JPH0360366A - Switching regulator - Google Patents

Abstract

PURPOSE:To protect smoothing capacitors on the secondary side of a transformer from dielectric breakdown by a method wherein, when a voltage between the output lines of a rectifier smoothing circuit exceeds a certain value, the output lines are short-circuited by a short-circuiting circuit. CONSTITUTION:When induced voltages on the secondary winding N2 and the auxiliary winding Nb of a transformer are elevated, the charging voltage of a capacitor C3 is increased. If the charging voltage exceeds the Zener voltage of a Zener diode ZD2, the diode ZD2 is turned ON. A thyristor SCR is turned ON by the discharge of a capacitor C4 and short-circuits the output lines of a rectifier smoothing circuit composed of a diode D2 and the capacitor C3. The source voltage of a switching control circuit 1 becomes almost 0 volt and the operation is discontinued while an output terminal OUT is at a ground potential. As a result, the induced voltages on the secondary winding N2 and the auxiliary winding Nb of the transformer become 0 volt, so that the dielectric breakdown of the capacitors C2 and C3 can be avoided.

Description

DETAILED DESCRIPTION OF THE INVENTION (Al Industrial Field of Application) The present invention relates to a switching regulator used in a constant voltage power supply circuit or the like.

(b) Prior Art In general, separately excited oscillation type switching regulators use switching regulator ICs whose basic configuration is a PWM control circuit using a triangular wave oscillator and a comparator as a switching control circuit.

A circuit diagram of a conventional switching regulator using such a switching regulator IC is shown in FIG. In the figure, 1 is a switching control circuit consisting of a switching regulator IC, 2 is a transformer, and a switching transistor Q is connected to the primary winding Nl of the transformer 2.
1 is connected. The switching transistor Q1 is controlled on/off by the OUT output of the switching control circuit 1. Also, 4 is a diode bridge,
Rectify AC input. Capacitor C1 smoothes this and supplies DC voltage to the primary winding Nl of the transformer. Diode D in the secondary S&%N2 of the transformer! A rectifying and smoothing circuit consisting of a capacitor C2 and a capacitor C2 is connected thereto. Connected to the output of this rectifying and smoothing circuit is a load supply voltage detection circuit comprising a voltage dividing circuit comprising resistors R2 and R3, a shunt regulator SR, and a photocoupler light emitting diode 3a. A diode D2 is connected to the auxiliary winding Nb of the transformer.
A rectifying and smoothing circuit consisting of a capacitor C3 and a capacitor C3 is connected, and its output is supplied as a power source for the switching control circuit 1. Note that at startup, power is supplied from a constant voltage circuit consisting of a resistor R1 and a Zener diode ZD1.

A reference voltage is generated from a terminal Vr of the switching control circuit 1, and the output of a voltage dividing circuit constituted by resistors R4 and R5 and a phototransistor 3b of a photocoupler is applied to a signal input terminal El.

In the circuit shown in Figure 3, the shunt regulator S
R causes the light emitting diode 3a to emit light with an amount of light depending on the load supply voltage. Accordingly, the input voltage of the input terminal El of the switching control circuit 1 is determined. The switching control circuit 1 controls the switching transistor Ql on/off with a duty ratio according to the voltage of the input terminal El. Since the duty ratio control of the switching transistor in response to fluctuations in the load supply voltage is negative feedback control, the load supply voltage is stabilized according to the voltage division ratio of the resistors R2 and R3.

(C) Problem to be Solved by the Invention However, in the conventional switching regulator using the switching regulator IC constituting the switching control circuit as described above, when the input circuit of the switching control circuit is in an open state, For example, resistor R3
If the shunt regulator S becomes open or the shunt regulator
When the input section of R is disconnected, or when the EI input terminal of the switching control circuit 1 itself becomes open, the input voltage at the input terminal El of the switching control circuit 1 drops to the lowest voltage. Therefore, switching transistor Q1
The pulse width of the on-period becomes maximum, causing a malfunction in which the output voltage jumps. As a result, an overvoltage is supplied to the load, which not only causes malfunction or failure of the load, but also causes breakdown of smoothing capacitors c2 and C3 provided on the secondary side of the transformer.

