GB2138644A - Power Transistor Protection - Google Patents

Abstract

A device for protecting a power transistor (Q6) against overload current includes a resistor R18, a comparator A1 receiving on its non-inverting input a voltage which is proportional to the current flowing in the resistor R18, and on its inverting input a reference voltage derived from a zener diode Z1 and a resistor chain R9, R10, a transistor Q2 for shunting the base electrode of the power transistor Q6, and a circuit including transistors Q3, Q4 operable from the output of the comparator A1 for turning on the shunting transistor Q2 when the voltage across the resistor (R18) exceeds the reference voltage. Transistors Q3 and Q4 function to raise the value of the voltage at the inverting input of comparator A1 once the comparator has detected overload of the power transistor Q6 so that the shunting transistor Q2 remains conductive even though the current through the power transistor is reduced. The comparator may be formed as a differential amplifier (Fig. 2) including an emitter coupled pair of transistors (Q7, Q8). In a further embodiment (Fig. 3) in which Q4 and Q5 are omitted on current overload the power transistor Q6 is not completely turned off and Q3 conducts to reduce the reference voltage (V2) applied to the inverting input of the differential amplifier (Q7, Q8) so that the shunting transistor Q2 remains on. The device is applicable to electronic ignition circuits for internal combustion engines. <IMAGE>

Description

SPECIFICATION Power Transistor Protection The present invention relates to devices for protecting a power transistor against overload current. More particularly, although not exclusively, it relates to the protection of power transistors used in electronic ignition systems for internal combustion engines.

- One of the problems with electronic ignition systems which are sold for the "do-it-yourself" market, is that should the battery be connected directly across the power transistor, the power transistor will be severly damaged due to excessive current flow through the collectoremitter circuit.

Such a problem cannot be solved by the use of fuses as these devices are far too slow in operation, which means that permanent damage will have occurred to the power transistors before the fuse blows.

One solution is to monitor the emitter current through the power transistor by means of a resistor in the emitter circuit, and use the voltage developed there across to directly turn on a transistor connected between the base electrode of the power transistor and earth. Such a device whilst providing adequate protection is disadvantageous in that it is not held in the "conductive" state to hold the power transistor fully off. This is because as soon as the transistor conducts to turn off the power transistor, the monitoring voltage across the emitter resistor is reduced which causes the transistor to turn off. In actual practice this system tends to stabilize in a position where the power transistor is sufficiently on to allow sufficient voltage to be developed across the emitter resistor to keep the transistor conductive.In this condition the power transistor will pass a high current with even a very small emitter resistor.

The disadvantage of such transistor protective system is that there is considerable current flow through the power transistor, which could cause serious damage thereto, if allowed to continue for any length of time.

A further system for protecting a power transistor is disclosed in our British Patent Specification No. 2,056,808, which discloses a system in which a power transistor is protected against being damaged by means of overload current by a device which monitors the current flow therethrough and switches the power transistor off when the current exceeds a predetermined value. When the voltage across a resistor in the emitter circuit of the power transistor exceeds a given value it causes a pair of transistors to latch on in thyristor mode, which clamps the voltage at the base electrode of the power transistor to a value which holds the power transistor non-conductive. A capacitor connected between the base of one of the transistors and the base of the power transistor improves the immunity of the circuit from responding to noise or other line borne transients.An emitter coupled stage improves the current sense capability of the circuit so as to make the trip current level substantially independent of temperature.

According to the present invention there is provided a device for protecting a power transistor against overload current including: means for deriving a voltage proportional to the current flowing in said power transistor; means for providing a reference voltage; means for comparing said derived voltage with said reference voltage; and means for reducing the current flow in said power transistor when said desired voltage exceeds the reference voltage.

Preferably said means for deriving a voltage which is proportional to the current flowing in the power transistor is a resistor in the emitter circuit of said power transistor.

The comparator means may be either a pair of transistors arranged as a differential amplifier or a comparator having inverting and non-inverting inputs.

In the case of the differential amplifier, the transistors are of PNP configuration having commoned emitter electrodes. In the case of a comparator, the reference voltage is applied to the inverting input and the derived voltage to the non-inverting input.

The means for reducing the current flow is preferably a transistor which shunts the base electrode of the power transistor to earth when said transistor is rendered conductive by the comparator means. In one form the power transistor is completely cut-off when said comparator means operates, and in an alternative form, the current flow is reduced to a very low value.

Means may be provided to either raise the value of the derived voltage or reduce the value of the reference voltage, once the comparator means has operated, so that a reduction in the current flow in the power transistor does not cause the shunting transistor to become nonconductive during the remainder of the period that the power transistor would have been conductive had the protective device not become operative.

The present invention will now be described in greater detail by way of example with reference to the accompanying drawings, wherein: Figure 1 is a circuit diagram of one preferred form of device for protecting a power transistor which is used in an electronic ignition system for an internal combustion engine; Figure 2 is a circuit diagram of a modified form of the circuit shown in Figure 1; and Figure 3 is a circuit diagram of a second preferred form of device for protecting a power transistor in which the protective circuit when operative does not completely cut off the power transistor.

