JPH077847A - Overcurrent protecting circuit - Google Patents

Abstract

PURPOSE:To simplify the selection of fuses and to protect overcurrent without changing the characteristics of a circuit by detecting the voltage drop by overcurrent of a semiconductor switch for turning on/off the power output line of a power circuit having a control terminal, and stopping the operation of the power circuit. CONSTITUTION:The saturated on-resistance of an FET 9 is considered to be 0.3OMEGA for example, and maximum load current is to be 1A for convenience. On this occasion, when voltage is supplied from the output of the FET 9, and the FET 9 and a transistor(Tr) 10 are on, and the output current of the FET 9 becomes 3A for example, a voltage drop of 0.9V is generated in the FET 9. Since a Tr 11 is turned on and gets a saturation voltage of about 0.1V at this time, a voltage of about 0.8V is applied between the base and the emitter of a Tr 12. Accordingly, the Tr 12 can be turned on sufficiently, and VCC1 of a PWM-IC 1 is the output voltage of a switching circuit or more. Consequently, a Tr 23 is turned on by the drive of the base of a Tr 14. As the result of this, the operation of the Ic 1 is stopped, and the FET 9 is turned off.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an overcurrent protection circuit.

[0002]

2. Description of the Related Art FIG. 3 shows an example of a conventional technique and is an example of a power supply circuit using a voltage regulator. As an overcurrent protection element, a fuse 19 is provided on the primary circuit side of the power supply circuit, that is, the circuit side before the voltage regulator, and an overcurrent detection resistor 16 is provided on the secondary circuit side, that is, after the voltage regulator 15. The fuse 19 is not blown by the steady output current of the power supply circuit, of course, does not blow even at the maximum load or inrush current, has a long life, and is blown when an abnormality such as a circuit short circuit occurs. Overcurrent detection resistor 16
Is set so that the voltage drop due to overcurrent has a value that can be recognized by the voltage regulator 15. For example, if the voltage regulator 15 determines that there is an abnormality when the voltage drop across the overcurrent detection resistor 16 is 1 V or more, the overcurrent detection resistor is 0.33Ω if an abnormality occurs when a current of 3 A flows. An overcurrent protection circuit will be described using a conventional power supply circuit.

The input voltage is applied to the input terminal IN of the voltage regulator 15 through the fuse 19. The voltage regulator steps down or boosts and outputs an arbitrary output voltage to the output terminal O.
It can be output from the UT. The output voltage is applied to the external load 1 by the semiconductor switch 17 via the overcurrent detection resistor 16.
8 are supplied. The semiconductor switch is turned on / off by an external input. F as a semiconductor switch
ET and transistors are usually used. The output is supplied to the external load 18 when the semiconductor switch 17 is turned on, and the voltage obtained by subtracting the voltage drop due to the overcurrent detection resistor 16 is supplied to the external load 18 as the output voltage. This can be read by the input terminal IPK of the voltage regulator 15, and the output can be stopped when the voltage difference between the output terminal OUT and the input terminal IPK reaches the voltage determined inside the voltage regulator 15. The voltage at which the output can be turned off is fixed inside the voltage regulator 15, but the overcurrent detection level can be adjusted by selecting the resistance value of the overcurrent detection resistor 16 to an arbitrary value. The fuse 19 can be blown when an overcurrent state occurs in the circuit before the voltage regulator 15 or when the overcurrent detection resistor 16 is abnormal.

[0004]

However, the above-mentioned prior art has some problems.

The method of preventing the overcurrent by inserting the fuse into the power supply line has a problem that it is difficult to select the fuse. In order to select a fuse, it is necessary not to blow when the output current at the maximum load is continuously supplied, and not to be blown even if the inrush current is repeatedly applied. Furthermore, when an abnormal current flows due to a circuit abnormality, the fuse must be blown immediately to stop the operation of the power supply circuit. Usually, there is a problem that the overcurrent protection cannot be performed by the fuse unless there is a difference of about 2 to 3 times between the current flowing through the fuse when supplying the output current at the maximum load and the current at the time of abnormality. In particular, when a circuit is constructed by using a semiconductor switch for turning on / off the power supply line on the secondary side by inserting a fuse in the primary side of the power supply circuit, the problem becomes remarkable. The semiconductor switch inserted in the secondary side acts as a buffer that separates the power supply circuit from the load circuit, and the supply current on the power supply circuit side will be There may be a phenomenon that the current is not so different from that at the maximum load. In this case, it is necessary to add a circuit for detecting an overcurrent to an arbitrary position because a fuse inserted on the primary side cannot deal with it.

