CIRCUIT ARRANGEMENT

Abstract: The invention relates to a circuit arrangement for igniting and operating an electrodeless discharge lamp provided with a half-bridge commutator having two switching elements each having an emitter electrode and a control electrode and a parasitic capacitance in between, each switching element being alternately switched to a conducting state by means of a resonant control circuit. The resonant control circuit is coupled to an oscillator by way of a transformer. According to the invention, the resonant circuit is provided with capacitor means which form part of the oscillator.

Description: Circuit arrangement. The invention relates to a circuit arrangement for igniting and operating an electrodeless discharge lamp provided with a bridge commutator having two switching elements each having an emitter electrode (source) and a control electrode (gate) and a parasitic capacitance in between, said switching elements being alternately switched to a conducting state by means of a resonant control circuit which is coupled to an oscillator via a transformer. A circuit arrangement of the type described in the opening paragraph is known from US 5,200,672. The known lamp is suitable for operating a low-pressure mercury discharge lamp. In the known circuit arrangement, the lamp is operated by means of a frequency of 13.56 MHz. Such a high operating frequency requires the use of a crystal oscillator for realizing a sufficiently stable frequency. An electrodeless discharge lamp which is frequently used in practice is operated at an operating frequency between 1 and 3 MHz, which imposes less stringent requirements on the stability of the operating frequency. In the known circuit arrangement, each control electrode is directly connected to a relevant secondary winding of the transformer. A drawback of the known circuit arrangement is that pulsatory currents during the periods when each switching element is switched to a conducting state lead to distortion of the output voltage of the transformer. It is an object of the invention to provide a measure to obviate said drawback. According to the invention, this object is achieved in that a circuit arrangement of the type described in the opening paragraph is characterized in that the resonant control circuit incorporates capacitor means which form part of the oscillator. The circuit arrangement according to the invention appears to have the advantage that no noticeable distortion of the voltage at the output of the transformer occurs under the influence of the pulsatory gate currents of the switching elements, so that the control of the period of conductance of the switching elements is maintained. Moreover, it is sufficient to use an oscillator which, in addition to a switch, comprises only inductive and capacitive elements. The circuit arrangement is preferably dimensioned in such a way that the bridge commutator is operated at a frequency of between 2.5 and 3 MHz. In a further preferred embodiment of the circuit arrangement according to the invention, each switching element is connected to a relevant part of the resonant control circuit, and each relevant part is provided with a capacitor forming part of the capacitor means and having a capacitance which is at least 5 times larger than that of the parasitic capacitance between the control electrode and the emitter electrode of the relevant switching element. These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter. In the drawing: Fig. 1 is a circuit diagram of a circuit arrangement according to the invention, and Fig. 2 shows further details of a part of the circuit diagram of Fig. 1. In Fig. 1, the reference I denotes a switched-mode power supply (SMPS) for igniting and operating an electrodeless discharge lamp L connected to lamp terminals 3, 4. The SMPS which is suitable to be supplied from, for example a public mains via connection terminals 1, 2, is provided with a bridge commutator. In the embodiment described, the SMPS comprises a pre-conditioner PRC which converts the AC voltage supplied by the mains into a DC voltage with which the bridge commutator is supplied. The pre-conditioner PRC also ensures that a mains current is obtained which, in a good approximation, is sinusoidal. It is thus possible to realize a high power factor. The bridge commutator comprises two switching elements BSl, BS2 each with emitter electrodes El, E2 and control electrodes Gl, G2, respectively. The switching elements are alternately switched to a conducting state. The reference II denotes control means for controlling the bridge commutator. The means II, which are shown in more detail in Fig. 2, comprise an oscillator OS and a resonant control circuit RS. The oscillator OS comprises an oscillating circuit with an oscillator capacitor COS and an oscillator inductance LOS. The oscillator capacitor COS is connected, at one end, to ground and, at the other end, to a source electrode OSS of an oscillator switch SOS which is connected to a power supply voltage VI by way of a drain electrode OSD. A gate electrode OSG of the oscillator switch SOS is connected to the oscillating circuit COS, LOS by means of a capacitor Cl and an inductive winding LS. The inductive winding LS and the oscillator inductance LOS constitute a primary winding of a transformer T. The coupling between oscillator OS and the resonant control circuit RS is realized by means of a secondary winding of the transformer T. The gate electrode OSG of oscillator switch SOS is connected by means of a parallel arrangement of a resistor Rl and a diode Dl to the power supply voltage VI for starting the oscillator when the power supply voltage is connected. Limitation of the voltage amplitude of the voltage across the primary winding LOS takes place by limiting the voltage at the gate electrode OSG via a circuit comprising a resistor R2, a diode D2 and a zener diode ZD. The resonant control circuit RS comprises secondary windings LSI, LS2 of transformer T, which are coupled oppositely with respect to each other. Together with a capacitor CEl, winding LSI constitutes a resonant circuit as the relevant part of the resonant control circuit which is connected between the control electrode Gl and a source electrode El of switching element BSl. A parasitic capacitance CPl is present between the source electrode El and the control electrode Gl. Analogously, a secondary winding LS2 constitutes, together with a capacitor CE2, a resonant circuit as the relevant part of the resonant control circuit which is arranged between control electrode G2 and source electrode E2 of switching element BS2. Analogously, a parasitic capacitance CP2 is present between source electrode E2 and control electrode G2. The capacitors CEl and CE2 have a capacitance which is chosen to be such that its value considerably exceeds that of the parasitic capacitances CPl and CP2 and thus has a predominant influence on the oscillation frequency. The capacitors CEl and CE2 thus form part of the capacitor means of the resonant control circuit and hence of the oscillator. In a practical realization of the described embodiment of the circuit arrangement according to the invention, the circuit arrangement is suitable for connection to a power supply mains of 220 V, 50 Hz and for operation of an electrodeless fluorescence lamp with a nominal power of 165 W. The lamp thus operated has a luminous flux of 12 klm The switching elements BSl and BS2 are implemented as FETs, type IRFIBC306, made by International Rectifier. The parasitic capacitance between the source electrode and the control electrode is dependent on the prevailing drain-source voltage and varies between 1300 pF and 660 pF. Capacitors CEl and CE2 have a capacitance of 6.6 nF each. Transformer T has a ferrite core on which a primary winding consisting of 5 turns as LOS and 2 turns as LS is provided. The oscillator inductance LOS has a value of 370 nH. The mutually oppositely coupled secondary windings LSI and LS2 consist of 3 turns each. The oscillator switch SOS is a FET, type BSP122 made by Philips. The oscillator OS also comprises the oscillator capacitor COS having a value of 1.8 nF and capacitor Cl of 1 nF. Capacitor Cl is used for DC decoupling the gate OSG, thereby enabling the oscillator OS to start via resistor Rl . In the presence of the power supply voltage VI, Cl is charged via Rl. As soon as the voltage across Cl has become sufficiently high, the oscillator switch SOS reaches its linear operating range and constitutes an amplifier having a large gain factor. Due to omnipresent noise, the oscillator OS thus starts oscillating. In the embodiment described, resistor Rl has a value of 4.7 kOhm and the zener diode ZD has a zener voltage of 13 V. In the operating state of the circuit arrangement, the connected lamp is operated at a frequency of 2.65 MHz, realizing a power factor of 0.98.

