WO2007072260A1

WO2007072260A1 - Device for driving a discharge lamp, and switching circuit for use in such device

Info

Publication number: WO2007072260A1
Application number: PCT/IB2006/054551
Authority: WO
Inventors: Hubertus C. Miermans
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2005-12-21
Filing date: 2006-12-01
Publication date: 2007-06-28
Also published as: TW200733817A

Abstract

A switch circuit (100) comprising two switch terminals (101, 102) and at least one control terminal (103) is capable of switching a lamp current. Responsive to a control signal received at its control terminal (103), the circuit operates in either a first operational state (CLOSED) or a second operational state (OPEN). The switch circuit comprises: a transformer (110) comprising a primary winding (111) and a secondary winding (112), wherein the secondary winding (112) is coupled in series between said two switch terminals (101, 102); a closed loop current path (120) connected in series with the primary transformer winding (111); at least one controllable switch (121) connected in series with said closed loop current path (120), the controllable switch (121) having a control input (121a) coupled to the control terminal (103) of the switch circuit.

Description

Device for driving a discharge lamp, and switching circuit for use in such device

FIELD OF THE INVENTION

The present invention relates in general to a device for driving a discharge lamp, specifically a fluorescent lamp, and to a switching circuit for use in such device.

BACKGROUND OF THE INVENTION

A fluorescent lamp comprises, in general, a transparent vessel, typically glass, usually of a tubular shape, having two electrodes disposed at opposite ends of such tube. The tube contains a specific gas atmosphere, typically comprising more than 50% Argon. In operation, an electrical power source is connected to the electrodes, such that a discharge is caused in said atmosphere. During discharge operation of the lamp, the voltage over said electrodes has a typical value, indicated as lamp voltage, and the current through the lamp has a typical value, indicated as lamp current. Although the lamp current can typically be controlled by a driving power source, or driver, a lamp has typical nominal operating voltage and operating current values which depend on lamp type. The lamp is intended to be continuously operated at nominal operational parameters, i.e. nominal voltage and nominal current, and under those nominal operational conditions the lamp generates a typical light intensity according to design specifications.

There are situations where it is desirable that the fluorescent lamp is operated in a switched mode, in which the lamp is alternatively switched ON and OFF at a certain switching frequency.

For instance, it may be desired that the light output is reduced, i.e. that the lamp is dimmed. When the lamp is operated in a switched mode, the lamp generates light only during the ON-periods and generates no light during the OFF-periods. The switching frequency is typically selected to be 100 Hz or more, in order to prevent undesirable flicker phenomena. Then, the human eye only observes an average light intensity which depends on the duty cycle, i.e. the ratio of the ON-periods with respect to the total switching period. In another example, a fluorescent lamp may be arranged in an array of multiple fluorescent lamps, and it may be desirable to alternatively switch the lamps on and off. An example of such application is a scanning backlight unit such as used for instance in an LCD television. Switched mode operation of a lamp, or another type of load, is typically executed by arranging a controllable switch in series with the lamp: by switching the switch either OPEN or CLOSED, the lamp is switched OFF and ON. An obvious implementation of a controllable switch would be a transistor, for instance a MOSFET. However, simply arranging a transistor in series with the lamp leads to problems. Specifically, the switch must be arranged at the "hot side" of the lamp, and should be able to withstand the very high voltages such as required for ignition, typically higher than 1 kV. Further, the switch controller should be able to withstand such very high voltages.

An important aspect of the present invention is to provide a switch design that eliminates or at least reduces the above problems.

