JP2013128343A - Overcurrent protection circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an overcurrent protection circuit capable of blocking a current before a fuse blows at the time of overcurrent by easily simulating fuse characteristics without performing temperature estimation by square calculation.SOLUTION: An overcurrent protection circuit includes: a switch element 9 interposed between a power-supply device 20 and a load 8; branch circuits 10 and 11 passing a current according to a flowing current of the switch element 9 through a resistor 17; voltage detection means 2 detecting a voltage value related to a voltage across the resistor 17; and means 2 determining whether or not the detected voltage value is higher than a predetermined value. The overcurrent protection circuit turns off the switch element 9 when the detected voltage value is higher than the predetermined value. The overcurrent protection circuit further includes: an integration circuit having a second resistor 16 and a capacitor 18 and integrating a voltage across the resistor 17; and a series circuit connected in parallel to the second resistor 16 and in which a diode 13 forward-connected from the resistor 17 side, a reverse-connected Zener diode 14, and a third resistor 15 are connected. The voltage detection means 2 detects a voltage value across the capacitor 18.

Description

  The present invention includes a switch element interposed between a power supply device and a load, and a branch circuit that allows a current corresponding to the current flowing through the switch element to flow through the resistor, and a voltage value related to the voltage across the resistor is obtained. The present invention relates to an overcurrent protection circuit that detects and turns off a switch element when the detected voltage value is higher than a predetermined voltage value.

When operating a load mounted on a vehicle, a motor, a lamp, or the like may be operated using a switch, a relay, or the like. At this time, a fuse for protecting the electric wire is provided in the uppermost stream on the power source side of the electric wire connecting them.
In such a case, the motor may lock and a large current may flow. Even in such a case, in order to prevent the fuse and the electric wire from fusing, it is necessary to match the lock current. The capacity of the fuse and the diameter of the electric wire are determined so as not to generate smoke.

  Patent Document 1 discloses an overheat protection device that protects an electric wire connected to a load from overheating. The overheat protection device stores a charge according to a current value and a time when the current flows, a comparison unit that compares the first signal having a magnitude proportional to the load current and the second signal that periodically changes, In addition, there is provided a thermal equivalent circuit that discharges the accumulated electric charge with time when the current stops. In addition, when the first signal exceeds the second signal, the switching means for supplying a current having a magnitude proportional to the load current to the thermal equivalent circuit is compared with the first voltage generated in the thermal equivalent circuit and a predetermined threshold voltage. And an overheat cutoff means for stopping the power supply to the load when the first voltage exceeds the threshold voltage. The second signal has a periodic waveform in which the time during which the first signal exceeds the second signal is n times when the magnitude of the first signal is n times.

  In Patent Document 2, a semiconductor switch connected between a power source and a load, a current detection unit that detects a current flowing in an electric wire between the semiconductor switch and the load, and a current value detected by the current detection unit, There is disclosed a power supply device including a current conversion unit that calculates and outputs a square current value whose magnitude corresponds to the square of the current value. The power supply device further includes a heat equivalent means for inputting the square current value and outputting a temperature rise equivalent value corresponding to the magnitude of the temperature rise of the semiconductor switch or a temperature equivalent value corresponding to the temperature, and a temperature rise And an abnormality determining unit that determines that an abnormality occurs when the equivalent value or the temperature equivalent value exceeds a first abnormality determination value that is set based on a limit temperature of the semiconductor switch. The semiconductor switch driving means turns off the semiconductor switch when it is determined as abnormal by the abnormality determining means.

JP 2011-182544 A JP 2009-142146 A

As described above, conventionally, the capacity of the fuse and the diameter of the electric wire are determined in accordance with the lock current, but since it is adjusted to the lock current, compared to the current that constantly flows to the motor, The capacity and diameter are set with a margin. For this reason, a fuse function or the like for cutting off the current before smoke and fusing occur by estimating the temperature of the fuse and the electric wire from the value of the flowing current and the like is considered.
For example, the current value flowing through is monitored to estimate the temperature of the fuse and the electric wire, etc., and the fuse and electric wire are protected by cutting off the current with a relay, switch or the like before smoking or blowing.

However, in order to estimate the temperature of the electric wire and fuse, it is necessary to calculate the square of the current value, such as the square of the current value × time, and a microcomputer is necessary to realize this square. And there are problems such as an increase in circuit scale.
For example, in the invention disclosed in Patent Document 1 described above, a characteristic that approximates the square characteristic is realized without using a microcomputer or a square arithmetic circuit, but a circuit such as a triangular wave generator is required. is there. In the invention disclosed in Patent Document 2, the electric wire temperature is estimated without using a microcomputer, but a square arithmetic circuit is required.

