JPH0851772A - Protective circuit for switching power supply - Google Patents

Abstract

PURPOSE:To reduce the quantity of heat generated from a three-terminal regulator under overload. CONSTITUTION:The protective circuit for switching power supply comprises a voltage monitoring circuit 7 for detecting voltage drop at the output terminals of three-terminal regulators 11, 12 in a switching power supply having a plurality of DC outputs 8-10 including DC outputs 9, 10 connected with the input terminals of the three-terminal regulators 11, 12, and an overload control circuit 6 for controlling the current of a light emitting diode D9 in a photocoupler 3 when the voltage monitoring circuit 7 detects voltage drop at the output terminals of the three-terminal regulators 11, 12 to cause intermittent switching operation of a switching circuit 1.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a switching power supply which stabilizes an output voltage by feeding back the output voltage using a photocoupler.
The present invention relates to a protection circuit for a switching power supply that is stabilized by using a terminal regulator together.

[0002]

2. Description of the Related Art In a self-excited or separately excited switching power supply, the output voltage is stabilized by feeding back the output voltage using a photocoupler. However, even in a switching power supply having multiple DC outputs with different voltages, the voltage to be fed back in order to stabilize the output voltage is
Limited to the output voltage of one DC output. Therefore, in a switching power supply having a plurality of DC outputs, the voltage of the DC output having the largest output power is fed back as a voltage representative of the plurality of DC outputs. As a result, the DC output voltage that has not been fed back is subjected to open loop control, and the voltage stability is reduced. However, since the output power is small, there is a case where high voltage stability is required even for a DC output that is open-loop controlled. As described above, in the case where the DC output is open-loop controlled and high voltage stability is required, a configuration in which a three-terminal regulator is connected to the DC output has been adopted.

[0003]

As described above, when the three-terminal regulator is connected to the DC output whose voltage is relatively stabilized to obtain high voltage stability, the output terminal of the three-terminal regulator is 3 when overloaded
Only the protection circuit inside the terminal regulator operates. That is, since the voltage of the DC output that supplies power to the three-terminal regulator is only stabilized by the open loop control, if the current value is such that the protection circuit inside the three-terminal regulator operates, the DC output The voltage is not controlled to be a voltage near 0. Therefore, the electric power supplied as the DC output is entirely converted into heat inside the three-terminal regulator, which increases the amount of heat generated by the three-terminal regulator. This increase in the amount of heat generation may cause thermal destruction of the three-terminal regulator, so in order to prevent thermal destruction of the three-terminal regulator,
There was a problem that a large radiating fin had to be attached to the three-terminal regulator.

The present invention was devised to solve the above-mentioned problems, and an object of the present invention is to provide a switching operation intermittently when the output voltage of a three-terminal regulator decreases due to overload. The purpose is to provide a protection circuit for a switching power supply that can reduce the amount of heat generated by a three-terminal regulator when overloaded. Another object of the present invention is to provide a protection circuit for a switching power supply, which is capable of intermittently performing a switching operation with a simplified circuit. Further, an object of the invention of claim 3 is also to provide at least two three-terminal regulators,
It is an object of the present invention to provide a protection circuit for a switching power supply, which can reduce the amount of heat generated by an overloaded three-terminal regulator without causing the circuit to be complicated. Claim 4
An object of the described invention is to provide a protection circuit for a switching power supply, which can optimize the overload detection level.

[0005]

In order to solve the above problems, a protection circuit for a switching power supply according to the invention of claim 1 is provided with a plurality of DC outputs, and the DC output is increased by increasing the output current of a photocoupler. Stabilizes the voltage of the DC output by performing feedback indicating an increase in voltage error,
A voltage monitoring circuit that applies at least one DC output of the DC output to a switching power supply connected to an input terminal of a three-terminal regulator and detects a voltage drop at the output terminal of the three-terminal regulator; An overload control circuit for intermittently performing the switching operation of the switching circuit by controlling the output current of the photocoupler by controlling the current of the light emitting diode of the photocoupler when detecting the voltage drop of the output terminal. It has a configuration including and.

