DC-AC CONVERTER FOR IGNITING AND SUPPLYING A GAS DISCHARGE LAMP

Abstract: DC-AC converter for igniting and supplying a gas discharge lamp (1), which converter has two input terminals (C, D) to be connected to a DC voltage source, said terminals (C, D) being interconnected by means of a series arrangement with a load circuit comprising at least the lamp (1) and an induction coil (5), and a first semiconductor switching element (6) including a freewheel diode, said load circuit being bridged by a circuit including a second semiconductor switching element (7) with a freewheel diode, said semiconductor switching element (6, 7) being provided with control circuits for rendering said switching elements alternately conducting, whilst the control circuit (9) of switching element (7) has a voltage measuring point which is connected to a rectifier element (10) connected to the control circuit (8) of the first switching element (6).

Description: The invention relates to a DC-AC converter for igniting and supplying a gas discharge lamp, which converter has two input terminals to be connected to a DC voltage source, said input terminals being interconnected by means of a series arrangement with a load circuit and a first semiconductor switching element including a freewheel diode, said load circuit, comprising at least the discharge lamps and an induction coil, being bridged by a circuit including a second semiconductor switching element and also having a freewheel diode, said semiconductor switching elements each provided with control circuits for rendering said switching elements alternately conducting. A converter of this type is known from WO-A-83/00587. The above-mentioned semiconductor switching elements are rendered conducting and non-conducting by means of their respective control electrodes. The freewheel function can be provided using a type of semiconductor element having an integral diode element or using a separate diode element arranged in parallel therewith. The said WO-A-83/00587 describes half bridge converter with a transformer which is present in the load circuit (in which the lamp is incorporated), which transformer has two secondary windings. These windings form part of the control circuits of the semiconductor switching elements. The switching elements are rendered alternately conducting and non-conducting by means of the transformer and the control circuits, respectively. In WO-A-83/00587 a converter for supplying a discharge lamp with a rectifier element is present in the starting circuit of the converter only. During continuous lamp operation, however said rectifier element is of no further significance. It has been found that it is difficult to adjust the oscillator frequency of the system in a reproducible manner to a fixed value due to the use of a transformer. This is a drawback, notably when the system is used in lamps which are manufactured in a bulk manufacturing process. Moreover, the transformer described in voluminous and costly, whilst the associated control circuits comprise a comparatively large number of components. The integration of the circuit in a compact discharge lamp (such as an "SL" lamp) is then troublesome. It is an object of the invention to provide a DC-AC converter obviating the drawbacks of the known converter whilst reducing the number of components required in the circuit to a minimum. According to the invention a converter of the type described in the opening paragraph is therefore characterized in that during the continuous and stable operation of the lamp in the control circuit of the second switching element a voltage measuring point is present which is connected via a rectifier element to the control circuit of the first switching element, said voltage measuring point determining when said switching element is conducting. During the continuous and stable operation of the lamp the control circuit of the second switching element functions as a main control circuit and the control circuit of the first switching element functions as an auxiliary control circuit. The instantaneous state of conductance of the second switching element is fixed in this auxiliary control circuit via the rectifier element. Based on the value of the voltage in the control circuit of the second switching element, the first switching element is brought to a conducting state which is opposed to that of the second switching element. Only one induction coil is required in the converter according to the invention. The use of a voluminous and cosily transformer in the converter (with special requirements being imposed on the electrical insulation between the primary and secondary windings) is obviated. In comparison with the known converter, the number of electrical components required in the circuit is reduced. In addition integration of components (for example, with help of a "surface mounted device" technique) becomes more practical. This renders the converter more suited for use in a compact discharge lamp to replace an incandescent lamp for general illumination purposes. In a preferred embodiment the rectifier element is connected to the central tap functioning as a voltage measuring point of an LC oscillatory circuit in the control circuit of the second semiconductor switching element, whilst the coil of the LC circuit is magnetically decoupled from the induction coil, said central tap being connected to the load circuit by means of the capacitor of the LC circuit. The frequency at which the switching elements are rendered conducting can be adjusted exactly by means of an oscillatory circuit. In addition, this frequency is very stable. In a practical embodiment of the converter according to the invention the control circuit of the first semiconductor switching element comprises a circuit connected to one input terminal for switching on the switching element, and a separate circuit for switching off said switching element, said latter circuit including a third semiconductor switching element which is conducting during substantially the same periods as the second switching element. The first switching