SUMMARY OF THE INVENTION

According to an important aspect of the present invention, a switch circuit comprises a transformer having a primary winding and a secondary winding. The secondary transformer winding is coupled in series with the lamp. The primary transformer winding is short-circuited by a closed loop which includes a controllable switch. The primary transformer winding has less turns than the secondary transformer winding, so that a voltage in the secondary transformer winding is transformed to a lower value in the primary transformer winding; as a consequence, the controllable switch may be implemented by a standard transistor switch such as a MOSFET.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects, features and advantages of the present invention will be further explained by the following description with reference to the drawings, in which same reference numerals indicate same or similar parts, and in which:

Fig. 1 is a block diagram schematically illustrating a lamp driving circuit;

Fig. 2 is a block diagram schematically illustrating a lamp driving circuit capable of switching the lamp ON and OFF;

Fig. 3 is a block diagram schematically illustrating a first embodiment of a switch circuit according to the present invention;

Fig. 4 is a block diagram schematically illustrating a second embodiment of a switch circuit according to the present invention;

Fig. 5 is a block diagram schematically illustrating an embodiment of a lamp driving circuit capable of supplying a pulsed current to a lamp; Fig. 6 is a block diagram schematically illustrating an embodiment of another lamp driving circuit capable of supplying a pulsed current to a lamp;

Fig. 7 is a block diagram schematically illustrating an embodiment of a lamp driving circuit for driving a plurality of lamps.

DETAILED DESCRIPTION OF THE INVENTION

Fig. 1 is a block diagram schematically illustrating a circuit for driving three lamps Ll, L2, L3. In a manner as known per se, a series arrangement of two MOSFET switches 2 and 3 receives DC power from a DC power source 1. A switch controller 4 controls the two switches 2 and 3, so that alternatively one is open and the other is closed and vice versa. At the switch-over moments, a short dead-time is implemented, during which both switches are closed, in order to prevent short-circuiting and to achieve zero-voltage switching. A capacitor 5 is connected to the node between said two switches 2 and 3. The arrangement of DC source 1, switches 2 and 3, controller 4, and capacitor 5 can be considered as implementing an AC power source 8 having output terminals 6 and 7. Lamps Ll, L2, L3 are driven in parallel from these output terminals 6 and 7.

It is desirable to be able to switch lamps Ll, L2, L3 ON and OFF individually, either as simple ON/OFF selection, or as duty cycle switching for dimming purposes. Switching the power source 1 or 8 will result in all lamps Ll, L2, L3 being switched simultaneously. Thus, each lamp Ll, L2, L3 should be provided with an individual controllable switch in series.

Fig. 2 is a block diagram schematically illustrating a basic circuit illustrating this for one lamp L only, coupled to output terminals 6 and 7 of AC power source 8. In series with the lamp L, a controllable switch circuit 100 is arranged. The controllable switch circuit 100 has two switch terminals 101 and 102, and a control terminal 103. The first switch terminal 101 is coupled to one output terminal 6 of the AC power source 8, the second switch terminal 102 is coupled to the lamp L. The switch circuit 100 has a first operative state that will be indicated as "closed", in which a substantially conductive path is provided between the two switch terminals 101 and 102, and a second operative state that will be indicated as "open", in which a substantially non-conductive or at least high- impedance path is provided between the two switch terminals 101 and 102. The switch circuit 100 is in either one of these two operative states, depending on an input control signal received at its control terminal 103. An obvious choice for embodying such switch circuit would be a transistor, specifically a FET. However, a problem in this situation is that the lamp is a discharge lamp, specifically a fluorescent tube. Once such lamp is off, it may need a very high voltage for re- ignition, typically higher than 1 kV, and the switch circuit should be capable of withstanding such high voltage over its switch terminals 101, 102. It will be very difficult, and at any rate very expensive, to meet this requirement with a FET. Further, the switch circuit must be located at the high- voltage side of the lamp L ("hot" side), so a controller providing the control signal for the switch circuit must also be capable of withstanding the high ignition voltage. Thus, an aim of the present invention is to provide a switch which is relatively simple to implement, relatively inexpensive, capable of withstanding high voltages over its switch terminals, and capable of providing a good high- voltage separation from its switch terminals to its control terminal.

Fig. 3 is a block diagram, schematically illustrating the essential buildup of a switch circuit 100 in accordance with the present invention. The switch circuit 100 proposed by the present invention comprises a transformer 110 comprising a primary winding 111 and a secondary winding 112. One end of the secondary winding 112 is connected to the first switch terminal 101, the opposite end of the secondary winding 112 is connected to the second switch terminal 102. The primary winding 111 has its ends connected to a closed loop 120. A controllable switch 121 is incorporated in this closed loop 120, this controllable switch 121 having a control terminal 121a connected to the control terminal 103 of the switch circuit 100. Basically, this controllable switch 121 can be implemented by a transistor, specifically a FET, more specifically a MOSFET.