According to a second aspect of the present invention, there is provided a protection circuit for a switching power supply, wherein a current is supplied to the anode of the light emitting diode of the photocoupler from a positive terminal of a smoothing capacitor for smoothing the DC output. An error detection circuit for detecting the voltage error is connected to the cathode of the light emitting diode of the photocoupler, the cathode of the light emitting diode of the photocoupler is connected to the collector of the overload control circuit, and the emitter thereof is It is grounded, and its base is a first transistor connected to the positive terminal through a first resistor and one terminal is a first transistor.
Of the delay capacitor connected to the base of the transistor and the other terminal to the ground, and the second capacitor connected to the base of the first transistor and the other terminal to the second terminal.
And a second transistor whose collector is connected to the base of the first transistor, whose emitter is grounded, and whose base is connected to the detection point, and one terminal of which is connected to the base of the first transistor. Is connected to the output terminal of the three-terminal regulator, and the other terminal is connected to the detection point.

The protection circuit for a switching power supply according to a third aspect of the present invention is applied to a switching power supply in which the three-terminal regulator is provided for each of at least two DC outputs of the plurality of DC outputs, 3 above
The diode is provided corresponding to each of the terminal regulators, the cathode of which is connected to the output terminal of the corresponding three-terminal regulator, and the anode of which is connected to the detection point.

According to a fourth aspect of the present invention, there is provided a protection circuit for a switching power supply, which is provided with a Zener diode whose anode is connected to the base of the second transistor and whose cathode is connected to the detection point. .

[0009]

The operation of the invention according to claim 1 will be described below. When the output of the 3-terminal regulator is overloaded, the voltage monitoring circuit detects a voltage drop at the output terminal of the 3-terminal regulator. At this time, the overload control circuit intermittently performs the switching operation by controlling the current value of the light emitting diode of the photocoupler. As a result, power is supplied to the input terminal of the three-terminal regulator when the switching operation is performed, and power is not supplied to the input terminal of the three-terminal regulator when the switching operation is stopped. Therefore, the average power supplied to the input terminal of the three-terminal regulator is reduced. That is, 3
When the output of the terminal regulator becomes overloaded and the protection circuit inside the three-terminal regulator operates, the power supplied to the three-terminal regulator decreases.

The operation of the invention according to claim 2 will be described below.
When the output of the three-terminal regulator becomes overloaded and the internal protection circuit operates, the second transistor turns off. Therefore, the terminal voltage of the delay capacitor becomes approximately 0.7 V after a certain period of time. After this time, the first transistor is turned on and a large current flows through the light emitting diode of the photocoupler while the charge remains in the smoothing capacitor. When a large current flows through the light emitting diode, the output current of the photocoupler increases, so that the switching operation on the primary side stops. That is, the switching operation is stopped during the period in which the charge charged in the smoothing capacitor supplies the base current to the first transistor. Then, when the charge stored in the smoothing capacitor is discharged, the switching operation is restarted, and the charging of the smoothing capacitor is started. However, since the base voltage of the first transistor is a delay capacitor, the voltage rise is delayed. Therefore, the switching operation is not stopped, and the smoothing capacitor is sufficiently charged. That is, the switching operation is performed in the period until the voltage of the delay capacitor becomes approximately 0.7V. Then, when the voltage of the delay capacitor reaches approximately 0.7 V, the switching operation is stopped again. After that, the same process is repeated.

The operation of the invention according to claim 3 will be described below.
One of the at least two 3-terminal regulators
When the output of the terminal regulator becomes overloaded, the protection circuit inside the overloaded 3-terminal regulator operates. Therefore, the output voltage of the three-terminal regulator in which the protection circuit operates decreases, and a current flows from the detection point to the output terminal side of the three-terminal regulator via the diode. As a result, the voltage at the detection point is lowered, and the second transistor is turned off. As a result, the switching operation becomes intermittent.

The operation of the invention according to claim 4 will be described below.
When a voltage obtained by adding the Zener voltage of the Zener diode to the base voltage of the second transistor of about 0.7 V is set as the detection voltage, the voltage at the detection point is higher than the detection voltage in order to turn on the second transistor. There is a need. That is, the reference voltage for switching the second transistor on and off is set by the Zener voltage of the Zener diode.

[0013]

An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a circuit diagram showing an electrical connection of an embodiment of a protection circuit for a switching power supply according to the present invention.

