GB2238437A

GB2238437A - Transistor driver circuits

Info

Publication number: GB2238437A
Application number: GB8926401A
Authority: GB
Inventors: Robert Sidney Ireland
Original assignee: Plessey Co Ltd
Current assignee: Plessey Co Ltd
Priority date: 1989-11-22
Filing date: 1989-11-22
Publication date: 1991-05-29
Also published as: GB8926401D0

Abstract

A driver circuit for a field-effect transistor Tr3, in which a driver transistor Tr1 is arranged to be rendered conducting in order to maintain the field-effect transistor Tr3 in a non-conducting state, in which the conducting path of the driver transistor includes a relatively high impedance power source for the circuit to limit current drain and in which a high current gain arrangement TR2, D2 is arranged to be switched into the gate circuit of the field-effect transistor Tr3 effectively to substantially reduce the impedance of the relatively high impedance power source and thereby ensure rapid switching on of the transistor when the driver transistor Tr1 is rendered non-conducting. A negative gate voltage may be required to turn off an FET Tr3 subject to radiation particles. <IMAGE>

Description

IMPROVEMENTS RELATING TO TRANSISTOR DRIVER CIRCUITS This invention relates to transistor driver circuits and relates more specifically to driver circuits for power field-effect transistors (MOSFETS).

In order to minimise the build up of heat in power field-effect transistors it is important to effect very rapid switching between on and off states of the transistors by the transistor driver circuit. This rapid switching which may be of the order of micro-seconds necessitates a low impedance power source for the field-effect transistor which has hitherto resulted in an undesirably high current drain through the driver transistor for the field-effect transistor when the latter is in the non-conducting condition. This relatively high current drain is particularly disadvantageous when the fieldeffect transistor is powered from a battery power source or when used in power conscious applications.

According to the present invention there is provided a driver circuit for a field-effect transistor which overcomes the disadvantage referred to above, in which a driver transistor is rendered conducting in order to maintain the field-effect transistor in a non-conducting state, in which the conducting path of the driver transistor includes a relatively high impedance power source for the circuit and in which a high current gain means is arranged to be switched into the gate circuit of the field-effect transistor effectively to substantially reduce the impedance of the relatively high impedance power source for the field-effect transistor and thereby ensure rapid switching on of the transistor when the driver transistor is rendered conducting.

In carrying out the present invention the high current gain means may comprise a normally non-conducting triggering transistor which is connected across the relatively high impedance of the power source and which is rendered conducting when the driver transistor is switched off and which acts as an emitter follower circuit to provide high current gain between the base emitter and the collector emitter junctions of the triggering transistor so as effectively to substantially reduce the value of the relatively high impedance of the power source and thus ensure fast operation of the field-effect transistor.

By way of example the present invention will now be described with reference to the accompanying drawings in which; Figure 1 shows a known MOSFET drive circuit; Figure 2 shows one MOSFET driver circuit according to the present invention; Figure 3 shows another MOSFET driver circuit according to the present invention.

Referring firstly to Figure 1, a MOSFET TR3 has a driver circuit comprising a driver transistor TR1 having its collector and emitter connected to the gate and source electrodes, respectively, of the MOSFET TR3. The gate electrode is connected to a DC voltage source (e.g. battery) through a low impedance R2 (1K or 2K ohms) and a Zener diode D1 is connected as illustrated between the gate and source electrodes of the MOSFET TR3, with the purpose of limiting the gate-source voltage to a nominal 10 volts.

For the purpose of holding the MOSFET TR3 in the nonconducting condition a positive input voltage is applied to the base of the driver transistor TRI to hold the transistor TRI conducting in saturation. The consequential saturation current flowing through the driver transistor TRl will be determined by the ohmic value of the resistor R2 and thus represents a current drain on the DC supply (e.g.

battery) in the non-conducting condition of the MOSFET TR3.

In order to switch on the MOSFET TR3 the base voltage of the driver transistor TR1 is reduced by means of a voltage step input signal to zero volts which renders the transistor TR1 non-conducting.

The voltage at the gate electrode of the MOSFET TR3 accordingly rises at a rate dependent upon the ohmic value of the resistor R2 and the inherent capacitance C (indicated by the dotted lines) of the MOSFET TR3. As will be appreciated, the lower the ohmic value of the resistor R2 the sooner the gate voltage of the MOSFET will reach the value necessary to effect switching on of the MOSFET TR3. By keeping the value of resistor R2 low, fast switching of the MOSFET of the order of micro-seconds can be achieved to keep the build up of heat in the MOSFET to a minimum but the current drain by the driver transistor TR1 in the off state of the MOSFET TR3 is undesirably high.

Referring now to Figure 2 of the drawings, this shows one embodiment of the present invention for overcoming the disadvantages of the known driver circuit of Figure 1.

In Figure 2 the components corresponding to those of Figure 1 bear the same designations. As can be seen the driver circuit additionally comprises a triggering transistor TR2 which has its collector connected to the DC voltage supply (battery) and its emitter connected to the gate electrode of the MOSFET TR3. The base of the triggering transistor TR2 is connected to the junction between the resistor R2 and the collector of the driver transistor TRi. A diode D2 is connected between the gate of the field-effect transistor TR3 and the junction of the resistor R2 and the collector of the driver transistor TRi.