  The present invention has been made in view of the circumstances as described above, and by simply simulating the fuse characteristics without performing temperature estimation using a square operation, the fuse is blown at the time of overcurrent. Another object of the present invention is to provide an overcurrent protection circuit capable of interrupting current.

  An overcurrent protection circuit according to a first aspect of the present invention includes a switch element interposed between a power supply device and a load, a branch circuit that allows a current corresponding to the current flowing through the switch element to flow through the resistor, and both ends of the resistor. A voltage detection unit that detects a voltage value related to the voltage; and a unit that determines whether or not the voltage value detected by the voltage detection unit is higher than a predetermined voltage value, and determines that the unit is higher than the predetermined voltage value. In the overcurrent protection circuit configured to turn off the switch element, an integration circuit having a second resistor and a capacitor, integrating the voltage across the resistor, and in parallel with the second resistor And a series circuit to which a diode connected in order from the resistor side, a Zener diode connected in reverse, and a third resistor are connected, and the voltage detecting means detects a voltage value across the capacitor. Characterized in that are configured to.

  In this overcurrent protection circuit, the switch element is interposed between the power supply device and the load, and the branch circuit allows a current corresponding to the current flowing through the switch element to flow through the resistor. The voltage detection means detects a voltage value related to the voltage across the resistor and the determination means determines whether the detected voltage value is higher than a predetermined voltage value, and the determination means is higher than the predetermined voltage value. Is determined, the switch element is turned off. An integrating circuit having a second resistor and a capacitor integrates the voltage across the resistor, and a series circuit in which a diode connected in order from the resistor side, a Zener diode connected in reverse, and a third resistor are connected to the second resistor. Connected in parallel. The voltage detection means detects the voltage value across the capacitor.

  In the overcurrent protection circuit according to a second aspect of the invention, the branch circuit is connected in parallel to the series circuit of the switch element and the load, is connected in series to the resistor, and is turned on / off by the same signal as the switch element. Second switch element, an on-resistance variable circuit that is interposed between the second switch element and the resistor and has a variable on-resistance, and the potential of the load-side terminal of the switch element and the resistance side of the second switch element A potential difference detection circuit for detecting a potential difference between the terminals, and by increasing / decreasing the ON resistance of the ON resistance variable circuit based on the potential difference detected by the potential difference detection circuit, the potential of the load side terminal is increased to the resistance side And a matching circuit for matching the potentials of the terminals.

  In this overcurrent protection circuit, the branch circuit is connected in parallel to the series circuit of the switch element and the load, and the second switch element is connected in series to the resistor and is turned on / off by the same signal as the switch element. . In the matching circuit, an on-resistance variable circuit having a variable on-resistance is interposed between the second switch element and the resistor, and a potential difference detection circuit is configured to detect the potential of the load side terminal of the switch element and the resistance side of the second switch element. Detects the potential difference between terminals. The on-resistance of the variable on-resistance circuit is controlled to increase or decrease based on the potential difference detected by the potential difference detection circuit, so that the potential of the resistance-side terminal matches the potential of the load-side terminal.

  According to the overcurrent protection circuit of the present invention, the current is interrupted before the fuse is blown at the time of overcurrent by simply simulating the fuse characteristics without performing temperature estimation using a square operation. It is possible to realize an overcurrent protection circuit that can be used.

It is a block diagram which shows the structure of embodiment of the overcurrent protection circuit which concerns on this invention. It is a characteristic view which shows the fusing characteristic of a fuse. FIG. 6 is a characteristic diagram illustrating an example of characteristics of an integration circuit in which an RC-only integration circuit and a nonlinear circuit A are connected in comparison with an example of fuse blowing characteristics.

Hereinafter, the present invention will be described with reference to the drawings illustrating embodiments thereof.
FIG. 1 is a block diagram showing a configuration of an embodiment of an overcurrent protection circuit according to the present invention.
This overcurrent protection circuit supplies power to, for example, a vehicle power window motor that is a load 8, and includes a fuse 7, an N-channel MOS (Metal Oxide Semiconductor) FET (Field Effect Transistor) 9, a current sensor circuit 6, A current processing circuit 5, a control circuit 1, a determination circuit 2, and a constant voltage source 19 are provided.

