DC-AC CONVERTER FOR IGNITING AND SUPPLYING A DISCHARGE LAMP

Abstract: DC-AC converter for igniting and supplying a low-­ pressure discharge lamp such as an electrodeless low-­ pressure gas discharge lamp, which converter is provided with two input terminals (9, 10) intended to be connected to a d.c. voltage source which input terminals are connected together by means of a series arrangement of a semiconduc­ tor switching element (12) provided with a control electrode, a first coil (14) and a parallel circuit with the discharge lamp 15 and incorporated in one of its branches, the control elec­ trode and a main electrode of the controlled semiconductor switching element (12) being connected together by means of a secondary winding (21) of a transformer (17) whose primary winding (16) forms part of the parallel circuit. Ac­ cording to the invention the converter incorporates a starter circuit comprising a resistor (28) connected between a main electrode and the control electrode of the semiconductor switching element (12) and a capacitor (29) arranged between the control electrode and one end of the secondary winding (21) of the transformer (17).

Description: The invention relates to a DC AC converter for igniting and supplying a low-pressure discharge lamp, which converter is provided with two input terminals intended to be connected to a d.c. voltage source, the input terminals being connected together by means of a series arrangement of a semiconductor switching element having a control electrode, a first coil and a parallel circuit with the discharge lamp being incorporated in one of its branches, the control electrode and a main electrode of the controlled semiconductor switching element being connected together by means of a secondary winding of a transformer whose primary winding forms part of the parallel circuit, a starter circuit being incorporated in the converter. A DC AC converter of this type is known from the published NL-A-8004175. With this converter, for example, an electrodeless low-pressure discharge lamp having an operating frequency of at least 0.5 MHz or a compact low-pressure mercury discharge lamp is supplied. It has been found that the converter has a high efficiency. In order to start the converter a starter circuit is incorporated in the control circuit of a semiconductor switching element consisting inter alia of a series arrangement of a resistor and a capacitor connecting the input terminals of the converter together. Furthermore the starter circuit has a branch comprising a bidirectional breakdown element (diac) which is connected at one end to a junction between the said resistor and capacitor and at the other end to the control electrode of the semiconductor switch. It has been found that the use of the capacitor and the said breakdown element in the starter circuit often gives rise to an increased radio interference level. In fact, the said capacitor is charged over and over again via the resistor up to the breakdown level of the diac whereafter a start pulse in the operating converter is supplied. Furthermore it has been found that the breakdown current of the diac at higher temperatures increases in the course of the operating time. For reliable starting a circuit is to be designed for a fairly high breakdown current which also flows through the said resistor. In that case extra thermal losses occur. This is a drawback, particularly if the converter and the lamp are integrated to form a compact unit. It is an object of the invention to provide a DC-AC converter having such a starter circuit that interference signals are reduced as much as possible and which obviates the use of a breakdown element. To this end a DC-AC converter of the type described in the opening paragraph according to the invention is characterized in that the starter circuit includes a resistor which is connected between the main electrode and the control electrode of the semi-conductor switching element, and a capacitor which is arranged between the control electrode and one end of the secondary winding of the transformer. The starter circuit of the converer according to the invention obviates the use of a bidirectional breakdown element. During operation of the converter no interference signals are generated on the supply mains. The energy dissipation in the starter circuit is also greatly reduced during operation. The number of components required is further reduced as compared with the known circuit. Because of these properties the entire circuit of the converter can easily be integrated in a lamp base of a compact lamp such as in the base of a fluorescent electrodeless low-pressure mercury discharge lamp. The invention is based on the recognition that upon switching on the converter the capacitor arranged between the control electrode and a main electrode of the switching element is first charged until the voltage on the control electrode is sufficiently high to render the switching element conducting. As a result a much higher current suddenly flows through the resonant circuit which is incorporated in the series arrangement. This resonant circuit is thy parallel arrangement of the primary transformer winding and a capacitor on the one hand and the first coil. This parallel arrangement also incorporates the winding of an electrodeless discharge lamp. The resonant circuit starts oscillating because of the flow of the suddenly much higher current. The primary winding incorporated in this circuit then takes over the drive of the semiconductor switching element. Any further starting pulses are then no longer applied to the switching element, The capacitance of the capacitor in the starter circuit is chosen to be such that its impedance at the operating frequency of the converter is low. Preferably a value at which the thermal losses during operation are negligible is chosen for the starting resistor. The said semiconductor switching element is preferably a MOS-FET. The invention will be further described by way of example with reference to the accompanying drawing. The drawing shows an embodiment of a DC-AC converter according to the invention with an electrodeless low-pressure mercury vapour discharge lamp connected thereto. The input terminals intended to be connected to an a.c. voltage source (220 V, 50 Hz) are denoted by 1 and 2. These terminals are connected via a current-limiting resistor 3 and a fuse 4 to a full-wave rectifier 5. The output voltage of this rectifier is smoothed by means of the capacitor 6. Together with the highfrequency capacitor 7 and coil 8 capacitor 6 constitutes a mains interference suppression filter. The output terminals 9 and 10 of the rectifier are also the input terminals for the DC-AC converter, capacitor 11 of which constitutes the RF-d.c. voltage source. The input terminals 9 and 10 are connected together by means of a first series arrangement of a semiconductor switching element (power MOS-FET) 12, a capacitor 13, a coil 14 and a parallel arrangement of a coil 15 at one end and a series connection of a primary winding 16 of a current transformer 17 and a capacitor 18 at the other end. Coil 15 is coupled to the discharge path in an electrodeless discharge lamp, diagrammatically denoted by 19. Furthermore the circuit incorporates a second semiconductor switching element (power MOS-FET) 20, which is arranged in series with 12 and is connected to terminal 10. The primary winding 16 is bridged by a resistor 30 for optimizing the phase of the feedback signal. The transformer 17 has two secondary windings 21 and 22. Winding 21 forms part of the control circuit of the switching element 12 and connects the control electrode of 12 to a main electrode thereof. The ends of winding 21 are connected by a voltage-limiting circuit consisting of a series arrangement of two oppositely arranged Zener diodes 23 and 24. Winding 22 forms part of the control circuit of switch 20 and is bridged via capacitor 25 by the series arrangement of the oppositely arranged Zener diodes 26 and 27. The starter circuit for the converter forms part of the control circuit of the POWER-MOS-semiconductor switching element 12. The starter circuit includes a resistor 28 which is connected between a main electrode and the control electrode of the element 12, as well as a capacitor 29 which is arranged between the said control electrode and the end of the secondary winding 21. The circuit described operates as follows. If the terminals 1 and 2 are connected to an a.c. voltage source, the capacitors 6, 7 and 11 are quickly charged via the rectifier 5 up to the peak value of the a.c. voltage originating from the a.c. voltage source. The d.c. voltage is present across the series arrangement 28, 29, 21, 13, 14 and 15. The capacitors 29 and 13 are charged via resistor 28 until the voltage across capacitor 29 reaches a threshold at which the semiconductor switching element 12 becomes conducting. Then a much higher current flows through the elements 12, 13 and 14 as well as in the parallel circuit 15, 16 and 18. An oscillation is then produced in this circuit whereafter the transformer 17 renders the semiconductor element 12 non-conducting and renders 20 conducting. This produces a current through capacitor 11 whereafter 20 becomes non-conducting again and 12 becomes conducting again and so forth. In a practical embodiment the abovementioned circuit elements approximately have the values as shown in the Table below: The electrodeless lamp (19) which is coupled to a circuit including the elements shown in the table above has a power of 18 W and is further described in the published NL Patent Application 8 400 409. The coil 15 is formed as a winding around a ferromagnetic core which is arranged centrally in a spherical discharge vessel. The RC-time of the series resistor (3) of the rectifier 5 and the smoothing capacitor 6 is shorter than the resonance time of the starter circuit 28 and 29. The starter circuit does not operate until after the d.c. voltage source has reached the peak value of the mains voltage. Generally favourable results were achieved with the above-mentioned circuit if the first-mentioned resonance time was a factor of 500 to 1500 shorter than the second. At the values shown in the Table the first resonance time was approximately a factor of 1300 shorter. In an alternative embodiment the parallel circuit incorporates a low-pressure mercury vapour discharge lamp having a hook-shaped bent discharge tube instead of the electrodeless lamp 19. Coil 15 is then replaced by a series arrangement of the two electrodes present on the ends of the tube whilst a resistor having a positive temperature coefficient is arranged between the juxtaposed supply wires of the two electrodes.