In the figure, a switching circuit 1 is a block for switching a current flowing through a primary coil 51 of a transformer 5 based on a voltage generated in a base coil 52. A current limiting circuit for limiting the current flowing through the element is provided. Switching control circuit 2
Is a block that controls the switching operation of the switching circuit 1. Then, when the output current sent from the emitter of the transistor Q3 of the photocoupler 3 increases, it is assumed that the output voltage has risen, and the period in which the switching circuit 1 supplies the current to the primary coil 51 is shortened, so that the secondary coil 53 receives the current. Reduce the voltage generated. Further, when the output current exceeds a certain value, the operation of the switching circuit 1 is stopped.

The photocoupler 3 is an element for feeding back to the primary side the voltage of the DC output 8 generated in the secondary coil 53, rectified by the diode D6, and then smoothed by the smoothing capacitor C3. From the positive terminal of the smoothing capacitor C3 to the anode of D9,
Current is supplied via the resistor R4. Error detection circuit 4
Is a block that controls the current flowing through the light emitting diode D9 to which the cathode is connected, based on the difference between the internally generated reference voltage and the divided voltage of the DC output 8. When the voltage of the DC output 8 is lowered due to overload, a protection circuit for increasing the current flowing through the light emitting diode D9 and reducing the voltage generated in the secondary coil 53 is included.

Further, each of the two sets of secondary coils 54, 55 is provided with rectifying diodes D7, D8 and smoothing capacitors C4, C6. The resistor R5
Is a fuse resistance for protection, and the Zener diode D3 is for absorbing an abnormal voltage. As described above, the block and the element whose configuration has been described above are included. In FIG.
The configuration excluding the capacitors 1 and 12 and the capacitors C5 and C7 is a self-excited switching power supply having three DC outputs 8 to 10, which is known in the prior art.

The three-terminal regulators 11 and 12 are blocks for stabilizing the voltages of the DC outputs 9 and 10 corresponding to each other with high accuracy.
The capacitors C5 and C7 are connected.

The voltage monitoring circuit 7 is a three-terminal regulator 1.
When the voltage of at least one of the output terminals 1 and 12 drops, this block detects the voltage drop. When the voltage drop is detected, the transistor Q2 is turned off to turn off the three-terminal regulators 11 and 12. Indicates the voltage drop at the output terminal. That is, the voltage monitoring circuit 7
Its collector is connected to the base of the first transistor Q1, its emitter is grounded, and its base is connected to the detection point 1 via the diode D1 and the Zener diode D2.
And a second transistor Q2 connected to 3.
In addition, one terminal corresponds to the corresponding three-terminal regulator 1
Connected to the output terminals of 1 and 12, the other terminal is the detection point 1
And a third resistor R6, R7 connected to
The cathodes are connected to the output terminals of the corresponding three-terminal regulators 11 and 12, and the anodes are provided with diodes D4 and D5 connected to the detection point 13. The resistor R3
Is an element for lowering the impedance of the base of the second transistor Q2, and the capacitor C2 is an element for preventing malfunction due to noise.

In the overload control circuit 6, the voltage monitoring circuit 7 is 3
This block controls the output current of the emitter of the transistor Q3 by controlling the current of the light emitting diode D9 of the photocoupler 3 when detecting the decrease of the output voltage of the terminal regulators 11 and 12. Then, by controlling the output current of the transistor Q3, the switching operation of the switching circuit 1 is intermittently performed via the switching control circuit 2. That is, in the overload control circuit 6, the cathode of the light emitting diode D9 is connected to the collector, the emitter is grounded, and the base is connected to the plus terminal of the smoothing capacitor C3 via the first resistor R1. It has a first transistor Q1. In addition, one terminal is connected to the base of the first transistor Q1 and the other terminal is grounded, and one terminal is connected to the base of the first transistor Q1 and the other terminal is grounded. Second resistor R2
It has and.

The base current flows through the second transistor Q2 because the voltage at the detection point 13 is the Zener voltage of the Zener diode D2, the forward voltage of the diode D1, and the base voltage of the second transistor Q2. Is higher than the added value of. Therefore, when the Zener voltage is changed, the detection level of the voltage drop changes. Therefore, in this embodiment, by appropriately selecting the Zener voltage, the detection level of the voltage drop is set to the most optimum value according to the output voltage of the three-terminal regulators 11 and 12.