In order to maintain the MOSFET TR3 in the non-conducting (i.e.

off) state the input to the driver transistor TR1 is held at a positive value such that the current flow through the resistor R1 into the base of transistor TR1 is sufficient to maintain the driver transistor in saturation. For example, with the ohmic value of resistor R1 at 10K ohms and an input of + 5 volts, 0.43 mA will flow into the base of the driver transistor TR1. Assuming this transistor TR1 has a gain hFE of typically 150, a current of 64mA may flow into the collector of the transistor TR1 before its collector emitter voltage begins to rise from saturation.

The collector of the transistor TR1 is connected to the positive DC supply, through the resistor R2, to the base of the triggering transistor TR2 and the anode of diode D2. With the transistor TR1 saturated, the base of the triggering transistor TR2 is held at 0 volts.

Transistor TR2 is acting as an emitter follower, with its emitter connection always displaying a voltage nominally 0.7 volt lower than the voltage on the base so that in this state the emitter is at 0 volts and transistor TR2 is non-conducting. The diode D2 ensures that the emitter of transistor TR2 can rise to no more than 0.7 volts above the base voltage and provides a current path from the gate of the power MOSFET transistor TR3 to its DC supply power source. This ensures that the power MOSFET TR3 is maintained in the off state with a maximum gate source voltage of 0.7 volt. In this off state of the MOSFET the circuit dissipates power in the resistors R1 and R2. With a transition between the off to on state of the MOSFET TR3 required.

the turn off/on input to the driver transistor TRl is taken to 0 volts by means of a voltage step. This removes the current flow through the resistor R1 into the base of transistor TR1 rendering the latter non-conducting. The voltage on the base of C transistor TR2 then begins to rise towards the positive supply voltage pulled high by the resistor R2. The inherent input capacitance of the power MOSFET transistor TR3 being fully discharged holds the gate source voltage of the MOSFET at the off state value of 0.7 volt maximum. The rising collector voltage of TRI therefore reverse biases the diode D2 and renders the triggering transistor TR2 conducting as an emitter follower circuit. This circuit offers the advantage of high current gain between the base emitter and the collector emitter junctions where; IE = hFE x IB This current is fed into the gate source capacitance of the power MOSFET TR3 from the low impedance source represented by the emitter follower transistor TR2, causing the gate to source voltage to rise rapidly following the rising voltage on the base of the conducting triggering transistor TR2. Eventually the base voltage of the conducting transistor TR2 reaches the voltage of Zener diode D1 and stops rising. The emitter of conducting transistor TR2, therefore, holds a minimum voltage level of VZDl - 0.7 volt and the maximum voltage of VZD1 + 0.7 volt. In the on state of the MOSFET TR3 only small currents are required to maintain the voltage level on the gate electrode.

As will be appreciated, the effect of the emitter follower circuit provided by the triggering transistor TR2 is theoretically to allow the ohmic value of the resistor R2 to be hFE times as large as a resistor that would normally be required to be connected directly to the gate of the power MOSFET TR3 (see Fig I) in order to provide fast switching thereof. In practice a multiplication factor of 10 to 15 can be achieved which enables the power source impedance and thus the current drain by the driver transistor TR1 to be kept at a very small level.

Referring now to Figure 3 of the drawings, this shows another embodiment of the invention that is suitable for use in cases where the MOSFET transistor TR3 may be subjected to radiation particles and a negative gate voltage may be required to switch off the MOSFET TR3. This requirement results from the build up of an internal charge between the gate and source electrodes of the MOSFET when the latter is irradiated. The voltage internally of the MOSFET is accordingly higher than the external gate voltage so that when the gate voltage drops to zero the internal voltage may still be sufficient to keep the MOSFET turned on. The negative voltage is accordingly applied to the gate electrode of the MOSFET TR3, as shown in the Figure 3 arrangement, in order to accommodate this difference in voltage.

Claims

1. A driver circuit for a field-effect transistor, in which a driver transistor is arranged to be rendered conducting in order to maintain the field-effect transistor in a non-conducting state, in which the conducting path of the driver transistor includes a relatively high impedance power source for the circuit and in which a high current gain means is arranged to be switched into the gate circuit of the field-effect transistor effectively to substantially reduce the impedance of the relatively high impedance power source for the field-effect transistor and thereby ensure rapid switching on of the transistor when the driver transistor is rendered non-conducting.

2. A driver circuit is claimed in claim 1, in which the high current gain means comprises a normally non-conducting triggering transistor which is connected across the relatively high impedance of the power source and which is rendered conducting when the driver transistor is switched off and acts as an emitter follower circuit to provide high current gain between the base emitter and collector emitter junctions of the transistor so as effectively to substantially reduce the value of the high impedance of the high impedance power source and thereby ensure fast operation of the field effect transistor.

3. A driver circuit as claimed in claim 2, in which the base of the triggering transistor is connected to the junction of the collector driver transistor and the relatively high impedance of the power source and the emitter is connected to the gate of the field-effect transistor which is also connected to said junction through a suitably poled diode, the voltage across the driver transistor being controlled by a Zener diode.

4. A driver circuit for a field-effect transistor substantially as hereinbefore described with reference to Figure 2 of the accompanying drawings.

5. A driver circuit for a field-effect transistor substantially as hereinbefore described with reference to Figure 3 of the accompanying drawings.