GB2095943A - Transistor drive circuit - Google Patents

Abstract

A transistor drive circuit comprises a switched mode power supply 15 and a transistor push-pull circuit 26 connected in series to provide base current for a high power bi-polar transistor 10. The switched mode circuit 15 includes a switch 19 and the transistor base current may be controlled by controlling the mark space ratio of this switch. In order to switch off the transistor 10, the push-pull circuit 26 is switched off and a turn-off circuit 46 applies a current pulse with reverse voltage across the base-emitter junction of transistor 10. <IMAGE>

Description

SPECIFICATION Transistor drive circuit This invention relates to a transistor drive circuit and particularly, but not exclusively, relates to a drive circuit for a high voltage bi-polar transistor.

In one known drive circuit, base current is provided by a pair of transistors connected to the transistor so as to form a triple Darlington transistor.

This circuit suffers from the disadvantage that the voltage drop associated with the resulting Darlington transistor causes a power loss which is significant at high currents. In another known circuit the base current is provided by a voltage source which is connected to the base through a current limiting resistor. This circuit suffers from the problem that there is a power loss associated with the current limiting resistor, this power loss also being significant at high currents.

It is an object of this invention to provide a new drive circuit in which these disadvantages are overcome or reduced.

According to this invention there is provided a transistor drive circuit comprising a transistor, and a lossless current regulator circuit and an inverter circuit connected together to form a series circuit to provide base current to the transistor.

This invention will now be described in more detail with reference to the accompanying drawings in which Figures land 2 are circuit diagrams of transistor drive circuits embodying this invention.

Referring now to Figure 1 there is shown a drive circuit for a high voltage bi-polartransistor 10. The collector and emitter of transistor 10 are connected to a pair of terminals 12 and 14 which in use are connected to appropriate junctions in the circuit of which the transistor 10 forms a part. The transistor 10 may, for example, be part of a DC chopper circuit, the chopper circuit itself forming part of a DC motor control circuit.

The drive circuit includes a switched mode power supply 15 which comprises a pair of low voltage DC rails 16, 18. The rail 16 is connected through a switch 19, which may be a transistor, to the cathode of a diode 21, the anode of which is connected to the rail 18. The cathode of diode 21 is connected through an inductor 22 to a rail 24. In operation, the switch 19 is driven on and off by a control circuit 28 and the switch 19, diode 21 and inductor 22 and function as a current regulator circuit, the value of the current being determined by the mark space ratio of switch 19.

The drive circuit also includes a push-pull transistor circuit 26 in which the rail 24 is connected to a central tap of the primary winding 30 of a transformer 32. One end of the winding 30 is connected to the collector of an NPN transistor 34, the emitter of which is connected to the rail 18, and the other end of winding 30 is connected to the collector of an NPN transistor 36, the emitter of which is also connected to rail 18. The Q output of an oscillator 38 is connected to the base of transistor 34 and 0 output is connected to the base of transistor 36. Oscillator 38 is switched on and off by control circuit 28. When the oscillator 38 is switched on the transistors 34 and 36 are switched on alternately.

The central tap of the secondary winding 40 of transformer 32 is connected to the emitter of transistor 10 and the two ends of winding 40 are connected to the respective anodes of a pair of diodes 42 and 44 the cathodes of which are connectd to the base of transistor 10.

When the oscillator 38 is turned on, the secondary winding 40 provides base current to the transistor 10 thereby turning it on, the value of the base current being determined by the mark space ratio of the switch 19. As the direction of magnetizing transformer 32 is continuously reversed, saturation of the core of the transformer 32 is avoided. The size of the transformer may be kept small by choosing a sufficiently high frequency for oscillator 32.