In the method of inserting an overcurrent detection resistor in series with a load to detect and prevent an abnormal current, an overcurrent detection resistor is inserted in a power supply line to the load, which causes a change in circuit characteristics. There's a problem. Since the overcurrent detection resistor has a voltage drop by itself, there is a problem that heat generation is large and a resistor having a large size is required. For example, if the voltage drop in the voltage regulator that is determined to be abnormal is 1 V, and if it is desired to determine that there is an abnormality when the output current is 3 A or more, the value of the overcurrent detection resistor is 0.33Ω. in this case,
The output current at the maximum load continues to flow through the overcurrent detection resistor, and the maximum output current value is less than 3A. If a current of 3 A flows through a resistance of 0.33 Ω, it consumes about 3 W. Therefore, as the overcurrent detection resistor, a resistor having a large outer size such as a cement resistor is used, which causes a space problem. Further, there is a problem that the heat generation for consumption of 3 W is considerably large.

[0007]

In order to solve the above problems, the overcurrent protection circuit of the present invention has a control terminal capable of stopping the operation of the power supply circuit and outputs the output of the power supply circuit. In a power supply circuit having means for controlling by a semiconductor switch having a resistance component, means for driving the semiconductor switch, and means for detecting a voltage drop of the semiconductor switch due to an output current of the power supply circuit, The operation of the power supply circuit is stopped by detecting the voltage drop of the semiconductor switch due to the output.

[0008]

By using the above configuration of the present invention, overcurrent protection can be performed only by detecting the voltage drop due to the current in the on / off circuit of the power supply line using the semiconductor switch inserted in the power supply circuit. .

[0009]

FIG. 1 shows an embodiment of the present invention. In this embodiment, a step-up type switching regulator having an output voltage higher than the input voltage is configured, and a circuit for performing on / off using a P-channel FET in the output line is shown. The PWM-IC1 has an internal oscillator and has about 10
It can operate at a frequency of 0 KHz. The PWM-IC1 has a reference voltage source of 2.5V inside, and has an open collector type output terminal OUT, an input terminal IN for output feedback, and a PWM.
-Has FB as an IC control terminal. As the power supply voltage of the PWM-IC1 itself, VCC1 having a voltage equal to or higher than the output voltage is supplied from any place. PWM-I
The input terminal IN of C1 is internally inputted to the voltage comparator and compared with the internal reference voltage source 2.5V. The output terminal OUT is an open collector output of an NPN transistor internally. The input terminal FB is a control terminal of the PWM-IC1 and is internally pulled up so that the operation of the PWM-IC1 can be stopped by setting the terminal to the GND level. When the operation of the PWM-IC 1 is stopped by the input terminal FB, the voltage immediately before the FET 9 becomes the voltage obtained by removing the path loss of the line, that is, the loss of the fuse 13, the coil 2, and the diode 5 from the input voltage.

The PWM-IC 1, the coil 2, the transistors 3, 4, the diode 5, and the smoothing capacitor 6 constitute a step-up type switching regulator. PW
The M-IC1 divides the output voltage divided by the voltage dividing resistors 7 and 8 into P
The pulse width of the pulse train output from the output terminal OUT is controlled so as to make the output voltage constant by comparing with the reference voltage inside the WM-IC1. When the output voltage is + 5V, since the internal reference voltage source is 2.5V, the voltage dividing resistors 7 and 8 having the same value are selected. PWM-IC1
A pulse train is output from the output OUT of the device at a frequency of about 100 KHz to drive the transistor 3 to switch the transistor 4, thereby smoothing the boosted voltage by the smoothing circuit configured by the diode 5 and the smoothing capacitor 6. A boosted output voltage can be obtained. If the output voltage from the smoothing circuit rises or falls for some reason, the PW is compared by comparing the input voltage of the input terminal IN of the PWM-IC1 with the internal reference voltage source 2.5V.
Since the pulse width of the pulse train output from the output terminal OUT of the M-IC1 increases / decreases, the smoothed output voltage is controlled to be constant.

