Overcurrent protection circuit and method

Abstract: In an overcurrent protection circuit, when a control section (7) turns on a switch (3) in response to a signal from a microcomputer (5) so that power is supplied from a power source (1) to an IC card (4), a signal controlling section (6) prevents the overcurrent protection circuit from functioning even if a current greater than a predetermined level is detected by a current detecting section (2), until a voltage of the IC card is stabilized. When the control section (7) turns off the switch (3) in response to a signal from the microcomputer (5), power supply from the power source (1) to the IC card (4) is stopped, thereby preventing a malfunction of the overcurrent protection circuit, even if the microcomputer (5) goes out of control or malfunctions.

Description: The present invention relates to an overcurrent protection circuit and an overcurrent protection method for protecting an electric or electronic device, such as an IC (Integrated Circuit) incorporated in an IC card, from an overcurrent due to abnormality in the circuits of the device. An electric or electronic device, such as an IC card, incorporates an overcurrent protection circuit for protecting the device from overcurrent due to abnormality in the circuits of the device. Conventionally, a type of overcurrent protection circuit in an electric or electronic device is formed of a combination of a microcomputer and a logical circuit for controlling operations of the circuits in the device. Such an overcurrent protection circuit having a microcomputer and a logical circuit may malfunction when the microcomputer goes out of control or malfunctions or power is supplied thereto. DE 38 42 921 A1 discloses a circuit according to the preamble of claim 1, wherein the predetermined detection level is raised at the initial power supply of the load. US 4,819,117 and DE 43 16 185 A1 each disclose an overcurrent protection circuit, wherein a predetermined overcurrent detection level is adapted at the initial power supply of the load. It is an object of the present invention to provide an overcurrent protection circuit and method preventing the circuit from malfunctioning when the microcomputer gets out of control or power is first supplied to the circuit. This object is achieved by a circuit according to claim 1 and a method according to claim 4. Further developments of the invention are given in the dependent claims. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: FIG. 1 is a block diagram showing a schematic structure of an overcurrent protection circuit according to the present invention; FIG. 2 is a circuit diagram of the overcurrent protection circuit of the present invention; and FIG. 3 is a diagram for explaining true values of the flip-flop circuit shown in FIG. 2. An embodiment of the present invention will be described with reference to the accompanying drawings. FIG. 1 shows a schematic structure of an overcurrent protection circuit according to the present invention. The overcurrent protection circuit comprises a power source 1 serving as power supplying means, a current detecting section 2 serving as current detecting means, a switch 3 serving as switching means, a load 4 such as an electric or electronic device, a microcomputer 5, a signal controlling section 6 and a control section 7. The load 4 is, for example, an IC incorporated in an IC card. Power is supplied to the IC card connected to the circuit of the present invention provided in an IC card reader/writer. In the following description, the load is referred to as the IC card 4. The control section 7 turns on the switch 3 in response to a signal supplied from the microcomputer 5, so that power can be supplied to the IC card 4. After the power is supplied, the current detecting section 2 detects a current flowing through the IC card 4. The control section 7 determines whether the amount of the current is greater or smaller than a preset current, and controls ON/OFF of the switch 3. The signal controlling section 6 operates so that the control section 7 may not be influenced, even if the microcomputer outputs an abnormal signal, when it goes out of control or malfunctions. FIG. 2 shows a structure of the overcurrent protection circuit. The overcurrent protection circuit has the elements described above with reference to FIG. 1. The current detecting section 2 comprises a current detecting resistor R1, reference voltage resistors R2 and R3, and a voltage comparator IC1. The switch 3 comprises a power cutoff transistor Q1. The computer 5 outputs an output 1 through a terminal 9 and an output 2 through a terminal 10. The signal controlling section 6 comprises a transistor Q3, a power limiting resistor R4, a differential circuit 8 having a resistor R5 and a capacitor C1, an overvoltage protecting diode D1, and a buffer IC4. The control section 7 comprises a transistor Q2, a flip-flop circuit IC2, an inverter IC3 and an adjusting resistor R6. The power limiting resistor R4 limits a charge current supplied to the capacitor C1, when a low pulse of the output 1 from the terminal 9 rises in the microcomputer 5. The overvoltage protecting diode D1 prevents an overvoltage of a terminal R of the flip-flop circuit IC2, when the capacitor C1 is charged. An operation of the overcurrent protection circuit shown in FIG. 2 will now be described. First, the microcomputer 5 causes the output 2 from the terminal 10 to be high (Hi). As a result, the base terminal of the transistor Q3 becomes high in level, so that the transistor Q3 is turned on. In the voltage comparator IC1, the negative terminal becomes low (Lo) in level, since the transistor Q3 is on, and the level of the output voltage is high. Since the output voltage of the voltage comparator IC1 is high, a terminal PR of the flip-flop circuit IC2 is high in level. Further, the microcomputer 5 causes the output 1 from the terminal 9 to be low, causing a terminal R of the flip-flop IC2 to be low via the buffer IC4. In the flip-flop circuit IC2, since the terminal PR is high in level and the terminal R is low in level, the terminal Q is low. The inverter IC3 inverts a low signal output from the terminal Q to a high signal, which is input to the base terminal of the transistor Q2. As a result, the transistor Q2 is turned on. When the transistor Q2 is on, the transistor Q1 is also on, so that power is supplied from the power source 1 to the IC card 4. In this embodiment, the microcomputer 5 causes the negative terminal of the voltage comparator IC1 to be low in level, while it is keeping the output 2 from the terminal 10 at the high level. Therefore, the current detecting section 2 does not work until the capacitor (not shown) of the IC card 4 is charged. Thus, the overcurrent protection circuit is prevented from malfunctioning. After the voltage of the IC card 4 is stabilized, the microcomputer 5 causes the output 2 from the terminal 10 to be low in level, so that a voltage is applied to the negative terminal of the voltage comparator IC1 through the reference voltage resistors R2 and R3. In the voltage comparator IC1 for comparing voltages, the voltage at the positive terminal is varied by a voltage drop due to a current flowing through the current detecting resistor R1. If the IC card 4 disorders, when the current flowing through the current detecting resistor R1 is increased and the voltage of the positive terminal of the voltage comparator IC1 becomes smaller than that of the negative terminal thereof, the voltage comparator IC1 outputs a voltage of low level, which causes the PR terminal of the flip-flop circuit IC2 to be low in level. As a result, since the terminal PR and the terminal R of the flip-flop circuit IC2 are low in level, the terminal Q thereof is high in level. The inverter IC3 inverts a high signal output from the terminal Q to a low signal, which is input to the base terminal of the transistor Q2, thereby turning off the transistor Q2. When the transistor Q2 is off, the transistor Q1 is also off, with the result that power supply from the power source 1 to the IC card 4 is stopped. Thus, if the IC card 4 disorders, the power supply from the power source 1 to the IC card 4 is stopped by the transistor Q1. FIG. 3 shows true values of the flip-flop circuit IC2 described above. For example, in the flip-flop circuit IC2, when the terminal R (Reset) is high and the terminal PR (Preset) is low, the terminal Q is high. The inverter IC3 inverts a high signal output from the terminal Q to a low signal, which is input to the base terminal of the transistor Q2, thereby turning off the transistor Q2. When the transistor Q2 is off, the transistor Q1 is also off, with the result that power supply from the power source 1 to the IC card 4 is stopped. When the power supply to the IC card 4 is stopped, the amount of current flowing through the current detecting resistor R1 is reduced. Accordingly, the voltage at the positive terminal of the voltage compactor IC1 is increased and becomes higher than the voltage at the negative terminal thereof. Therefore, the voltage comparator IC1 outputs a voltage of high level, which is input to the terminal PR of the flip-flop circuit IC2. However, in the flip-flop circuit IC2, even if a signal input to the terminal PR is changed, the state of the terminal Q remains unchanged, unless a low signal is input to the terminal R. Therefore, the power supply of the IC card 4 is kept stopped. At this time, if the microcomputer 5 goes out of control or malfunctions, the output 1 from the terminal 9 is a low signal. However, since only low pulses of the output 1 from the terminal 9 are input through the differential circuit 8 to the terminal R of the flip-flop circuit IC2, the overcurrent protection circuit is prevented from malfunctioning. As described above, according to the embodiment of the present invention, the overcurrent protection circuit is prevented from malfunctioning when power is supplied to the IC card (load) and from stopping when the microcomputer goes out of control.

Claims: An overcurrent protection circuit, comprising power supply means (1) for supplying power, switch means (3) for switching power supply and stopping the power supply to a load (4) connected to the power supply means; current detecting means (2) for detecting whether a current exceeds a predetermined detection level when power is supplied from the power supply means to the load, stop means (7) for stopping the power supply by switching the switch means when the current detecting means detects that the current exceeds the predetermined detection level, and a microcomputer (5) for outputting an instruction signal for switching the switch means, characterized by the load being an IC card (4), the microcomputer (5) being adapted for outputting first and second instruction signals (OUTPUT 2, OUTPUT 1) for switching the switch means, first control means (6, Q3) for controlling the operation of the current detecting means to be stopped until a load voltage of the IC card is stabilized, when the power is supplied from the power supply means to the IC card by switching the switch means in accordance with the first instruction signal (OUTPUT 2) outputted from the microcomputer; and second control means (6, IC4, 8, D1) for controlling the operation of the stop means to keep the power supply stopped unless, in use of malfunction of Failure of the microcomputer, the second instruction signal (OUTPUT 1) is output from the microcomputer power supply and therefore can be stopped by the switching means in accordance with a signal outputted form the current detecting means (2) indicating a detection of a current exceeding the predetermined detection level to prevent a malfunction due to a false signal from the microcomputer. The overcurrent protection circuit according to claim 1, characterized in that the first control means (6) is adapted to stop the operation of the current detecting when the power is first supplied from the power supply means to the IC card by switching the switch means in accordance with a first instruction signal outputted from the microcomputer and it is adapted to control the current detecting means to operate at the predetermined detection level in accordance with a second instruction signal outputted from the microcomputer when the load voltage of the IC card supplied with the power is stabilized. The overcurrent protection circuit according to claim 1 or 2, characterized in that the second control means (6) comprises a differential circuit (8) for keeping the power supply from the power supply means to the IC card stopped unless a specific instruction signal (low pulse) is output from the microcomputer. A method for controlling an overcurrent protection circuit for an IC card reader/writer according to one of claims 1 to 3, the method comprising stopping the operation of the current detecting means when the power is first supplied from the power supply means to the IC card by switching the switch means in accordance with an instruction signal outputted from the microcomputer, and controlling the current detecting means to operate at the predetermined detection level in accordance with an instruction signal outputted from the microcomputer when the load voltage of the IC card supplied with the power is stabilized.

Inventors: KATSUMI TAKAHIRO, KAWAGISHI TOSHIYUKI, NOGUCHI MINORU, SUZUKI TOSHIYUKI, KATSUMI, TAKAHIRO, KAWAGISHI, TOSHIYUKI, NOGUCHI, MINORU, SUZUKI, TOSHIYUKI
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1998
Main classification: H02H3
IPC codes: H02H1, H02H3