GB2236919A

GB2236919A - Overload and short-circuit protection of electric output drivers

Info

Publication number: GB2236919A
Application number: GB8921229A
Authority: GB
Inventors: Asko Juntunen
Original assignee: Kone Elevator GmbH
Current assignee: Kone Elevator GmbH
Priority date: 1989-10-04
Filing date: 1989-09-20
Publication date: 1991-04-17
Anticipated expiration: 2009-09-20
Also published as: GB2236919B; GB8921229D0; FR2652687B1; FR2652687A1

Abstract

Output drive circuits (Q31, Q41 ... Q37, Q47) are coupled between a supply voltage (Vcc) and respective outputs (O1...O7). When an input is applied at a terminal (I1), transistors (Q1, Q61) are turned on, a transistor (Q11) is prevented from conducting, and the drive circuit (Q31, Q41) is energised. If the voltage at the output (O1) falls, transistor (Q11) turns on, and the drive circuit is rapidly turned off. An oscillator circuit (IC, Q50) produces short pulses at intervals to monitor the fault condition. Each pulse turns on transistor (Q1) to energise the drive circuit (Q31, Q41) by way of transistor (Q61), and conduction will continue if the fault has been removed. Otherwise, the driver will again be rapidly turned off. Zener diode (D8) provides a stable bias voltage (Vd). <IMAGE>

Description

PROCEDURE AND DEVICE FOR OVERLOAD AND SHORT-CIRCUIT PROTECTION OF OUTPUT DRIVERS The present invention relates to a procedure and device for overload and short-circuit protection of output drivers.

The output drivers (transistors, FETs etc.) of a control output used to control the electric power supplied into a load will break down if overloaded or short-circuited. A device for overload and short-circuit protection is proposed e.g. in US patent publication 4695915. This device uses no reference value, which means that the overload current limit cannot be set to a desired value. Besides, the current limit changes with variations in the voltage supplying the load, because the variations change the base current.

Also, the output transistor requires a large control current, resulting in large thermal losses. For this reason it is practically impossible to integrate several outputs in a single IC. Moreover, the device is not turned on immediately when the control signal is applied. Instead, a separate external pulse is required.

The object of the present invention is to eliminate the drawbacks mentioned and to achieve a protection system that requires no power-consuming and heat-generating resistors in series with the load. Such resistors cause thermal losses that make it impossible to integrate several protector units in the same housing. The procedure of the invention for overload and short-circuit protection of output drivers is characterized in that at least one output driver is controlled in a normal situation by a reference voltage which is set independtly of external control, whereas in an overload or short-circuit situation the control signal is applied for short instants only, and that when the load current of the output driver exceeds a certain limit, the reference value begins to fall rapidly.

The characteristics of the rest of the embodiments of the procedure of the invention and of the device designed for applying the procedure are presented in the other claims.

The invention provides the important advantage that the output driver is completely shut off even in a momentary overload situation, which means that the generation of heat is immediately prevented. Furthermore, the invention has a provision for monitoring the presence of an overload or short-circuit situation by means of very short pulses applied at certain intervals, and in order for the driver to be turned on again, the load current must be lower than in an overload situation. This arrangement ensures that overheating can never occur. Thus it is possible to integrate several (e.g. 7) outputs in a single circuit. Moreover, the circuit can be directly controlled by a microprocessor. In addition, the control circuit can be implemented either as a drain-type or a source-type circuit.

The circuit protects the output driver against any form of short-circuit or overload condition. It is practically impossible for the driver to be overheated, resulting in improved reliability and longer life. The circuit uses no power resistors and no heat-generating components. This makes it possible to create an integrated circuit comprising e.g. eight control outputs, thus obviating the need for complicated casing solutions. It is also possible to produce a circuit that is pin-compatible with the currently used circuit, so that short-circuit protection can be effected by replacing the old circuit with a new one.

In the following, the invention is described by the aid of examples, with reference to the drawings attached, wherein: Fig. 1a is a block diagram illustrating the principle of short-circuit and overload protection in the case of a source-type control output.

Fig. ib is a block diagram illustrating the principle of short-circuit and overload protection in the case of a drain-type control output.

Fig. 2 shows a source-type overload and short-circuit protector.

Fig. 3 shows a drain-type overload and short-circuit protector.

Fig. 4 represents the operational cycles of the circuit in a normal situation.

Fig. 5 represents the operational cycles of the circuit during an overload or short-circuit condition.

