JP2003319648A - Switching power supply - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a switching power supply comprising a self-excited ringing choke converter. <P>SOLUTION: The switching power supply comprises the self-oscillation ringing choke converter. The power supply is provided with a constant-voltage control circuit connected with the secondary winding output of a switching transistor; a current detection resistor which detects a load current; a current comparison circuit portion which detects a voltage generated in the current detection resistor and makes comparison; a timer count portion connected with the current comparison circuit; a latch control circuit which is connected with the timer count portion and outputs a signal to lower a secondary winding output voltage; a photocoupler which controls the latch control circuit and the secondary winding output voltage; and a driver circuit connected with the photocoupler. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

The present invention relates to a switching power supply composed of a self-excited ringing choke converter.

[0001]

2. Description of the Related Art Recently, a large number of self-exciting ringing choke converters, which have a simple circuit and are inexpensive, have been adopted as small-capacity switching power supplies. For example, JP 2001-3
This ringing choke converter is disclosed in Japanese Patent No. 7219, and a load current exceeding the rated value may momentarily flow depending on the characteristics of the main body. is there.

However, when a steady overcurrent flows to the output due to a device failure or the like, a temperature fuse is provided for detecting the abnormal rise of the internal temperature for protection.

As a result, when an overcurrent flows, the temperature fuse is blown, and the output circuit of the switching power supply is cut off to make it unusable, which often prevents an excessive temperature rise of the equipment.

[0004]

However, when the thermal fuse is used, the cost of the parts and the number of mounting steps are increased, which leads to an increase in the product cost. Also,
There is a problem that once the thermal fuse is blown, it cannot be used unless repaired.

[0005]

In order to solve the above problems, a switching power supply of the present invention comprises a switching transformer having a primary winding, a secondary winding, and a control winding, and one of the primary winding. The drain of the switching transistor is connected, one of the control windings is connected to the gate of the switching transistor through a series circuit of a resistor and a capacitor, and the output of the secondary winding generated by the oscillation of the switching transistor is output as an error amplifier. In a switching power supply consisting of a self-excited oscillation type ringing choke converter that detects the output of the switching transistor, a constant voltage control circuit connected to the secondary winding output of the switching transistor and a current detection resistor that detects the load current. And a current comparison circuit section that detects and compares the voltage generated in the current detection resistor, and the current comparison circuit. And a latch control circuit unit for outputting a signal for reducing the output voltage of the secondary winding connected to the timer count unit and the latch control circuit unit for controlling the output voltage of the secondary winding. A photocoupler and a driver circuit section connected to the photocoupler are provided, and the current comparison circuit section detects that a current exceeding a specified value has flown into the current detection resistor, and the timer count section counts a specified time. At this time, the driver circuit section is operated by the signal from the latch control circuit section to maintain the state in which the current of the photocoupler is increased and the secondary winding output voltage is decreased, and the current is regulated again. The feature is that the output voltage is maintained even when the value becomes less than the value.

By adopting such a configuration, even when a current more than the rated value constantly flows, switching is performed without adding a protection component such as a temperature fuse and repairing after the protection operation. The power supply can be surely protected.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the accompanying drawings to provide an understanding of the present invention.
The embodiments described below are examples embodying the present invention, and do not limit the technical scope of the present invention.

FIG. 1 is an electric circuit diagram of the switching power supply according to this embodiment, FIG. 2 is a characteristic diagram of output voltage-output current showing a state in which the output voltage is lowered during the protection operation, and FIG. 3 is a constant voltage control during the protection operation. FIG. 7 is a characteristic diagram of output voltage-output current showing a state of switching from to constant current control.

In FIG. 1, T1 is a switching transformer, which is composed of a primary winding, a secondary winding, and a control winding. Q1 is a switching transistor, the drain is on one side of the primary winding, the gate is a capacitor C3, the resistor R
5 to one side of the control winding and the source to the other side of the control winding.

