GB1596100A - Overload protection circuit for a transistor - Google Patents

Description

(54) OVERLOAD PROTECTION CIRCUIT FOR A TRANSISTOR (71) We, HONEYWELL GESELL SCHAFT MIT BESCHRAENKTER HAFTUNG, a German Company of D600 Frankfurt am Main, West Germany, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to an overload protection circuit for a transistor, and is an improvement in or modification of the protection circuit described and claimed in our patent No. 1,462,446. This earlier circuit provides good results so long as the output current of the output stage switching transistor does not exceed a value of about 200 Ma.If higher load currents of for example 1 ampere are required on the output side, and if the output voltage is to be kept relatively low at such high load currents, then the circuit does not achieve its object since positive temperature coefficient resistors having small resistance values at ambient temperature and having a small size are not available.

It is an aim of the invention to provide an improved overload protection circuit in which the output voltage can be kept low at high current loads and in which the transistor drawing the load current is not subject to thermal overload.

According to the invention, there is provided an overload protection circuit for a transistor, the circuit including a resistor having a positive temperature coefficient of resistance, the resistor being connected with good thermal conduction to the transistor to be protected, a voltage limiter for limiting the input voltage of said circuit, and a control transistor for controlling the current flowing through the transistor to be protected, the voltage limiter being arranged between the base of the control transistor and a reference potential, the resistor being arranged between the emitter of the control transistor and the collector of the transistor to be protected when arranged in a common collector circuit or the base of the transistor to be protected when arranged in a common emitter circuit.

Embodiments of the invention will now be described by way of example only with reference to the accompanying drawings, in which: Figure 1 shows an overload protection circuit according to the present invention, the transistor to be protected being in a common collector circuit and being of a conductivity type different from that of the control transistor, and Figure 2 shows a different embodiment of overload protection circuit according to the present invention, the transistor to be protected being in a common emitter circuit and being of the same conductivity type as the control transistor.

Referring to Figure 1, the overload protection circuit includes a voltage limiter in form of a twin diode D1 connected between the base of a control transistor Ql and the reference potential. The twin diode Dl limits the input voltage to a maximum value V1. The emitter of the control transistor Qi is connected to the collector of the transistor Q21 to be protected via a resistor RPTC having a positive temperature coefficient of resistance and a low resistance value. The collector of the control transistor Q1 is connected to the base of the transistor Q21 to be protected.

The emitter of the transistor Q21 is connected to the operating voltage +V, via a load resistor RL and a base resistor RB is connected in parallel to the base-emitter path of the transistor Q21. The resistor RPTC is in conductive connection (shown by dotted lines) with the collector of the transistor Q21 to be protected to achieve a low thermal resistance between the resistor RPTC and the housing of the transistor Q21.

Since the resistor RPTC is arranged in the control circuit of the transistor Q21 to be protected and since this resistor RPTC consumes only a low power, its dimensions can be kept small with provides a small thermal time constant.

Assuming that the current 1E1 flowing through the resistor RPTC is very small in relation to the collector current Ic, the resistance value of the resistor RPTC is relatively small and that the load current 1L approximately corresponds to the collector current Ic, the equation for the loop being Vl=VBEI+IE1 . RpTC+(IEl+IC)RG (1) and is reduced to the following equation for the load current k::

It is therefore apparent that the load current 1L is substantially proportional to the difference between the input voltage Vl given by the voltage limiter Dl, and the base-emitter-voltage VBEI of the control transistor Ql, and is inversely proportional to the value of the negative feedback resistor RG. The output voltage Vo ,t also at high load currents of for instance 1 ampere has a low value.

According to Figure 2 the input voltage of the control transistor Ql is limited by a voltage limiter in form of a twin diode D2.

The emitter of the control transistor Ql is connected to the base of the transistor Q22 to be protected by the resistor RPTC having a positive temperature coefficient of resistance and the collector of the control transistor Ql is connected to the collector of the transistor Q22 to be protected. Both collectors are connected by the load resistor R to the positive operating voltage +VB.

The base of the transistor Q22 to be protected lies over the base resistor RB at the reference potential. The emitter of the transistor Q22 is also connected to the reference potential by means of the negative feedback resistor RG. With regard to this circuit, equation

is valid under the assumption that the emitter current 1E of the transistor Q22 to be protected is very large with regard to the emitter current IEI of the control transistor Ql flowing through the resistor having the positive temperature coefficient, that the load current 1L approximately corresponds to the emitter current 1E and that the resistor RPTC has a low resistance value at ambient temperature.

