The present invention relates to protective devices for power elements included in integrated circuits, and more particularly, to devices for protection against overcurrents and overvoltages at the output as a result, for example, of a short circuit, in final power transistors included in monolithically integrated amplifier circuits.
Such protective devices are integrated together with the circuit including the power element to be protected. Therefore, their fabrication must be simple and economical and, above all, they must not cause power losses that would limit the dynamic operation of the power element.
In addition, they must have a high degree of reliability to ensure protection.
A type of protective device of the prior art that satisfies such requirements has a circuit diagram comprising at least one active element which is thermally coupled to the power element to be protected, said active element being connected to a control circuit element which provides for the turn-off of the integrated circuit with which said power element is associated when the active element detects a dangerous temperature level, which is an indication of an excessive power dissipation under overvoltage or overcurrent conditions.
However, this type of protection, however efficient and reliable, is unsuitable when the abnormal operating conditions are only transitory, because in every case, the device remains turned off without outside intervention.
Therefore, even if more complicated from the circuit point of view, protective devices with an operating threshold which is not related to the level of thermal dissipation but is rather related to the level of the electrical magnitudes, i.e.--current and voltage, relative to the element to be protected, are more commonly employed. On the basis of their levels, such magnitudes can be properly adjusted without the necessity of turning off the integrated circuit.
Such protective devices usually include circuit elements for the detection and processing of the value of the current flowing through the power element and the voltage supplied to the power element in order to effect above, the activation of a threshold circuit so as to reduce the value of the current flowing through the power element to a maximum value corresponding to a predetermined threshold level as a function of the value of the voltage supplied to the power element.
The circuit diagram of this type of protective device, normally employed in monolithically integrated amplifier circuits, is illustrated in FIG. 1 of the drawings. It protects a final bipolar power transistor of the NPN type denoted in the figure by the symbol T.sub.1 and pertaining to an integrated circuit not shown in detail in the figure.
The emitter of the transistor T.sub.1 is connected to an output terminal U for the connection to the load by means of an electrical conductor having a distributed resistance with an accurately determined overall valve R.sub.1. By way of example, this may be physically realized by means of a gold wire having dimensions with extremely close tolerances.
For greater clarity, the overall resistance R.sub.1 of such a conductor is shown in the figure as a lumped resistance. The collector of transistor T.sub.1 is connected to the positive terminal +Vcc of a supply voltage source and the base of the transistor T.sub.1 is connected to the emitter of a second bipolar transistor T.sub.2 of the NPN type whose collector is also connected to the positive terminal +Vcc.
The current signal whose power is to be amplified is supplied to an input terminal IN connected to the base of transistor T.sub.2 which controls transistor T.sub.1.
The emitter of transistor T.sub.1 is likewise connected, through a diode D.sub.1, to the base of a bipolar transistor T.sub.3 of the NPN type, whose emitter is also connected to the output terminal U through an electrical conductor having a distributed resistance R.sub.2 of a much smaller value designated in the figure as being a lumped resistance for greater clarity.
The base of transistor T.sub.3 is connected to the positive terminal +Vcc through a biasing resistor R.sub.3.
The collector of transistor T.sub.3 is connected to the base of a bipolar transistor T.sub.5 of the PNP type, which is also connected to the positive terminal +Vcc through a constant current generator A.sub.1.
The emitter and collector of transistor T.sub.5 are respectively connected to the input terminal IN and the negative terminal -Vcc of the supply voltage source.
Diode D.sub.1, transistor T.sub.3, and resistor R.sub.3 perform the function of processing the current flowing through resistor R.sub.1 and the voltage between the collector and the emitter of the final power transistor T.sub.1.
Any increase in the voltage or the output current of transistor T.sub.1 results in an increase in the base current and, thereby, in the collector current of transistor T.sub.3. Transistor T.sub.5 and constant current generator A.sub.1 together form a threshold circuit. Until the value of the collector current of transistor T.sub.3 drops below the value of the constant current generated by constant current generator A.sub.1, transistor T.sub.5 is inoperative and the signal current supplied to the input IN is amplified without modification and transferred to the output U, but--due to an excessive increase in the voltage or current at the output of transistor T.sub.1 --as soon as the current flowing through the collector of transistor T.sub.3 exceeds the value of the reference current generated by constant current generator A.sub.1, transistor T.sub.5 starts to conduct.
The transistor T.sub.5 is controlled by the current resulting from the differential between the reference current and the collector current of transistor T.sub.3 and, thus, a part of the signal current present at the input IN is absorbed by transistor T.sub.5 . Therefore, transistor T.sub.1, being controlled by a lower current, reduces its level of conduction relative to the voltage supplied thereto to the value by which the collector current of transistor T.sub.3 is reduced to the limit of the current generated by constant current generator A.sub.1.
Thus, the output current does not exceed, in relation to the voltage supplied to transistor T.sub.1, the automatically checked maximum value determining the operation of the protection circuit.
The protective circuit described hereinabove is designed in such a way that said relationship between the current flowing through the final power transistor and the voltage supplied thereto, depending on the supply and on the load conditions, is of the exponential type, as can easily be determined by analytical means due to the presence of a diode in the circuit for processing the magnitudes under control.
By appropriately choosing the value of the circuit components, said relationship can be represented on the plane which is characteristic of the power transistor with a curve which approaches, in part, the curve of the maximum power that can be dissipated by the transistor and, in part, by that of the secondary breakdown of the transistor, but remaining at every point below said curves.
It is well known that the curve of the maximum power that can be dissipated is a branch of a hyperbole having as asymptotes the coordinated axes of the characteristic plane, that is to say, a curve which is a locus of the points in which EQU V.times.I=constant,
while the secondary breakdown curve has a typical shape for any type of power transistor.
It will be understood that by appropriately designing the protective circuit, only one diagram of which is indicated by way of illustrative example, those skilled in the art can arrive at the relationship on the characteristic plane which is more suited to the type of final transistor and to the type of application.
In the event of a short circuit at the output, the collector-emitter biasing voltage, without negligible voltage drops across the resistive elements of the circuit, equals the supply voltage, even if during a brief initial transient, the voltage across the transistor may exceed the supply voltage for inductive loads.
Therefore, following such a possible transient, the maximum electrical power in the transistor equals the product of the supply voltage and the maximum current therein allowed by the protective circuit in relation to the value of such a voltage.
The maximum allowable value of the current that can flow through the power transistor, on which depends the circuit values of the protective circuit, is determined by the physical characteristics.
In general, for reasons of economy, users of integrated circuits including power elements determine the values of the external dissipator elements through the heat developed by such elements according to the requirements of normal operating conditions, because brief periods of high heat dissipation are well tolerated.
However, prolonged short circuits are dangerous, because they may damage the integrated circuit or may cause the surrounding material to overheat and burn as a result of the heat developed which is insufficiently dissipated to the outside.
On the other hand, it is inconvenient to reduce the maximum current level in the power element by lowering the operating threshold of the protection, because this would unnecessarily limit the dynamic performance of the circuit under normal operating conditions.