JPH08228473A - Protective circuit of switching power source - Google Patents

Abstract

PURPOSE: To quicken the function of the operation of a protective circuit by sending out an abnormality signal only when the switching waveform to a main switch element and the voltage waveform of a flywheel diode are discordant, and breaking a switching circuit from the power source. CONSTITUTION: Energy being accumulated in an inductance L1 on output side while a main switching element is on is supplied to load RL while the main switching element Q1 is off. In that case, the switching circuit (composed of an IC for switching signal, a transistor Q2, and resistors R1-R3) is broken from the power source E by comparing the switching signal waveform of the main switching element Q1 with the voltage waveform of a flywheel diode D1, and sending out an abnormality signal only when both waveforms are discordant, and short-circuiting the input side of the switching power source E. Hereby, the function of the protective circuit can be quickened, and the influence of thermal breakage to other parts by the delay of heat conduction can be avoided.

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, for example, a short-circuit protection circuit for a flywheel diode on the output side of a chopper circuit.

[0002]

2. Description of the Related Art Although the prior art is not particularly shown,
Basically, the invention part A is removed from the circuit diagram of the embodiment of the present invention shown in FIG.

Although the embodiment is shown by a chopper circuit, when the flywheel diode D1 is short-circuited, the main switching element Q1 carries a current with a voltage, and its base signal is turned off. If the input power is not shut off, the main switching element will be damaged by heat.

(2) However, since there is a time lag until thermal destruction, there is a possibility that the printed circuit board or the like will be burned due to overheating of the main switching element and the damaged portion will be enlarged.

As a countermeasure against this, a main switching element,
A thermally coupled temperature fuse is provided, and the input side is shut off by this temperature fuse.

However, as the drawbacks of these conventional measures, the temperature fuse is expensive, and the fixing method in which the thermal fuse of the temperature fuse is improved has a problem in workability such as attaching an adhesive. In addition, there was a problem that the heat conduction was delayed and the thermal damage to other parts spread.

[0007]

SUMMARY OF THE INVENTION In order to solve the above problems, the present invention is not a method of detecting a failure mode by heat but a short circuit of a flywheel diode is detected by an electric signal mode.
The function of the protection circuit operation is accelerated. Therefore, an inexpensive and highly reliable protection circuit for a switching power supply can be provided.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a circuit diagram of a switching power supply provided with a protection circuit of the present invention, which is shown as a chopper circuit as an example. FIG. 2 shows waveforms at various parts when the flywheel diode is short-circuited.

In FIG. 1, a DC power source E, a main switching element Q1, a flywheel diode D1, an inductance L1, a smoothing capacitor C1 and a load RL constitute a main circuit of a chopper circuit.

Switching signal IC, transistor Q
The switching signal circuit is composed of 2 and the resistors R1, R2, and R3. The IC detects the output voltage and performs voltage control (3).

An output overvoltage detection protection circuit is constituted by the thyristor THY1, Zener diode D3 and resistors R8 and R9, and the circuit is disconnected by the input short circuit at the time of overvoltage.

The above-mentioned conventional circuit is a known technique and its operation is omitted. The portion related to the present invention is a short circuit mode detection circuit A portion surrounded by a dotted line in the figure, and the operation of this portion will be described with reference to the waveforms in FIG.

In the portion A of the figure, D2 is a diode and C2 is a diode.
Are capacitors, Q3, Q4 are transistors and R4, R5,
R6 and R7 are resistors. And (a), (b) in the figure,
Figure 2 shows the waveforms in the failure mode for the parts shown in (c) and (d).
Shown in

In a normal operation state, the switching signal IC outputs a signal as shown in FIG. 2A to turn on and off the main switching element Q1. Therefore, the cathode voltage of the flywheel diode D1 is shown in Fig. 2 (b).
It has a waveform like.

The two signals are compared and detected in the short circuit mode detection circuit A.

It is assumed that the flywheel diode D1 is short-circuited at the third pulse in FIG. At that point, the pulse waveform of (b) disappears, so that the emitter potential of the transistor Q3 remains 0 volt,
By the switching signal of (a), the capacitor C2,
Therefore, the potential of (c) is repeatedly charged.

When the base potential of the transistor Q3, that is, the potential of (c) is charged up to about 0.7 V (4), the transistor Q3 is turned on, and therefore the thyristor TH.
The gate signal of Y1 is applied as shown in (d), the thyristor THY1 is turned on, and the input side is short-circuited. As a result, the fuse F on the input side is shut off, and the circuit is disconnected from the power source and protected.

The circuit including the thyristor THY1 also functions as an overvoltage detection protection circuit and also has an overvoltage protection function of shorting the input side and disconnecting the circuit from the power supply when the output voltage is an overvoltage.

Although not shown, it is also possible to stop the switching signal by inputting the signal in which the transistor Q3 is turned on by the abnormal mode signal to the switching signal IC.

[0020]

The short circuit mode detection circuit of the present invention can be mounted on a substrate as an inserter component, or can be a hybrid IC.
As a result, the number of assembly steps can be reduced, and an inexpensive and highly reliable switching power supply can be provided.

[Brief description of drawings]

FIG. 1 is a switching power supply including a protection circuit of the present invention (one embodiment shown by a chopper circuit).

[Fig.2] Waveform of each part in failure mode

[Explanation of symbols]

 E DC power supply F Phase Q1 Main switching element D1 Flywheel diode L1 Inductance (5) C1 Smoothing capacitor RL Load IC IC for switching signal Q2 Transistor R1, R2, R3 Resistor THY1 Thyristor D3 Zener diode R8, R9 resistance A Short circuit mode detection circuit according to the present invention D2 diode C2 capacitor Q3, Q4 transistor R4, R5, R6, R7 resistance

Claims (2)

[Claims] 1. A flywheel diode configured to supply energy stored in an output-side inductance to a load while the main switching element is on, to a load when the main switching element is off. In the switching power supply, the switching signal waveform to the main switching element and the flywheel diode voltage waveform are compared, and an abnormal signal is sent out only when the two are inconsistent signals, and the input side of the switching power supply is A switching power supply protection circuit characterized in that the switching circuit is cut off from the power supply by short-circuiting. 2. The protection circuit for a switching power supply according to claim 1, wherein the switching signal of the switching power supply is stopped by the abnormal signal.