Overload protection circuit

Abstract: The overload protection circuit 12 comprises a resistor bridge 20 with a low impedance sensing resistor r and high impedance resistors R1 to R4 which generates voltages Va and Vb at the inputs 32 and 34 of a comparator 22. Voltage Vb depend upon the value of the load current so that the comparator provides at its output 36 a control signal for opening the switching device 24 comprising a field effect transistor when the load current exceeds a maximum value. Resistor R5 reinforces the effect of the control signal. This circuit can be used in a system which comprises a central power supply unit 8 which powers a plurality of devices such as 4. The status of the circuit 12 is reported to the system 8 though line 14 and circuit 12 comprises a control circuit 26 which is responsive to set and reset control signals on lines 14 and 18 from the unit 8 to close or open the switch 26.

Description: The subject invention relates to an overload protection circuit and more particularly to such a circuit which can be used for protecting devices powered from a unique remote unit. Background Art Electronic circuits for protecting devices against overloads due to short circuits or any type of defaults already exist on the shelfs. Such a circuit having the features specified in the preamble of claim 1 is disclosed in DE-A-3 421 520. Circuits of this type should present a negligible resistance when in set status (close) in order to be able to protect devices powered from a unique remote unit. In addition there is a requirement at the remote powering unit to be aware of the status of the overload protection circuit in any device in order to take appropriate actions depending upon the reported status. Summary of the Invention Consequently, an object of the invention is to provide an overload protection circuit which presents a negligible resistance. Another object of the invention is to provide such an overload protection circuit which is simple and can be easily integrated in a chip. Another object of the invention is to provide such an overload protection circuit which is able to report its status to a remote unit and to be set or reset under control of this unit. The overload protection circuit according to the present invention is used for protecting a device powered from a power supply unit which provides a supply voltage and a load current to the device. This circuit is of the type described in DE-A-3 421 520, comprising : overload detecting means which includes: a resistor bridge having two legs connected in parallel between the supply voltage and a reference voltage and first and second output terminals, and generating a fixed voltage on the first output terminal and a variable voltage which depends upon the load current value on the second output terminal, a comparator having first and second inputs connected to the first and second output terminals of the resistor bridge, respectively, and an output, said comparator providing on its output a control signal which indicates whether an overload condition exists or not, solid state switch means having an input receiving the load current, an output connected to the device and a control input responsive to the control signal for closing or opening the switching means, whereby the load current is provided to the device or not, and a feedback means connected between the switching means and the resistor bridge to reinforce the effect of the control signal on the closing and opening of the switching means. The first leg of the resistor bridge comprising two high impedance resistors connected in series, and the second leg comprising a low-impedance sensing resistor connected in series with two high impedance resistors, the common points of the high impedance resistors in the first and second legs constituting the first and second output terminals of the resistor bridge. According to the subject invention the overload protection circuit is characterised in that the common point of the sensing resistor and high-impedance serially connected resistors in the second leg is connected to the input of the switching means, and the feedback means is connected between the output of the switching means and the second output terminal of the resistor bridge. According to a further characteristic of the present invention the control signal at the output of the comparator is indicative of the status of the switching means and is reported to the power supply means. In addition, the protection circuit can be used for protecting a device which is powered from a power supply means which comprises first and second control lines and provides on these line switching control signals for closing and opening the switching means, respectively. The overload protection circuit comprises a switching control means connected to the first and second control lines and responsive to the switching control signals on these lines to provide control voltages to the first and second inputs of the comparator, whereby the comparator generates closing or opening control signals on the control input of the switching means. The overload protection circuit according to the subject invention can be implemented in a system comprising a central power supply means which powers a plurality of devices, wherein each device is protected against overloads by an overload protection circuit so that an overload in any one of the devices does not impair the operation of the other devices. Brief Description of the Figures Figure 1 represents the block diagram of a system wherein the circuit according to the present invention can be implemented. Figure 2 represents a detailed schema of the circuit according to the present invention when the supply voltage is positive. Figure 3 represents a detailed schema of the circuit according to the present invention when the supply voltage is negative. Detailed Description of the Invention Figure 1 represents a system incorporating overload protection circuits according to the subject invention. This system is only shown for the sake of illustration and it will be obvious for the man skilled in the art to implement the overload protection circuit in any other system from the description of the overload protection circuit. The system shown in Figure 1 comprises a central unit 2, which can be any type of information processing unit or any other central control unit attached to remote devices 4-1 to 4-n through busses 6-1 to 6-n respectively. Busses 6-1 to 6-n carry the control and information signals which are useful for the operation of the remote devices, these signals are specific to the system and do not interact with the operation of the overload protection circuit of the present invention. The remote devices 4-1 to 4-n do not have their own power supply and are powered from the central power supply unit 8. Each remote device 4-1 to 4-n is powered from the power supply unit 8 through lines 10-1 to 10-n respectively. An overload protection circuit 12-1 to 12-n respectively, is associated to each remote device 4-1 to 4-n, to protect the device against overcurrents due to short circuits or any other failures. For example, the overload protection circuit 12-1 for device 4-1 is connected to line 10-1 and prevents the remote device 4-1 from being powered through line 11-1 if it detects an overcurrent due to a short circuit and it reports its status to the power supply system through line 14-1. The overload protection circuits can be set or reset by the power supply unit though lines 16-1 and 18-1. The powering lines 11-1 to 11-n from circuits 12-1 to 12-n to remote devices 4-1 to 4-n are partly dotted in Figure 1 to symbolize the fact that they can be quite long, more than 100 meters. In another implementation, the overload protection circuits 12-1 to 12-n can be close to the remote devices 4-1 to 4-n and lines 10, 14, 16, 18 can be quite long, and the provision of an overload protection circuit associated to each device allows the operation of the system not to be disturbed by an overload due to a short circuit in any device, since when a short circuit occurs in one device, the device is isolated from the other ones, from a power standpoint. The overload protection circuit 12 for any device 4, according to the subject invention is shown in Figure 2. Its major components are a resistor bridge 20, a comparator 22, a switch 24, a control circuit 26 and a feedback path 28. The power supply system 8 generates at node 30 in the resistor bridge a voltage V. In the implementation shown in Figure 2, the voltage V is positive. One leg of the resistor bridge 20 comprises a low value sensing resistor r in series with resistors R1 and R2 and the other leg comprises resistor R3 and R4. Resistor r, R1 and R2 in the first leg are serially mounted between node 30 and node 40 which is connected to the ground. Resistors R3 and R4 in the second leg are serially mounted between nodes 30 and 40. The common point A of resistors R3 and R4 is connected to the plus input 32 of comparator 22 and the common point B of resistors R1 and R2 is connected to the minus input 34 of comparator 22. The signal which is generated at the output of comparator 22 as will be described later on, is provided to the switch 24 in order to control its status, thus the status of the overload protection circuit is indicated by the signal at the output of comparator 22 which is connected to STATUS line 14. In a preferred embodiment of the invention, the switch comprises a field effect transistor FET which presents a very low resistance when closed. In the circuit of Figure 2, the transistor FET is of the P enhancement type. The source is connected to the common point 38 between resistor r and resistor R1, the drain is connected node 42 which is connected to the remote device 14. When the switch is closed a current I (load) is provided by overload protection circuit to the remote device 4 which is thus powered. The feedback path comprises resistor R5 which is connected between node 42 and node B. The control circuit 26 comprises two switches 44 and 46 which are operated by signals provided by the power supply unit 8th trough lines 14 and 18 to respectively set and reset the overload protection circuit, as will be described later on. The circuit shown in Figure 2 operates as follows. Resistor r is a sensing resistor which has a low value. The supply current I (load) depends upon the supply voltage +V and the load in the remote device. The resistor bridge generates a fixed voltage which is derived from +V at node A and a variable voltage which is derived from +V-rIs, where Is is the current in sensing resistor at node B. The resistances of resistors R1, R2, R3 and R4 are chosen in order that when I (load) = 0, the voltage Va at node A is higher than the voltage Vb at node B, which causes the comparator to provide an output signal to switch OFF the field effect transistor 24. The field effect transistor 24 is switched ON through the power supply unit 8 which provides a SET signal on line 16 to close switch 44. Thus, voltage Va is lower than voltage Vb, and comparator 22 provides a zero level signal on its output 36, which switches ON field effect transistor 24. When field effect transistor 24 is switched ON, resistor R5 is parallely mounted with resistor R1. As far as Is remains lower than a threshold current It, Va is lower than Vb and the field effect transistor remains ON, When Is becomes higher than It, Va becomes higher than Vb and the field effect transistor switches OFF, and the remote device is protected against the overcurrents higher than It. Due to the feedback path arrangement comprising the resistor R5, when the field effect transistor 24 is switched ON, voltage Vb decreases and when the field effect transistor is switched OFF, voltage Vb increases. This results in an hysteresis effect which reinforces the control effect of the sense current Is upon the switching ON and OFF of the field effect transistor. Consequently, the sensing resistor can have a low value. In a preferred embodiment of the invention the resistors have the following values: R1 = 54.9 Kiloohms R2 = R3 = R4 = 49.9 Kiloohms R5 = 274 Kiloohms r = 1.6 ohms The capacitor C connected across nodes A and B prevents the overload protection circuit from being sensitive to transient overload conditions. Other locations for capacitor C will provide unequal switch on and switch off same constants. The functions of switches 44 and 46 can be implemented by any type of switching devices, such as field effect transistors or bipolar transistors. In a preferred embodiment of the invention this function is performed by TTL open-collector gates. Figure 3 represents the overload protection circuit when implemented to protect a remote device operating with a negative supply voltage -V. The components of this circuit are referenced by the reference numbers of Figure 2 added to 100, i.e. switch 124 in Figure 3 has the same function as switch 24 in Figure 2. In that implementation, switch 124 comprises a FET transistor of the N enhancement type. The resistors and capacitor in Figure 3 have the same references as in Figure 2 with a prime sign. The operation of circuit of Figure 3 is similar to the operation of the circuit of Figure 2, so that it is not described again. Only the operation of the control circuit 126 is different. Both switches 144 and 146 are closed in normal operation mode. They are open one at a time to set or reset the circuit which causes the voltages at node A′ or B′ to be changed by means of resistors 100 and 102 connected between a positive reference voltage +Vref and node 140.

Claims: An overload protection circuit for protecting a device (4) powered from a power supply means (8) providing to the device a supply voltage and a load current, of the type comprising : . overload detecting means (20, 22; 120, 122) including: a resistor bridge (20; 120) having two legs connected in parallel between the supply voltage and a reference voltage and first (A; A′) and second (B; B′) output terminals, and generating a fixed voltage on the first output terminal (A; A′) and a variable voltage which depends upon the load current value on the second output terminal (B; B′) a comparator (22; 122) having first and second inputs connected to the first and second output terminals of the resistor bridge, respectively, and an output (36), said comparator providing on its output a control signal which indicates whether an overload condition exists or not, solid state switching means (24; 124) having an input receiving the load current, an output connected to the device (4) and a control input responsive to the control signal on the output (36) of the comparator for closing or opening the switching means, whereby the load current is provided to the device or not, and a feedback means (R5; R′5) connected between the switching means (24; 124) and the resistor bridge (20; 120) to reinforce the effect of the control signal on the closing and opening of the switching means, the first leg of the resistor bridge (20; 120) comprising two high impedance resistors (R3, R4; R′3, R′4) connected in series, and the second leg comprising a low-impedance sensing resistor (r; r′) connected in series with two high impedance resistors (R1, R2; R′1, R′2), the common points of the high impedance resistors in the first and second legs constituting the first and second output terminals of the resistor bridge, characterised in that the common point of the sensing resistor (r; r′) and high-impedance serially connected resistors (R1, R2; R′1, R′2) in the second leg is connected to the input of the switching means (24; 124), and the feedback means (R5, R′5) is connected between the output of the switching means (24; 124) and the second output terminal (B; B′) of the resistor bridge. An overcurrent protection circuit according to claim 1, characterized in that the switching means (24; 124) comprises a field effect transistor the first electrode of which receives the load current, the second electrode of which is connected to the device and the gate electrode of which is connected to the output of the comparator. An overload protection circuit according to claim 2 characterized in that the supply voltage is positive with respect to the reference voltage and in that the field effect transistor is of the P enhancement type. An overload protection circuit according to claim 2 characterized in that supply voltage is negative with respect to the reference voltage and the field effect transistor is of the N enhancement type. An overload protection circuit according to any one of claims 1 to 4 characterized in that the feedback means (R5; R′5) comprises a high-impedance resistor connected between the output of the switching means (24; 124) and the second output terminal (B; B′) of the resistor bridge. An overload protection circuit according to any one of claims 1 to 5 characterized in that the control signal at the output (36) of the comparator (22; 122) is indicative of the status of the switching means (24; 124) and is reported to the power supply means (8). An overload protection circuit according to any one of claims 1 to 6 for protecting a device (4) which is powered from a power supply means (8) which comprises first and second control lines (16, 18) and provides on these lines switching control signals for closing and opening the switching means (24; 124), respectively, said overload protection circuit being characterized in that it further comprises: switching control means (26; 126) connected to the first and second control lines (16; 18) and responsive to the switching control signals on these lines to provide to the first and second inputs of the comparator (22; 122) control voltages whereby the comparator generates closing or opening control signals on the control input of the switching means (24; 124). A system comprising a central power supply means (8) which powers a plurality of devices characterized in that each device is protected against overloads by an overload protection circuit according to any one of claims 1 to 7.

Inventors: FERMIER YVAN, FERRY MICHEL, JACQUART CHRISTIAN, VACHEE PIERRE, FERMIER, YVAN, FERRY, MICHEL, JACQUART, CHRISTIAN, VACHEE, PIERRE
Applicants: IBM, INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1990
Main classification: H02H3
IPC codes: H02H3, G06F1, H02J13