JPH08115135A - Overcurrent detecting circuit - Google Patents

Abstract

PURPOSE: To suppress an output current at the time of output short-circuiting and to shorten the time for output recovery after the cancel of short-circuiting with extremely simple configuration. CONSTITUTION: Corresponding to the voltage increase for a resistor 5 with current increase, a drooping characteristic a-b is provided by controlling a transistor 6 to turn it on. The output of a shunt regulator 15 is increased by a further voltage drop, and a fold-back drooping characteristic b-c is provided by turning on a constant voltage diode 14 and a transistor 12. At the time of output recovery, the output of the shunt regulator 15 is lowered, the transistor 12 is turned on and the output voltage is rapidly recovered.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an overcurrent detection circuit,
In particular, the present invention relates to an overcurrent detection circuit that detects and protects an overcurrent of a stabilized DC power supply circuit.

[0002]

2. Description of the Related Art FIG. 2A is a diagram showing an example of a DC stabilized power supply circuit having a conventional overcurrent detection circuit of this type. The input voltage is input to the series circuit of the overcurrent detection resistor 5 and the P-channel series transistor 9 to be stabilized and output to the load (not shown) as the stabilized voltage V0.

A control PNP transistor 6 is provided in order to detect an overcurrent of the series transistor 9 and perform an off control of the transistor 9. This transistor 6 has an emitter and a collector connected to a series connection point of a series circuit of resistors 2 to 4 inserted in parallel with an input voltage.

The base of the transistor 6 is connected to the series connection point of the resistors 7 and 8, and the series connection circuit of these resistances 7 and 8 is provided between the source of the series transistor 9 and the ground. .

The capacitors 1 and 10 are input / output capacitors.

When the current I0 flowing in this circuit increases,
The voltage applied to the resistor 5 rises, the voltage between the base and the emitter of the transistor 6, which is represented by the sum of the voltages applied to the resistor 5, the resistor 7, and the resistor 2, rises to the on-voltage, and the transistor 6 is turned on. Therefore, the transistor 6
Since the voltage between the collector and the emitter of is decreased, the voltage applied to the resistor 3 is decreased. Therefore, the total of the voltages applied to the resistors 2 and 3 decreases, and the voltage between the gate and the source of the transistor 9 having the same potential as this voltage decreases, and the transistor 9 gradually turns off.

In order to keep the transistor 9 off, it is necessary to keep the base-emitter voltage of the transistor 6 on. Considering that the input voltage to the overcurrent detection circuit is constant, the voltage applied to the resistors 7 and 2 is constant. Further, it can be seen that the output current I0 flowing through the resistor 5 becomes constant because the voltage applied to the resistor 5 is made constant and the base-emitter voltage of the transistor 6 is kept at the ON voltage.

The drooping characteristic of this circuit is the vertical droop shown in FIG. 2B, but as a drawback, a voltage approximately equal to the input voltage is applied between the drain and source of the transistor 9 when the output is short-circuited, and this voltage and the output current. The loss obtained by multiplying by is large, and the load on the transistor becomes large. Also, in order to suppress the temperature rise due to loss, heat dissipation must be considered,
If a heat sink is required, it is disadvantageous because it requires a large mounting area. In order to solve this, there is a method of reducing the output current when the output is short-circuited by making the drooping characteristic fold-down.

FIG. 3 shows this drooping type overcurrent detection circuit.
It shows in (A). In FIG. 3A, the same partialization as in FIG. 2A is indicated by the same reference numeral, and the description thereof will be omitted.

In this example, the resistor 8 in FIG. 2A is omitted, and a resistor 11 is provided between the base of the transistor 6 and the drain of the transistor 9 so that the transistor 9 is vertically suspended until the transistor 9 is turned off. Although the operation is the same, the voltage between the drain and the source of the transistor 9 increases, and thus the voltage applied to the resistor 7 also increases.

The base-emitter voltage of the transistor 6 is represented by the sum of the voltages applied to the resistors 5, 7 and 2 as described above. Since the base-emitter voltage of the transistor 6 is maintained at the on-voltage, the resistance is reduced. As the voltage applied to 7 increases, the voltage applied to the resistor 5 decreases, and the output current I0 decreases. Therefore, the drooping characteristic is the drooping fold shown in FIG.

There are various techniques having this type of fold-down characteristic, and they are disclosed in JP-A-2-28164 and JP-A-57-28164.
No. 152021, Japanese Utility Model Laid-Open No. 56-64125, and the like, respectively.

