NZ202517A

NZ202517A - Transistor switching circuit and dc-dc converter using such a circuit

Info

Publication number: NZ202517A
Application number: NZ20251782A
Authority: NZ
Inventors: M Goodman
Original assignee: Int Standard Electric Corp
Priority date: 1981-11-19
Filing date: 1982-11-17
Publication date: 1985-08-16
Also published as: GB2110493B; GB2110493A

Description

202517 fpriority Date(s): . Jfl'JJ?>• AComptote Specification Filed: t~l CIbss! #• • fj 6 AUG 1985 Publication Date: P.O. Journal, Nc: .....

NEW ZEALAND THE PATENTS ACT, 1953 .COMPLETE SPECIFICATION "DC-DC CONVERTER' WE, INTERNATIONAL STANDARD ELECTRIC CORPORATION, a Corporation of the State of Delaware, United States of America, of 320 Park Avenue, New York 22, New York, United States of America, hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement %j; • • M. Goodman 5 fj'fjeV \ DC-DC CONVERTER The present invention relates to transistor switching circuits especially to such circuits for use in DC-DC converters.

In certain DC-DC converters there is a power transistor, which is driven by the input DC to generate pulses which-it drives through the primary winding of a power transformer. The secondary of this transformer provides the DC output via a rectifier and filter arrangement. The output voltage is determined at least in part by the turns ratio of the transforrner. 'The power transistor is controlled to generate these pulses by a so-called, modulating circuit which often includes an oscillator. In order to obtain efficient converter operation it is i essential to correctly drive the power transistor.

An object of the invention is to provide an arrangement.in which rapid turn of the power transistor occurs.

According to the invention there is provided a transistor switching circuit, in which the currenc flowing in the collector-emitter path is transformer coupled to the base-emitter path to provide the necessary base current when the transistor is rendered conductive; in which there is a further winding on the transformer which is connected in series with a diode and is normally non-conductive, said diode being connected to the collector of the transister, and in 2025 t which when the transistor turns on the d.i4 progressively rendered conductive so that current is progressively diverted from the base-emitter circuit of the transistor to flow in said further winding, 5 thus enabling the transistor to operate in a quasi-saturated mode, allowing improved turn off to be obtained.

Embodiments of the invention will now be described with reference to the accompanying drawing, 10 j.n which Figs. 1 and 2 are highly simplified circuits explanatory of the basis of this invention, while Fig. 3 shows how a circuit such as that of Fig. 2 is used in a DC-DC converter where the transistor to be switched is the converter's power transistor. 15 The principle of the invention will be explained with reference to Fig. 1, where T is the power transistor to be switched. The initial switching impulse is applied to the transistor by means (not-shown) as a result of vhich the transistor is rapidly 20 turned on to initiate the conduction of collector current.

With a current transformer system drive as shown the controlling ampere turns is 1^ N^, where is the number of turns of the transformer winding 25 connected in the emitter-collector path. This current is initiated by the switch-on referred tc above. By transformer ..action this feeds N2 into the base and N2 is the number of turns of the transformer winding in the base circuit of T. Therefore the 30 collector voltage of T tends to fall, and as this voltage moves downwards, a diode D in series with a third winding of the transform, becomes forward biassed. This removes a current via the winding which has turns. Hence the base current of the 35 transistor is reduced, so that the transistor is now operating in a quasi-saturated state.

Such an arrangement enhances the switching 2025 1 speed, and allows high frequency operation by reducing leakage effects in the base circuit since the base winding is directly coupled to the base, whereas in conventional circuits other components are used between 5 the base winding and the base.

The arrangement of Fig. 2 operates in substantially the same way as does that of Fig. 1 so that no specific description.thereof is needed.

Fig. 3 shows how the circuit of Fig. 1 is 10 used in a DC-DC converter, where the transistor TR is the power transistor of the converter and is controlled, or modulated, by means (not shown) which can follow established practice. Txl is the control transformer whose use has been described with 15 reference to Fig. 1, and Tx2 is the power transformer. The secondary of this transformer feeds a rectifier D1-D2 the output of which is the DC output via a filter Ll-Cl.

Claims

202517 What we claim is:-

1. A transistor switching circuit, in which the current flowing in the collector-emitter path is transformer coupled to the base-emitter path to provide necessary base current when the transistor is rendered conductive, in which there is a further winding on the transformer which is connected in series with a normally non-conductive diode, said diode being connected to the collector of the transistor, in which when the transistor turns on the diode is progressively rendered conductive so that current is progressively diverted frcm the base-emitter path of the transistor to flow in said further winding, thus enabling the transistor to operate in a quasi-saturated mode, allcwing improved turn off to be obtained.

2. A circuit as claimed in claim 1, and in which the series cctnbina-tion of the diode and the further winding is connected between the collector and the base of the transistor.

3. A circuit as claimed in claim 1, and in which the series combination of the diode and the further winding is connected between the collector and the emitter of the transistor.

4. A circuit as claimed in claim 1, 2 or 3, and in which the transistor is the pcwer transistor of a DC-DC converter.

5. A transistor switching circuit substantially as described with reference to Fig. 1, Fig. 2 or Fig. 3 of the accompanying drawing.

6. A DC-DC converter substantially as described with reference to Fig. 3 of the accompanying drawing. INTERNATIONAL STANDARD ELECTRIC CORPORATION P.M. Conrick - Authorized Agent P5/1/1466