Conventionally, an overvoltage protection circuit is sometimes provided on the primary or secondary side of a transformer to protect the load from overvoltage. , a shutdown circuit that uses a switch element such as Cyrisk and its gate control circuit to cut off power input or power output by the output of an overvoltage detection circuit, requires a relatively large number of components, and requires a high withstand voltage or A switch element with a large current capacity was required.

An object of the present invention is to configure a circuit that prevents the output voltage from rising when the circuit connected to the input of the switch control circuit becomes open for some reason using an extremely simple circuit. An object of the present invention is to provide a switching regulator which prevents voltage breakdown of a smoothing capacitor provided in the switching regulator.

(d) Means for Solving the Problems The present invention includes a transformer, a switching transistor that cuts off the current flowing through the primary winding of the transformer, a switching control circuit that controls on/off the switching transistor, and a secondary winding of the transformer. In a switching regulator having a rectifying and smoothing circuit that rectifies and smoothes the electromotive force of a winding or an auxiliary winding and supplies power to the switching control circuit, when the voltage between the output lines of the rectifying and smoothing circuit exceeds a certain value, A feature is that a short circuit is provided to short-circuit the output lines.

(e) Function In the switching regulator of the present invention, a rectifying and smoothing circuit is provided in the secondary winding or auxiliary winding of the transformer as an auxiliary power supply circuit for one switching control circuit, and between the output lines of this rectifying and smoothing circuit, A short circuit is provided that shorts when the voltage between the output lines exceeds a certain value. Therefore, if the circuit that provides the load supply voltage detection signal to the error amplification circuit provided in the switching control circuit becomes open, the power supplied to the transformer from the transformer primary winding increases, and the secondary The electromotive voltage of the next winding and the auxiliary winding increases, but when the output voltage of the rectifier and smoothing circuit exceeds a certain value, the short circuit short-circuits the output lines of the rectifier and smoothing circuit. As a result, the power supply voltage of the switching control circuit becomes approximately 0, and the on/off control of the switching transistor is stopped. Therefore, the electromotive voltage in the secondary winding or the auxiliary winding of the transformer becomes zero, thereby preventing breakdown of the smoothing capacitor.

(f) Embodiment FIG. 1 shows a circuit diagram of a switching regulator which is an embodiment of the present invention.

In FIG. 1, 1 is a switching control circuit consisting of a switching regulator IC, and 2 is a transformer. 4
is a diode bridge that rectifies AC input such as a commercial AC power supply, and C1 is a smoothing capacitor that smooths it.
The rectified and smoothed output is supplied to the primary winding N1 of the transformer. A switching transistor Q1 is connected in series to the primary winding N1 of the transformer 2. A PWM-controlled rectangular wave signal is output from the OUT terminal of the switching control circuit l, and this signal is supplied to the gate of the switching transistor Ql. A rectifier and smoothing circuit including a rectifier diode DI and a smoothing capacitor C2 is connected to the secondary winding N2 of the transformer 2.

Further, a load supply voltage detection circuit consisting of resistive voltage dividing circuits R2 and R3, a shunt regulator SR, and a photocoupler light emitting diode 3a is provided between the output terminals.

Further, a rectifying and smoothing circuit including a diode D2 and a capacitor C3 is connected to the auxiliary winding Nb of the transformer, and supplies a power supply voltage to the switching control circuit 1.

The resistor R1 and the Zener diode ZDI are a constant voltage circuit that supplies a power supply voltage to the switching control circuit 1 via the diode D3 at the time of startup. A voltage dividing circuit made up of R4, R5 and a phototransistor 3b of a photocoupler is connected between the reference voltage output terminal Vr of the switching control circuit 10 and the ground (GND), and the divided voltage value thereof is applied to the input terminal Fl.

