JPH02269470A - Switching regulator - Google Patents

Abstract

PURPOSE:To prevent the breakdown of dielectric strength of a capacitor for smoothing installed on the secondary side of a transformer by connecting a Zener diode conducting when the output voltage of a rectifying smoothing circuit exceeds a fixed value between an output from the rectifying smoothing circuit and an input to a maximum duty-ratio limiting circuit. CONSTITUTION:A maximum duty-ratio limiting circuit is composed of a comparator 11, to which a triangular wave signal and the input signal of a terminal DB are input, and a gate circuit 14 generating the AND signal of the output signal of the comparator 11. A rectifying smoothing circuit consisting of smoothing capacitors C2, C3, etc., is connected to the secondary winding N2 of a transformer 2, and a rectifying smoothing circuit made up a capacitor C4, etc., is connected to the secondary winding N3 of the transformer. A Zener diode ZD2 is connected between an output from the rectifying smoothing circuit and an input to the maximum duty-ratio limiting circuit. The Zener diode ZD2 conducts when the output voltage of the rectifying smoothing circuit exceeds a fixed value. Accordingly, the breakdown of dielectric strength of the capacitors C2, C4 for smoothing connected on the secondary side is prevented.

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, in separately excited oscillation type switching regulators, the switching regulator type 1c whose basic configuration is a PWM control circuit using a triangular wave oscillator and a comparator as a switching control circuit is used.

FIG. 4 shows a circuit diagram of a conventional switching regulator using such a switching regulator LC. In the figure, 1 is a switching control circuit consisting of a switching regulator IC, 2 is a transformer,
A switching transistor Q1 is connected to the primary winding N1 of the transformer 2, and Ql is controlled on/off by the E output of the switching control circuit 1. Further, 4 is a diode bridge which rectifies the AC input, smoothes it with a capacitor C1, and supplies a DC voltage to the primary 5kiN1 of the transformer. A rectifying and smoothing circuit including a diode D1, capacitors C2 and C3, and a choke coil L is connected to the secondary winding N2 of the transformer. 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 rectifying and smoothing circuit consisting of D2 and C4 is connected to the other winding N3 on the secondary side of the transformer, and its output is supplied as a power source to 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 the terminal REF of the switching control circuit 1, and the resistors R4 and R5 and the phototransistor 3 of the photocoupler
The output of the voltage divider circuit configured by b is the signal input terminal NI
given to.

In the circuit shown in Figure 4, 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. In response to this, the input voltage of the input terminal Nl of the switching control circuit 1 is determined. The switching control circuit 1 controls the switching transistor Q1 on/off with a duty ratio according to the voltage of the input terminal Nl. 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.

(C1 Problem to be Solved by the Invention However, in the conventional switching regulator using the switching regulator IC that constitutes the switching control circuit as described above, when the input circuit of the switching control circuit is in an open state, e.g. Resistor R3
If the shunt regulator S becomes open or the shunt regulator
When the input section of R is disconnected, the input voltage at the input terminal Nl of the switching control circuit 1 drops to the lowest voltage. Therefore, the pulse width during the ON period of the switching transistor Q1 becomes maximum, causing a malfunction in which the output voltage jumps. As a result, an overvoltage is supplied to the load, causing not only malfunction or failure of the load, but also problems such as breakdown of smoothing capacitors C2, C3, and 04 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 cuts off the power input or output using the output of the overvoltage detection circuit using a switch element such as Cyrisk and its gate control circuit, and requires a relatively large number of components.

An object of the present invention is to configure a circuit that prevents the output voltage from rising even if the circuit connected to the input of the switch control circuit becomes open for some reason, using an extremely simple circuit, so that the secondary side of the transformer 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.

(Means for Solving the d1 Problems) This invention provides a switching transistor on the primary side of a transformer for switching on and off the primary winding current of the transformer,
A rectifying and smoothing circuit is provided on the secondary side of the transformer, a triangular wave oscillator, a comparator that compares the output voltage of the triangular wave oscillator with the output voltage of the error amplifier circuit and generates a PWM signal, and an input voltage increaser. Accordingly, the PWM
In a switching regulator including a maximum duty ratio limiting circuit that reduces a maximum duty ratio of a signal and provided with a switching control circuit that controls on/off of the switching transistor, conduction occurs when the output voltage of the rectifying and smoothing circuit exceeds a certain value. The present invention is characterized in that a Zener diode is connected between the output of the rectifying and smoothing circuit and the input of the maximum duty ratio limiting circuit.