The operation of this switch circuit 100 is as follows. In a first operative state, the controllable switch 121 is closed, i.e. substantially conductive. Thus, a current can flow in the closed loop 120. If the secondary transformer winding 112 receives an alternating current, it induces an alternating current in its primary winding 111, which induced current can flow freely because the primary winding 111 and its closed loop 120 form no hindrance. Thus, the secondary transformer winding 112 constitutes a low impedance for alternating currents. In a second operative state, the controllable switch 121 is open, i.e. substantially non-conductive. Thus, no current can flow in the closed loop 120, hence the primary transformer winding 111 can not allow any induced current. Consequently, no alternating current can flow in the secondary transformer winding 112. In other words: the secondary transformer winding 112 constitutes a high impedance for alternating currents: the switch circuit 100 is open.

As schematically shown in Fig. 3, the number of turns Nl of the primary transformer winding 111 is less than the number of turns N2 of the secondary transformer winding 112. As a consequence, the switch 121 in series with the primary transformer winding 111 is loaded with reduced voltage as compared to the secondary transformer winding 112 in series with the lamp L. The fact that the current induced in the primary transformer winding 111 is consequently higher than the current in the secondary transformer winding 112 is not of high importance, since MOSFETs are capable of withstanding high currents.

In view of the fact that the switch 121 should be able to completely block an alternating current, switch 121 should be of a bipolar type. In principle, this is not a problem, since a MOSFET as such is bipolar. However, in practice high-power MOSFETs are typically provided with a body-diode parallel to the drain-source path, making the MOSFET always conductive in the direction from source to drain. This problem can be solved by using two switches in anti-series arrangement, as illustrated in Fig. 4. A node A of the closed loop 120 is connected to a reference voltage, typically ground. A first switch 121 (MOSFET) is coupled between a first end I l ia of the primary transformer winding 111 and said node A, having its source terminal directed to said node A. A second switch 122 (MOSFET) is coupled between a second end 111b of the primary transformer winding 111 and said node A, having its source terminal directed to said node A. The control terminals (gates) 121a and 122a of said switches 121 and 122, respectively, are connected together to the control terminal 103 of the switch circuit 100. Thus, both switches 121 and 122 are always switched ON or OFF simultaneously, always one of the switches blocking the current in closed loop 120 depending on the direction of the current.

Fig. 5 is a block diagram schematically illustrating an embodiment of a driving circuit 1000 capable of supplying a pulsed current to a lamp L, or in fact to another type of load. The driving circuit 1000 has output terminals 1101, 1102 for connecting a lamp. The driving circuit 1000 comprises a power source 20, specifically an AC power source, having two output terminals 21, 22. Between the first power source output terminal 21 and the first output terminal 1101 of the driving circuit 1000, the secondary transformer winding 112 of a switching circuit 100 as described above is coupled. The driving circuit 1000 further comprises a control circuit 30, having a control output terminal 33 coupled to the control input terminal 103 of the switching circuit 100.

By providing a block signal with a certain duty cycle, the control circuit 30 is capable of switching the lamp L ON/OFF with the same duty cycle. In the embodiment of Fig. 5, the power source 20 has its second output terminal 22 coupled directly to the second output terminal 1102 of the driving circuit 1000. Fig. 6 illustrates a variation, where a second switching circuit 200, of a same design and preferably identical to the first switching circuit 100, has its secondary transformer winding 212 coupled between the second power source output terminal 22 and the second driving circuit output terminal 1102. The second switching circuit 200 has a control input terminal 203 coupled to a control output terminal of the control circuit 30. The control circuit 30 is designed to generate control signals so that the two switching circuits 100, 200 are switched substantially at the same time.

It is possible that the control circuit 30 has two separate control output terminals coupled to the two separate switching circuits 100, 200. Conveniently, however, the control circuit 30 has one control output terminal 33 coupled to both switching circuits 100, 200, as shown.

