JPH01122366A - Switching power source circuit - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed Description of the Invention] [Object of the Invention] (Industrial Application Field) The present invention relates to a switching power supply circuit having an overvoltage protection circuit that detects overvoltage on the secondary side and stops switching operation on the primary side. In particular, the present invention relates to an improvement in a switching power supply circuit having a protection circuit using a photocoupler as an overvoltage detection element.

(Prior Art) Generally, an overvoltage protection circuit in a switching power supply circuit using a converter transformer has high insulation between the secondary side and the primary side, and therefore is configured using an optical coupling element for overvoltage detection.

FIG. 2 shows an example of a conventional switching power supply circuit. In the figure, 21 is a commercial AC power supply, 22 is a rectifier, 23 is a converter transformer, Q21 is a switching transistor,
24 is an optical coupling element for overvoltage protection. A commercial AC power source 21 is connected to a predetermined pair of input terminals of a rectifier 22 via a power switch 25 .

The converter transformer 23 has an input winding L21 and a drive winding L22 on the negative side, and two output windings L23 on the secondary side.
．． 24. Of the pair of output terminals of the rectifier 22, the positive voltage output terminal is connected to the collector of the switching transistor Q21 via the input winding L21, and the negative voltage output terminal is connected to one end of the drive winding 122. The other end of the drive winding L22 is a diode 02.
1 to the base of the switching transistor Q21. Further, the output winding L23 derives an output voltage to the output terminal pair 26.27 via a rectifier circuit consisting of a diode D22 and a capacitor C21, and the output winding L23
24 derives an output voltage to a pair of output terminals 28 and 29 via a rectifier circuit consisting of a diode D23 and a capacitor C22.

The switching transistor Q21 has two transistors Q22. The collector of Q23 is connected, and the emitter is connected to the negative voltage output terminal of the rectifier 22 via a resistor R21.

The two transistors Q22. The emitters of Q23 are commonly connected, and the common connection point is connected to the negative voltage output terminal of the rectifier 22. Q22 is used for overvoltage protection, and Q23 is used for output stabilization.

That is, the optical coupling device 24 in this case is an insulating coupling device consisting of a triac Q24 and a light emitting diode [24], one end of the triac Q24 is connected to the base of the transistor Q22, and the other end of the triac Q24 is connected to the resistor R22. It is connected to the positive polarity voltage output terminal of the rectifier 22 via. Further, the light emitting diode D24 has an anode connected to the secondary output terminal 26, and a cathode connected to the constant voltage diode D26. It is connected to an output end 27 that is paired with the output end 26 through a series connection of a resistor R23.

On the other hand, 30 is a voltage stabilizing circuit, and 31 is an optical coupling element including a phototransistor Q25 and a light emitting diode D25. The voltage stabilizing circuit 30 inputs the voltage generated across the output terminal pair 28 and 29, and causes a current proportional to the input voltage to flow through the light emitting diode D25 as an output. Thereby, the light emitting diode D25 drives the phototransistor Q25 according to the voltage across the output terminal pair 28 and 29. The emitter of the phototransistor Q25 is connected to the base of the transistor Q23, and the collector is connected to the other end of the drive winding L22 via a diode D27. As a result, when the secondary output voltage increases, the phototransistor Q25 becomes conductive, and the base current supplied to the switching transistor Q21 through the diode [21] can be shunted by the transistor Q23 and reduced. This lowers the secondary side output voltage and performs a stabilizing operation.

The negative voltage output terminals 27.29 of each pair of secondary output terminals 26.27 and 28.29 are respectively connected to the common terminal of the commercial AC power supply 21 and the same potential point.

In the above configuration, as already explained, the voltage stabilizing circuit 30 stabilizes the output voltage by controlling the base current of the switching transistor Q21. On the other hand, the overvoltage protection circuit composed of the optical coupling element 24 operates as follows.

The secondary side output voltage for overvoltage detection is output terminal 26.
28 may be used for detection, but in this configuration, the voltage at the output terminal 26 is detected. For example, output winding L23 of converter transformer 23. Or when L24 is short-circuited, the voltage at the output terminal 26 becomes an overvoltage, the constant voltage diode D2G becomes conductive, and then the light emitting diode D24
conducts. As a result, triac Q24 becomes conductive and transistor Q22 turns on. When the transistor Q22 turns on, the base current of the switching transistor Q21 flows from the collector to the emitter of the transistor Q22, stopping the operation of the switching transistor Q21. Therefore, no voltage is generated in the output winding L23, thereby preventing damage to the load circuit due to overvoltage.