The AC input power source passes through the current fuse F1 and the line filter L1 and is full-wave rectified by the diodes D1 to D4.

The output of the secondary winding is smoothed and rectified by a diode D21 and a capacitor C22, and then L21 and C23.
After reducing the switching noise by the filter of
It is being output.

The IC 1 is a constant voltage / constant current control IC, and includes a current detection unit, a voltage detection unit, a constant current control circuit unit, a constant voltage control circuit unit, a timer count unit, a latch operation control unit, and a photocoupler driver circuit. It is composed of parts.

The output voltage is divided by resistors R22 and R23 and compared with a reference voltage inside IC1. When the voltage increases, the current of the photocoupler increases to stabilize the voltage.

The power source of IC1 is an output rectifying diode D21.
After being rectified and smoothed by, the signal is input to the power supply terminal of IC1. The photocoupler driven by the IC1 is connected between the collector and the base of the transistor Q2, whose collector and emitter are connected to each other, and the emitter and the collector of the switch transistor Q1, respectively.

The capacitor C4 connected between the base and emitter of Q2 is discharged from R7 when the switch transistor Q1 is OFF, and is photocoupler P when Q1 is ON.
It is charged by HC1. When the charging voltage of the capacitor C4 rises to the operating voltage of the transistor Q2, Q2 turns on and the switching transistor Q1 turns off.

That is, the time during which Q1 remains ON is C
It is the same as the time until Q2 is turned on by charging 4 and can be controlled by the current of the photocoupler PHC1.

The transistor Q3 is a transistor at the time of output short circuit or overload protection, and has a resistor R7 and a capacitor C5 connected to the control winding of the switching transformer T1.
The base terminal is connected to the series circuit connection point of.

When the switching transistor Q1 is OFF, the voltage of the capacitor C5 is discharged through R7 by a negative voltage corresponding to the secondary side output voltage generated in the control winding, and when Q1 is OFF, it is charged through R7 by the positive voltage of the control winding. To be done.

Therefore, the charging voltage of C5 is equal to that of the transistor Q.
When the operating voltage rises to 3, the gate voltage of Q1 decreases and Q1 turns off.

That is, the time during which Q1 remains ON is P
It is determined by the shorter of the time until Q2 is turned on by the HC1 current and the time until Q3 is turned on by R7.

During normal operation, Q3 is set to a time longer than the rated output of the switching power supply so that it does not turn on. However, when the overload protection operation state is entered due to an output short circuit and when the control by Q2 does not work for some reason. Then, the control shifts to the protection control by Q3.

Specific operation contents in the above configuration will be described below.

When the output current on the secondary side flows through the current detecting resistor R21, a voltage VR is generated across the resistor. The IC1 detects the voltage across the IC1 and compares it with the internal reference voltage in the current comparison circuit inside the IC. If the output current is less than the predetermined value set in advance, it is not detected by the current comparison circuit unit and the output is turned off. Therefore, when the output current is less than or equal to the specified value, the output remains in the OFF state even if the output load changes, and there is no change in operation.

Next, when the output current flows over the specified value, the output is turned on in the current comparison circuit section inside the IC1. At this time, the timer counting unit inside the IC1 starts counting.

At this time, when the output current is less than the specified value while the output current of the specified value or more flows and the timer count section is counting, the output of the current detection comparison section is turned off. Then, the count of the timer count unit is stopped, and the count time at that time is reset.

There is no change in the operation of the equipment even if an output current of a specified value or more flows within the specified time provided in the timer counting section. Therefore, the device does not detect that an overcurrent has flown, even if a current of a predetermined value or more transiently flows due to an inrush current when the load of the main body changes.

The timer counting section continues counting, and when the specified time is reached, the output of the timer counting section is turned on. When the latch control circuit section detects the signal, the output is turned on and the signal is sent to the driver circuit section.