Fig. 7 schematically shows an embodiment of a driving circuit 7000 for driving a plurality of lamps Ll, L2, L3, etc., or in fact a plurality of another type of loads. In the embodiment of Fig. 7, only three of such lamps are shown, but the driving circuit can also be implemented in an embodiment for driving two lamps or for driving four or more lamps. In any case, it is noted that one single lamp can always be replaced by two or more lamps in parallel, but such group of lamps can only be switched in common. The embodiment of driving circuit 7000 of Fig. 7 can be compared to the embodiment of Fig. 5. AC power source 20 is a common power source for all lamps, having a first output terminal 21 and a second output terminal 22. The driving circuit 7000 has a first set of output terminals 7101, 7102 for connecting a first lamp Ll, a second set of output terminals 7201, 7202 for connecting a second lamp L2, a third set of output terminals 7301, 7302 for connecting a third lamp, etc. Between output terminal 7101 and first common power source output terminal 21, the secondary transformer winding 112A of a first switching circuit IOOA is connected. Likewise, a switching circuit 200A has its secondary winding 212A connected between first common power source output terminal 21 and output terminal 7201, and a switching circuit 300A has its secondary winding 312A connected between the common output terminal 21 and output terminal 7301. On the other side of the lamps, switching circuits 10OB, 200B, 300B have their respective secondary winding 112B, 212B, 312B connected between second common power source output terminal 22 and the output terminals 7102, 7202, 7302, respectively. A control circuit 30 has three output terminals 33A, 33B, 33C. The first control output terminal 33 A is connected to the control inputs 103 A and 103B of switching circuits IOOA and 10OB, respectively. The second control output 33B is connected to the control inputs 203 A and 203B of the switching circuits 200A and 200B, respectively. The third control output 33C is connected to the control input terminals 303A and 303B of switching circuits 300A and 300B, respectively.

In a possible embodiment, the controller 30 has three operative states. In a first operative state, the first output terminal 33 A is at a high level whereas the second and third output terminals 33B and 33C are at a low level, so that the first lamp Ll is ON and the second and third lamps L2 and L3 are OFF. In a second operative state, the second control output terminal 33B is at a high level while the two other control output terminals 33 A and 33C are at a low level, so that the second lamp L2 is ON and the other lamps L2 and L3 are OFF. In a third operative state, the third control output terminal 33C is at a high level while the two other control output terminals 33 A and 33B are at a low level, so that the third lamp L3 is ON while the two other lamps Ll and L2 are OFF. Thus, the control circuit 30 can drive the lamps Ll, L2, L3 in a scanning manner. The power source 20 always "feels" the load of one lamp Ll, L2, L3 at a time.

It should be clear to a person skilled in the art that the present invention is not limited to the exemplary embodiments discussed above, but that several variations and modifications are possible within the protective scope of the invention as defined in the appending claims. For instance, the number of lamps connected to the common power source can be easily increased by connecting more branches of a lamp and at least one switching device to the output terminals 21 and 22, while the control terminals of such switching devices are connected to a new output terminal of the control device 30. Such driving circuit with a plurality of fluorescent tubes is suitable for use in a scanning backlight system for an LCD-panel, or for an ambilight system for a flat TV, etc.

Further, although Fig. 7 shows only one lamp (for instance Ll) connected between two corresponding output terminals (for instance 7101 and 7102), it is noted that it is possible to connect a group of two or more lamps in parallel between such two corresponding output terminals, in which case the lamps in the groups are all switched ON and OFF simultaneously.