In Japanese Patent Laid-Open No. 2-28164, in a switching power supply, a second reference voltage lower than a first reference voltage is used.
There is shown a technique for comparing the reference voltage and the input voltage of the second voltage detection circuit with each other and performing overcurrent protection based on the comparison result to obtain the overcurrent drooping characteristic shown in FIG. 3B.

According to Japanese Unexamined Patent Publication No. 57-152021, the same switching power supply has constant current characteristics while the DC stabilized output voltage is relatively high, and fold-back characteristics when the output voltage is low. A technique for obtaining a protection circuit having a small internal loss when the output is short-circuited is shown.

Further, Japanese Utility Model Laid-Open No. 56-64125 discloses a switching power supply device having an overcurrent protection circuit for controlling an overcurrent at the time of overload, in which a voltage detection signal is generated by a voltage divider circuit of a resistor and a constant voltage diode. Therefore, there is disclosed an overcurrent protection circuit having a fold-back characteristic in which these are used as input signals of a comparison amplifier.

[0016]

In the case of the fold-back characteristic shown in FIG. 3B, the loss at the time of output short circuit can be suppressed to a small level, but when this is released after the output short circuit,
The recovery time of the output voltage is determined by the time constants of the resistors 7 and 11 and the capacitor 10 in FIG. Therefore, there is a drawback that the recovery time of the output voltage cannot be shortened sufficiently.

In the conventional techniques described in the above publications,
Although a configuration having a fold-back characteristic in an overcurrent protection circuit of a switching regulator is disclosed, in both cases, the circuit configuration is complicated, and regarding the early recovery of the recovery time of the output voltage after the load short circuit described above, There is a drawback that the output voltage cannot be promptly restored again because it is neither disclosed nor suggested.

An object of the present invention is to provide an overcurrent detection circuit which has a very simple structure and which can reduce the loss when an output short circuit occurs and can improve the recovery time after the short circuit is released.

[0019]

According to the present invention, a series transistor element that stabilizes an input voltage and supplies a stabilized voltage to a load, a current detection resistor that detects a current of the series transistor element, and a current detection element. An overcurrent detection circuit including a control transistor element for turning off the series transistor element according to a voltage across a resistor, the detection means detecting a voltage proportional to the stabilized output voltage, and the detected voltage as a reference. An overcurrent characterized by including a shunt regulator element as an input and control means for conducting a control when the output voltage of the shunt regulator element becomes a predetermined value or more to control the voltage across the current detection resistor to decrease. A detection circuit is obtained.

[0020]

[Function] When the output voltage becomes a certain value or less due to overcurrent, the anode-cathode voltage of the shunt regulator element is correspondingly increased, and the increased voltage realizes the fold-back characteristic and cancels the overcurrent. When the output voltage rises and reaches the internal reference voltage of the shunt regulator element, the voltage between the anode and the cathode is lowered to instantly raise the output voltage.

[0021]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1A is a circuit diagram of an embodiment of the present invention, and the same parts as those in FIGS.

In FIG. 1A, an input voltage is input to a series circuit of an overcurrent detection resistor 5 and a P-channel series transistor 9 to be stabilized and output to a load (not shown) as a stabilization voltage V0. It

In order to detect the overcurrent of the series transistor 9 and perform the off control of the transistor 9, the PNP is used.
Control transistor 6 is provided. This transistor 6 has an emitter and a collector connected to a series connection point of a series circuit of resistors 2 to 4 inserted in parallel with an input voltage.

The base of the transistor 6 is connected to the series connection point of the resistor 7 and the NPN transistor 12, and the resistance 7 and the transistor 12 are connected between the series connection point of the resistor 5 and the transistor 9 and the ground. Are connected in series.

A resistance voltage dividing circuit for detecting the output voltage is provided and comprises resistors 16 and 17. This voltage-divided output serves as a reference input to the shunt regulator 15, and the anode-cathode voltage that is the output of the shunt regulator 15 is applied to the series circuit between the Zener diode 14 and the base-emitter of the transistor 12. There is.

A resistor 13 is provided between the connection point of the cathode of the shunt regulator 15 and the cathode of the Zener diode 14 and the connection point of the resistor 5 and the transistor 9 in series.

Reference numerals 1 and 10 are input and output capacitors,
The P-channel field effect element is used as the series transistor 9 in order to minimize the loss in the DC voltage line.

