SWITCH MODE POWER SUPPLY

Abstract: The invention relates to a switch mode power supply provided with controlled main switching means which are switched to the conducting and non-conducting state periodically by means of a switching signal and which are provided with a control electrode and two main electrodes. According to the invention, the switch mode power supply is characterized in that a ceramic bead is directly electrically connected to one of the main electrodes of the controlled main switching means. Preferably, the bead is a ferrite bead. The bead on the one hand effectively counteracts the occurrence of a high-frequency interference signal and on the other hand further reduces switching losses.

Description: Switch mode power supply. The invenuon relates to a switch mode power supply provided with controlled main switching means which are switched to the conducting and non-conducting state periodically by means of a switching signal and which are provided with a control electrode and two main electrodes. Such a supply is known inter alia from electronic ballasts of the EMC 035-S01 and EMC 070-S01 types, make Philips, designed for igniting and operating metal halide lamps. The known ballasts are equipped with two DC-DC converters, one of the upconverter or boost converter type and one of the downconverter or Buck converter type. Another known type is the flyback converter. This type of converter is also known under the designation Buck-boost converter. Each of these DC-DC converter types is suitable per se for use as a switch mode power supply, and are indeed widely used in, for example, supply units and drives for tools and domestic appliances, supplies for lighting equipment, and the like. The controlled main switching means are preferably switched into the conducting and non-conducting state with a frequency above 20 kHz so as to avoid noise pollution. Switching frequencies of a few hundreds of kHz are quite usual here. The use of high frequencies is furthermore advantageous because it implies that inductive means, which always form part of the converters and supplies, can be comparatively small. A comparatively fast switching from the conducting into the non¬ conducting state of the switching means and vice versa is generally desirable because it has a strongly reducing effect on the switching losses accompanying the switching. This fast switching, however, has the consequence that high-frequency signals are generated. In the known circuit arrangement, the control electrode of the main switching means in each of the converters is provided with a branch comprising a parallel arrangement of a rectifier and an ohmic impedance in order to safeguard a sufficient delay in the switching of the switching means to the conducting state. Furthermore, each converter is provided with a so-called dV/dt capacitor for counteracting radio interference. This has the disadvantage, however, that the energy stored in the capacitor is dissipated in the switch. The invention has for its object to provide a measure for further reducing the switching losses on the one hand and for counteracting interference signals accompanying fast switching on the other hand. According to the invention, a switch mode power supply of the kind mentioned in the opening paragraph is for this purpose characterized in that a ceramic bead is directly electrically connected to one of the main electrodes of the controlled main switching means. The presence of the bead in one of the main electrode branches effectively counteracts the occurrence of a high-frequency interference signal caused by fast switching of the switching means and also further promotes the reduction of switching losses. The energy stored in the bead in fact promotes a faster charging of the parasitic capacitance of the charging means during switching of the controlled switching means to the non-conducting state, and thus a faster switching of the switching means. At the same time, the bead controls the current rise through the switching means during switching to the conducting state, which contributes to a reduction in high-frequency interference signals. This renders it possible both to dispense with the dV/dt capacitance and to simplify the control circuit through the elimination of the parallel arrangement of rectifier and ohmic impedance. A further advantage of the use of the bead is the property that this bead has a considerable ohmic impedance for high frequencies. Oscillations of parasitic capacitances and self-inductances at high frequencies are strongly damped thereby. Preferably, the ceramic bead is a ferrite bead, because of the suitable magnetic properties of ferrite. The fast switching particularly relates to FETs. The use of a bead accompanied by the elimination of the parallel arrangement in the control circuit may indeed give rise to such a high voltage across the FET that the latter enters a zener state, but the energy stored in the bead will be insufficient, because of the very low self-inductance value of the bead, for reaching and exceeding the avalanche energy of the FET. In principle, the bead can be placed in series with any of the two main electrodes, i.e. drain and source. Preferably, the bead is connected in series with the drain of the FET. The use of snubbers is known for bipolar transistors as a means of reducing switching losses and counteracting interference signals. The self-inductances to be used for this already have a high parasitic capacitance owing to the windings required such that the relevant self-inductances are ineffective in limiting interference signals of high frequency. In addition, the combined use of a self-inductance, diode, resistor, and capacitor is necessary for practical snubbers