JPS5810923A - Switching power supply protecting circuit - Google Patents

Abstract

PURPOSE:To constitute a protection circuit with high reliability by simple constitution, by inserting a detecting resistor in series with an input current/voltage smoothing capacitor, detecting a voltage across the resistor, and controlling the switching of a switching output transistor (TR). CONSTITUTION:A protection circuit A including a detection resistor R9, an input DC voltage smoothing capacitor C1 and a thyristor TH and the like is provided to a switching power supply circuit. When a load is shortened, a high voltage across the resistor R9 is produced with the discharge current from the capacitor C1 to turn on a diode D9. Then, a current flows from a resistor R0 to a connecting point B via the diode D9 and a capacitor C5 is charged to a negative potential. The charge of the capacitor C5 flows as the gate current of the thyristor TH via a resistor R11 to turn on the thyristor TH. Then, a terminal (d) of a control circuit 2 is grounded, the amplitude of a rectangular pulse P1 is decreased, and the switching of a TRQ2 is turned off.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a protection circuit for a switching iiEM circuit,
The purpose is to provide a device that is highly reliable during startup and can safely protect the switching transistor in a load short-circuit condition. First, a typical conventional switching LLE source circuit is
This will be explained using Figure 1. Figure 1 is a circuit diagram of an insulated flyback converter type, and shows a fishing AC 11E i [
ii The output voltage of (1) is rectified through a circuit consisting of diodes H to D4, smoothed by a capacitor 6, and is applied as an unregulated DC voltage to one end of the winding L1 of the transformer 1. The other end of the line Ll is repeatedly switched by the switching transistor Ql under the condition that the output voltage of the secondary m of the transformer Tz is stabilized.

Also, the square wave pulse generated by the switching transistor Yu repeatedly turning on and off in the winding L3 of the transformer 1-2 is as follows.

Rectified and smoothed by diode Da and capacitor C3, control 1
The rectified and smoothed output of the diode D- and the capacitor c3, which is supplied to the terminal -) as the ionization voltage of the circuit 1 and the circuit (2), is supplied to the resistor R3 as a feedback voltage to stabilize the output voltage of this switching power supply circuit. and R4, and is applied to the terminals of the control circuit (2) as a detection voltage. Further, the rectified and smoothed output of the diode D- and the capacitor C1 is applied to the series circuit of the Zener diode D1 and the resistor R, and the terminal voltage of the Zener diode D1 becomes ! ! It is applied as a reference voltage to the terminal () of the lk1 circuit (2), detects the error voltage between the control circuit (in the illusion, the terminals) and (c), and generates a square wave pulse P controlled by this error voltage. ! output from the terminal). This square wave pulse Pl is voltage-divided by resistors R1 and R8 and then drives transistor Q2.

id switching transistor Q1 is formed by the transistor Q, a transformer TI, and a collector resistor Rz)
Driven by the drive path, the transformer T
The secondary output voltage of χ is controlled to be always constant. Note that the square wave pulse PI generated in the winding tz of the transformer T1 is rectified and smoothed by a diode DI and a capacitor c1, and is applied to the drive circuit as a voltage #1. When starting up the switching circuit, power is supplied to the control circuit (2) and the drive circuit through resistors R and R-, respectively, and the switching circuit is started. The output voltage is determined by the winding ratio of the windings L1 and L4 of the transformer T, and a DC stabilized voltage that has been rectified and smoothed by the diode DS and the capacitor C4 is applied to the load (3).

Since the conventional switching power supply circuit is configured in this manner, when the load (3) is switched, an excessive current flows through the switching transistor Q1 and there is a risk of damage to the switching transistor Q1. Therefore, conventionally, a 6-noodle protection circuit has been used, but no one has been proposed that has a simple configuration and has a high @axis property that does not malfunction at the time of startup.

FIG. 2 shows an embodiment of the present invention that can avoid the above-mentioned problems, and the following description will be given based on this FIG. 2.

