JPH08256474A - Overvoltage protective circuit - Google Patents

Abstract

PURPOSE: To provide an overvoltage protective circuit which protects load from the abnormal rise of output voltage caused by the fault of a power circuit, pertaining to a self excitation switching power source. CONSTITUTION: This overvoltage protective circuit is provided with an on period limiting circuit, which limits the on period of a switching transistor 4, a negative rectifier circuit 11, which takes out negative voltage from the feedback winding N3 of a switching transformer 5, and a detection circuit 12 which detects the output voltage of the negative rectifier circuit 11 falling to a certain voltage or under. When the output voltage goes up, the output voltage of the rectifier circuit 11 drops proportionally, and when it reaches a certain voltage or under, the detection signal is transmitted from the detection circuit 12 to the on period limiting circuit 8 so as to limit the on period of the switching transistor 4.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a color television receiver,
Related to power supply circuits such as color display devices,
The present invention relates to an overvoltage protection circuit for a self-excited switching power supply that protects a load circuit from an increase in output voltage due to a failure of a power supply circuit, that is, an overvoltage.

[0002]

2. Description of the Related Art A conventional protection circuit for a self-excited switching power supply circuit encloses a detection resistor in series with a switching element,
When the load circuit fails and goes into an overload state, the voltage drop of the detection resistor increases.A circuit is provided to limit the ON period of the switching element to reduce the output voltage, that is, an overcurrent with drooping characteristics. It was a protection circuit. Such a known example is described in Japanese Utility Model Publication No. 1-127331.

[0003]

In the above-mentioned prior art, although the load circuit is protected in the overload state, the load cannot be protected from the abnormal rise of the output voltage due to the failure of the power supply circuit, that is, the overvoltage.

An object of the present invention is to provide an overvoltage protection circuit which protects a load from overvoltage.

[0005]

In order to achieve the above object, the present invention detects a negative rectifier circuit for rectifying a negative voltage of a feedback winding of a switching transformer, and detecting that the negative rectified voltage becomes a certain voltage or less. And a turn-on period limiting circuit that limits the turn-on period of the switching element that receives the detection signal.

[0006]

[Operation] When an unregulated power supply is input to the power supply circuit, power is supplied to the switching element via the starting resistance to turn on the switching element, and thereafter, power is supplied from the feedback winding of the switching transformer to cause self-excited oscillation. start. Further, by controlling the on / off time ratio of the switching element, the stabilized power supply is supplied to the output. On the other hand, by negatively rectifying the feedback winding, a negative voltage proportional to the output voltage is taken out.

When a failure occurs in the power supply circuit and the output voltage increases, the negative rectified voltage also decreases in proportion to it. When the negative rectified voltage falls below a certain voltage, the detection circuit detects it and sends a detection signal to the ON period limiting circuit to limit the ON period of the switching element, that is, decrease the ON / OFF time ratio to increase the output voltage. suppress.

[0008]

Embodiments of the present invention will be described below with reference to FIGS.

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

In FIG. 1, 1 is an unregulated power supply, 2 is a switch, 3 is a starting resistor, 4 is a switching transistor which is a switching element, 5 is a switching transformer,
6 is a detection resistor that detects an overload state, 7 is a bias circuit that applies a bias from the feedback winding N3 of the switching transformer 5 to the switching transistor 4, and controls the on / off time ratio. 8 is a voltage drop of the detection resistor 6. Becomes larger, the ON period limiting circuit that limits the ON period of the switching transistor 4, 9 is a rectifying circuit that rectifies the output voltage of the secondary winding N2 of the switching transformer 5, 10 is a load circuit, and 11 is the output of the feedback winding N3. A negative rectifier circuit that rectifies the voltage to a negative voltage, and 12 is a detection circuit that detects that the voltage of the negative rectifier circuit 11 becomes a certain voltage or less.

In FIG. 1, when the switch 2 is turned on, a current flows from the unstabilized power source 1 to the switching transistor 4 via the starting resistor 3 to turn on the switching transistor 4. After that, the feedback winding N3 of the switching transformer 5
A bias is applied to the switching transistor 4 via the bias circuit 7, self-excited oscillation starts, and a voltage is output to the secondary winding N2 of the switching transformer 5 to rectify the voltage.
Is rectified by and is supplied to the load 10. Further, the bias circuit 7 controls the on / off time ratio of the switching transistor 4 so that the output voltage becomes constant. On the other hand, the feedback winding N3 oscillates by self-excited oscillation, so that the feedback winding N3 is wound in a polarity opposite to that of the secondary winding N2. In order to obtain a voltage proportional to the output voltage of the rectifying circuit 9, the negative rectifying circuit 11 rectifies the negative voltage. It When a failure occurs in the power supply circuit, for example, the control function of the bias circuit 7 is stopped, and when the on / off time ratio increases, the output voltage of the rectifier circuit 9 increases. At the same time, the negative rectifier circuit 1
The output voltage of 1 also drops proportionally. When the output voltage of the negative rectifier circuit 11 becomes a certain voltage or less, the detection signal is transmitted from the detection circuit 12 to the ON period limiting circuit 8, and the ON period of the switching transistor 4 is limited to reduce the ON / OFF time ratio. The rise of the output voltage of the circuit 9 is suppressed.

FIG. 2 shows an ON period limiting circuit 8 and a detection circuit 1.
2 is a specific circuit example of No. 2.

2 is different from FIG. 1 in that 13 is a transistor for limiting the ON period of the switching transistor 4, 14 is a Zener diode for detecting that the voltage of the negative rectifier circuit 11 is below a certain voltage, and 15 is A transistor that detects that a current flows through the Zener diode 14 and operates the on-period limiting transistor 13;
Reference numeral 16 is a resistor that prevents the transistor 15 from malfunctioning due to the leakage current of the Zener diode 14.

In FIG. 2, when the output voltage of the rectifier circuit 9 rises and the output voltage of the negative rectifier circuit 11 decreases due to the failure of the power supply circuit, and the voltage across the Zener diode 14 exceeds the Zener voltage, a current flows through the Zener diode 14. The transistor 15 detects the flowing current, operates the on-period control transistor 13 and limits the on-period of the switching transistor 4, thereby suppressing an increase in the output voltage of the rectifier circuit 9.

[0015]

According to the present invention, a load can be protected by suppressing an increase in output voltage due to a failure of a power supply circuit.

[Brief description of drawings]

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

FIG. 2 is a specific circuit diagram of an ON period limiting circuit 8 and a detection circuit 12 of FIG.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Unstabilized power supply, 2 ... Switch, 3 ... Start-up resistance, 4 ... Switching transistor, 5 ... Switching transformer, 6 ... Detecting resistance, 7 ... Bias circuit, 8 ... ON period limiting circuit, 9 ... Rectifier circuit, 10 ... Load circuit, 11 ... Negative rectification circuit, 12 ... Detection circuit.

Claims (1)

[Claims] 1. A self-excited switching power supply circuit for switching a power supplied from an unregulated power supply circuit to a primary winding of a switching transformer by a switching element to obtain a stabilized voltage on a secondary winding of the switching transformer, An on period limiting means for limiting an on period of the switching element, a rectifying means for rectifying a negative voltage of the feedback winding of the switching transformer, and a detecting means for detecting that the negative rectified voltage becomes a predetermined voltage or less are provided. An overvoltage protection circuit characterized in that