Let us first consider a source-type control output driver (transistor, FET etc.) which will break down if overloaded or short-circuited. The block diagram in Fig. 1 illustrates the principle of operation of the short-circuit and overload protection circuit for said driver. When an input signal is switched on, the control logic 1' issues a short sensing pulse to the driver while at the same time monitoring the output for the presence of a short-circuit or overload condition. If no short-circuit or overload condition prevails, the control logic 1' applies to the driver 3' a control signal determined by the contol reference value 2', and the driver is thus turned on. However, if a short-circuit or overload condition prevails, the overload and short-circuit monitoring circuit 4' interrupts the control signal from the logic to the driver.After the overload or short-circuit condition has disappeared, the driver is again turned on by a pulse from an oscillator 2'. The oscillator monitors the duration of the short-circuit or overload condition by giving, via the control logic, short sensing pulses to the driver at regular intervals, and as soon as no overload/short-circuit condition is detected, the driver is allowed to continue operating in accordance with the input.

Fig. 2 represents the control units of source-type drivers for seven inputs I1...17, the control units for inputs I1 and I7 being shown in the figure. All these control units are identical in construction and operation, so only the unit for input I1 will be described here. In the figure, the last digit of the reference number for components of input I1 is 1, and 7 for components of input I7.

The output driver consists of two transistors Q31 and Q41 connected as a Darlington pair. The emitter of the Darlington pair is connected to the supply voltage +Vcc while the collector is connected to the output 01. In the circuit controlling the output driver, the emitter of the first transistor Q1 is connected to the collector of an auxiliary transistor Q21, the collector to the base of a second transistor Q11, and the base to a point between the first resistor R71, the other end of which is connected to the supply voltage +Vcc, and a second resistor R61, the other end of which is connected to the collector of the oscillator transistor Q50. The emitter of the auxiliary transistor Q21 is connected to the supply voltage +Vcc. The emitter and base of Q21 are connected by a third resistor R1. The input signal I1, via a fourth resistor R21, is applied to the base of a third transistor Q51, the emitter of which is connected to ground and the collector via capacitor C1 to the base of the first transistor Q1. The collector of Q51 is also connected, via a fifth resistor R41, to the base of the auxiliary transistor Q21 and, via a sixth resistor R31, to the base of a fourth transistor Q61. The emitter of the second transistor Q11 is connected to the collector of the auxiliary transistor, the-base via a seventh resistor Ril to the output 01, and the collector to the base of the output driver and, via an eighth resistor R5, to the emitter of the fourth transistor Q61.In each unit, the collector of the fourth transistor Q61 is connected to a point between a Zener diode D8 and a ninth resistor R10, the other terminal of the Zener diode being connected to the supply voltage +Vcc while the other end of the resistor is grounded, the reference voltage Vd being obtained from said point.

The circuit includes an oscillator which is common to all control units and consists of a capacitor C2 and two Schmitt triggers IC. Connected across the Schmitt triggers are a resistor R9 and a series circuit of a diode D9 and a resistor R8. The output of the amplifier is connected via resistor R50 to the base of transistor Q50, whose emitter is grounded.

When the voltage at the input I1 is increased above the threshold, the third transistor Q51 starts to conduct, thus also turning on the auxiliary transistor Q21. The first transistor Q1 is now turned on by a signal obtained via capacitor C1. Thus the second transistor Q11 receives a base signal that prevents it from conducting. As a result of these actions, the fourth transistor Q61 starts to conduct, thus applying a control voltage to the base of the output driver. Diode D8 constitutes a bias regulator ensuring that small variations in the supply voltage will not affect the control of the output stage. The overload and short-circuit monitoring circuit consists of the second transistor Q11 and the seventh resistor R11.When the voltage of the output signal 01 falls, the result is that the second transistor Q11 begins to conduct, thus altering the base voltage of the output driver so that the latter starts to turn off. This again has the effect that the output voltage 01 falls rapidly further. In this manner, the output driver is shut off very quickly. The flow of leakage current is prevented by means of the auxiliary transistor Q21, the third resistor R1 and the fifth resistor R41.

The oscillator circuit outputs a short pulse at regular intervals to monitor the presence of e.g. a short-circuit condition. The oscillator pulse turns on the first transis- tor Q1, thereby preventing conduction by the second transistor Q11. The output driver is now controlled by the fourth transistor Q6i. If the short-circuit condition has disappeared, the output voltage at 01 will not fall and the output driver thus remains in the conducting state. When the input I1 is driven into the low state, the third transistor Q51 ceases to conduct, thereby turning off the fourth transistor Q61. Thus deprived of its base control signal, the output driver is turned off.