Claims: CLAIMS: 1. A circuit arrangement for igniting and operating an electrodeless discharge lamp provided with a bridge commutator having two switching elements each having an emitter electrode and a control electrode and a parasitic capacitance in between, said switching elements being alternately switched to a conducting state by means of a resonant control circuit which is coupled to an oscillator via a transformer, characterized in that the resonant control circuit incorporates capacitor means which form part of the oscillator. 2. A circuit arrangement as claimed in claim 1, characterized in that each switching element is connected to a relevant part of the resonant control circuit, and in that each relevant part is provided with a capacitor forming part of the capacitor means and having a capacitance which is at least 5 times larger than that of the parasitic capacitance between the control electrode and the emitter electrode of the relevant switching element. 3. A circuit arrangement as claimed in claim 1 or 2, characterized in that the bridge commutator is operated at a frequency of between 2.5 and 3 MHz.

Inventors: JACOBS RONNY A A M, VAN DEN BOSSCHE ALEX P M, JACOBS, RONNY, A., A., M., VAN DEN BOSSCHE, ALEX, P., M.
Applicants: KONINKL PHILIPS ELECTRONICS NV, KONINKLIJKE PHILIPS ELECTRONICS N.V.
Application year: 1999
Main classification: H05B41
IPC codes: H02M7, H05B41