JPH0622447A - Self-reset overcurrent protective circuit - Google Patents

Abstract

PURPOSE:To obtain a self-reset overcurrent protective circuit realizing positive protection against overcurrent. CONSTITUTION:A load current Io is fed from a DC power supply +B through a transistor TR11 and a resistor R13 to a load Rd. When voltage across the resistor R13 exceeds base-emitter voltage, transistor TR13 is turned ON and, transistor TR is also turned ON. Consequently, transistor TR12 is turned OFF thus turning the transistor TR11 OFF. Even if the transistor TR11 is turned OFF, a monitor current is fed from the DC power supply +B through a resistor R17 and a transistor TR15 to the load Rd. When the potential difference across the load RD due to the monitor current exceeds Zener voltage of a Zener diode ZD12, the transistor TR12 is turned ON again to turn the transistor TR11 ON thus performing self-resetting.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a self-reset type overcurrent protection circuit for supplying and driving DC power to loads such as various electronic devices.

[0002]

2. Description of the Related Art An example of a conventional overcurrent protection circuit is shown in FIG.
Shown in. This overcurrent protection circuit consists of DC power supply + B and load R
It is provided between d and. Power switch etc. are turned on,
When a DC voltage is supplied from the DC power source + B, a current for charging the capacitor C which has been in the discharged state up to that point flows. This current is supplied to the capacitor C via the emitter-base of the PNP transistor TR1, the parallel circuit of the resistor R1 and the resistor R2. Since the base current of the transistor TR1 flows, current also flows in the collector and the output voltage rises. This output voltage is divided by the resistors R3 and R4 to turn on the NPN transistor TR2.
Set to N state. When the transistor TR2 is turned on, the base current of the transistor TR1 changes to the transistor T1.
It flows between the collector and emitter of R2, and the transistor TR
The ON state of 1 continues stably.

A load current Io flows from the collector of the transistor TR1 through the resistor R5 to the load Rd. The voltage generated across the resistor R5 by the load current Io is PNP.
When the forward voltage between the emitter and the base of the transistor TR3 is exceeded, a base current flows between the emitter and the base of the transistor TR3 via the resistor R6, and the transistor TR3 is turned on. When the transistor TR3 is turned on, its collector current passes through the resistor R7 and NP
It flows between the base and emitter of the N-transistor TR4 to turn on the transistor TR4. Transistor TR
4 is turned on, the resistor R4 turns on the transistor TR4.
Is short-circuited due to the conduction between the collector and the emitter, and the potential difference across the resistor R4 is reduced. The voltage across the resistor R4 is the voltage between the base and emitter of the transistor TR2, and when this voltage decreases, the collector current of the transistor TR2, that is, the base current of the transistor TR1 also decreases. When the base current of the transistor TR1 decreases, the collector current also decreases and its output voltage also decreases. The voltage across the resistor R4, which divides this output voltage, further decreases, and the base current of the transistor TR2 further decreases. In this way, once the transistor TR3
After being turned on, the base voltage of the transistor TR2 is lowered by the positive feedback action, and finally the transistor TR2 is turned off. Transistor TR
When 2 is turned off, the base current of the transistor TR1 charges the capacitor C again, and when this voltage rises, the base current of the transistor TR1 decreases, and finally the emitter-base potential difference of the transistor TR1 can be secured. Then, the transistor TR1 is turned off. In this way, the load current I supplied to the load Rd
When o becomes excessively large, the transistor TR1 is turned off to provide overcurrent protection.

In the overcurrent protection circuit shown in FIG. 5, the potential difference generated across the resistor R9 by the load current Io is PN.
When the emitter-base voltage of the P-transistor TR5 is exceeded, the voltage generated across the resistor R10 due to the collector current of the transistor TR5 increases. When this voltage increases, the emitter of the PNP transistor TR1
The base-to-base voltage becomes smaller, and the collector current of the transistor TR1, that is, the load current Io also becomes smaller. When the load current Io decreases, the potential difference across the resistor R9 also decreases, the collector current of the transistor TR5 also decreases, and the potential difference across the resistor R10 also decreases. When the voltage across the resistor R10, that is, the base voltage of the transistor TR1 decreases, the potential difference between the emitter and the base of the transistor TR1 increases, and the load current Io can be increased. Therefore, the maximum value of the load current Io is
The equilibrium state is reached when the potential difference generated in the resistor R9 is near the emitter-base voltage of the transistor TR5.