In the overcurrent detection circuit of FIG. 1A, the operation is the same as that of the conventional vertical drooping circuit until the overcurrent is detected and the output voltage drops to a certain value. It becomes like between a and b of (B).

However, when the output voltage becomes lower than a certain value,
This is detected by the output voltage detection resistors 16 and 17,
Shunt regulator 1 that is equal to the voltage across resistor 17
The voltage between the reference and the anode of No. 5 decreases, and the cathode-anode voltage of the shunt regulator 15 increases.

The operation of the shunt regulator 15 will be briefly described. The reference-anode voltage is compared with the internal reference voltage. When the reference-anode voltage is lower than the internal reference voltage, the cathode-anode voltage is increased. Also in this circuit, the voltage between the reference and the anode becomes lower than the internal reference voltage, so that the voltage between the cathode and the anode is increased.

When this voltage exceeds the sum of the voltage of the constant voltage diode 14 and the base-emitter voltage of the transistor 12, the transistor 12 is turned on and a current flows between the resistor 7 and the base-emitter of the transistor 12. , The voltage applied to the resistor 7 gradually increases.

As described above, since the base-emitter voltage of the transistor 6 is kept at the ON-voltage, the base-emitter voltage of the resistors 5 and 7 is applied to the resistor 5 when the voltage applied to the resistor 7 rises. Since the voltage drops, the output current I0 also drops. Therefore, the drooping characteristic is b in FIG.
It becomes as shown in between -c.

From the above, the drooping characteristic is a combination of the vertical drooping ab and the hang-up bc as shown in FIG.

After the output is short-circuited, the recovery time of the output voltage when the short-circuit is released depends on the output voltage detection resistance 1
When the voltage detected by 6 and 17 instantly increases and reaches the internal reference voltage of the shunt regulator 15, the cathode-anode voltage of the shunt regulator 15 decreases, the transistor 12 is turned off, and the resistor 7 is applied. The voltage drops. As a result, the drooping characteristic shifts from the fold-back droop to the vertical droop, and the output waveform rises instantaneously.

[0035]

As described above, in the overcurrent detection circuit according to the present invention, the loss of the circuit at the time of the output short circuit is suppressed to a low level, and the drooping characteristic and the fold-down characteristic are maintained even in the recovery time after the output short circuit is released. By combining them, there is an effect that the recovery can be performed in a short time as compared with the case where only the fold-back is applied. Further, the above-mentioned effects can be realized with a simple configuration using only the shunt regulator element.

[Brief description of drawings]

1A is a circuit diagram of an embodiment of the present invention, and FIG. 1B is a diagram showing a characteristic example of this circuit.

FIG. 2A is a diagram showing an example of a conventional overcurrent protection circuit, and FIG. 2B is a diagram showing a characteristic example of this circuit.

FIG. 3A is a diagram showing another example of a conventional overcurrent protection circuit, and FIG. 3B is a diagram showing a characteristic example of this circuit.

[Explanation of symbols]

 1,10 Capacitors 2-5,7,13,16,17 Resistance element 6 Controlling transistor element 9 Series transistor element 14 Constant voltage diode 15 Shunt regulator element

Claims (3)

[Claims] 1. A series transistor element for stabilizing an input voltage to supply a stabilized voltage to a load, a current detection resistor for detecting a current of the series transistor element, and the series according to a voltage across the current detection resistor. An overcurrent detection circuit including a control transistor element for turning off a transistor element, the detection means detecting a voltage proportional to the stabilized output voltage, a shunt regulator element having the detected voltage as a reference input, and An overcurrent detection circuit, comprising: a control unit that conducts when the output voltage of the shunt regulator element exceeds a predetermined value to control the voltage across the current detection resistor to decrease. 2. The current detection resistor has a first resistance element connected in series with the series transistor element, and a second resistance element whose one end is connected to a connection point between the first resistance element and the series transistor element. 2. The overcurrent detection circuit according to claim 1, wherein the control transistor element is formed of a resistance element, and the control transistor element has an emitter and a base connected to both ends of a series connection circuit including the first and second resistance elements. . 3. The control means includes a constant voltage diode element, a transistor element, and a third resistance element,
The constant voltage diode element and the base / emitter of the transistor element are connected in series at both ends of the shunt regulator element, and are connected between the output point of the shunt regulator element and the connection point, and The overcurrent detection circuit according to claim 2, wherein a collector output of the transistor element is a base input of the control transistor element.