Claims: 1. A DC-AC converter for igniting and supplying a low-pressure discharge lamp (19), which converter is provided with two input terminals (9, 10) intended to be connected to a d.c. voltage source, said input terminals (9, 10) being connected together by means of a series arrangement of a semiconductor switching element (12) having a control electrode, a first coil (14) and a parallel circuit with the discharge lamp (19) being incorporated in one of its branches, the control electrode and a main electrode of the controlled semiconductor switching element (12) being connected together by means of a secondary winding (21) of a transformer whose primary winding (16) forms part of the parallel circuit, a starter circuit being incorporated in the converter, characterized in that the starter circuit includes a resistor (28) which is connected between the other main electrode and the control electrode of the semiconductor switching element, and a capacitor (29) which is arranged between the control electrode and one end of the secondary winding of the transformer. 2. A DC-AC converter as claimed in Claim 1, in which the input terminals are connected to a rectifier including a smoothing capacitor, characterized in that the resonance time formed in the series arrangement of said capacitor and a resistor of the rectifier is shorter than the resonance time of the resistor and the capacitor of the starter circuit.

Inventors: HOUKES, HENK
Applicants: N.V. PHILIPS' GLOEILAMPENFABRIEKEN
Application year: 1986
Main classification: H05B41
IPC codes: H02M7, H05B41