The operation of the embodiment having the above configuration will be described. When no overload occurs in all outputs, the base current flows through the second transistor Q2,
Transistor Q1 is turned off. Therefore, the error value of the DC output 8 is fed back to the primary side by the photocoupler 3 and the error detection circuit 4, and the switching control circuit 2 controls the switching circuit 1. As a result, the DC output 8 is
Stabilizes to the set voltage.

In this state, if the output voltage of the three-terminal regulator 12 becomes 0V due to a short circuit, a current flows from the detection point 13 to the diode D5, and the voltage at the detection point 13 becomes approximately 0.7V. As a result, the second transistor Q2 has a base current of 0 and is turned off. Therefore, the terminal voltage of the delay capacitor C1 becomes approximately 0.7 V after a certain period of time. After this time, during the period when the charged charge remains in the smoothing capacitor C3,
The first transistor Q1 is turned on, and a large current flows through the light emitting diode D9. When a large current flows through the light emitting diode D9, the output current of the transistor Q3 increases, so that the switching control circuit 2 stops the operation of the switching circuit 1. That is, the smoothing capacitor C3
The switching operation on the primary side is stopped during the period in which the charged electric charges of 1 supply the base current to the first transistor Q1.

When the charge stored in the smoothing capacitor C3 is discharged, the current value of the light emitting diode D9 becomes 0 and the output current of the transistor Q3 becomes 0, so that the operation of the switching circuit 1 is restarted and the smoothing capacitor C3 is restarted.
Charging of 3 is started. However, the first transistor Q1
Since the base voltage of 1 is delayed by the delay capacitor C1, the voltage rise is delayed. Therefore, no current flows through the light emitting diode D9, so that the switching circuit 1 does not stop operating. Therefore, the smoothing capacitor C3 is sufficiently charged. That is, the switching circuit 1 performs the switching operation during the period until the voltage of the delay capacitor C1 becomes approximately 0.7V. When the voltage of the delay capacitor C1 becomes approximately 0.7 V, a large current flows through the light emitting diode D9, so that the switching circuit 1 stops its operation again. After that, the same process is repeated.

That is, when the output voltage of the three-terminal regulator 12 falls below the detection level set by the Zener voltage due to overload, the switching circuit 1
Repeats the switching operation and the switching stop. Therefore, the average power introduced to the input terminal of the three-terminal regulator 12 decreases. As a result, the calorific value of the three-terminal regulator 12 does not have a large value. The same is true when the output voltage of the three-terminal regulator 11 is below the detection level, and the switching circuit 1 intermittently performs the switching operation. Therefore, 3-terminal regulator 1
The average power supplied to the first input terminal decreases, and the heat generation amount of the three-terminal regulator 11 does not have a large value.

[0025]

The protection circuit for the switching power supply according to the first aspect of the present invention is provided with a plurality of DC outputs, and by performing feedback that indicates an increase in the voltage error of the DC output due to an increase in the output current of the photocoupler. The voltage of the DC output is stabilized, and at least one DC output of the DC outputs is applied to the switching power supply connected to the input terminal of the three-terminal regulator. A voltage monitoring circuit for detecting a voltage drop at the output terminal of the three-terminal regulator is provided. Further, when the voltage monitoring circuit detects a voltage drop at the output terminal of the three-terminal regulator, the switching operation of the switching circuit is intermittently controlled by controlling the output current of the photocoupler by controlling the current of the light emitting diode of the photocoupler. It is equipped with an overload control circuit. Therefore, when the output of the 3-terminal regulator becomes overloaded and the protection circuit inside the 3-terminal regulator operates,
Since the average power supplied to the terminal regulator is reduced, it is possible to reduce the heat generation amount of the three-terminal regulator during overload.

