two semiconductor switches, until the body diode of the one semiconductor switch is in an at least substantially non-conducting state such that after switching-on the other of the two semiconductor switches, no or only a minimum current can flow through the body diode of the one semiconductor switch.

2. Method according to claim 1, wherein said two semiconductor switches each comprise a control terminal and two switching terminals, which are controlled conducting or closed and non-conducting or open via the control terminal, and wherein the conducting or non-conducting state of the body diode of a semiconductor switch is detected by measuring the voltage level over the two switching terminals of the semiconductor switch.

3. Method according to claim 1, wherein a switch-on signal received for switching-on one of the semiconductor switches is logically AND combined with a signal indicating a non-conducting state of the body diode of the other of the semiconductor switches, and wherein the resulting AND combined output signal is fed to the one semiconductor switch for switching-on the same.

4. Circuit arrangement for driving a gas discharge lamp by means of at least one half- bridge comprising two series connected semiconductor switches (Tl, T2) with body diodes, for commutating a supplied DC power (1) to a lamp current, comprising a detector (6, 8) for detecting an at least substantially non-conducting state of the body diode of one of the two semiconductor switches (Tl; T2), and inhibiting elements (9, 10) for inhibiting the supplying of a received switch-on signal to the other semiconductor switch (T2; Tl) until an at least substantially non-conducting state of the body diode of the one semiconductor switch (Tl; T2) has been detected.

5. Circuit arrangement according to claim 4, wherein the inhibiting elements are provided in the form of logical elements (9, 10) for logically AND combining the received switch-on signal for the other of the two semiconductor switches (T2; Tl) with the detector signal of the body diode of the one semiconductor switch (Tl; T2).

6. Circuit arrangement according to claim 4, wherein the detector is provided in the form of a high side voltage detection unit (6) for detecting a voltage level over the body diode of a high side semiconductor switch (Tl) and a low side voltage detection unit (8) for detecting a voltage level over the body diode of a low side semiconductor switch (T2).

7. Circuit arrangement according to claim 6, comprising a high to low side transmitter unit (7) for referencing the output signal of the high side voltage detection unit (6) to a reference potential of the low side voltage detection unit (8).

8. Circuit arrangement according to claim 6, comprising a first logical AND gate (9) for logically AND combining the output signal of the low side voltage detection unit (8) with a supplied switch-on signal for the high side semiconductor switch (Tl), and a second logical AND gate (10) for logically AND combining the output signal of the high side voltage detection unit (6) with a supplied switch-on signal for the low side semiconductor switch (T2).

9. Circuit arrangement according to claim 6, wherein the high side voltage detection unit (6) and the low side voltage detection unit (8) is provided for comparing the detected voltage level over the body diode of the related semiconductor switch (Tl; T2) with a predetermined voltage level or threshold.

10. Lamp driver (20, 30) for generating bipolar or alternating high speed drive pulses, especially for driving a high power DBD lamp (11), comprising a circuit arrangement according to one of claims 4 to 9.