Components having the same functions as those in FIG. 111 are given the same reference numerals, and their explanations will be omitted.

In FIG. 2, the part surrounded by the broken line is the added protection circuit with the switching voltage H circuit lζ of g t V. R- is a detection resistor, which is interposed between the negative terminal of the capacitor C1 for smoothing the input DC voltage and a reference level (hereinafter referred to as the ground point). rH is a thyristor connected to the terminal shaft of the control circuit (2) so as to be able to switch the amplitude of the square wave pulse P1 generated therein. D- and MHI
is a diode and a resistor connected in series, and this series circuit consists of a diode D- and a resistor connected in series so that the polarity is opposite to the input DC voltage.
The cathode of the resistor R1oa) is connected to the connection point (3) between the capacitor CI and the detection resistor R9, and one end of the resistor R1oa) opposite to the one end (G) of the connection side with the diode Ds is connected to the ground point. Cs is a capacitor for charging and discharging, and the resistor R
to & connected in parallel. A resistor 1 is a resistor connected between the gate of the thyristor 1 and the ground point. The anode of the thyristor positive is connected to the terminal (d) of the Komami U path (2), and the cathode is connected to the connection side end (a) of the paper joint 1G.

Because of this configuration, when the negative current is applied, a current flows out from the capacitor (l), causing a high voltage drop across the detection resistor R#, turning on the diode D9.

When diode DI is turned on, resistance w! A current flows from the intestine to the connection point () through the diode D-, and the capacitor Cm is charged to a positive potential. The charge of the capacitor CI flows through the resistor R11 as a gate current of the thyristor pin, and the thyristor T)I is turned on. When the thyristor 1 is turned on, the droplet (d) of the control circuit (2) is connected to the ground point via the thyristor 1 and the resistor R, so the amplitude of the square wave pulse P decreases and the switching of the transistor is turned off. and the switching output transistor is also turned off. Therefore, it can be safely protected even when the load is short-circuited.

Note that at the time of startup, a current flows in the charging direction to the capacitor CI, and at this time, the potential of the detection resistor is on the positive side and the diode D9 is turned off, so there is very little risk of malfunction. Also, capacitor Cs and resistor RI
Due to the way the time constant of G is selected, even if the diode D is turned on due to J#f, the capacitor Cs is charged to a positive potential in this special number, so the thyristor T)16s will not turn on, making it highly reliable. .

As described above, according to the present invention, in a switching power supply circuit, a detection resistor is inserted in series with an input current/voltage smoothing capacitor, and the voltage across the detection resistor is detected to control switching of a switching output transistor.
Since the switching output transistor is configured to be protected in the event of a load short circuit, it is possible to realize a highly reliable protection circuit that is simple in configuration and does not malfunction at startup.

[Brief explanation of drawings]

Fig. 1 is a block diagram of a conventional switching power supply, and Fig. 2 is a block diagram of a switching power supply equipped with a protection circuit according to an embodiment of the present invention. Q... Switching output transistor, A... Protection circuit, C4... Input DC current smoothing capacitor, R
9...Detection resistor, D,...Diode, Rlo, R
11...Resistance, C. ... Capacitor, 1... Thyristor [semiconductor switching element], P, ... Square wave pulse, (3) -...
Load, Agent Yoshihiro MorimotoFigure 1Figure 2

Claims (1)

[Claims] 1. In a switching power supply circuit, a detection resistor is inserted in series with an input DC voltage smoothing capacitor, and the voltage across the detection resistor is detected to control switching of a switching output transistor, A switching power protection circuit, characterized in that the switching output transistor is protected in the event of a short circuit. L A series circuit of a resistor of $11 and a diode is connected in parallel with a detection resistor connected in series to the input DC voltage smoothing capacitor, with the diode having a different polarity with respect to the input DC voltage, and the first A switching power supply protection circuit according to claim 1, characterized in that a capacitor is connected in parallel to a resistor, and the discharge current of the capacitor controls a semiconductor switching element to turn off a switching output transistor. .