A drain of the FET 9 is connected to the power supply line from the battery (power supply device) 20 through the fuse 7, and a load 8 is connected between the source of the FET 9 and the ground line.
The power supply line is connected to the drain of an N-channel MOS FET 10 through a fuse 7, and the drain of an N-channel MOS FET (ON-resistance variable circuit) 11 is connected to the source of the FET 10. A resistor 17 is connected between the source of the FET 11 and the ground line.

The output terminal of the operational amplifier (potential difference detection circuit) 12 is connected to the gate of the FET 11, the source of the FET 9 is connected to the inverting input terminal of the operational amplifier 12, and the source of the FET 10 is connected to the non-inverting input terminal. The operational amplifier 12 is supplied with power by a power circuit (not shown).
The FET 11 and the operational amplifier 12 constitute a matching circuit, and the FETs 10, 11 and the operational amplifier 12 constitute a current sensor circuit 6.

Between the source of the FET 11 and the ground line, an integrating circuit that includes a resistor 16 and a capacitor 18 and integrates the voltage across the resistor 17 is connected. A non-linear circuit A in which a forward-connected diode 13, a reverse-connected Zener diode 14, and a resistor 15 are connected in series is connected in parallel to the resistor 16 of the integrating circuit from the source side of the FET 11.
The resistors 16 and 17, the capacitor 18, and the non-linear circuit A constitute the current processing circuit 5.

  A terminal voltage (a voltage between both ends) of the capacitor 18 of the integrating circuit is supplied to a determination circuit (voltage detection means, determination means) 2, and a constant voltage from a constant voltage source 19 is also supplied to the determination circuit 2. The determination circuit 2 determines whether the constant voltage value (predetermined voltage value) and the terminal voltage value of the capacitor 18 are high or low, and when determining that the terminal voltage value of the capacitor 18 is higher than the constant voltage value, supplies power to the load 8. The control circuit 1 is supplied with a shut-off signal instructing shut-off.

  The control circuit 1 performs on / off control of the FETs 9 and 10 with the same signal based on a control signal from the outside, and turns off the FETs 9 and 10 when a cutoff signal is given from the determination circuit 2. The control circuit 1 can also perform PWM (Pulse Width Modulation) control of the FETs 9 and 10 with a duty ratio determined based on a control signal from the outside.

The operation of the overcurrent protection circuit having such a configuration will be described below.
When receiving a control signal for turning on the load 8 from the outside, the control circuit 1 turns on the FETs 9 and 10 with the same signal.
The operational amplifier 12 detects the potential difference between the sources of the FETs 9 and 10, and when the potential of the source of the FET 10 is higher, the on-resistance of the FET 11 is reduced by an output corresponding to the potential difference. As a result, the current flowing through the FET 10, the FET 11, and the current processing circuit 5 (branch circuit) increases, and the potential of the source of the FET 10 becomes lower due to the voltage drop of the FET 10.

When the potential of the source of the FET 10 is lower, the operational amplifier 12 increases the on-resistance of the FET 11 by an output corresponding to the potential difference. As a result, the current flowing through the FET 10, FET 11, and current processing circuit 5 decreases, and the potential of the source of the FET 10 becomes higher due to the voltage drop of the FET 10.
The operational amplifier 12 repeatedly converges this series of operations at high speed, so that the source potential of the FET 10 is instantaneously substantially matched with the source potential of the FET 9. At this time, the ratio of the on-resistance value of the FET 10 to the sum of the on-resistance value of the FET 11 and the combined resistance value of the current processing circuit 5 matches the ratio of the on-resistance value of the FET 9 and the resistance value of the load 8. The current Is flowing through the processing circuit 5 is a current corresponding to the current IL flowing through the load 8, and Is = IL / k (k is a constant).

FIG. 2 is a characteristic diagram showing the fusing characteristics of the fuse 7, and the fuse fusing characteristics indicate the continuous flow time (several hundred seconds to several seconds) when the fuse 7 is blown for each current value. The current value here is the current value IL flowing through the load 8 and is measured by the voltage value across the resistor 17 of the branch circuit.
Here, the fuse blowing characteristics shown in FIG. 2 are simulated by an integration circuit using only R (resistance) and C (capacitance) like the resistor 16 and the capacitor 18, and the current value is measured by the voltage value across the capacitor 18. Try. In this case, if the time constant RC is relatively large, the continuous conduction time that reaches the current value (the voltage value across the resistor 17) indicated by the fuse blowing characteristic becomes too long, and the fuse value is measured before the current value is measured. 7 will blow out.