Referring first to Figure 1, part of the ignition system comprises a power transistor Q4 and a first stage transistor Q1. The input circuit comprises resistors R1 to R4, the resistor R2 being connected between the input and the base electrode of the transistor Q1. The resistor R1 supplies base current to the transistors Q1 and Q5 via resistors R2 and R3 respectively. A resistor R5 is connected between the collector electrode of the transistor Q1 and the supply voltage V5, and a diode D1 is connected between the base electrode of the power transistor Q6 and earth to protect the power transistor against negative going transients on the line.

The device for protecting the power transistor Q6 against current overload receives a stabilized voltage supply from a zener diode Z1 which is in series with a resistor R6 across the voltage supply V5. A resistor R1 8 between the emitter electrode of the power transistor Q6 and earth, and acts as a monitoring device for the current flowing through the power transistor.

The device for protecting the power transistor Q6 against current overload comprises a voltage comparator Al, transistors Q2 to Q5, resistors R7 to R17 and capacitors C1 to C2. The noninverting input to the comparator Al is connected to the junction between the emitter electrode of the power transistor and the current monitoring resistor R18 through the resistor R 17. The inverting input to the comparator Al is connected to the junction of a potential divider chain comprising resistors R9 and R10 connected across the stabilized supply provided by the zener diode Z1. The capacitor C2 is connected in parallel with the resistor R10 to form a filter network. The capacitor C1 is connected between the non-inverting input to the comparator Al and earth.The resistor R17 and capacitor C1 provide a filter network to stop false triggering due to transient voltages appearing at the non-inverting input to the comparator Al. It is important that the time constant of the filter circuit comprising the components C1, R17 should be substantially greater than the time constant provided by the other filter network.

Also connected to the non-inverting input of the comparator Al is the resistor R16 and the emitter-collector path of the transistor 04. The emitter electrode of the transistor Q4 is connected directly to the stabilized supply provided by the zener diode Z1, whilst the base electrode is biased by means of the resistor R1 4 connected to the stabilized supply.

The collector-emitter path of the transistor Q2 is connected between the base electrode of the power transistor Q6 and earth, and when this transistor is conductive it short-circuits the base current of the power transistor Q6 to earth thus turning it off completely. The base electrode of the transistor Q2 is connected to the junction between the output of the comparator Al and earth.

The collector-emitter path of the transistor Q3 and the resistor R15 is in series between the base electrode of the transistor Q4 and earth. The base electrode of the transistor 03 is connected firstly to the output of the comparator Al through a resistor R12 and the collector electrode of the transistor Q5, whose emitter electrode is earthed.

The base electrode of the transistor OS is supplied with input signals through the resistor R3.

The resistor R1 1 is connected across the base and emitter electrodes of the transistor Q5 and the resistor R 13 together with the resistors R7 and R8 from a potential divider chain across the zenered supply voltage to the power transistor protection circuit.

In operation, current flow in the power transistor Q6 sets up a voltage across the resistor R 1 8. A voltage V1 produced at the junction between resistors R16 and R17 which is supplied to the non-inverting input of the comparator Al is compared with a reference voltage V2 derived at the junction between the resistors R9 and R10 and applied to the inverting input of the comparator Al.

Under the condition when the input is low so that the transistors Q1 and Q5 are nonconductive, and the voltage V1 exceeds V2 indicating a current overload in the power transistor, the comparatpr supplies current at its output, which flows through the resistors R8 and R12. Allthe current flowing through the resistor R12 is injected into the base electrode ofthe transistor 03, whereas a part of the current flowing through the resistor R 8-flows through the resistor R7, and the base current to the transistor Q2 is provided only after the required baseemitter voltage is et up across the resistor R7.

Accordingly, the transistor 03 is turned on slightly earlier than the transistor Q2. When the transistor Q3 becomes conductive it causes the transistor Q4 to become conductive, which takes the junction between the collector electrode of the transistor Q4 and the resistor R16 to a VCESAT below emitter voltage of the transistor 04. The resulting current which flows through the resistors R 1 6, R 17 and R 1 8 increases the voltage V 1, thus holding the protective circuit in the operative state, even if the voltage across the resistor R18 should drop below the critical value before the transistor Q2 becomes conductive to switch of the power transistor Q6.

When the transistor Q2 becomes conductive after the delay due to the build-up of the bias voltage on its base electrode, the transistor shunts the base electrode of the power-transistor Q6 thus turning it fully off. The power transistor 06 is held in the non-conductive state because the voltage V1 on thenon-inverting input of the comparator Al is held at the increased value above V2 due to the fact that the transistor Q4 remains conductive.