FIG. 6 shows operating characteristics of the overcurrent protection circuit shown in FIGS. 4 and 5. The load current Io is the maximum current Iot
Up to, the output voltage Vo is a substantially constant voltage Vo1. In the circuit shown in FIG. 4, when the load is short-circuited, the load current Io decreases according to the current cutoff type fold-back characteristic shown by the two-dot chain line in FIG. The current cutoff foldback characteristic is adjusted by the value of the resistor R8 connected to the collector circuit of the transistor TR3. Depending on the circuit shown in FIG. 5, the drooping characteristic shown by the solid line in FIG. 6 can be obtained. Further, a circuit having a fold-back characteristic shown by a broken line in FIG. 6 and a drooping fold-back characteristic shown by a chain line can be realized.

[0006]

In the conventional protection circuit shown in FIG. 4, once an excessive current is detected,
According to the current cutoff type fold-back characteristic shown in FIG. 6, the protection state is established. This protection state continues until the electric charge stored in the capacitor C shown in FIG. 4 is discharged. Therefore, in order to supply the DC power to the load Rd again, it is necessary to discharge the electric charge stored in the capacitor C. The diode D shown in FIG. 4 is provided to discharge the electric charge of the capacitor C when the DC power supply + B is turned off. The circuit shown in FIG. 4 cannot supply DC power to the load Rd unless the DC power supply + B is turned off once to discharge the electric charge stored in the capacitor C and then the DC power supply + B is turned on again.

The protection circuit shown in FIG. 5 has the drooping characteristic shown in FIG. 6 and does not have the above-mentioned problem at the time of current restoration due to the positive feedback action for realizing the foldback characteristic. However, since the maximum current Iot continues to flow when the load Rd is short-circuited, the power supply voltage Vo1 is applied as it is between the emitter and collector of the transistor TR1 shown in FIG. 5, and the power loss between the emitter and collector increases. This power loss causes the transistor TR1 to generate heat, and if it continues for a long time, the transistor TR1 may be damaged. A large capacity transistor is used as the transistor TR1,
Although it is possible to withstand heat generation if the heat dissipation conditions are improved, excessive current continues to flow, increasing power loss, which is dangerous.

An object of the present invention is to provide a self-recovery type overcurrent protection capable of reliably performing protection from an overcurrent, reducing power loss in the protection state, and easily returning from the protection state. It is to provide a circuit.

[0009]

According to the present invention, a current control circuit for controlling a current supplied from a DC power source to a load and a current supplied to the load from the current control circuit are detected, and a value of the supplied current is predetermined. When it exceeds the maximum current value, it is connected in parallel with the current control circuit and a current limiting circuit that controls so that the supply current from the current control circuit is reduced, and supplies a predetermined monitoring current from the DC power supply to the load. Constant current circuit to detect the voltage generated in the load by the monitoring current,
A self-recovery type overcurrent protection circuit, comprising: a current recovery circuit that releases a supply current decreasing state of a current control circuit by a current limiting circuit when the generated voltage exceeds a predetermined recovery voltage.

[0010]

According to the present invention, the self-reset type overcurrent protection circuit includes a current control circuit, a current limiting circuit, a constant current circuit,
A current recovery circuit is included. The current control circuit controls the current supplied from the DC power supply to the load. The current limit circuit
The current supplied to the load from the current control circuit is detected, and when the value of the supplied current exceeds a predetermined maximum current value, the supplied current is controlled to be in a reduced state. The constant current circuit is connected in parallel with the current control circuit and supplies a predetermined monitoring current from the DC power supply to the load. The current recovery circuit detects the voltage generated in the load by the monitoring current, and when the generated voltage exceeds a predetermined recovery voltage, cancels the supply current decreasing state of the current control circuit by the current limiting circuit.

