GB2317065A

GB2317065A - Reduced power dissipation in FET under reverse polarity

Info

Publication number: GB2317065A
Application number: GB9618489A
Authority: GB
Inventors: Kevin Trevor Talbot
Original assignee: MG Rover Group Ltd
Current assignee: MG Rover Group Ltd
Priority date: 1996-09-05
Filing date: 1996-09-05
Publication date: 1998-03-11
Also published as: GB9618489D0

Abstract

A vehicle headlamp 36 is connected in series with a MOSFET 38 between first and second lines 30, 32, connected to ground and the positive terminal of a battery 34 respectively. In this condition, a first oscillator circuit 40 and a voltage pump 42 produce a positive gate-source voltage on the MOSFET to turn on the headlamp. If the battery is connected up the wrong way round so that the second line 32 is at a negative potential, a second oscillator circuit 60 drives the voltage pump 42 thus maintaining a positive gate-source voltage. This ensures that the MOSFET will be turned on when the battery is wrongly connected, thus preventing the need for a large heat sink normally required for the MOSFET because of the high resistance it exhibits when connected backwards without a positive gate-source voltage.

Description

Electrical Circuits The present invention relates to electrical circuits which supply power from a DC source to a load and include a transistor to control the power supply to the load.

An example of a typical known circuit is shown in Figure 1. Two lines 10, 12 are connected to the positive terminal of a vehicle battery and ground respectively, and are therefore held at 12V and 0V. A headlamp bulb 14 is connected in series with an n-channel MOSFET 16 between the two lines 10, 12. A voltage multiplier 18 is also connected between the two lines 10, 12 and is arranged to apply a voltage to the gate g of the MOSFET under the control of a microprocessor 20. During normal operation the drain d of the MOSFET is connected to the 12V line 10 as shown, and the source sc and substrate sb are connected to the bulb 14. When the MOSFET is turned on it has a resistance of about 20 mQ and therefore, for a bulb of about 5A, dissipates about 500mW of power. This means that the MOSFET can, for normal operation, be mounted on a PCB with minimal heat sink.

If the vehicle battery is accidentally connected up the wrong way round, the second line will still be at 0V, but the first line will be at -12V. When the MOSFET is connected 'backwards' in this way it behaves as a diode which will continuously pass current. When connected 'backwards' a MOSFET can still be turned on by applying a positive gate-source voltage thereby reducing its resistance substantially. However, in conventional circuits, the voltage multiplier will either have its power cut off by a diode safety device, or will supply a negative voltage to the gate. In either case the circuit will not be able to turn on the MOSFET. In this situation the MOSFET will produce a voltage drop of about 1.2V and will therefore produce a power of about 6W. This requires a relatively large heatsink which will require an undesirable amount of space and material.

The present invention therefore provides a power circuit for a powering a load from a DC power source, the circuit comprising first and second lines, at least one of which is normally connected to the power source so as to produce a potential difference between them, a field effect transistor having its source and drain connected in series with the load between said lines such that, with the power source normally connected, a normal drain-source voltage is produced, and a control circuit powered from said lines which, when the power source is normally connected, can apply a gate-source voltage of the same sign as the drainsource voltage to turn on the transistor, and which includes a voltage pumping means so that, if the potential difference between the two lines is reversed, the control circuit can produce a gate-source voltage of the opposite sign to the drain-source voltage to turn on the transistor.

A preferred embodiment of the present invention will now be described, by way of example only, with reference to Fig. 2 of the accompanying drawings which shows a power circuit for a vehicle headlamp system in accordance with the invention.

The circuit includes a first line 30 connected to ground (0V) and a second line 32 normally connected to the positive terminal of a vehicle battery 34 which is at 12V.

A vehicle headlamp bulb 36 is connected between the two lines 30, 32 in series with an n-channel MOSFET 38 which acts as a switch to turn the headlamp bulb 36 on and off.

The MOSFET 38 has its source connected to the bulb 36 and its drain connected to the second line 32.

The input to the gate of the MOSFET is controlled by a control circuit which includes a first oscillator circuit 40 and a voltage pump 42. The first oscillator circuit comprises a Schmidt trigger 44, a resistor R1 and a capacitor C1, the Schmidt trigger 44 having power inputs 46, 48 connected respectively to a 5V supply derived from the second line 32, and the first line 30. The input 50 to the Schmidt trigger is combined by a logic AND gate with a control input 52, so the control input acts to turn the first oscillator circuit 40 on and off. The output from the first oscillator circuit 40 is connected to a first side C2a of a capacitor C2.

A capacitor C3 and resistor R2 are connected in parallel between the source and gate of the MOSFET 38. A pair of diodes D1, D2 are also connected between the source and gate of the MOSFET 38, both arranged to allow current to flow towards the gate. The second side C2b of the capacitor C2 is connected to a point between the two diodes D1, D2.