In the above discussion with reference to Fig. 3, a possible embodiment of controller 30 has been described where the duty cycle of each lamp is equal to 1/N, N being the total number of lamp branches. In the embodiment discussed, at all times always one lamp is burning, a lamp always being switched ON simultaneously with another lamp being switched OFF. However, such mode of operation is not essential to the invention. It is possible that the duty cycle is less than 1/N, in which case there will be moments when not one lamp is burning. Or, it is possible that the duty cycle is higher than 1/N, in which case there will be moments when two or more lamps are burning. For instance, it is possible that the duty cycle is equal to 2/N, and that always two lamps are burning at the same time. Then, it is possible that, at a switching moment, the two currently burning lamps are switched OFF while simultaneously two other lamps are switched ON, but it is also possible that, at a switching moment, a first burning lamp is switched OFF, a second burning lamp remains ON, and a third lamp is switched ON, while at a next switching moment the second burning lamp is switched OFF, the third lamp remains ON, and a fourth lamp is switched ON, and so on.

Fig. 5 illustrates a preferred detail of the switch circuit 100, which detail is also preferred in the embodiments of Figs. 6 and 7 but not shown there for sake of simplicity. In practice, the secondary winding 112 of the transformer 110 will always have a certain parasitic capacitance in parallel, which may for instance be in the order of about 50 pF. In the OPEN state of the switch circuit 100, no current can flow through the secondary winding 112, but the parasitic capacitance can cause a non-zero output voltage to be present between output terminals 1101 and 1102, where lamp L is connected. In the case of a cold cathode fluorescent lamp, the lamp has a very high impedance in its OFF state, so the load L does not reduce said output voltage. It may then be that the level of the output voltage exceeds the threshold where the lamp ignites. In order to prevent this, a protection capacitor 130 has one terminal connected to the second switch terminal 102, i.e. the switch terminal directed towards the lamp connection output terminal 1101. This protection capacitor 130 has its other terminal connected to zero, or another low voltage level, so as to constitute a capacitive voltage divider with the said parasitic capacitance. If the capacitance value of this protection capacitor 130 is selected sufficiently high, the output voltage between output terminals 1101 and 1102 can be reduced sufficiently, as should be clear to a person skilled in the art. It is noted that the protection capacitor 130 may be part of the switch circuit 100, as shown, but it is also possible that the protection capacitor 130 is external to the switch circuit 100.

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

Claims

CLAIMS:

1. Switch circuit (100) capable of switching a lamp current, the circuit comprising two switch terminals (101, 102) and at least one control terminal (103), the circuit being responsive to a control signal received at its control terminal (103) by operating in either a first operational state (CLOSED) in which a substantially conductive path is provided between the two switch terminals (101, 102) or a second operational state (OPEN) in which a substantially high- impedance or non-conductive path is provided between the two switch terminals (101, 102); wherein the switch circuit comprises:

- a transformer (110) comprising a primary winding (111) and a secondary winding (112), wherein the secondary winding (112) is coupled in series between said two switch terminals

(101, 102);

- a closed loop current path (120) connected in series with the primary transformer winding

(i n);

- at least one controllable switch (121) connected in series with said closed loop current path (120), the controllable switch (121) having a control input (121a) coupled to the control terminal (103) of the switch circuit.

2. Circuit according to claim 1, wherein said controllable switch (121) comprises a MOSFET.

3. Circuit according to claim 1, comprising at least a first controllable switch (121) and a second controllable switch (122) connected in series with said closed loop current path (120), the two controllable switches (121, 122) having their control inputs (121a, 122a) connected together; wherein the two controllable switches (121, 122) are arranged in an anti-series arrangement.

4. Circuit according to claim 3, wherein the primary transformer winding (111) has a first end (11 Ia) and a second end (11 Ib); wherein the first controllable switch (121) is coupled between the first primary winding end (11 Ia) and a reference voltage node (A), and wherein the second controllable switch (122) is coupled between the second primary winding end (11 Ib) and this reference voltage node (A).

5. Circuit according to claim 1, wherein the number of turns (Nl) of the primary transformer winding (111) is less than the number of turns (N2) of the secondary transformer winding (112).

6. Circuit according to claim 1, further comprising a protection capacitance (130) connected between one switch terminal (102) and zero voltage.

7. Driving circuit (1000) for supplying a pulsed current to a load (L), the driving circuit having output terminals (1101, 1102) for connecting the load; the driving circuit comprising:

- a power source (20) having a first output terminal (21) and a second output terminal (22); - a switching circuit (100) according to claim 1, having its secondary transformer winding

(112) coupled between the first power source output terminal (21) and the first driving circuit output terminal (1101);

- a control circuit (30) having a control output terminal (33) coupled to the control terminal (103) of the switching circuit (100).