Furthermore, a constant voltage circuit and a SIRISK SCR, each consisting of a Zener diode ZD2 and a resistor R6, are connected between the output line of the rectifying and smoothing circuit consisting of a diode D2 and a capacitor C3, and the ground. Also, this thyristor SC
The operation of the circuit shown in FIG. 1 in which a capacitor C4 is connected between the gate of R and the cathode (ground) is as follows.a Normally, the switching transistor Q1 is controlled by PWM control output from the switching control circuit l. The transformer is turned on/off by a rectangular wave signal, and the current flowing through the primary winding of the transformer is interrupted. As a result, electromotive voltages are generated in the secondary winding N2 and the auxiliary winding Nb of the transformer, and are rectified and smoothed, respectively. The amount of light emitted from the light emitting diode 3a of the photocoupler changes depending on the load supply voltage, thereby changing the on-resistance of the phototransistor 3b of the photocoupler, and changing the voltage applied to the El terminal of the switching control circuit 1. Since the duty ratio control of the switching transistor Ql with respect to fluctuations in the load supply voltage is negative feedback control, the load supply voltage is stabilized according to the voltage division ratio of the resistors R2 and R3.

If for some reason the load supply voltage detection circuit provided at the input end of the switching control circuit 1 and the circuits connected thereto, such as the resistor R3 and the shunt regulator SR, become open, or if the switching control circuit 1 When the input terminal El of the circuit 1 is in an open state, the voltage applied to the input terminal E2 is extremely reduced, so that the on-duty ratio of the switching transistor Q1 reaches its maximum value. This allows the transformer's secondary winding N
2 and the auxiliary winding Nb increase, and the capacitor C
3 charging voltage increases. When this charging voltage exceeds the Zener voltage of the Zener diode ZD2, Zn2 becomes conductive and a gate current flows to the gate of the thyristor SCR.

At this time, the charge stored in the capacitor C4 is discharged as a gate current. As a result, the thyristor SC
R is turned on, and the output line of the rectifying and smoothing circuit consisting of D2 and C3 is short-circuited. Therefore, the power supply voltage of the switching control circuit becomes approximately Ov, and the operation is stopped while keeping the output terminal OUT at the ground potential. As a result, the electromotive voltage of the secondary winding 2 and the auxiliary winding Nb of the transformer becomes O, thereby preventing voltage breakdown of the capacitors C2 and C3.

If the thyristor SCR is partially turned on, the diode bridge 4. Since a holding current flows through the resistor R1 and the diode D3, the on state is maintained. After that, if you turn off the power switch (not shown) and cut off the AC input, the thyristor SCR will turn off, and after repairing the faulty part,
The circuit will be restored by turning the power switch back on.

In the example shown in Figure 1, the short circuit is configured with a constant voltage circuit using a Zener diode and a switching element such as a silice, but it is also possible to configure a short circuit without using a Zener diode as shown below. can. Second
In the figure, resistors R7 and R8 are a voltage dividing circuit, and a gate current flows through the thyristor SCH according to the divided output voltage. Since the breakover voltage of Cyrisk changes depending on the gate current, when the output voltage of the rectifying and smoothing circuit consisting of the diode D2 and the capacitor C3 exceeds a certain value, the breakover voltage decreases due to the increase in the gate current, and the circuit is turned on.

(Effects of the IE Invention According to this invention, even if the load supply voltage detection circuit provided on the input side of the switching control circuit and a part of the circuit connected thereto are opened, the secondary side of the transformer is This prevents the output voltage of the rectifier and smoothing circuit provided from rising, and prevents voltage breakdown of the smoothing capacitor.Also, since the auxiliary power supply of the switching control circuit is only short-circuited, the low withstand voltage and low current capacity can be reduced. A switch element can be used. Furthermore, since no overvoltage detection circuit or the like is provided on the secondary side of the transformer, a photocoupler such as a photocoupler is not required, and miniaturization and cost reduction can be achieved.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram of a switching regulator that is an embodiment of the present invention. FIG. 2 is a circuit diagram of the main parts of a switching regulator according to another embodiment. FIG. 3 is a circuit diagram of a conventional switching regulator. 1-Switching control circuit, 2-Transformer, Ql-Switching transistor, C2, C3,-Smoothing capacitor, 5-Short circuit. Outhole

Claims (1)

[Claims] (1) A transformer, a switching transistor that intermittents the current flowing through the primary winding of the transformer, a switching control circuit that controls on/off the switching transistor, and a rectifier that rectifies the electromotive force of the secondary winding or auxiliary winding of the transformer. In a switching regulator having a rectifying and smoothing circuit that smooths and supplies power to the switching control circuit, a short circuit is provided that shorts the output lines when the voltage between the output lines of the rectifying and smoothing circuit exceeds a certain value. A switching regulator characterized by: