Transistor Protection Circuit

This invention relates to a transistor protection circuit comprising:

an output transistor of a first conductivity type comprising a collector, base and emitter,

a driver transistor of the first conductivity type comprising a collector, base and emitter, the emitter being coupled to the base of the output transistor,

a current-sensing transistor of the first conductivity type comprising a collector, base and emitter, having its base and emitter coupled to the base and the emitter, respectively, of the output transistor, its collector current being a measure of the collector current of the output transistor, and

means coupled to the current-sensing transistor for limiting the drive current to the output transistor depending on the collector current of the current-sensing transistor.

Such a circuit may be used, for example, in audio and video circuitry and in motor-control circuits.

Such a circuit is known from inter alia, U.S. Pat. No. 4,330,757. In this circuit the current through the output transistor is measured by means of a current-sensing transistor whose base-emitter junction is arranged in parallel with that of the output transistor, the collector current of the current-sensing transistor being a fraction of the collector current of the output transistor. By means of a current mirror the collector current of the current-sensing transistor is reflected to a resistance element which is arranged in parallel with the base-emitter junction of a clamping transistor, the collector of the clamping transistor being connected to the base of the driver transistor. In the case of an excessive collector current of the current-sensing transistor the clamping transistor is driven into conduction so that, what would otherwise be a part of the base current of the driver transistor is drained via the clamping transistor.

A disadvantage of this known circuit arrangement is that in integrated form it exhibits a high degree of instability in the protection loop. These instabilities are caused by phase shifts in the current mirror arranged in the protection loop, which current mirror comprises PNP-transistors which have a comparatively low cut-off frequency.

Therefore, it is an object of the invention to provide a transistor protection circuit which has a stable protection loop and which is suitable for manufacture in integrated form. To this end a transistor protection circuit of the type specified in the opening paragraph is characterized in that the current limiting means are provided with:

a current generator for supplying a reference current, the generator output being coupled to the collector of the current-sensing transistor, and

a semiconductor P/N junction which is coupled to the output of the current generator, and which is driven to become forward-biased when the collector current of the current-sensing transistor is substantially equal to the reference current from the current generator. The invention utilizes the principle that in the event that the collector current of the current-sensing transistor becomes larger than the reference current from the current generator, the voltage at the output of the current generator changes from a high to a low value, thereby turning on a semiconductor junction by means of which the drive current to the output transistor is limited. The protection loop exhibits no instabilities because the protection loop only comprises transistors of one conductivity-type. Moreover, the arrangement is highly suitable for fabrication as an integrated circuit.

The semiconductor P/N junction may be coupled to the output of the current generator via an emitter-follower transistor of the first conductivity type comprising a collector, base, and emitter, which follower transistor has its base coupled to the output of the current generator and its emitter coupled to the semiconductor junction. In this case the semiconductor junction may comprise the collector-base junction of the driver transistor, the collector of the driver transistor being coupled to the emitter of the follower transistor. When the protection circuit is actuated the driver transistor is bottomed, thereby limiting the base current of the output transistor. As an alternative the semiconductor junction may comprise a diode, or a transistor connected as a diode, having its cathode coupled to the emitter of the follower transistor and its anode coupled to the base of the driver transistor. In the latter case, when the protection circuit becomes operative a part of what would otherwise be the base current of the driver transistor is drained via the diode so that the current through the output transistor is limited. As a modification the cathode may be connected directly to the output of the current generator.

The current generator for supplying the reference current may comprise a constant current source or a reference resistor in series with a reference-voltage source. In order to limit the base current of the output transistor when the output transistor is bottomed a limiting resistor may be arranged in the collector line of the follower transistor.

The invention will now be described in more detail, by way of example, with reference to the accompanying drawings, in which:

FIG. 1 shows a first embodiment,

FIG. 2 shows a first variant of the circuit shown in FIG. 1,

FIG. 3 shows a second variant of the circuit shown in FIG. 1,

FIG. 4 shows a third variant of the circuit shown in FIG. 1,

FIG. 5 shows another embodiment, and

FIG. 6 shows another embodiment.