The drive circuit also includes a turn-off circuit 46 which includes a transformer 40 having a primary winding 48 and a secondary winding 52. One end of the secondary winding 52 is connected to the emitter of transistor 10. The other end is connected to the cathode of a diode 54 the anode of which is connected to the cathode of a diode 56, and the anode of diode 56 is connected to the base of transistor 10. The diodes 54 and 56 are provided to prevent the base current flowing through winding 52. One end of primary winding 48 is connected to a +10V power supply rail and the other end is connected to the collector of an NPN transistor 58.

The emitter of transistor 58 is connected through a resistor 60 to a OV power supply rail and its base is connected to this rail through a resistor 62. The base of transistor 58 is also connected to the emitter of an NPN transistor 64, the collector of which is connected through a resistor 65 to the +1 0V rail. The base of transistor 64 is connected to the output of a monostable 66 which is controlled by control circuit 28. The base of transistor 64 is connected to the collector of an NPN transistor 68, the emitter of which is connected to the OV rail and the base of which is connected to the emitter of transistor 58.

When a high signal is supplied to the base of transistor 64, transistor 58, 64 and 66 operates as a current source.

When it is desired to turn-off transistor 10, the oscillator 38 is turned-off and monostable 66 is triggered. Consequently, a current pulse is applied with reverse voltage across the base-emitterjunc- -tion of transistor 10 thereby turning it off.

If it is desired to vary the base current with the collector current this may be achieved by sensing the collector current and adjusting the mark space ratio of switch 19 accordingly. Alternatively, the collector-emitter voltage drop may be sensed and the base current adjusted to maintain this at a desired level.

It is to be appreciated that in the drive circuit described above the major part of the drive circuit includes the control circuit 28 and the rails 14 and 18 are isolated from the output circuit of transistor 10.

In an alternative arrangement, the current control circuit 15 is connected between the push-pull arrangement 26 and the transistor 10.

Referring now to Figure 2 there is shown a modification to the circuit of Figure 1 and in Figure 2 the same reference numerals are used for like elements.

In the arrangement shown in Figure 2, the secondarywinding of transformer 32 is provided with a pair of over-windings 80 and 81 which are connected respectively through a pair of diods 82 and 83 to a negative supply rail 84 for the turn-off circuit 46. The rail 84 replaces the OV rail of Figure 1. The central tap of the secondary winding 40 is connected to a positive supply rail 85 for the turn-off circuit 46. The rail 35 replaces the 1 OVrail of Figure 1 and rails 85 and 84 are connected by a capacitor 86. Also in the arrangement shown in Figure 2 the monostable 66 is connected to the base of transistor 64 through an isolating transformer 87 and a resistor 88, and the collector of transistor 58 is connected directly to the base of transistor 10.

In the arrangement of Figure 2 there is no need to provide power supply rails for the turn-off circuit 46.

Also the transformer 87 may be smaller than the transformer 50.

In a further modification, the over-windings 80 and 31 may be replaced by a further secondary winding for transformer 32. Also, the transformer 87 may be replaced by an opto-isolator.

It is to be understood that in the arrangements shown in both Figures 1 and 2 base current may be supplied continuously to the base of transistor 10 and so these arrangements are suitable for applications such as a motor control circuit where the transistor 10 conducts continuously.

Claims (6)

1. A transistor drive circuit comprising a transistor, and a-lossless current regulator circuit and an inverter circuit connected together to form a series circuit to provide base current to the transistor.

2. A transistor drive circuit as claimed in Claim 1 in which the inverter circuit is connected between the current regulator circuit and the base of the transistor.

3. Atransistor drive circuit as claimed in Claim 1 or Claim 2 in which the inverter circuit comprises a push-pull transistor circuit.

4. A transistor drive circuit as claimed in Claim 1 or Claim 2 in which the lossless current regulator comprises a switched mode power supply circuit.

5. A transistor drive circuit as claimed in Claim 1 or Claim 2furtherincluding acircuitforturning-off the transistor.

6. A transistor drive circuit substantially as hereinbefore described with reference to and as shown in the accompanying drawings.