if they are to be effective in switching both to the conducting and to the non-conducting state. The above and further aspects of the invention will be explained in more detail below with reference to a drawing of an embodiment of a switch mode power supply according to the invention, in which Fig. 1 is a diagram of a switch mode power supply of the upconverter type according to the prior art, Fig. 2 is a diagram of a switch mode power supply of the downconverter type according to the prior art, Figs. 3 and 4 are diagrams of switch mode power supplies according to the invention of the upconverter and downconverter types, respectively, and Fig. 5 is a diagram of a switch mode power supply according to the invention of the flyback converter type. Corresponding parts have been given the same reference numerals in Figs. 1 to 5. A switch mode power supply comprises input terminals 1, self-inductance means L, switching means S, rectifying means D, and output terminals 2. The output terminals are often interconnected by a voltage buffer in the form of a buffer capacitor 3. The controlled main switching means are provided with two main electrodes 7, 8 and a control electrode 6. The control electrode 6 of the main switching means S is provided with a branch comprising a parallel arrangement of a rectifier and an ohmic impedance 9 in order to safeguard a sufficient delay in switching of the switching means to the conducting state, and thus to counteract the generation of EMI. The switch mode power supply shown in Fig. 1 is an upconverter or boost converter. The main switching means S are shunted here by a capacitor 5 acting as a dV/dt capacitance. The capacitor 5 acting as the dV/dt capacitance is placed between the two main electrodes 7, 8 in the embodiment shown in Fig. 2. This relates to a downconverter or Buck converter. In the switch mode power supplies according to the invention, a ceramic bead 10 is included in series with the main electrode 7 of the controlled main switching means S, and electrically directly connected thereto. In a practical realization of an upconverter or boost converter as shown in Fig. 3 or a downconverter as shown in Fig. 4, the controlled main switching means are formed by a type STP4NA60F1 FET, make Thomson-SGS. The bead used is a ferrite bead, type BL02RN2, make Murata. The ohmic impedance thereof rises from 40 Ω at a frequency of 5 MHz to 140 Ω at 300 MHz. The self-inductance value is 1.6 μU. In a practical application, the practical realizations of the upconverter or boost converter and the downconverter as described above each form part of a circuit arrangement for igniting and operating a high-pressure discharge lamp, for example of the CDM35W type, make Philips, with a power rating of 39 W. The lamp has a nominal lamp voltage Via of 90 V. The circuit arrangement is suitable for connection to a supply source, for example a voltage source of 220 V, 50 Hz, and comprises a rectifier arrangement which converts the AC voltage supplied by the supply source into a DC voltage. The DC voltage thus formed serves as a supply for the upconverter or boost converter, for which purpose the rectifier device is connected to the input terminals of said upconverter or boost converter. The upconverter or boost converter supplies a DC voltage of 400 V at its output terminals, which are at the same time the input terminals of the downconverter. When the lamp is not ignited, the open voltage at the output terminals of the downconverter is 380 V. The downconverter acts as a controlled current generator for an ignited lamp during stable operation, the voltage at the output terminals of the downconverter being approximately 90 V in that case. The lamp is included in a commutator network in order to counteract cataphoresis in the lamp, said network being connected to the output terminals of the downconverter. The upconverter or boost converter and the downconverter each switch with a frequency between 70 kHz and 200 kHz during operation of the circuit arrangement. Switching-off of the current through the converter to 0 A takes place very quickly, within a time interval of 40 ns, in the downconverter during switching of the controlled main switching means to the non-conducting state. The voltage between the main electrodes rises to 400 V in 50 ns in that case. Switching-off of the current to 0 A takes place in 20 ns in the upconverter, and the voltage rise to 400 V takes 100 ns. During switching to the conducting state, the voltage drop to 0 V between the main electrodes of the controlled main switching means takes place in 20 ns. The rise of the current is somewhat delayed here, inter alia as a result of the inductive means, including the ferrite bead, present in the relevant converter.

Claims: CLAIMS: 1. A switch mode power supply provided with controlled main switching means which are switched to the conducting and non-conducting state periodically by means of a switching signal and which are provided with a control electrode and two main electrodes, characterized in that a ceramic bead is directly electrically connected to one of the main electrodes of the controlled main switching means. 2. A switch mode power supply as claimed in Claim 1 , characterized in that the controlled main switching means are formed by a FET and the main electrodes are formed by a drain and a source. 3. A switch mode power supply as claimed in Claim 2, characterized in that the bead is directly electrically connected to the drain of the FET.

Inventors: DEURLOO OSCAR JAN, LEMMERS WILLY, DEURLOO, OSCAR, JAN, LEMMERS, WILLY
Applicants: PHILIPS ELECTRONICS NV, PHILIPS NORDEN AB, PHILIPS ELECTRONICS N.V.
Application year: 1997
Main classification: H02M1
IPC codes: H02M1, H02M3, H03K17, H05B41