GB2334393A

GB2334393A - MOSFET driver circuit

Info

Publication number: GB2334393A
Application number: GB9803208A
Authority: GB
Inventors: Ray Mumford; Nicholas Vardill
Original assignee: Individual
Current assignee: Individual
Priority date: 1998-02-17
Filing date: 1998-02-17
Publication date: 1999-08-18
Anticipated expiration: 2018-02-17
Also published as: GB2334393B; GB9803208D0

Abstract

A MOSFET driver circuit which uses a chain of video transistors (10, 12, 14), connected between the positive and negative sides (+v, -v) of a power supply, firstly to receive input drive signals from a pre-amplifier receiving feedback signals from the MOSFETs and secondly to provide outputs to the MOSFET gates via gate stoppers (46, 48) which have resistive values sufficiently high to limit the current drawn by the video transistor chain to a power rating not exceeding the rating of the transistors.

Description

MOSFET Driver Circuit This invention relates to a MOSFET driver circuit.

In a conventional MOSFET driven circuit, low power drive signals are typically fed to the MOSFET gates from a chain of conventional amplifying transistors via resistive gate steppers. Due to the development of back e.m.f. signals which can impinge on the transistor chain, huge driving power is used to reduce back e.m.f. spikes and the like to negligible proportions. The effective resistance of the gate stoppers therefore has to be relatively low. The use of huge power tends to aggravate the problem of inefficiency, which causes, for example in audio amplifiers, the speaker coils to lag behind the high power loudspeaker drive signals.

According to the invention, there is provided a MOSFET driver circuit comprising a chain of video transistors connected between positive and negative sides of a power supply, the video transistor chain receiving driving signals as inputs and providing control signals as outputs to MOSFET gates via resistive gate stoppers, wherein the gate stoppers have resistance values which are effective to so limit the current drawn by the video transistor chain that the power rating of the video transistors is not exceeded.

Video transistors are considerably more accurate in respect of linearity and electrical noise than conventional transistors but have not hitherto found application in MOSFET driving circuits because of the apparent problem of high power requirements. It has now been found, however, that a video transistor chain can be operated very stably at relatively low power, below 1.OW and preferably at a power of about 0.1W, to drive the MOSFET gates, so that speaker coil lag is substantially eliminated in audio amplifiers for example, and efficiency improves in this and other applications. Back e.m.f. signals have no effect because of the high resistance values of the gate stoppers, which prevent these signals impinging on the video transistor chain.

The invention is further described with reference to the accompanying circuit diagram.

The illustrated MOSFET driver circuit comprises a chain of three video transistors 10, 12, 14. The emitter of video transistor 10 is connected to the positive power line through a 47 ohm resistor 16 and its collector is connected to the emitters of video transistor 12, the collector of which is connected to the collector of video transistor 14, which in turn has its emitter connected to the negative power line via a 47 ohm resistor 18.

Transistor 10 receives a ripple free power supply signal at its base, via a 1.0K resistor 20. Any ripple appearing on the positive power supply line alters the gain of the video transistor 10, thus causing the complete transistor chain to rise and fall in harmony with the supply rail, thus cancelling any power supply ripple at the MOSFET drive. Transistor 12, which is a balancing transistor, has its base linked to its emitter and its collector through 220 ohm resistors (one variable) 22, 24. Resistors 22, 24 set the gain of the transistor 12 to govern the potential differences between the transistors 10 and 14. Transistor 14 receives at its base a low power control signal on line 26 from a pre-amplifier circuit forming no part of the present invention, this pre-amplifier circuit receiving feedback signals from the MOSFETs (not shown) on line 28. The collector of transistor 14 is connected to the control signal line 26 via a 10PF capacitor 30, the line 26 also being coupled to the negative power supply line by a 100PF capacitor 32. Capacitors 30, 32 limit the frequency range of the device and assist stability.

In use, the output signals to the MOSFET gates, provided on lines 34, 36, swing up and down between the positive and negative power supply voltages dependently on the input control signals. Reversed diodes 38, 40 and 42, 44 couple the lines 34, 36 to the feedback signal line 28.

The outputs on lines 34, 36 are fed to the MOSFETs via gate stoppers 46, 48, also in accordance with conventional practice. However, the gate stoppers 46, 48 differ from conventional MOSFET gate stoppers by their high resistances.

In the present case, 4.7K resistors are used, compared to resistors not exceeding 600 ohms used in conventional MOSFET gate stoppers. The reference FB indicates ferrite beads for very high frequency limiting.

These high resistance MOSFET gates protect the video transistors 10, 12, 14 from any back e.m.f signals emanating from the driver unit. This is essential because, in contrast to chains of conventional transistors hitherto used in MOSFET driver circuits, the video transistor chain 10, 12, 14 operates at low power, and low current levels are necessary to keep power consumption down to less than 1.0W, in the present case approximately 0.1W.

The video transistor chain operates with high stability at this relatively low power, and the avoidance of the use of high energy means that lag of loudspeaker coils in audio amplifiers relative to the high power drive signals derived from 200V rated MOSFETs can be substantially eliminated. In audio amplifiers, a driving voltage is always applied to the loudspeaker coils, regardless of the behaviour of the loudspeaker coils. However, the MOSFET driver circuit of this invention is by no means limited to use in audio amplifiers, but finds application in inverters, transmitters, electric motor speed control, lighting level control, and like applications where there is a requirement for variable frequency and voltage control.

Claims

Claims 1. A MOSFET driver circuit comprising a chain of video transistors connected between positive and negative sides of a power supply, the video transistor chain receiving driving signals as inputs and providing control signals as outputs to MOSFET gates via resistive gate stoppers, wherein the gate stoppers have resistance values which are effective to so limit the current drawn by the video transistor chain that the power rating of the video transistors is not exceeded.
2. A circuit according to claim 1, wherein the video transistor chain is operated at a power not exceeding 1.0W.
3. A circuit according to claim 2, wherein the video transistor chain is operated at a power of approximately 0.1W.
4. A circuit according to any of claims 1 to 3, wherein the video transistor chain comprises a chain of three video transistors connected between positive and negative power lines.
5. A circuit according to claim 4, wherein the central video transistor of the chain is a balancing transistor connected in a resistive circuit to govern the potential differences between the end transistors of the chain.
6. A circuit according to claim 3 or claim 4, wherein the video transistor connected to the negative power line is connected to a control signal input line from a pre-amplifier which receives feedback signals from the MOSFETs.
7. A circuit according to claim 6, wherein the said video transistor connected to the negative power line is connected to capacitors for limiting the frequency range of the driver circuit.
8. A circuit according to any of claims 3 to 7, wherein outputs to the MOSFETs are taken via ferrite beads from the collectors of the central video transistor and the video transistor connected to the positive power line.
9. A circuit according to claim 8, wherein the gate stoppers comprise 4.7K resistors.
10. A MOSFET driver circuit substantially as hereinbefore described with reference to the accompanying drawing.