element is in a conducting state whilst the periods when the first switching element is non-conducting are determined by means of the third switching element. The use of the third semiconductor switching element has the advantage that the relevant control circuit does not require any extra inductive elements. In a special embodiment the coil of the LC oscillatory circuit in the control circuit of the second switching element (in which the central tap of the circuit is connected to the rectifier element) is bridged by a variable impedance. The advantage of this embodiment is that the lamp can be dimmed by increasing the frequency at which the circuit oscillates. In a special embodiment the variable impedance comprises a series arrangement of a resistor and two oppositely arranged zener diodes also arranged in series. Together with a capacitor arranged between the lamp electrodes and the coil arranged in series with the lamp, the frequency of the LC oscillatory circuit is adjusted to a value near the resonance frequency of the oscillatory circuit constituted by the said capacitor and the coil in series with the lamp. A high voltage for igniting the lamp is available by choosing an appropriate starting frequency. The invention will now be described in greater detail by way of example with reference to the accompanying drawings in which Fig. 1 shows diagrammatically the circuit of the converter according to the invention with a discharge lamp connected thereto and Fig. 2 shows an embodiment of the circuit of the converter according to the invention. In Fig. 1 the reference numeral 1 denotes a U-shaped low-pressure mercury vapour discharge lamp. In a practical embodiment the lamp has four parallel discharge tubes arranged in a square and interconnected by bridges (see US-PS 4,374,340). The lamp has two electrodes (2 and 3, respectively). The references C and D denote the input terminals of the converter. These are intended to be connected to a DC voltage source such as a diode bridge with a smoothing capacitor (see Fig. 2). The terminals C and D are interconnected by means of a series arrangement with a load circuit (comprising a capacitor 4, the lamp 1, and an induction coil 5) and a first semiconductor switching element 6 with an integrated freewheel diode (6a) which is shown in a broken lineform. The load circuit is bridged by a circuit including a second semiconductor switching element 7 (with freewheel diode 7a). The two switching elements 6 and 7 are provided with control circuits 8 and 9 which are shown diagrammatically. With the aid of these control circuits the elements 6 and 7 are rendered alternately conducting and non-conducting. The control circuit 9 for the second switching element 7 has a voltage measuring point which is connected to the rectifier element (diode) 10, which element is connected to the control circuit 8 of the first switching element 6. The rectifier element functions as a sensor for the voltage at the said measuring point. Further electrode 2 is connected to D by capacitor 11. The control circuit 9 functions as a main control circuit, whilst control circuit 8 functions as an auxiliary control circuit. The instantaneous state of conductance of the switching element 7 is fixed in this circuit 8 via the rectifier element 10. Based on the value of the voltage in the control circuit of said switching element 7 the switching element 6 is brought to a state of conductance which is opposed to that of the switching element 7. This is realised in practice in the circuit according to Fig. 2. In Fig. 2 the same elements as those in Fig. 1 have identical reference numerals. The reference numerals 12 and 13 denote the input terminals to be connected to an AC voltage source (220 V, 50 Hz). The input terminal 12 is connected via a resistor 14 to an input terminal of a diode bridge 15. The two input terminals of the bridge are interconnected by means of the capacitor 16. The combination of the resistor 14 and the capacitor 16 constitutes an input filter. The output terminals of the bridge 15 are interconnected by means of the smoothing capacitor 17. Furthermore a smoothing coil 18 is connected in series between one terminal of the bridge and a first input terminal C of the DC-AC converter. This converter is actually connected to the ends of the combination of capacitor 17 and coil 18. The converter is in the form of a half bridge converter. The first pair of legs of this half bridge converter is constituted by a series arrangement of two branches each comprising a capacitor 4 and a capacitor 11, respectively. A second pair of legs is constituted by a series arrangement of two branches each comprising semiconductor switching elements 6 and 7, respectively (with an integrated freewheel function, see Fig. 1). The central branch of the converter is constituted by the connection of the point A (between 6 and 7) and B (between 11 and 4). The terminals C and D are interconnected via the load circuit already referred to in Fig. 1 (comprising the series arrangement of capacitor 4, lamp 1, coil 5 and switching element 6). This load circuit is bridged by a circuit including the second switching element 7. The first switching element 6 is associated with a control circuit (8) comprising a circuit which is connected to an input terminal (D) for switching on element 6. A separate circuit is provided which is connected to the central tap of an LC oscillatory circuit comprising the coil 19 and the capacitor 20. The coil 19 is electrically connected to coil 5 via the auxiliary winding 21 on coil 5. The circuit comprises the rectifier element 10 which is connected to the base of the auxiliary transistor 22. The base of this transistor is also connected to terminal C via the resistor 23. The collector of 22 is connected to the control electrode of 6. Furthermore the resistor 24 is connected between the collector of 22 and the terminal C. The control circuit of the second switching element 7 includes a resistor 25 coupled between the control electrode of element 7 and the central tap P of the LC oscillatory circuit (19 and 20). Furthermore