Next, let us consider a drain-type control output driver (transistor, FET etc.) which will break down if overloaded or short-circuited. In this case the overload/short-circuit protection is based on the principle illustrated by Fig.

ib, which fully corresponds to the principle in Fig. la.

The only difference is that in this case the load is connected to the supply voltage, whereas in the case of a source-type protection circuit the load is connected to ground.

Fig. 3 shows the control units for drain-type dirvers for seven inputs I1'...I7'. The control units are identical in construction and operation, and they all have the same components. The components in each unit are marked with reference numbers in which the last digit corresponds to the number of the input in question. Let us now consider the control unit of input I1' as an example.

In this case, too, the output driver consists of two transistors Q31' and Q4' connected as a Darlington pair. In the circuit controlling the output driver, the collector of the first transistor Q1' is connected to a resistor R11', whose other end is connected to the output 01, and to the base of a second transistor Q11', the emitter to the emitter of the second transistor Q11' and the base via a capacitor C1' to the input I1.The base of the output driver and the collector of the second transistor Q11' are connected via a second resistor R31' to a point between a third resistor R100', which is common to all drivers and connects to the supply voltage Vcc', and a Zener diode D8' connected to ground1 the reference voltage Vd' being obtained from this point. Connected between the input I1' and the second resistor R31' is a second diode D1'. The base of the auxiliary transistor Q21' is connected to the input via a fourth resistor R1', the collector to the emitter of the second transistor Q11' and the emitter together with the emitter of the output driver to ground.The base of the first transistor Q1' is connected to an oscillator via a fifth resistor R27'.

The oscillator is common to all the control units and consists of two Schmitt triggers IC' connected to a capacitor C8'. Connected across the Schmitt triggers are a resistor R9' and a series circuit of a diode D9' and a resistor R8'.

When the input I1' is driven high, the first transistor Q1' is turned on by a voltage applied to its base via the capacitor C1', thus preventing conduction by the second transistor Q11'. The current flowing through the second resistor R31' is now passed to the base of the output driver. If the collector of the output driver has the right load, then the collector voltage falls below a certain value, so that the second transistor Q11' receives no base voltage and the output driver remains in the conducting state. When the input is low, no leakage current can flow from the collector of the output driver through the first resistor R11' and the base of the second transistor Q11' to ground, because the auxiliary transistor Q21' is not conducting.

In case the collector of the output driver is short-circuited to the supply voltage, then, after the short pulse from capacitor C1' to the base of the first transistor, the second transistor is turned on because it receives a base voltage from the collector of the output driver. Consequently, the current flowing through the second resistor R31' is passed to ground via the second transistor, so that the output driver receives no control current and remains shut off.

Capacitor C1' forces the output into the state corresponding to the input directly without waiting for a pulse from the oscillator.

E.g. in a short-circuit situation the pulses supplied by the oscillator at regular intervals monitor the presence of the short-circuit condition. If the collector of the output driver is still in short-circuit connection to the supply voltage, then the control voltage applied to its base will not effect a sufficient reduction in the collector voltage, so that the output driver remains shut off.

When the short-circuit condition disappears, the pulse supplied by the oscillator will cause the collector voltage of the output driver to fall, so that the second transistor Q11' receives no control voltage and is therefore turned off. Thus, the current from the second resistor R31' flows into the base of the output driver, which therefore remains in the conducting state.

If desired, the output driver can be switched off by driving the input I1 into the low state, in which case the current from the second resistor R31' will not flow to the base of the output driver, which therefore ceases to conduct. Neither will the oscillator pulse result in the output driver being turned on.

The Zener diode D8' maintains a constant base voltage even if the supply voltage Vcc' varies.

Fig. 4 represents the operational cycles in a normal situation, and Fig. 5 the operational cycles in an overload or short-circuit situation, showing the actions at the following points of the circuit (see Fig. 1b): 1 input, 2 driver input, 3 driver output, 4 oscillator. The arrows represent the sequence of events. In a normal situation, the control unit input controls the driver input, which in turn controls the driver output. In Fig. 5, a short-circuit or overload condition appears at instant 1" and disappears at instant 2". During this condition, the oscillator sends a pulse which appears at the input of the driver and therefore also at its output. When the abnormal condition has disappeared, the pulse supplied by the oscillator has the effect of restoring normal operation.