(e) Effect In the switching regulator of the present invention, a Zener diode is connected between the output of the rectifier and smoothing circuit and the input of the maximum duty ratio limiting circuit, and this Zener diode is connected so that the output voltage of the rectifier and smoothing circuit remains at a constant value. When it exceeds, it becomes conductive. When the Zener diode conducts, the output voltage of the rectifier and smoothing circuit (more precisely, the voltage obtained by subtracting the Zener voltage of the Zener diode from the output voltage of the rectifier and smoother) is applied to the input of the duty ratio limiting circuit, and the maximum duty ratio of the PWM signal is is limited. This reduces the on-duty ratio of the switching transistor and reduces the output voltage of the rectifier and smoothing circuit. Therefore, even if the circuit that provides the load supply voltage detection signal to the error amplifier circuit becomes open for some reason, the output voltage of the rectifier and smoothing circuit is prevented from rising, and the smoothing circuit connected to the secondary side This prevents the voltage breakdown of the capacitor used.

(fl Embodiment) The basic configuration of a switching control circuit used in a switching regulator which is an embodiment of this invention and the voltage waveforms of each part thereof are shown in FIGS. 2 and 3. In FIG. 2, 10 is connected to terminals R and C. A triangular wave oscillator that generates a triangular wave signal at a frequency determined by a connected CR constant, 1
1 is a comparator that receives the triangular wave signal and the input signal of terminal DB; 13 is an error amplification circuit that amplifies the input voltage difference between terminals Nl and I; and 12 is a triangular wave signal and error amplification circuit 1.
14 is a gate circuit that generates an AND signal of the output signals of comparators 11 and 12, 15 is an output transistor, and 16 is a terminal V
This is a reference voltage generation circuit that generates a reference voltage from the voltage supplied between c c and GND, supplies it to each part, and outputs it to the outside from a terminal REF. Here comparator 1
1 and the gate circuit 14 correspond to the "maximum duty ratio limiting circuit" according to the present invention.

Each waveform of (a) to (d) shown in Fig. 3 is a in Fig. 2.
This is the voltage signal at the location indicated by wd. Error amplification circuit 13
If the output voltage of is vl shown in Fig. 3, comparator 1
The output signal C of No. 2 is a rectangular wave signal having a duty ratio as shown in FIG. 3 (C). However, when the input voltage of the DB terminal is the voltage shown by v2 in FIG. 3, the output signal of the comparator 11 becomes a rectangular wave signal with a low duty ratio as shown in FIG. 3(b). As a result, the output transistor 15 is driven at a low duty ratio as shown in FIG. 3(d).

FIG. 1 shows a circuit diagram of a switching regulator that 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 E terminal of the switching control circuit 1, and this signal is supplied to the gate of the switching transistor Q1 via resistors R6 and R7.

A rectifier diode D1 is connected to the secondary winding 1JIN2 of the transformer 2.
, a rectifying and smoothing circuit including smoothing capacitors C2 and C3 and a choke coil L are connected. Also, between the output terminals are resistor voltage divider circuits R2 and R3, and a shunt regulator SR.
A load supply voltage detection circuit consisting of a photocoupler light emitting diode 3a is also provided. Further, a rectifying and smoothing circuit consisting of a diode D2 and a capacitor C4 is connected to the secondary winding N3 of the transformer, and supplies a power supply voltage to the switching control circuit l. The resistor R1 and the Zener diode ZD1 are a constant voltage circuit that supplies a power supply voltage to the switching control circuit 1 via the diode D3 during startup. The reference voltage output terminal REF of the switching control circuit 1 and the ground (GND
) is connected between R4, R5 and a phototransistor 3b of a photocoupler, and the voltage dividing circuit is applied to the input terminal Nl. Further, a resistive voltage dividing circuit consisting of R8 and R9 is connected between the reference voltage output terminal REF and the ground, and the voltage dividing value thereof is applied to the input terminal DB of the maximum duty ratio limiting circuit.

This terminal DB and the above-mentioned D2. A Zener diode ZD2 is connected between the outputs of the rectifying and smoothing circuit composed of C4.