a circuit of two oppositely arranged zener diodes 26 and 27 is present between point A and the control electrode of 7. Transistor 22 is also bridged by zener diode 22a. The coil 19 is bridged by a series arrangement of a resistor 28 and two oppositely arranged zener diodes 29 and 30. The converter is also provided with a starter circuit comprising a series arrangement of a resistor 31 and a bidirectional breakdown element (diac) 32 between the control electrode of 7 and a junction point of the resistors 33 and 34. A capacitor 35 is also connected between the said junction point and point A. The first ends of the electrodes 2 and 3 of the lamp are interconnected by means of capacitor 39. The other ends are interconnected by means of a parallel arrangement of resistor 38 having a positive temperature coefficient (PTC) and capacitor 37. Resistor 40 is arranged parallel across 11. The converter operates as follows. If the terminals 12 and 13 are connected to the supply main (220 V, 50 Hz), the capacitor 17 will be charged via the diode bridge 15. This results in the capacitors 4 and 11 also being charged via coil 18. The starting capacitor 35 will also be charged via the circuit 18, 33, 35 and A, D. When the voltage at the capacitor 35 reaches the threshold voltage of circuit element 32, said element 32 will become conducting and it will render the semiconductor switch 7 conducting via circuit element 31. The electrodes 2 and 3 of the lamp are then preheated (by means of the PTC resistor 38, see Netherlands Patent Application 8400923 laid open to public inspection). During the continuous and stable operation the control signal for rendering switching element 6 conducting is directly supplied by the voltage of the capacitors 4 and 11. An AC voltage at a frequency which is very accurately determined by coil 19 and capacitor 20 (the LC oscillatory circuit) is produced across capacitor 20 between the points P and A. The said voltage is responsible for switching off 7. Switching element 7 is switched off while a current is still flowing through coil 5, with a freewheel current flowing through 6. Consequently point A acquires the same potential as point D. If the voltage at point P becomes negative with respect to point A, the control of the auxiliary transistor 22 is offset via rectifier element 10 and 22 is turned off. However, as soon as the voltage difference between P and A is zero again, the auxiliary transistor 22 is turned on and 6 becomes non-conducting. Then 7 becomes conducting again, etc. Thus the voltage is measured in the control circuit 9 of switching element 7 and this measuring voltage determines when the element 6 is rendered conducting. For igniting the lamp the frequency of the oscillatory circuit can be adjusted by means of the elements 28, 29 and 30. In one embodiment the most essential circuit elements had the following values: capacitor 4: 220 nF capacitor 11: 220 nF capacitor 20: 10 nF capacitor 40: 11 µF capacitor 35: 22 nF coil 5: 3 mHenry coil 19: 680 µHenry ratio of turns of coil 5: coil 21 = 200 : 7 turns MOS-FET 6: type BST 78 MOS-FET 7: type BST 78 transistor 22: type BC 547 frequency LC circuit: 28 kHz. In this embodiment the lamp 1 ignited at a voltage of 600 V between the electrodes 2 and 3. The lamp is of the type having four interconnected discharge tubes arranged in a square (see, for example, Netherlands Patent Application 8600252, laid open to public inspection). The lamp efficiency was approximately 60 lm/Watt.

Claims: A DC-AC converter for igniting and supplying a gas discharge lamp (1), which converter has two input terminals (C, D) to be connected to a DC voltage source, said input terminals (C, D) being interconnected by means of a series arrangement with a load circuit and a first semiconductor switching element (6) including a freewheel diode, said load circuit, comprising at least the discharge lamp (1) and an induction coil (5), being bridged by a circuit including a second semiconductor switching element (7) and also having a freewheel diode, said first and second semiconductor switching elements (6, 7) being each provided with control circuits (8, 9) for rendering said switching elements (6, 7) alternately conducting, characterizing in that, during the continuous and stable operation of the lamp (1), in the control circuit (9) of the second switching element (7) a voltage measuring point (P) is present which is connected via a rectifier element (10) to the control circuit (8) of the first switching element (6), said voltage measuring point (P) determining when said first switching element (6) is conducting. A DC-AC converter as claimed in Claim 1, characterized in that voltage measuring point (P) is a central tap in an LC oscillatory circuit in the control circuit (9) of the second semiconductor switching element (7), whilst the coil (9) of the LC circuit is magnetically decoupled from the induction coil (5), said measuring point (P) is connected to the load circuit by means of the capacitor (20) of the LC circuit. A DC-AC converter as claimed in Claim 1 or 2, characterized in that the control circuit (8) of the first semiconductor switching element (6) comprises a circuit connected to one input terminal (C) for switching on the switching element (6), and a separate circuit for switching off said switching element (6), said latter circuit including a third semiconductor switching element (22) which is conducting during substantially the same periods as the second switching element (7). A DC-AC converter as claimed in Claim 2 or 3, characterized in that the coil (19) of the LC oscillatory circuit in the control circuit (9) of the second switching element (7) is bridged by a variable impedance. A DC-AC converter as claimed in Claim 4, characterized in that the variable impedance comprises a series arrangement of a resistor (28) and two oppositely arranged zener diodes (29,30).

Inventors: HOUKES, HENK, LAMMERS, FRANCISCUS HUBERTUS THEODORUS, VELDMAN, PAUL ROBERT
Applicants: N.V. PHILIPS' GLOEILAMPENFABRIEKEN
Application year: 1988
Main classification: H05B41
IPC codes: H05B41, H02M7