The output voltage from the smoothing circuit obtained as described above is input to the FET 9 which is a semiconductor switch. When the base input level of the transistor 10 is high level, the FET 9 is turned on and a voltage is supplied to the output of the FET 9. When the base input of the transistor 10 is low level, the transistor 10 is turned off, the FET 9 is turned off and F
The output voltage from ET9 will be cut off.

Now, consider the case where a voltage is supplied from the output of the FET 9, that is, the case where the transistor 10 and the FET 9 are on. The current supplied from the output of the FET 9 to the load changes from zero to the maximum load current depending on the state of the load. In this power supply circuit in a normal state, the output voltage of the FET 9 does not drop abnormally even if the maximum load current is supplied. In the normal state, if the FET 9 is sufficiently turned on and is in the saturation region, the ON resistance of the FET 9 is low
The difference between the input voltage and the output voltage of 9 is small. If the saturation ON resistance of the FET 9 is 0.3Ω and the maximum load current is 1 A, the potential difference between the input and output of the FET 9 is 0.3V. That is, even if the maximum load current is supplied, as long as the base input voltage of the transistor 10 does not drop to such an extent that the transistor 10 cannot be turned on, or the current does not flow to the extent that the voltage drop of the FET 9 becomes a problem, the input and output of the FET 9 are The potential difference does not increase.

However, if the output current from the FET 9 becomes 3 A, a voltage drop of 0.9 V will occur in the FET 9 having a saturation ON resistance of 0.3Ω. Transistor 1 at this time
Since transistor 1 is on, transistor 1 is on and has a saturation voltage of about 0.1 V.
Therefore, a voltage of about 0.8 V is applied between the base and the emitter of. At this time, the transistor 12 can be sufficiently turned on,
Since the VCC1 of the PWM-IC1 is equal to or higher than the output voltage of the regulator, that is, the smoothing circuit, driving the base terminal of the transistor 14 turns on the transistor 23. The operation of the PWM-IC1 is stopped, the output voltage becomes lower than the input voltage, and the FET 9 is turned off. As described above, overcurrent can be prevented.

FIG. 2 shows another embodiment of the circuit from the output part of the regulator shown in FIG. 1 to the latter half. The output of the FET 9 is + 5V, and the CPU circuit 20 that performs arbitrary processing
And to the peripheral circuit 21. The CPU circuit 20 outputs a high-level or low-level signal from the output terminal CNT to drive the base terminal of the transistor 10. The base input of the transistor 10 is at the GND level when the output terminal CNT of the CPU circuit 20 is at the low level, and when the output terminal CNT of the CPU circuit 20 is at the high level, approximately the same voltage as the VCC of the CPU circuit 20 is output. When it is desired to stop the supply of the VCC of the CPU circuit 20, the CPU circuit 20 may output the output terminal CNT to the low level, that is, the GND level. Next CPU
To start the circuit 20, the push switch 22 is pushed to supply VCC to the CPU circuit 20, and the CPU circuit 20 outputs a high level to the transistor 10 from the output terminal CNT. Since the processing capability of the CPU is fast with respect to the operation of a human, the transistor 10 is driven before the human releases the push switch 22, so that VCC is continuously supplied to the CPU circuit 20.