In this second circuit, the resistor RPTC cannot be conductively connected to the collector. However it is thermally connected to the base-collector-barrier layer representing the main temperature source. Also with this circuit, it is possible to draw relatively large load currents without any danger of thermal overload occurring.

WHAT WE CLAIM IS: 1. An overload protection circuit for a transistor, the circuit including a resistor having a positive temperature coefficient of resistance, the resistor being connected with good thermal conduction to the transistor to be protected, a voltage limiter for limiting the input voltage of said circuit, and a control transistor for controlling the current flowing through the transistor to be protected, the voltage limiter being arranged between the base of the control transistor and a reference potential, the resistor being arranged between the emitter of the control transistor and the collector of the transistor to be protected when arranged in a common collector circuit or the base of the transistor to be protected when arranged in a common emitter circuit.

2. The circuit of Claim 1, wherein a base resistor is connected in parallel with the base emitter path of the transistor to be protected.

3. The circuit of Claim I or 2, wherein the control transistor is of the NPN type and the transistor to be protected is of the PNP type when arranged in a common collector circuit or of the NPN type when arranged in a common emitter circuit.

4. The circuit of any one of the preceding claims, wherein the first mentioned resistor is connected with good thermal conduction to the collector of the transistor to be protected.

5. An overload protection circuit for a transistor substantially as herein described with reference to Figure 1 or Figure 2 of the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (5)

**WARNING** start of CLMS field may overlap end of DESC **. consumes only a low power, its dimensions can be kept small with provides a small thermal time constant. Assuming that the current 1E1 flowing through the resistor RPTC is very small in relation to the collector current Ic, the resistance value of the resistor RPTC is relatively small and that the load current 1L approximately corresponds to the collector current Ic, the equation for the loop being Vl=VBEI+IE1 . RpTC+(IEl+IC)RG (1) and is reduced to the following equation for the load current k:: It is therefore apparent that the load current 1L is substantially proportional to the difference between the input voltage Vl given by the voltage limiter Dl, and the base-emitter-voltage VBEI of the control transistor Ql, and is inversely proportional to the value of the negative feedback resistor RG. The output voltage Vo ,t also at high load currents of for instance 1 ampere has a low value. According to Figure 2 the input voltage of the control transistor Ql is limited by a voltage limiter in form of a twin diode D2. The emitter of the control transistor Ql is connected to the base of the transistor Q22 to be protected by the resistor RPTC having a positive temperature coefficient of resistance and the collector of the control transistor Ql is connected to the collector of the transistor Q22 to be protected. Both collectors are connected by the load resistor R to the positive operating voltage +VB. The base of the transistor Q22 to be protected lies over the base resistor RB at the reference potential. The emitter of the transistor Q22 is also connected to the reference potential by means of the negative feedback resistor RG. With regard to this circuit, equation is valid under the assumption that the emitter current 1E of the transistor Q22 to be protected is very large with regard to the emitter current IEI of the control transistor Ql flowing through the resistor having the positive temperature coefficient, that the load current 1L approximately corresponds to the emitter current 1E and that the resistor RPTC has a low resistance value at ambient temperature. In this second circuit, the resistor RPTC cannot be conductively connected to the collector. However it is thermally connected to the base-collector-barrier layer representing the main temperature source. Also with this circuit, it is possible to draw relatively large load currents without any danger of thermal overload occurring. WHAT WE CLAIM IS:

1. An overload protection circuit for a transistor, the circuit including a resistor having a positive temperature coefficient of resistance, the resistor being connected with good thermal conduction to the transistor to be protected, a voltage limiter for limiting the input voltage of said circuit, and a control transistor for controlling the current flowing through the transistor to be protected, the voltage limiter being arranged between the base of the control transistor and a reference potential, the resistor being arranged between the emitter of the control transistor and the collector of the transistor to be protected when arranged in a common collector circuit or the base of the transistor to be protected when arranged in a common emitter circuit.

2. The circuit of Claim 1, wherein a base resistor is connected in parallel with the base emitter path of the transistor to be protected.

3. The circuit of Claim I or 2, wherein the control transistor is of the NPN type and the transistor to be protected is of the PNP type when arranged in a common collector circuit or of the NPN type when arranged in a common emitter circuit.

4. The circuit of any one of the preceding claims, wherein the first mentioned resistor is connected with good thermal conduction to the collector of the transistor to be protected.

5. An overload protection circuit for a transistor substantially as herein described with reference to Figure 1 or Figure 2 of the accompanying drawings.