8. Driving circuit according to claim 7, wherein the load (L) is a lamp, specifically a fluorescent tube.

9. Driving circuit according to claim 7, wherein the power source (20) is an AC source.

10. Driving circuit according to claim 7, wherein the second power source output terminal (22) is coupled directly to the second driving circuit output terminal (1102).

11. Driving circuit according to claim 7, further comprising a protection capacitance (130) connected between the first driving circuit output terminal (1101) and zero voltage.

12. Driving circuit according to claim 7, further comprising:

- a second switching circuit (200) according to claim 1 , having its secondary transformer winding (212) coupled between the second power source output terminal (22) and the second driving circuit output terminal (1102); wherein the control circuit (30) has a control output terminal (33) coupled to the control terminal (203) of the second switching circuit (200); and wherein the control circuit (30) is designed to switch the two switching circuits (100, 200) substantially simultaneously and in phase.

13. Driving circuit according to claim 12, wherein the two control terminals (103,

203) of the two switching circuits (100, 200) are coupled to the same control output terminal (33) of the control circuit (30).

14. Driving circuit (7000) for driving a plurality of loads (Ll; L2; L3), the driving circuit having a plurality of first and second output terminals (7101, 7102; 7201, 7202; 7301,

7302) for connecting the respective loads; the driving circuit comprising:

- a common power source (20) having a first output terminal (21) and a second output terminal (22);

- a plurality of first switching circuits (10OA; 200A; 300A) according to claim 1, each first switching circuit having its secondary transformer winding (112 A; 212A; 312A) coupled between the first power source output terminal (21) and a respective one (7101; 7201; 7301) of the first driving circuit output terminals;

- a control circuit (30) having control output terminals (33A; 33B; 33C) coupled to the respective control terminals (103 A; 203 A; 303A) of the respective switching circuits (10OA; 200A; 300A).

15. Driving circuit according to claim 14, wherein the loads (Ll; L2) are lamps, specifically fluorescent tubes.

16. Driving circuit according to claim 14, wherein the common power source (20) is an AC source.

17. Driving circuit according to claim 14, wherein the second power source output terminal (22) is coupled directly to the second driving circuit output terminals (7102; 7202; 7302).

18. Driving circuit according to claim 14, further comprising:

- a plurality of second switching circuits (10OB; 200B; 300B) according to claim 1, each second switching circuit having its secondary transformer winding (112B; 212B; 312B) coupled between the second power source output terminal (22) and a respective one (7102; 7202; 7302) of the second driving circuit output terminals; wherein the control circuit (30) has control output terminals (33 A; 33B; 33C) coupled to the respective control terminals (103B; 203B; 303B) of the respective second switching circuits (200A; 200B); and wherein the control circuit (30) is designed to switch the first and second switching circuits (10OA, 10OB; 200A, 200B; 300A, 300B) corresponding to a same load (Ll; L2; L3) substantially simultaneously and in phase.

19. Driving circuit according to claim 18, wherein the two control terminals (103A, 103B; 203A, 203B; 303A, 303B) of the first and second switching circuits (10OA, 10OB; 200A, 200B; 300A, 300B) corresponding to a same load (Ll; L2; L3) are coupled to the same control output terminal (33A; 33B; 33C) of the control circuit (30).

20. Driving circuit according to claim 14, wherein the control circuit (30) is designed to provide control signals at its control output terminals (33A; 33B; 33C) in such a manner that always when one currently active set of output terminals (7101, 7102; 7201, 7202; 7301, 7302) is disconnected from the power source, one currently inactive set of output terminals (7201, 7202; 7301, 7302; 7101, 7102) is actively coupled to the power source simultaneously.

21. Scanning backlight system for an LCD panel, comprising a plurality of fluorescent tubes and a driving circuit according to claim 14.

22. Ambilight system for a flat TV, comprising a plurality of fluorescent tubes and a driving circuit according to claim 14.