Therefore, if the monitoring current is set to a small value, the power loss of the constant current circuit does not increase even when the load is short-circuited, and the supply current of the current limiting circuit is in a reduced state. The power loss of the current control circuit is also small. When the short-circuited state of the load is eliminated, the voltage generated in the load increases due to the monitoring current. When the generated voltage exceeds the recovery voltage, the current recovery circuit cancels the supply current decreasing state of the current control circuit, so that the supply current to the load can be self-recovered.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows an electric circuit configuration of an embodiment of the present invention. The current control circuit that supplies the load current Io from the DC power source + B to the load Rd includes a PNP transistor TR11 and an NPN transistor TR12. The PNP transistor TR13 and the NPN transistor TR14 are included in the current control circuit which detects that the supply current Io becomes excessive and causes the current control circuit to be in the supply current decreasing state. The constant current circuit that supplies the monitoring current from the DC power source + B to the load Rd includes PNP transistors TR15 and TR16, a field effect transistor TR17, and a zener diode ZD11. The Zener diode ZD12 is included in the current restoration circuit that releases the supply current decreasing state when the voltage generated by the monitoring current flowing through the load Rd exceeds the restoration voltage.

A resistor R1 is provided between the emitter and base of a transistor TR11 whose emitter is connected to a DC power source + B.
1 are connected in parallel. A resistor R is provided between the base of the transistor TR11 and the collector of the transistor TR12.
12 are connected. A resistor R13 is connected between the collector of the transistor TR11 and the load Rd. The emitter of the transistor TR13 is connected to the connection point between the collector of the transistor TR11 and the resistor R13. A resistor R14 is connected between the base of the transistor TR13 and the load Rd. A resistor R is provided between the collector of the transistor TR13 and the base of the transistor TR14.
15 are connected. A resistor R16 is connected between the connection point between the collector of the transistor TR13 and the resistor R15 and the emitter of the transistor TR14.

A resistor R17 is connected between the emitter of the transistor TR11 and the emitter of the transistor TR15. The base of the transistor TR15 is connected to the emitter of the transistor TR16, and
5 and the collector of the transistor TR16 are commonly connected. In this way, the field effect transistor T is provided at the base of the transistor TR16 connected in Darlington.
The constant current circuit by R17 is connected. Transistor T
The connection point between the base of R16 and the field effect transistor TR17 is connected to the DC power source + B via the Zener diode ZD11. A series circuit of a Zener diode ZD12 and a resistor R18 is connected in parallel to the load Rd. The connection point between the Zener diode ZD12 and the resistor R18 is connected to the base of the transistor TR12 and the collector of the transistor TR14 via the resistor R19.

In the circuit shown in FIG. 1, the maximum value of the load current Io is the detection current Iot expressed by the following equation (1).

[0016]

[Equation 1]

Here, Vbe13 is a transistor TR
13 is a base-emitter voltage, and R13 represents a resistance value of the resistor R13. The load current Io is the detection current Iot
When it exceeds, the base current of the transistor TR13 flows, and thus the collector current also flows. This collector current becomes a base current of the transistor TR14 via the resistor R15 and turns on the transistor TR14. When the transistor TR14 is turned on, the transistor TR12 is turned off and the transistor TR12 is turned on.
The base current of 11 also stops flowing, and the transistor TR1
1 is in the OFF state. Transistor TR11 is off
In the process of transitioning to the state, the collector voltage of the transistor TR11 decreases, and the decreased voltage causes positive feedback in which the current supplied from the Zener diode ZD12 to the transistor TR12 via the resistor R19 also decreases. The current flowing through the resistor R13 decreases and the transistor T
Even if R13 is not in the ON state, the transistor TR1
The OFF state of 1 and TR12 continues.

A monitoring current Ios having a constant current value flows between the DC power source + B and the load Rd via the resistor R17 and the transistor TR15. The value of this monitoring current Ios is obtained by the following equation (2).