FIG. 1 shows a first embodiment of the invention. In this transistor protection circuit transistor T.sub.1 is the output transistor to be protected. This transistor can supply a current I.sub.o to a load 3, represented in broken lines, via the collector terminal 2. The emitter of transistor T.sub.1 is connected to the negative power-supply terminal 4, which in the present example is connected to ground. The transistor T.sub.1 is driven by a driver transistor T.sub.2 having an emitter connected to the base of transistor T.sub.1. An input current I.sub.i is applied to the base of transistor T.sub.2. In order to prevent the output transistor T.sub.1 from being damaged by an excessive current, the output current I.sub.o must be limited to a maximum value. The protection circuit comprises a current-sensing transistor T.sub.3 whose base-emitter junction is arranged in parallel with that of transistor T.sub.1. The base-emitter area A.sub.3 of transistor T.sub.3 is smaller than the base-emitter area A.sub.1 of transistor T.sub.1, so that the collector current of transistor T.sub.3 is equal to a fraction A.sub.3 /A.sub.1 of the collector current of transistor T.sub.1. The collector of transistor T.sub.3 is coupled to the positive power-supply terminal 6 by means of a current generator 5 which supplies a reference current I.sub.ref. Furthermore, the collector of transistor T.sub.3 is connected to the base 7 of a follower transistor T.sub.4 having an emitter connected to the collector of driver transistor T.sub.2 and a collector connected to the positive power-supply terminal 6. If the protection circuit is not activated an output current I.sub.o will flow in transistor T.sub.1. This current depends on the input current I.sub.i and on the current-gain factors of the transistors T.sub.2 and T.sub.1. The collector current of transistor T.sub.3 is then equal to a fraction of the output current I.sub.o. As long as the collector current of T.sub.3 is smaller than the reference current I.sub.ref from the current generator 5 the transistor T.sub.3 presents a load to the current generator 5. As a result of this, the voltage at the base 7 of transistor T.sub.4 is comparatively high. The voltage of the emitter of transistor T.sub.4 follows the volume on the base 7 with a difference corresponding to one base-emitter junction voltage. As a result, the collector of transistor T.sub.1 is at such a voltage that transistor T.sub.1 conducts normally. The protection circuit is activated at the instant that the collector current of transistor T.sub.3 exceeds the reference current I.sub.ref. The current generator 5 then presents a load to transistor T.sub.3. At this instant the voltage at the base 7 of transistor T.sub.4 changes to a comparatively low value. The voltage on the emitter of the transistor T.sub.4 and hence on the collector of transistor T.sub.2 then becomes so low that the base-collector junction of transistor T.sub.2, which is normally cut off, is forward-biased. Transistor T.sub.2 is then bottomed so that transistor T.sub.2 practically ceases to function as a current amplifier. The emitter current of transistor T.sub.2 is then of the same order of magnitude as the input current I.sub. i. Since, upon activation of the protection circuit, the collector current of transistor T.sub.3 is equal to the reference current I.sub.ref and the collector current of transistor T.sub.1 is equal to a fraction A.sub.1 /A.sub.3 of the collector current of transistor T.sub.3, the maximum output current will be ##EQU1##

In the foregoing the base current which transistor T.sub.4 draws from the current I.sub.ref has been ignored. However, this base current is generally so small that its influence on the maximum output current is minimal. Transistors T.sub.1 and T.sub.4 have current-gain factors .beta. which in practice are of the order of magnitude of 100. For a maximum output current of, for example, I.sub.omax =500 mA the maximum collector-emitter current of the transistors T.sub.2 and T.sub.4 may be approximately 5 mA, so that the base current of transistor T.sub.4 will be approximately 1/20 mA. For a ratio A.sub.1 /A.sub.3 between the emitter areas.apprxeq.500 the collector current of transistor T.sub.3 will be substantially 1 mA, which is large relative to the base current of transistor T.sub.4.

Instead of adjusting the ratio between the collector currents of the transistors T.sub.1 and T.sub.3 by means of the ratio between their emitter areas, this ratio may be adjusted by arranging a resistor in the emitter line of one of the transistors T.sub.1 and T.sub.3 or by arranging a resistor in both emitter lines. Upon arranging resistors in the emitter lines the ratio I.sub.omax /I.sub.ref becomes temperature-dependent unless the ratio between the resistor values is inversely proportional to the ratio between the emitter areas.

FIG. 2 shows a first variant of the circuit shown in FIG. 1, similar components bearing the same reference numerals. The current generator 5 for supplying the reference current I.sub.ref comprises a current source. This current source comprises a reference resistor 8 which, in series with a PNP-transistor T.sub.5 arranged as a diode, is clamped between the negative and the positive power-supply terminals 4 and 6. If the resistance of the resistor 8 is sufficiently high the current in this branch is almost wholly determined by this resistor. By means of a transistor T.sub.6, whose base-emitter path is connected in parallel with that of transistor T.sub.5, this current is reflected to the collector of transistor T.sub.3. The present embodiment employs PNP-transistors T.sub.5 and T.sub.6, but these transistors do not affect the stability of the circuit because they do not form part of the feedback loop comprising the transistors T.sub.3, T.sub.4 and T.sub.2.

The circuit operates in the same way as that in FIG. 1. If the collector current of transistor T.sub.3 is smaller than the collector current I.sub.ref of transistor T.sub.6, transistor T.sub.3 with its high collector-impedance constitutes a load for the current source transistor T.sub.6 and the voltage at point 7 is comparatively high. However, if the collector current of transistor T.sub.3 is larger than the collector current I.sub.ref of transistor T.sub.6, transistor T.sub.6 with its high collector impedance presents a load to the transistor T.sub.3, which then functions as the current-source transistor. The voltage at point 7 is then comparatively low so that transistor T.sub.2 is bottomed.

A second variant of the circuit shown in FIG. 1 will be described with reference to FIG. 3, in which similar components bear the same reference numerals as in FIG. 1. The current generator in this variant comprises a reference resistor 9, of which one terminal is connected to the collector of transistor T.sub.3 and the other terminal is connected to a reference voltage V.sub.ref. The reference current I.sub.ref is substantially equal to the quotient of the reference voltage and the resistance value of the resistor 9 if the reference voltage is not too low and the resistance of said resistor is sufficiently high. The circuit operates in the same way as that shown in FIGS. 1 and 2, the reference resistor forming a load for transistor T.sub.3 when the protection circuit is activated so that the voltage on point 7 changes and transistor T.sub.2 is bottomed. By an appropriate choice of the reference voltage the operation of the protection circuit, and consequently the maximum output current I.sub.o, can be made substantially temperature-independent. More particularly, it can be arranged that a voltage with a positive temperature coefficient appears across the reference resistor, which coefficient is substantially equal to the positive temperature coefficient of the reference resistor, so that the reference current is substantially temperature-independent.

A third variant of the circuit shown in FIG. 1 is shown in FIG. 4, in which similar components bear the same reference numerals. The current generator in this variant comprises a reference resistor 9 to which a reference voltage V.sub.ref is applied. When the circuit is integrated the NPN-transistor T.sub.1 gives rise to a parasitic PNP-transistor T.sub.1 ', whose collector is connected to the substrate of the integrated circuit, whose base is connected to the collector of transistor T.sub.1 and whose emitter is connected to the base of transistor T.sub.1. This transistor T.sub.1 ' is shown in broken lines in the Figure. When transistor T.sub.1 is bottomed its collector voltage drops below its base voltage so that the base-emitter junction of the parasitic transistor T.sub.1 ' is driven into conduction. The drive current of transistor T.sub.1, which is supplied by transistor T.sub.2, may than become fairly large, which demands an unnecessary amount of power. In order to limit this drive current a limiting resistor 10 is arranged in the collector line of transistor T.sub.4, which limiting resistor is coupled to the positive power-supply terminal 6 via a transistor T.sub.7 whose base is at the reference voltage V.sub.ref. It is to be noted that as an alternative the limiting resistor may be connected directly to the positive power-supply terminal 6.

Another protection circuit in accordance with the invention will be described with reference to FIG. 5 in which idential parts bear the same reference numerals as in FIG. 1. The emitter of transistor T.sub.4 is no longer connected to the collector of transistor T.sub.2, but T.sub.4 is still arranged as an emitter-follower, the current source 12 being arranged in its emitter line. The cathode of a diode 13 is connected to the emitter of transistor T.sub.4, the anode of said diode being connected to the base of transistor T.sub.2. Instead of a diode it is possible to employ a transistor with a short-circuited base-collector junction. If the collector current of the transistor T.sub.3 is smaller than the current I.sub.ref from the current generator 5, the voltage on the base 7 and hence on the emitter of transistor T.sub.4 is comparatively high, namely higher than the voltage on the base of driver transistor T.sub.2. Diode 13 is then cut off. If the collector current of transistor T.sub.3 exceeds the current I.sub.ref the protection circuit is activated so that the voltage on the base 7 of transistor T.sub.4 decreases substantially. The voltage on the emitter of transistor T.sub.4 then becomes lower than the voltage on the base of the driver transistor T.sub.2 so that diode 13 is turned on. As a result of this a part of the current I.sub.i which would otherwise be fed to the base of the driver transistor is drained via the diode 13. Thus, the output current of transistor T.sub.1 is limited to a maximum value I.sub.omax =I.sub.ref.A.sub.1 /A.sub.3.

Another transistor protection circuit in accordance with the invention will be described with reference to FIG. 6, in which idential parts bear the same reference numerals as in FIG. 5. The cathode of the diode 13 is now connected directly to the output 7 of the current generator 5 instead of via a follower transistor. The circuit operates in the same way as that shown in FIG. 5. When the protection circuit is activated a part of what would otherwise be the base current of transistor T.sub.2 flows through the collector of transistor T.sub.3 via diode 13. This current is small relative to the reference current I.sub.ref so that its influence on the maximum value of the output current of transistor T.sub.1 may be ignored.

The invention is not limited to the embodiments described but many variations are possible within the scope of the invention as defined by the claims. For example, it is possible to replace the simple emitter-follower transistor by a Darlington transistor or to arrange a diode in series with the base-emitter junction of the follower transistor. Moreover, the current generator may be constructed in various ways.