Now, a high level is output from the output terminal of the CPU circuit 20, the transistor 10 is driven, and the FET 9
Is turned on, and VCC is supplied to the CPU circuit 20 and the peripheral circuit 21. Transistor 1 of overcurrent detection circuit
The voltage at which 2 turns on is 0.8V when the voltage drop of FET9 is 0.8V, and when the ON resistance of FET9 is 0.3Ω, VCC
If the total current consumption is 0.5 A, the voltage drop in the FET 9 is 0.15 V, so the transistor 12 will not turn on. Considering a case where an abnormality such as a circuit short circuit occurs somewhere in the peripheral circuit 21, an excessive current is F
Since it starts flowing to ET9, the voltage of VCC starts to drop.
Therefore, the high-level voltage of the output terminal CNT of the CPU circuit 20 also drops, and the saturation voltage of the transistor 10 rises, resulting in an increase in the on-resistance of the FET 9. That is, the FE due to the voltage of VCC decreasing before the voltage drop of the FET 9 due to the overcurrent increases.
An increase in the on-resistance of T9 will occur. Therefore, since the voltage drop of the FET 9 becomes large with a current smaller than the original overcurrent, the transistor 11 of the overcurrent protection circuit,
Without 12, the state in which VCC is short-circuited occurs even though the current flowing into the FET 9 does not increase abnormally. For example, if the saturation voltage of the transistor 10 increases and the ON resistance of the FET 9 rises to about 0.5Ω, the voltage drop of the FET 9 reaches 0.8V with a current of 1.6A. This is because the output terminal CNT of the CPU 20 is V
This is due to the dependence on CC and the low output voltage VCC of + 5V.
Since there are 1 and 12, the transistors 14 and 23 are driven and the operation of the power supply circuit can be stopped when the voltage drop becomes 0.8 V or more. If the operation of the power supply circuit is stopped, the output voltage of the power supply circuit drops below the input voltage of the power supply circuit, so that it is not supplied to VCC and the operation of the circuit can be stopped, which is safe.

As described above, overcurrent protection can be performed by detecting the voltage drop of the FET 9 and stopping the power supply circuit. In the embodiment shown in FIG. 2, the output current from the power supply circuit is not so different from that when the maximum load current is being supplied even when there is an abnormality. Therefore, on the power supply circuit side,
It is not possible to recognize the abnormality of the circuit after 9 or overcurrent. It is very difficult to detect an overcurrent in this circuit with a fuse or an overcurrent detection resistor, and the voltage drop of the FET 9 is most suitable for detecting the overcurrent. The voltage drop of FET9, which can be judged as abnormal, is
It can be adjusted by adjusting the ON voltage of 2. The saturation ON resistance of the transistor 11 is set to 0.1 V, and the transistor 1
If the ON voltage of 2 can be adjusted from 0.5V to 2V, the voltage drop that can be judged as abnormal is from 0.6V to 2.1V.
Can be adjusted up to. Actually, the ON voltage of the transistor 12 can be adjusted by the constants of the base resistance of the transistor 12 and the bias resistance between the base and the emitter.

[0017]

By using the present invention, an overcurrent protection circuit can be easily constructed by only detecting a voltage drop due to an overcurrent in a semiconductor switch circuit that turns on / off a power supply output line of a power supply circuit having a control terminal. The effect is that you can do it. Further, there is an effect that selection of a fuse, which has been difficult in the past, can be simplified, and there is an effect that overcurrent protection can be performed without changing circuit characteristics like an overcurrent protection circuit having an overcurrent detection resistor inserted. . Moreover,
The configuration of the circuit for detection has the effect of being extremely simple and compact, and has the effect of allowing the detection overcurrent value to be adjusted arbitrarily. Further, unlike the case where the overcurrent detection resistor is inserted, there is an effect that there is no heat generation.

[Brief description of drawings]

FIG. 1 is a power supply circuit diagram showing an embodiment of the present invention.

FIG. 2 is a configuration diagram in which the present invention is applied to a CPU circuit.

FIG. 3 is a configuration diagram showing a conventional example.

[Explanation of symbols]

 1 PWM-IC 2 Coil 3, 4, 10, 11, 12, 14, 23 Transistor 5 Diode 6 Smoothing capacitor 7, 8 Voltage dividing resistor 9, 17 Semiconductor switch 13, 19 Fuse 15 Voltage regulator 16 Overcurrent detection resistor 18 Load 20 CPU circuit 21 Peripheral circuit 22 Push switch

Claims (1)

[Claims] 1. A means for controlling the output of the power supply circuit by a semiconductor switch having a resistance component, which has a control terminal capable of stopping the operation of the power supply circuit, a means for driving the semiconductor switch, and a power supply circuit In a power supply circuit having means for detecting a voltage drop of the semiconductor switch due to an output current, the operation of the power supply circuit is stopped by detecting a voltage drop of the semiconductor switch due to an output of an overcurrent of the power supply circuit. Current protection circuit.