It is obvious to a person skilled in the art that different embodiments the invention are not restricted to the examples described above, but that they may instead be varied within the scope of the following claims.

Claims

1. Procedure for overload and short-circuit protection of output drivers, c h a r a c t e r i z e d in that at least one output driver (Q31,Q41;Q31',Q41') is controlled in a normal situation by a reference value (+Vd) set independently of external control (I1-I7; I1'-I7'), whereas in an overload or short-circuit situation the control signal is applied for short instants only, and that when the load current of the output driver exceeds a set limit, the reference value begins to fall rapidly.

2. Procedure according to claim 1, c h a r a c t e r i z e d in that, when the input voltage is switched on, the control logic (1') gives a sensing pulse to the driver (3') to check the output for an overload or shortcircuit condition and prevents the driver from receiving a control signal during such a condition, and that the duration of the overload or short-circuit condition is monitored by means of sensing pulses issued via the control logic (1') to the driver (3') at regular intervals, the driver (3') being allowed to continue operating in accordance with the input as soon as the overload or short-circuit condition has disappeared.

3. Procedure according to claim 1 or 2, c h a r a c t e r i z e d in that the reference value fed into the output driver is reduced by limiting the control current flowing into its base in a short-circuit or overload situation by an arrangement which provides an alternative path, besides the path through the output driver, for the control current, at least part of said current being diverted in an overload or short-corcuit situation into said alternative path.

4. Procedure according to any one of the claims 1 - 3, c h a r a c t e r i z e d in that the current in the alternative path flows through a switch (Q11;Q11') controlled by the collector or emitter voltage of the output driver.

5. Device for overload and short-circuit protection of output drivers, designed for implementing the procedure of claim 1, c h a r a c-t e r i z e d in that, for each driver (3'), the device has a control logic (1') for controlling the driver by means of a reference value (+Vd;+Vd') set independently of external control (11-17; Ii'-17') and obtained from a reference voltage unit (2'), an overload and short-circuit monitoring circuit (4') which prevents the flow of control current to the driver during an overload or short-circuit condition, and an oscillator (5') which monitors the duration of the short-circuit or overload condition by means of sensing pulses given to the driver via the control logic.

6. Device according to claim 5, c h a r a c t e r i z e d in that, when the input voltage is switched on, the control logic issues a sensing pulse to detect whether an overload or short-circuit condition prevails.

7. Device according to claim 5 or 6, c h a r a c t e r i z e d in that the device is provided with controllable solid-state switches, of which, in a normal situation, the first switch (Q1) is turned on by preventing conduction by a second switch (Q11) when the input (I1-I7) is pulled up and a third switch (Q51) starts to conduct, turning on the first switch, so that a fourth switch (Q61) starts to conduct, turning on the output driver, and with a resistor (R11) which, together with the second switch, forms an overload and short-circuit monitoring circuit in which, when the output voltage (01-07) falls in an overload or short-circuit situation, said second switch starts to conduct, thus diverting the control current from the base of the output driver, and with an oscillator which is common to all drivers and issues sensing pulses at regular intervals, which pulses turn the first switch on, with the effect that the second switch ceases to conduct and the output driver receives its control current through the fourth switch (Q61).

8. Device according to claim 5 or 6, c h a r a c t e r i z e d in that the device is provided with controllable solid-state switches, of which, when the input voltage is switched on in a normal situation, the first switch (Q1') is turned on, thereby preventing conduction by the second switch (oil'), so that the control current is allowed to flow into the base of the output driver while its collector voltage or equivalent falls below a certain level and the driver remains in the conducting state, whereas in an overload or short-circuit situation the second switch (Q11') is turned on, which has the effect that the control current is passed to ground via the second switch, so that the output driver receives no control current and therefore remains shut off, and that the overload and short-circuit monitoring circuit of the device consists of a resistor (R11') and the second switch (Q11'), and that the pulses fed into the drivers at regular intervals by the common oscillator serve to check whether the short-circuit or overload condition has disappeared, so that when e.g. a short-circuit condition disappears, the oscillator pulse will cause the driver collector voltage or equivalent to fall, in which case the second switch (Q11') receives no control current and is therefore turned off, so that the control current is allowed to flow into the base of the driver, thus keeping it in the conducting state.