[0019]

[Equation 2]

Here, Vzd11 is a Zener diode Z
Zener voltage of D11, Vbe15 and Vbe
Reference numeral 16 represents the base-emitter voltage of the transistors TR15 and TR16, respectively. R17 is a resistor R1
7 shows a resistance value of 7. Here, the value of the monitoring current Ios has a limit represented by the following inequality.

[0021]

Equation 3] _{Ios ≦ Idss17 × h FE 15 ×} h FE 16 where Idss17 shows the drain current of the field effect transistor TR17, h _FE 15 and h _FE 16 is a DC current amplification factor of the transistor TR15 and TR16, respectively. Further, when the load is normal, most of the load current Io flows between the emitter and collector of the transistor TR11 and via the resistor R13. Need a relationship.

[0022]

## EQU00004 ## R13 << R17 That is, the resistance value R13 of the resistor R13 needs to be sufficiently smaller than the resistance value R17 of the resistor R17.

As an example, when the excessive load current Io flows, the detection current Iot is about 2A and the monitoring current Ios is about 2A.
It is assumed that the power supply voltage of the DC power supply + B is 30 mA, the load resistance Rd in the normal state is 14 Ω.
Set the zener voltage of the zener diode ZD11 to 4.7.
V, zener voltage of zener diode ZD2 is 2.0
V, resistors R11, R14, R15, R16, R18, R
The resistance value of 19 is 10 kΩ and the resistance value of R12 is 120
Ω, the resistance R13 is 0.27 Ω, and the resistance R17 is 15 Ω. When the DC power supply + B is turned on, the load current is 233
mA flows. Approximately 3.26V across load resistance Rd
Potential difference occurs. Since this potential difference is larger than the sum of the Zener voltage of the Zener diode ZD12 and the base-emitter voltage of the transistor TR12, a current flows through the resistor R19 to the base of the transistor TR2 and the transistor TR12 is turned on. . When the transistor TR12 is turned on, the transistor TR1
A current also flows through the base of 1 through the resistor R12, the transistor TR11 is turned on, and the load current Io is supplied to the load Rd. At this time, since the voltage drop between the emitter and collector of the transistor TR11 and the voltage drop across the resistor R13 are small, a voltage close to 14V of the DC power supply + B is applied to the load Rd, and a current of about 1.0A flows. .

In the above state, when a part of the load resistance Rd is short-circuited and the load resistance Rd changes to 6Ω, the load current value tends to increase to about 2.3A. At this time, the potential difference generated across the resistor R13 is
The voltage becomes higher than the emitter-base voltage of about 0.7V of R13, the transistor TR13 is turned on, and the base current of the transistor TR14 flows through the resistor R15. When the base current flows through the transistor TR14, the transistor TR14 is turned on and the transistor TR12 is turned off. When the transistor TR12 is turned off, the transistor TR11
Also becomes the OFF state, and the load current Io decreases to the monitoring current Ios in accordance with the downward fold-down characteristic of FIG.

Even if the transistor TR11 is in the OFF state, the monitoring current Ios flows through the resistor R17 and the transistor TR15. When the load resistance Rd is 6Ω, the potential difference across the load resistance Rd is about 1.4V, and the zener voltage of the zener diode ZD12 does not exceed 2.0V. Therefore, current cannot flow through the zener diode ZD12, and the supply current decreases. Continues. Load resistance Rd
Becomes larger and the potential difference generated by the monitoring current Ios exceeds the Zener voltage of the Zener diode ZD12 and the base-emitter voltage of the transistor TR12, the base current of the transistor TR12 flows through the Zener diode ZD12 and the transistor TR1
2 is turned on, and the transistor TR11 is also turned on. In this way, when the condition of the overcurrent for the load resistance Rd is resolved, the supply current decreasing state from the transistor TR11 is released. In the process of releasing the protection state, the recovery current Ios is changed from the monitoring current Ios of FIG.
It becomes the upward fold-back characteristic to om.