When the signal is input, the driver control section switches from the non-saturated state to the saturated state by increasing the base current of the transistor Q2. Then, the collector-emitter voltage of the transistor Q2 sharply drops. Figure
As shown in 2, the secondary side output voltage sharply drops to the protection voltage V2.

At this time, the protection voltage V2 is the collector of Q2.
It is the sum of the voltage between the emitter and the forward voltage Vf of the photodiode of PHC1, the collector current of Q2 and the voltage generated between the resistor R24.

The output voltage drops to the protection voltage V2,
Therefore, when the output load is released, the output voltage maintains the state of the protection voltage V2. Even if a normal load is connected after that, the output voltage does not change, so prevent overcurrent from flowing through the load.

Further, as shown in FIG. 3, it is possible to similarly reduce the output voltage by switching from the constant voltage control to the constant current control.

This protection operation turns off the AC input,
It can be released by resetting IC1.

By adopting such a configuration, even when a current more than the rated value constantly flows, switching is performed without adding a protection component such as a thermal fuse and repairing after the protection operation. The power supply can be surely protected.

[0034]

As is apparent from the above description of the embodiments, according to the present invention, in a switching power supply composed of a ringing choke converter, even when a current more than a rated value constantly flows, a temperature fuse or the like is used. It is possible to reliably protect the switching power supply without adding a protection component and repairing after the protection operation.

[Brief description of drawings]

FIG. 1 is an electric circuit diagram of a switching power supply according to this embodiment.

FIG. 2 is a characteristic diagram of output voltage-output current showing a state in which the output voltage is reduced during protection operation.

FIG. 3 is an output voltage-output current characteristic diagram showing a state in which constant voltage control is switched to constant current control during protection operation.

[Explanation of symbols]

Q1 switch transistor Q2-Q3 transistors F1 fuse C1 to C23 capacitors D1 to D21 diode L1 line filter L21 line filter R1 to R24 resistance T1 switching transformer ZD1 Zener diode IC1 IC PHC1 photo coupler

   ─────────────────────────────────────────────────── ─── Continued front page    F term (reference) 5H730 AA20 AS01 BB43 BB52 CC01                       DD04 EE02 EE08 EE59 FD01                       FD31 FG02 XX03 XX15 XX23                       XX35 XX48

Claims (2)

[Claims] 1. A switching transformer including a primary winding, a secondary winding, and a control winding, and a drain of a switching transistor is connected to one of the primary winding, and one of the control winding has a resistor and a capacitor. A self-excited ringing choke converter connected to the gate of the switching transistor through a series circuit of the control circuit and detecting the output of the secondary winding generated by the oscillation of the switching transistor with an error amplifier to control the output. In the switching power supply, a constant voltage control circuit unit connected to the secondary winding output of the switching transistor, a current detection resistor that detects a load current, and a current comparison circuit that detects and compares the voltage generated in the current detection resistor. Section, a timer counting section connected to the current comparing circuit section, and a timer counting section connected to the timer counting section. And a latch control circuit section for outputting a signal for lowering the secondary winding output voltage, the latch control circuit section, a photocoupler for controlling the secondary winding output voltage, and a driver circuit section connected to the photocoupler. When the current comparison circuit unit detects that a current more than a specified value has flowed in the current detection resistor and the timer count unit counts a specified time, the driver circuit is output by a signal from the latch control circuit unit. Is operated to maintain the state in which the current of the photocoupler is increased and the secondary winding output voltage is decreased, and the output voltage is maintained even when the current again becomes equal to or less than the specified value. A characteristic switching power supply. 2. A secondary side output when a constant current control circuit unit is provided, the current comparison circuit unit detects that a current exceeding a specified value has flowed, and the timer counting unit counts a specified time. 2. The switching power supply according to claim 1, wherein the secondary output voltage is reduced by switching control from constant voltage control of the constant voltage control circuit unit to constant current control of the constant current control circuit unit.