However, the above-described conventional circuit has the disadvantage that when the power switch 25 is turned on, the triac Q24 remains conductive, resulting in a malfunction in which the output does not rise.

FIG. 3 is an explanatory diagram for explaining an example of the cause, and shows the formation part of the overvoltage protection circuit. In the figure, the capacitance CO shown by a dotted line between the triac side (output side) and the light emitting diode side (input side) is the input and output side of the optical coupling element 24.
This is the distributed capacitance between the output terminals.

In this way, the optical coupling element 24 has a distributed capacitance CO between the input and output terminals. Then, the timing when the power switch 25 is turned on is the peak value of the commercial AC power supply ~
When near the peak value, a potential difference occurs between both ends of the distributed capacitor MCO (between the input and output terminals of the optical coupling element 24) depending on the timing, so a current flows through the distributed capacitor CO due to this potential difference. . This current i is triac Q
24 becomes conductive, and the triac Q
24 becomes conductive. In this way, the transistor Q22 turns on at the same time as the power is turned on, and the switching transistor Q21 remains inactive, so that the output does not rise. Note that the distributed capacitance that causes the above-mentioned malfunction is not limited to the capacitance between the ends 1 and 3 in FIG. 3, but also involves the capacitance between the ends 2 and 3.

In addition, the above malfunction does not occur when the power is turned on, and even when the power supply circuit is in operation, if a person charged with static electricity operates the device, the static electricity is discharged into the device, and this causes the triac Q24 to conduct with current. Sometimes it happens.

(Problems to be Solved by the Invention) Conventional switching power supply circuits that use optical coupling elements in overvoltage protection circuits have problems on the output side when the power is turned on due to the capacitance distributed between the input and output terminals of the optical coupling elements. There is a problem in that the element becomes conductive and the switching transistor becomes inoperable, resulting in a malfunction in which the output does not rise.

It is an object of the present invention to provide a switching power supply circuit which eliminates the above-mentioned problems and allows the output voltage to rise reliably from when the power is turned on.

[Structure of the Invention] (Means for Solving the Problems) This invention converts an input DC voltage obtained by rectifying a commercial AC power source into an AC voltage by a primary side circuit including an input winding of a converter transformer and a switching element. In a switching power supply circuit that obtains an output DC voltage from a secondary circuit which is formed by connecting a rectifier circuit to the output winding of the converter transformer that induces this AC voltage, an insulating coupling element consisting of an input element and an output element is provided. an optical coupling element configured such that an input element drives an output element in response to an overvoltage of the output DC voltage, and can suppress a base current of the switching element through the operation of the output element; and the output element. The device is characterized in that it is provided with a capacitor connected in parallel with the optical coupling device to suppress the potential difference generated between the input and output of the optical coupling device.

(Function) According to the present invention, even if a current attempts to flow through the distributed capacitance M existing between the input and output of the optical coupling element, the capacitor provided in parallel with the output element accumulates the current first.
The output element does not become conductive inadvertently, and the output voltage can be reliably raised when the power is turned on.

(Example) The present invention will be explained below with reference to illustrated embodiments.

FIG. 1 is a circuit diagram showing an embodiment of a switching power supply circuit according to the present invention. In this figure, the same parts as in FIG. 2 are given corresponding symbols. 11 is a commercial AC power supply, 15
is the power switch, 12 is the rectifier, and 13 is the input winding L11.
．． Drive winding L12. A converter transformer with output windings L13 and L14, Qll is a switching transistor. The positive DC voltage obtained by the rectifier 12 is applied to the collector of the switching transistor Qll via the input winding L11, and the emitter of the switching transistor Q11 is applied to the rectifier 12 via the resistor R11.
is connected to the negative polarity voltage output terminal (referred to as reference potential point P).

Furthermore, the voltage generated in the drive winding L12 is applied to the base of the switching transistor Q11 via the diode D11.

On the other hand, output winding L13. A voltage corresponding to the on/off operation of the switching transistor Qll is generated in Li2, and the voltage of the output winding L13 is rectified and smoothed by the diode D12 and the capacitor C11, and the voltage is applied to the output terminal pair 16.
17 as an output DC voltage. Output winding L1
4, the voltage generated across diode D13° and capacitor C
It is rectified and smoothed by I2 and output as an output DC voltage to the output terminal pair 18 and 19.

