GB2110493A - Transistor switching circuit - Google Patents

Abstract

In a switching circuit to provide the necessary base driving conditions, a current control transformer (Tx1) couples the collector-emitter and base-emitter circuit. This transformer has a third winding in series with a diode (D) connected to the collector of the transistor (TR) and normally blocked. When the transistor (TR) conducts it unblocks the diode (D) so that current is diverted from the base via the third winding. This enables the transistor to work in a quasi-saturated mode, and thus allowing improved turn off to be obtained. In an alternative (Fig. 2) the diode-third winding combination is connected between the transistor's collector and base. The switching circuit is shown used in a DC-DC converter where a power transistor TR is switched on by a control or modulating circuit to drive a pulsed current through a power transformer (Tx2). The secondary of this transformer feeds the DC output via a rectifier (D1, D2) and filter L1- C1). <IMAGE>

Description

SPECIFICATION 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 transformer. 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 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 current 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 transistor, and in which when the transistor turns on the diode is progressively rendered conductive so that current is progressively diverted from the base-emitter circuit of the transistor to flow in said further winding, 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 refecnce to the accompanying drawing, in 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.

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 which the transistor is rapidly turned on to initiate the conduction of collector current.

With a current transformer system drive as shown the controlling ampere turns is l,N" where N, is the number of turns of the transformer winding connected in the emitter-collector path.

This current is initiated by the switch-on referred to above. By transformer action this feeds l2N2 into the base and N2 is the number of turns of the transformer winding in the base circuit ofT.

Therefore, the collector voltage cf 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 13 via the winding which has N3 turns. Hence the base current of the transistor is reduced, so that the transistor is now operating in a quasesaturated state.

Such an arrangement enhances the switching 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 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 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. Tx 1 is the control transformer whose use has been described with reference to Fig. 1, and Tx2 is the power transformer. The secondary of this transformer feeds a rectifier Dl -D2 the output of which is the DC output via a filter L1-C1.

Claims.

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 the necessary base current when the transistor is rendered conductive, in which there is 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 transistor, in which when the transistor turns on the diode is progressively rendered conductive so that current is progressively diverted from the base-emitter circuit of the transistor to flow in said further winding, thus enabling the transistor to operate in a quasi-saturated mode, allowing improved turn off to be obtained.

2. 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 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 power 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.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (6)

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION 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 transformer. 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 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 current 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 transistor, and in which when the transistor turns on the diode is progressively rendered conductive so that current is progressively diverted from the base-emitter circuit of the transistor to flow in said further winding, 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 refecnce to the accompanying drawing, in 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. 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 which the transistor is rapidly turned on to initiate the conduction of collector current. With a current transformer system drive as shown the controlling ampere turns is l,N" where N, is the number of turns of the transformer winding connected in the emitter-collector path. This current is initiated by the switch-on referred to above. By transformer action this feeds l2N2 into the base and N2 is the number of turns of the transformer winding in the base circuit ofT. Therefore, the collector voltage cf 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 13 via the winding which has N3 turns. Hence the base current of the transistor is reduced, so that the transistor is now operating in a quasesaturated state. Such an arrangement enhances the switching 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 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 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. Tx 1 is the control transformer whose use has been described with reference to Fig. 1, and Tx2 is the power transformer. The secondary of this transformer feeds a rectifier Dl -D2 the output of which is the DC output via a filter L1-C1. Claims.

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 the necessary base current when the transistor is rendered conductive, in which there is 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 transistor, in which when the transistor turns on the diode is progressively rendered conductive so that current is progressively diverted from the base-emitter circuit of the transistor to flow in said further winding, thus enabling the transistor to operate in a quasi-saturated mode, allowing improved turn off to be obtained.

2. 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 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 power 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.