When the transistor Ol is turned on by the input voltage going high, the power-transistor 06 has its base electrode held at substantially earth potential and thus remains off, but the transistor OS is now rendered non-conductive by the input pulse going low. The transistor Q5 in turn renders the transistor Q3 non-conductive. When the transistor 03 is turned off, the base voltage at transistor Q4 rises thus cutting off the transistor 04. As there is no longer any current flowing through the power transistor Q6, the voltage V1 drops substantially to zero so that the comparator Al becomes inoperative and the transistor Q2 is turned off.Thus, the next time that the transistor Q1 is turned off by the input going low, the power transistor 06 is allowed to turn-on and the circuit reverts to normal.

The modified circuit shown in Figure 2 only differs from that shown in Figure 1 in that the comparator is in the form of a pair of transistors Q7 and Q8 formed as a differential amplifier. The base electrode of the transistor 07 receives the reference voltage V2 from the potentiometer chain R9, R 10, whilst the base electrode of the transistor Q8 receives the voltage V1 from the resistors R17 and R16. The resistor R13 is connected to the commoned emitter electrodes of the differential amplifier, whilst the resistor R12 is connected between the collector electrode of the transistor 07 and the base electrode of the transistor Q3.

The operation of the modified circuit shown is exactly the same as the circuit shown in Figure 1.

Referring now to the second embodiment shown in Figure 3, like reference characters denote the same components as used in the circuits of Figures 1 and 2. It will be noted that this is a simplified circuit as the transistors Q4 and Q5 together with their associated components are omitted. In this arrangement when the voltage V1 exceeds the voltage V2, the power transistor is not completely cut-off.

The differential amplifier comprising transistors 07 and Q8 is arranged as in the modified circuit of Figure 2. A resistor R20 is connected between the junction point of the resistors R9 and R10 and the emitter electrode of the transistor Q3. A resistor R2 1 is connected between the base electrode of the transistor Q3 and the collector electrode of the transistor 07.

Under the conditions of current overload in the power transistor Q6, when the voltage V1 exceeds the reference voltage V2, the transistors Q3 and Q2 are turned on, the transistor 03 becoming conductive slightly before the transistor Q2. Conduction of the transistor 02 has the effect of reducing the reference voltage V2, the resultant reduction in voltage depending on the values of the resistors R9, R10 and R20, the lowest value being when R20 approaches zero resistance so that V2=VCESAT of the transistor 03.

The reduction of the magnitude of the reference voltage has the effect of turning the output of the differential amplifier hard on. The conduction of the transistor 02 shunts the base electrode of the power transistor Q6 until the voltage V1 across the resistor R18 has been reduced to a value which is slightly above the reduced reference voltage V2. In practice it is preferred that the current flowing through the power transistor 06 is reduced to less than one tenth of the current which triggers the overload protection device.

The current overload circuits described in Figures 1, 2 and 3 are applicable to any type of power transistor both for ignition circuits and all other circuits using power transistors outside the field of ignition.

The overload protection circuits also protect the output stage comprising the power transistor against potentially damaging load dump transients.

In addition the current overload circuits are equally applicable to ppto-electronic ignition systems such as the oiies disclosed in our British Patent Specification Nos. 1,497,346 and 1,539,454.

Claims (12)

1. A device for protecting a power transistor against overload current including: means for deriving a voltage proportional to the current flowing in said power transistor; means for providing a reference voltage; means for comparing said derived voltage with said reference voltage; and means for reducing the current flow in said power transistor when said desired voltage exceeds the reference voltage.

2. A device according to claim 1, wherein said means for deriving a voltage which is proportional to the current flowing in the power transistor in the emitter circuit of said power transistor.

3. A device according to claim 1 or 2, wherein the comparator means is a pair of transistors arranged as a differential amplifier.

4. A device according to claim 1 or 2, wherein the comparator means is a comparator having inverting and non-inverting inputs

5. A device according to claim 3, wherein the transistors forming the differential amplifier are of PNP configuration having commoned emitter electrodes.

6. A device according to claim 4, wherein the reference voltage is applied to the inverting input of the comparator and the derived voltage is applied to the non-inverting input of the comparator.

7. A device according to any one of the preceding claims, wherein the means for reducing the current flow is a transistor which shunts the base electrode of the power transistor to earth when said transistor is rendered conductive by the comparator means.

8. A device according to claim 7, wherein the power transistor is completely cut-off when said comparator means operates.

9. A device according to claim 7, wherein the current flow through the power transistor is reduced to a very low value when said comparator means operates.

10. A device according to claim 7, wherein means is provided to either raise the value of the derived voltage or reduce the value of the reference voltage, once the comparator means has operated, so that a reduction in the current flow in the power transistor does not cause the shunting transistor to become non-conductive during the remainder of the period that the power transistor would have been conductive had the protective device not become operative.

ii. A device for protecting a power transistor against overload current, constructed and arranged to operate substantially as herein described with reference to Figure 1, or Figure 2, or Figure 3 of the accompanying drawings.

12. An electronic ignition system for an internal combustion engine incorporating a device for protecting the power transistor against overload current according to any one of the preceding claims.