The operation of the circuit shown in FIG. 1 is as follows.Normally, the switching transistor Q1 is connected to the switching control circuit 1.
It is driven on/off by a PWM-controlled rectangular wave signal output from the transformer, and the current flowing through the primary winding N1 of the transformer is interrupted. This generates electromotive voltages in the secondary windings N2 and N3 of the transformer, which are rectified and smoothed, respectively. The light intensity of the light emitting diode 3a of the photocoupler changes depending on the load supply voltage, which changes the on-resistance of the phototransistor 3b of the photocoupler, and the voltage applied to the NI terminal of the switching control circuit 1 changes. Since the duty ratio control of the switching transistor Q1 with respect to fluctuations in the load supply voltage is negative feedback control, the resistors R2 and R
The load supply voltage is stabilized according to the voltage division ratio of 3. Also, if the load supply voltage is within a predetermined voltage range, D2
, the switching control circuit 1 from the rectifying and smoothing circuit using C4.
rated power supply voltage is generated. In such a steady state, the Zener diode ZD2 does not conduct, and the DB terminal of the switching control circuit 1 is connected to the reference voltage resistor R8,
A divided voltage by R9 is applied. This voltage is set relatively low and is not limited by the maximum duty ratio limiting circuit in steady state.

Here, if for some reason the load supply voltage detection circuit provided on the input side of the switching control circuit l and the circuits connected thereto, such as the input side of the resistor R3 or the shunt regulator SR, become open, the switching control circuit The on-duty ratio of the switching transistor Q1 reaches its maximum value because the voltage applied to the input terminal NlO of the circuit 1 is a reference voltage divided by the resistors R4 and R5. As a result, the electromotive voltage of the winding N3 on the secondary side of the transformer increases, and the charging voltage of the capacitor c4 increases. When the charging voltage of the capacitor C4 with respect to the voltage of the DB terminal of the switching control circuit 1 exceeds the Zener voltage of ZD2, ZD2 becomes conductive and the charging voltage of the capacitor C4 (more precisely, the charging voltage of the capacitor C4 is applied to the DB terminal of the switching control circuit 1). A voltage obtained by subtracting the Zener voltage of ZD2 from the voltage is applied. The charging voltage of C4 is the terminal RE
Since it is the reference voltage output from F or a voltage exceeding this, the maximum duty ratio limiting circuit works to the maximum,
The on-duty ratio of switching transistor Q1 is reduced to the minimum. As a result, the output voltage of the rectifier and smoothing circuit provided on the secondary side of the transformer is restricted from rising, and voltage breakdown of the smoothing capacitors C2, C3, and C4 can be prevented.

(Effect of the 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 released, the secondary side of the transformer The installed rectifier and smoothing circuit q suppresses the output voltage jump and prevents voltage breakdown of the smoothing capacitor.Furthermore, it can be configured by simply adding an extremely simple circuit, resulting in smaller size and lower cost. becomes possible

[Brief explanation of drawings]

FIG. 1 is a circuit diagram of a switching regulator that is an embodiment of the present invention. FIGS. 2 and 3 are circuit diagrams and voltage waveform diagrams of various parts of a switching control circuit used in an embodiment of the present invention. FIG. 4 is a circuit diagram of a conventional switching regulator. 1-Switching control circuit, 2-Transformer, Ql-Switching transistor, C2, C3, C4-Smoothing capacitor, ZD2-Zener diode. Figure 42Figure 3

Claims (1)

[Claims] (1) A switching transistor is provided on the primary side of the transformer to intermittent the primary winding current of the transformer, a rectifying and smoothing circuit is provided on the secondary side of the transformer, and a triangular wave oscillator and an error in the output voltage of this triangular wave oscillator are provided. A comparator that generates a PWM signal by comparing the output voltage of the amplifier circuit, and a maximum duty ratio limiting circuit that reduces the maximum duty ratio of the PWM signal as the input voltage increases, and controls on/off the switching transistor. In the switching regulator equipped with a switching control circuit, a Zener diode that becomes conductive when the output voltage of the rectifying and smoothing circuit exceeds a certain value is connected between the output of the rectifying and smoothing circuit and the input of the maximum duty ratio limiting circuit. Switching regulator with special features.