Therefore, if the time constant RC of the integrating circuit is made relatively small, the continuous conduction time to reach the current value (voltage value across the resistor 17) indicated by the fuse blowing characteristic is shortened, which is much earlier than the fuse 7 is blown. The current value is measured, and the FETs 9 and 10 are turned off.
From the above, in this overcurrent protection circuit, when the current value is small, the above-described time constant RC becomes a characteristic of an integration circuit that is relatively small, and when the current value is larger than a predetermined value, it is connected in parallel to the resistor 16. Due to the non-linear circuit A, the capacitor 18 is rapidly charged, and the voltage at both ends rises quickly.

Thereby, the characteristic of the current processing circuit 5 (measured by the voltage across the capacitor 18) becomes the characteristic of the RC + A circuit in FIG. That is, in the required current value section, an integration circuit having a small time constant RC can be approximated to the fusing characteristics of the fuse, and a characteristic that can turn off the FETs 9 and 10 earlier than the fusing of the fuse is obtained.
Here, in FIG. 3, another example of the characteristics of the RC-only integration circuit (without A) and the non-linear circuit A are connected in comparison with the example (min, max) of the fuse blowing characteristics with a short and long fusing time. Another example (with A) of the characteristics of the two integration circuits having large and small time constant RC is shown.

FIG. 2 also shows an example of the operating current (lock current) that actually flows through the load 8. In the vicinity of the current value Ith for interrupting the operating current, the closest to the fusing characteristics of the fuse is shown. The characteristics of the CR + A circuit are adjusted.
The constant voltage source 19 outputs a constant voltage value Vth (= Ish × resistance value of the resistor 17) corresponding to the current value Ish of the branch circuit corresponding to the current value Ith. When the determination circuit 2 determines that the supplied terminal voltage of the capacitor 18 is higher than the constant voltage value Vth, the determination circuit 2 supplies a cut-off signal instructing cut-off of power supply to the load 8 to the control circuit 1.

In the case of the operating current shown in FIG. 2, since there is a period longer than the current value Ith, the terminal voltage of the capacitor 18 reaches the constant voltage value Vth earlier than the time shown in FIG.
The control circuit 1 turns off the FETs 9 and 10 when given a cutoff signal. The control circuit 1 returns to the normal control operation when a sufficient time has passed to cool the fuse 7 and the electric wire after turning off the FETs 9 and 10.

DESCRIPTION OF SYMBOLS 1 Control circuit 2 Determination circuit (Voltage detection means, determination means)
5 Current processing circuit 6 Current sensor circuit 7 Fuse 8 Load 9, 10 FET
11 FET (ON-resistance variable circuit)
12 Operational amplifier (potential difference detection circuit)
13 Diode 14 Zener Diode 15, 16, 17 Resistance 18 Capacitor 19 Constant Voltage Source 20 Battery (Power Supply Device)
A Non-linear circuit

Claims (2)

A switch element interposed between the power supply device and the load, a branch circuit that allows a current corresponding to the current flowing through the switch element to flow through the resistor, and a voltage detection that detects a voltage value related to the voltage across the resistor And a means for determining whether or not the voltage value detected by the voltage detection means is higher than a predetermined voltage value, and when the means determines that the voltage value is higher than the predetermined voltage value, the switch element is turned off. In the overcurrent protection circuit configured as follows,
An integrating circuit having a second resistor and a capacitor, integrating the voltage across the resistor, a diode connected in parallel to the second resistor, forwardly connected from the resistor side, a reversely connected Zener diode, and a third An overcurrent protection circuit comprising: a series circuit to which a resistor is connected; and the voltage detection means is configured to detect a voltage value across the capacitor.   The branch circuit is connected in parallel to a series circuit of the switch element and the load, is connected in series to the resistor, and is turned on / off by the same signal as the switch element, and the second switch element An on-resistance variable circuit interposed between the switch element and the resistor and having a variable on-resistance, and a potential difference detection circuit that detects a difference between the potential of the load-side terminal of the switch element and the potential of the resistance-side terminal of the second switch element And a matching circuit for matching the potential of the resistance side terminal with the potential of the load side terminal by increasing / decreasing the on resistance of the variable resistance circuit based on the potential difference detected by the potential difference detection circuit. The overcurrent protection circuit according to claim 1.

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