JPH0646560A - Voltage detecting circuit for switching power supply - Google Patents

Abstract

PURPOSE:To prevent overvoltage from occurring upon throw-in of input voltage. CONSTITUTION:The voltage detecting circuit 1 comprises resistors 11, 12 for dividing an output voltage 102, a shunt regulator 15 receiving voltage across the resistor 12, and the load of the shunt regulator 15, i.e., a photocoupler 13 and a resistor 14. The voltage detecting circuit 1 is also provided with a transistor 20 controlling the cathode voltage of the shunt regulator 15, a Zener diode 17 for feeding the transistor 20 with a working voltage, and resistors 18, 19. When an overvoltage appears as the output voltage 102, the transistor 20 functions immediately to lower the cathode voltage of the shunt regulator 15 forcibly. Consequently, a control circuit 3 functions to lower the output voltage 102. This circuitry suppresses overshoot of the output voltage 102 at the time of starting or the like.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a voltage detection circuit for detecting an output voltage which constitutes an output voltage control system of a switching power supply.

[0002]

2. Description of the Related Art Conventionally, as a voltage detection circuit for a switching power supply of this type, there is one shown in FIG. FIG. 2 shows a circuit diagram of a conventional example. The switching power supply switches an input voltage and converts the voltage to output an output voltage, and a voltage detection circuit 4 that detects a voltage change of the output voltage.
And a control circuit 3 for controlling the switching pulse width of the switching circuit 2 by the detection voltage of the voltage detection circuit 4 to keep the output voltage constant.

The voltage detection circuit 4 has resistors 41 and 42 for dividing the output voltage, a shunt regulator 45 having the voltage across the resistor 42 as an input, and a shunt regulator 4.
It is composed of a photocoupler 43 and a resistor 44 which serve as a load of 5. The operation of the voltage detection circuit 4 will be described below.
If the output voltage rises, the voltage across the resistor 44 rises, so the shunt regulator 45 that receives this voltage compares it with the internal reference voltage and drops the cathode voltage that is the output. The input diode current of the photocoupler 43, which is the series load, increases conversely by this amount and raises the detection voltage which is the output voltage of the output transistor. The voltage control circuit 3 receives the detected voltage and narrows the switching pulse width to control the output voltage in the lower direction. When the output voltage drops, the operation reverse to the above is performed to control the output voltage to increase, and the output voltage is always maintained constant.

[0004]

As described above, in the conventional example, the fluctuation of the output voltage is detected by the shunt regulator of the detection circuit. However, this shunt regulator responds to the fluctuation of the input voltage of the output voltage. Is slightly delayed. For this reason, the switching power supply is slightly delayed until a sudden output voltage change is controlled. Therefore, the output voltage rises sharply when the input power is turned on, and an overvoltage (overshoot) occurs until the output voltage is controlled, which causes a problem that the device on the load side is destroyed.

[0005]

A voltage detection circuit for a switching power supply according to the present invention includes first and second resistors for dividing an output voltage of the switching circuit, and the first and second resistors.
The shunt regulator for inputting the voltage at the connection point of the resistor and comparing it with the internal reference voltage and outputting the difference voltage, and the voltage in which the output voltage of the shunt regulator and the output voltage of the switching circuit are connected in series In the voltage detection circuit of the switching power supply, which is provided with a photocoupler for inputting the output voltage to the control circuit via the resistor of
Zener diodes connected in series and each having an output voltage of the switching circuit as an input, and fourth and fifth Zener diodes
And a transistor for inputting the voltage across the fifth resistor and controlling the output voltage of the shunt regulator with the output thereof.

[0006]

An embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a block diagram showing the configuration of this embodiment. The switching power supply switches an input voltage 101 and converts the voltage to output an output voltage 102, a voltage detection circuit 1 that detects a voltage change in the output voltage 102, and a switching circuit 2 that detects a voltage detected by the voltage detection circuit 1. And a control circuit 3 for controlling the switching pulse width and controlling the output voltage 102 to be constant.

The voltage detection circuit 1 has resistors 11 and 12 for dividing the output voltage 102, a shunt regulator 15 having a voltage across the resistor 12 as an input, a photocoupler 13 serving as a load of the shunt regulator 15, and a resistor 14. When,
The transistor 20 includes a transistor 20 that controls the cathode voltage of the shunt regulator 15, a Zener diode 19 that supplies an operating voltage to the transistor 20, and resistors 18 and 19.

Next, the operation of the voltage detection circuit 4 will be described. If the output voltage 102 rises, the voltage across the resistor 14 rises, so the shunt regulator 15 that receives this voltage compares it with the internal reference voltage and drops the cathode voltage that is the output. The input diode current of the photocoupler 13, which is the series load, increases conversely by this amount and raises the detection voltage which is the output voltage of the output transistor. The voltage control circuit 3 receives the detected voltage and narrows the switching pulse width to control the output voltage 102 in the direction of lowering it. When the output voltage 102 drops, the operation reverse to the above is performed to control the output voltage 102 to increase, and the output voltage 102 is always kept constant.

On the other hand, the transistor 20 operates by inputting the voltage across the resistor 19, and this input voltage is generated by the Zener diode 17 when the output voltage 102 becomes an overvoltage. That is, when the output voltage 102 becomes overvoltage, the transistor 20 operates immediately and forcibly lowers the cathode voltage of the shunt regulator 15. As a result, the control circuit 3 controls the output voltage 102 to decrease.
Output voltage 1 generated at the time of startup by this control
The 02 overshoot voltage is suppressed.

[0010]

As described above, according to the present invention, the overvoltage of the output voltage is forcibly suppressed by the simple circuit, so that the overshoot voltage generated at the time of starting the power supply can be prevented. Therefore, there is an effect that the device on the load side is not destroyed.

[Brief description of drawings]

FIG. 1 is a block diagram of an embodiment of the present invention.

FIG. 2 is a block diagram of a conventional example.

[Explanation of symbols]

 1 Voltage Detection Circuit 2 Switching Circuit 3 Control Circuit 11, 12, 14, 18, 19 Resistor 13 Photocoupler 15 Shunt Regulator 16 Capacitor 17 Zener Diode 20 Transistor

Claims (1)

[Claims] 1. A voltage difference between the first and second resistors for dividing the output voltage of the switching circuit and the connection point of the first and second resistors is input and compared with an internal reference voltage. A switching circuit including a shunt regulator for outputting the output voltage and a photocoupler for inputting a voltage in which the output voltage of the shunt regulator and the output voltage of the switching circuit are connected in series through a third resistor and outputting the output voltage to the control circuit. In a voltage detection circuit of a power supply, a Zener diode, a fourth and a fifth resistor connected in series, each of which receives an output voltage of the switching circuit as an input, and a voltage across the fifth resistor are input to A voltage detection circuit for a switching power supply, comprising: a transistor that controls an output voltage of the shunt regulator with an output.