A second oscillator circuit 60 comprises a Schmidt trigger 62, a resistor R3, and a capacitor C4, connected in the same way as the Schmidt trigger 44, resistor R1 and capacitor C1 of the first oscillator circuit 40, except that there is no control input and AND gate. Instead the power inputs 64, 66 of the second oscillator circuit 60 are connected across a capacitor C5 which is connected on one side to the first line 30, and via a diode D5 and resistor R4 to the second line 32. The diode is arranged to allow current to flow from the first line 30 to the second line 32, and therefore prevents a voltage from developing across the capacitor C5 unless the second line 32 is at a lower voltage than the first line 30. A Zener diode Z1 limits the voltage on the capacitor C5 to 5V. The output from the second oscillator circuit 60 is connected to one side C6a of a capacitor C6, the other side C6b of which is connected to a point between two diodes D3, D4, which are in parallel with the two diodes D1, D2 between the source and gate of the MOSFET 38.

When the battery 34 is connected up the normal way round, as shown in Figure 2, the first line 30 is at 0V and the second line is at 12V. The diode D5 prevents the capacitor C5 from charging, and the second oscillator circuit 60 is therefore inactive. If the first oscillator circuit 40 is turned on by making the control input 52 high, the oscillating signal produced is applied to the first side C2a of the capacitor C2. This alternately attracts and repels charge to and from the second side C2b of the capacitor C2, which alternately draws current through diode D1 and drives it through diode D2. Because R2 is large, this will result in capacitor C3 being charged up, producing a positive gate-source voltage sufficient to start current flowing through the MOSFET and the bulb 36.

This will result in the voltage on the source of the MOSFET becoming positive. Due to current flow through the diode D1 this positive source voltage will also be applied to the line 64. The oscillating current in this line will continue to pump current through D1 and D2 so a positive gate-source voltage is maintained. This keeps the MOSFET 38 turned on and therefore maintains current through the bulb 36.

If the battery 34 is connected backwards, the first line 30 will still be at 0V, but the second line will be at -12V.

The 5V supply to the Schmidt trigger will be inoperative because it is reverse-protected by a diode, and the first oscillator circuit 40 will not function. However, the diode D5 will allow the capacitor C5 to charge up to 5V, thus turning on the second oscillator circuit 60. This will apply an oscillating voltage to the capacitor C6 which will drive the voltage pump comprising diodes D3 and D4 to produce a positive gate-source voltage on the MOSFET sufficient to turn it on in the 'reverse' direction. This will result in the lamp 36 being turned on, and keep the power dissipated as heat in the MOSFET to a relatively low level.

It can be seen that the circuit described ensures that the MOSFET is turned on, and therefore only dissipates power of about 500mW, both with the battery connected up in the normal way, and when it is connected up backwards. It is therefore only necessary to provide a heat sink which can dissipate about 500mW of power.

Claims

1. A power circuit for a powering a load from a DC power source, the circuit comprising first and second lines, at least one of which is normally connected to the power source so as to produce a potential difference between them, a field effect transistor having its source and drain connected in series with the load between said lines such that, with the power source normally connected, a normal drain-source voltage is produced, and a control circuit powered from said lines which, when the power source is normally connected, can apply a gate-source voltage of the same sign as the drain-source voltage to turn on the transistor, and which includes a voltage pumping means so that, if the potential difference between the two lines is reversed, the control circuit can produce a gate-source voltage of the opposite sign to the drain source voltage to turn on the transistor.

2. A circuit according to claim 1 wherein the first line is connected to ground and the second line is normally connected to one terminal of the power source.

3. A circuit according to claim 1 or claim 2 wherein said one terminal is a positive terminal and the transistor is an n-channel transistor.

4. A circuit according to claim 2 wherein the control circuit includes a capacitor connected between the lines so that, when said potential difference is reversed, a voltage is produced on one side of the capacitor which is of the opposite sign, relative to the first line, to the voltage on the second line.

5. A circuit according to claim 4 wherein the control circuit includes a voltage limiter for limiting the voltage across the capacitor.

6. A circuit according to any foregoing claim wherein the voltage pump comprises an oscillator circuit and a pair of diodes.

7. A circuit according to claim 6 wherein the diodes are connected between the source and the gate.

8. A circuit according to claim 4 or claim 5 wherein the control circuit includes a diode between the second line and the capacitor which prevents the capacitor from charging when the power source is connected normally.

9. A circuit according to any foregoing claim wherein the field effect transistor is a MOSFET.

10. A power circuit for a vehicle component substantially as hereinbefore described with reference to Fig. 2 of the accompanying drawings.