According to a second aspect of the protection circuit for a switching power supply of the present invention, a current is supplied to the anode of the light emitting diode of the photocoupler from the positive terminal of a smoothing capacitor for smoothing the DC output, and at the same time the photocoupler An error detection circuit for detecting a voltage error is connected to the cathode of the light emitting diode. The collector of the overload control circuit is connected to the cathode of the light emitting diode of the photocoupler, the emitter thereof is grounded, and the base thereof is connected to the positive terminal of the smoothing capacitor via the first resistor. 1 transistor, one terminal of which is connected to the base of the first transistor and the other terminal of which is grounded; and one terminal of which is connected to the base of the first transistor and whose other terminal is grounded. And a second resistor. Further, in the voltage monitoring circuit, its collector is connected to the base of the first transistor, its emitter is grounded, its base is the second transistor connected to the detection point, and one terminal is the output of the three-terminal regulator. A third resistor connected to the terminal and the other terminal connected to the detection point. That is, the voltage monitoring circuit is composed of one transistor and one resistor, and the overload control circuit is composed of one transistor, two resistors, and one capacitor.
That is, the switching operation can be intermittently performed by a simplified circuit including only two transistors, three resistors, and one capacitor.

Further, the protection circuit for a switching power supply according to the invention of claim 3 is applied to a switching power supply in which at least two DC outputs of a plurality of DC outputs are provided with a three-terminal regulator. And 3
The diode is provided corresponding to each of the terminal regulators, the cathode of which is connected to the output terminal of the corresponding three-terminal regulator, and the anode of which is connected to the detection point. Therefore, when the output voltage of any one 3-terminal regulator decreases, a current flows from the detection point to the output terminal side of the 3-terminal regulator via the diode, and the second transistor is turned off. As a result, since the switching operation is intermittent, it is possible to reduce the heat generation amount of the overloaded three-terminal regulator.

Further, the protection circuit of the switching power supply according to the invention of claim 4 has a Zener diode whose anode is connected to the base of the second transistor and whose cathode is connected to the detection point. Therefore, the reference voltage for switching the second transistor on and off, which indicates the detection of overload, is set by the Zener voltage. Therefore, by using the Zener diode having the optimum Zener voltage, It is possible to optimize the detection level of.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an electrical connection of an embodiment of a protection circuit for a switching power supply according to the present invention.

[Explanation of symbols]

 1 Switching Circuit 3 Photocoupler 4 Error Detection Circuit 6 Overload Control Circuit 7 Voltage Monitoring Circuit 8, 9, 10 DC Output 11, 12 3 Terminal Regulator Q1 First Transistor Q2 Second Transistor C1 Delay Capacitor C3 Smoothing Capacitor D2 Zener Diode D9 Light emitting diode R1 First resistance R2 Second resistance R6, R7 Third resistance

Claims (4)

[Claims] 1. The DC output is stabilized by providing a plurality of DC outputs, and performing feedback that indicates an increase in a voltage error of the DC output due to an increase in an output current of a photocoupler. In a switching power supply in which at least one DC output is connected to the input terminal of a three-terminal regulator, a voltage monitoring circuit that detects a voltage drop at the output terminal of the three-terminal regulator, and the voltage monitoring circuit is a voltage at the output terminal. An overload control circuit for intermittently performing the switching operation of the switching circuit by controlling the output current of the photocoupler by controlling the current of the light emitting diode of the photocoupler when detecting the decrease is provided. A protection circuit for switching power supplies. 2. A current is supplied to the anode of the light emitting diode of the photocoupler from the positive terminal of a smoothing capacitor that smoothes the DC output, and the voltage error is applied to the cathode of the light emitting diode of the photocoupler. An error detection circuit for detecting is connected, the cathode of the light emitting diode of the photocoupler is connected to the collector of the overload control circuit, the emitter thereof is grounded, and the base thereof is connected via the first resistor. A first transistor connected to the positive terminal and one terminal connected to the base of the first transistor,
A delay capacitor whose other terminal is grounded, and one terminal connected to the base of the first transistor,
A second resistor whose other terminal is grounded, wherein the voltage monitoring circuit has a collector connected to the base of the first transistor, an emitter grounded, and a base connected to the detection point. 2. The switching device according to claim 1, further comprising a second transistor and a third resistor having one terminal connected to an output terminal of the three-terminal regulator and the other terminal connected to the detection point. Power protection circuit. 3. A switching power supply in which at least two direct current outputs of the plurality of direct current outputs are provided with the three-terminal regulators, respectively. The protection circuit for a switching power supply according to claim 2, further comprising a diode connected to an output terminal of the corresponding three-terminal regulator and having an anode connected to the detection point. 4. A protection circuit for a switching power supply according to claim 2, further comprising a Zener diode whose anode is connected to the base of the second transistor and whose cathode is connected to the detection point. .