One end of the drive winding L12 is connected to the reference potential point P, and the end on the base side of the switching transistor Q11 is connected to the reference potential point P.
It is connected to the collector of the phototransistor Q15 of the photocoupler 21 made up of the phototransistor Q15 and the light emitting diode [ ) 15 via the diode D17.

On the other hand, the output terminal pairs 18 and 19 are respectively connected to the voltage stabilizing circuit 2.
0 input end pair, and voltage changes occurring in the output winding L14 are detected.

The output terminal pair of the voltage stabilizing circuit 20 includes the optical coupling element 2.
1 light emitting diode [ ) 15 is connected. The emitter of phototransistor Q15 is connected to the base of transistor Q13. The emitter of the transistor Q13 is connected to the reference potential point P, and the collector is connected to the base of the switching transistor Q11.

14 is an optical coupling element using a triac Q14 as an output element and a light emitting diode D14 as an input element.

In this embodiment, the triac Q14 of this optical coupling element 14 is
A resistor R14 is connected in series with the resistor R14, and a capacitor CI3 for preventing malfunction is connected in parallel with this series connection.

The light emitting diode [) 14 of the optical coupling element 14 is a constant voltage diode D16. It is connected in series with a series circuit consisting of a resistor R13, and the series connection by these is connected to the output terminal pair 16.1.
Connected to 7. With this configuration, the output end pair 16.17
When the voltage between
16 becomes conductive and performs overvoltage detection. In addition, the above-mentioned resistor R14
In the series connection of the triac Q14 and the triac Q14, the end on the triac Q14 side is connected to the base of the transistor Q12, and the end on the resistor R14 side is connected to the positive voltage output terminal of the rectifier 12 via the resistor R12. The emitter of the transistor Q12 is connected to the reference potential point P, and the collector is connected to the base of the switching transistor Qll.

Since the present embodiment is configured as described above, when the power switch 15 is turned on, the voltage is applied between the input and output elements of the optical coupling element 14, but it is also applied to the capacitor C13.
Conventional current i (see FIG. 3) flowing through the distributed capacitance existing between the triac Q14 and the light emitting diode [14] is accumulated in the capacitor C13. Therefore, the triac Q14 is no longer conductive, and the output can be raised more reliably than when the power is turned on.

Further, when the protection operation according to this embodiment is activated, the triac Q14 becomes conductive, but at this time, the resistor R14
Triac Q14 is protected from being destroyed by an abnormally high current flowing through Q14.

In this way, the present embodiment allows the power to be turned on reliably without affecting the normal overvoltage protection operation.

Note that the optical coupling device 14 for overvoltage protection is not limited to one using a triac and a photodiode, but can also be applied to an element using a phototransistor and a light emitting diode.

[Effects of the Invention] As explained above, according to the present invention, it is possible to prevent malfunctions when the power is turned on and to reliably raise the output voltage.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing an embodiment of a switching power supply circuit according to the present invention, FIG. 2 is a circuit diagram showing an example of a conventional switching power supply circuit, and FIG. 3 is an operation diagram for explaining the operation of the conventional circuit. It is an explanatory diagram. 11... Commercial AC power supply, 12... Rectifier, 13...
・】Inverter transformer, 14... optical coupling element, QN・
...Switching transistor, C13...Capacitor, R14...Resistor, Q14...Triac, [
)14... Light emitting diode, D16... Constant voltage diode. 7F' Figure 3

Claims (1)

[Scope of Claims] The input DC voltage obtained by rectifying a commercial AC power supply is converted into an AC voltage by a primary side circuit including an input winding and a drive winding of a converter transformer, and a switching element, and the AC voltage is induced. A switching power supply circuit that obtains an output DC voltage from a secondary circuit formed by connecting a rectifier circuit to the output winding of a converter transformer, which has an input end and an output end, the output end is connected to the switching element, and the input a switching control circuit that stops the operation of the switching element by a voltage applied to the terminal; and an insulating coupling element comprising an input element and an output element, the input element being connected to a pair of output terminals of the output DC voltage,
An output element is connected to a derivation terminal of the input DC voltage and an input terminal of the switching control circuit, and the input element in response to the overvoltage of the output DC voltage drives the output element, and the switching control circuit is controlled by the operation of the output element. An overvoltage protection circuit that drives the circuit, and an overvoltage protection circuit connected in parallel to the output element and connected to the input/output of the optical coupling element.
A switching power supply circuit comprising: a bypass capacitor that suppresses a potential difference generated between output terminals;