The fold-back type characteristic according to the present embodiment shown in FIG. 2 exhibits a self-reset type characteristic having hysteresis as compared with the protection circuit of the conventional type shown by the broken line in FIG. Further, since the decreasing process and the increasing process of the supply current Io are performed by the positive feedback action of the transistors TR11 and TR12, they can be switched quickly, and the power loss during this period hardly occurs.

FIG. 3 shows a state in which the load circuit 12 is connected to the power supply circuit 11 according to the embodiment shown in FIG. 1 through the power supply line 13. The load circuit 12 includes a plurality of units 21, 22, 23, ..., 2n. When a load circuit 12 including a plurality of such units 21, 22, 23, ..., 2n is connected, an excessive current is generated and the power supply circuit 11
Suppose that the protective action is taken. With the DC power supply still being supplied from the power supply circuit 11, the units 21, 2
If the units 2, 23, ..., 2n are sequentially removed one by one, the output voltage value of the power supply line 13 returns to a normal state when the unit causing the excess current is removed. Therefore, it is very easy to find the failed unit. If the defective unit is removed, the other units can be operated as they are.

Further, according to this embodiment, it is not necessary to provide the capacitor C as shown in FIG. 4, and the load current can be changed quickly when the load current is decreased or increased. Further, the capacitor C shown in FIG. 4 requires an electrolytic capacitor having a relatively large capacity,
It is difficult to convert the circuit shown in FIG. 4 into a semiconductor integrated circuit. According to the embodiment shown in FIG. 1, it is easy to form a semiconductor integrated circuit.

[0029]

As described above, according to the present invention, when an excessive current flows such as when the load is short-circuited, the current limiting circuit controls so that the current supplied from the current control circuit to the load is reduced. . Since the protection is performed with the current supplied to the load reduced, the power loss of the current limiting circuit can be reduced even when the load is short-circuited, the heat generation amount can be reduced, and the load current from an excessive load current can be reduced. The protection can be ensured. Furthermore, by the constant current circuit,
When the monitoring current is supplied to the load and the voltage generated in the load exceeds the recovery voltage by the current recovery circuit, the supply current decreasing state of the current control circuit is released. By reducing the monitoring current, it is possible to prevent the power loss of the constant current circuit from increasing and the load condition is monitored. Can be restored.

Further, it is possible to surely protect from an excessive current,
Moreover, since it is possible to recover itself, it is one of the multiple loads.
When there is one cause of excessive current, it is easy to identify the load. That is, it is possible to determine that the load that is released from the protection state from the overcurrent is the cause of the overcurrent by sequentially removing each load, remove the load, and continue driving other loads. You can also do it.

[Brief description of drawings]

FIG. 1 is an electric circuit diagram of an embodiment of the present invention.

FIG. 2 is a graph showing operating characteristics of the embodiment shown in FIG.

3 is a block diagram showing a system configuration using a power supply circuit 11 of the embodiment shown in FIG.

FIG. 4 is an electric circuit diagram of a conventional overcurrent protection circuit.

FIG. 5 is an electric circuit diagram of a conventional overcurrent protection circuit.

FIG. 6 is a graph showing operating characteristics of the overcurrent protection circuits shown in FIGS. 4 and 5;

[Explanation of symbols]

 TR11 to TR17 transistor ZD11, ZD12 Zener diode Rd load 11 power supply circuit 12 load circuit 13 power supply line 21 to 2n unit

Claims (1)

[Claims] 1. A current control circuit for controlling a current supplied from a DC power supply to a load, and a current supplied to the load from the current control circuit is detected. When the value of the supplied current exceeds a predetermined maximum current value, the current is supplied. Connected in parallel with the current control circuit and the current limiting circuit that controls so that the supply current from the control circuit is reduced, and from the DC power supply to the load,
A constant current circuit that supplies a predetermined monitoring current and the voltage that is generated in the load by the monitoring current are detected, and when the generated voltage exceeds a predetermined reset voltage, the current limiting circuit's supply current decreasing state is canceled. A self-recovery type overcurrent protection circuit including a current recovery circuit.