JPH0731141A - Constant voltage circuit - Google Patents

Abstract

PURPOSE:To allow positive control of an output voltage by applying a reference voltage obtained by superposing a voltage induced in an auxiliary coil on the voltage induced in the secondary coil of a transformer to a feedback circuit for regulating the output voltage, a constant voltage control section, or a secondary control section. CONSTITUTION:A transformer 13 has a secondary coil 13b and an auxiliary coil 13c on the secondary. When a voltage and current detecting section 17 is applied with a reference voltage obtained by superposing a voltage induced in the auxiliary coil 13c on the voltage induced in the secondary coil 13b, a control signal is fed to a feedback circuit 15 and a constant voltage control section 12 based on the reference voltage in order to regulate the output voltage. Furthermore, the voltage and current detecting section 17 turns the secondary control section 16 ON/OFF. Consequently, the feedback circuit 15 and the constant voltage control section 12 are controlled positively without causing under- drive voltage. When the current increases over a reference value under heavy load on the constant voltage circuit 10, the secondary control section 16 is controlled to interrupt a dummy resistor, for example, thus decreasing the burden on the circuit.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a constant voltage circuit such as a switching power supply, a DC-DC converter, etc., and in particular, it can surely detect an output voltage or an output current and can reduce a load under high load. The present invention relates to a constant voltage circuit.

[0002]

2. Description of the Related Art Conventionally, a switching power supply is shown in FIG.
It is configured as shown in. That is, in FIG. 7, the switching power supply 1 converts an AC voltage, which is input from an AC power supply (not shown) via an input terminal, into a direct current by a rectifying / smoothing circuit 2 including a bridge rectifier 2a and a smoothing capacitor 2b, and further switches a switching transistor 3 The voltage is applied to the primary coil 4a of the transformer 4 via the output terminal while the secondary coil 4b of the transformer 4 is connected via the forward converter circuit 5 including the diodes 5a and 5b, the choke coil 5c and the capacitor 5d. The output voltage generated between the output terminals is input to the feedback circuit 6 including the voltage dividing resistor 6a and the shunt regulator 6b, so that the feedback circuit 6 passes through the photocoupler 7 based on the output voltage. Switching transistor by drive control circuit 8 So as to control the oscillation of.

In the feedback circuit 6, the output voltage is input to the shunt regulator 6b via the voltage dividing resistor 6a, and the shunt regulator 6b is
The light emitting unit 7a of the photocoupler 7 is driven. As a result, the light from the light emitting portion 7a enters the light receiving portion 7b of the photocoupler 7, and the drive control circuit 8 causes the switching transistor 3 to oscillate based on the output signal from the light receiving portion 7b. It is controlled so that the output voltage can be maintained at a predetermined voltage.

[0004]

However, in such a switching power supply 1, when the switching power supply 1 has a low voltage output, the reference voltage becomes low in the feedback circuit 6 by the shunt regulator 6b, which is sufficient. Since such a reference voltage cannot be obtained, the light emitting section 7a of the photocoupler 7 cannot be made to emit light, and thus the output voltage cannot be adjusted reliably.

Further, in such a switching power supply 1, a dummy resistor 9 is connected between the output terminals,
When the load is low, the dummy resistor 9 protects the circuit. However, when the load is high, the dummy resistor 9 becomes a burden on the circuit, which causes problems such as current consumption and heat generation by the dummy resistor 9. Will occur.

In view of the above points, the present invention makes it possible to reliably control the output voltage even in the case of a low voltage output, and to prevent the dummy resistor from becoming a burden on the circuit when the load is high. It is intended to provide a constant voltage circuit.

[0007]

In order to achieve the above object, according to the present invention, an input voltage is applied to a primary coil of a transformer via a constant voltage control section connected in series, and the transformer is also provided. The output terminal is connected to the secondary coil of, and the output voltage generated between the output terminals is input to the constant voltage control section by the feedback section, so that the output voltage becomes constant. In the constant voltage circuit designed to be held in the transformer, the transformer is provided with an auxiliary coil, and the voltage detected by detecting the voltage or current of the auxiliary coil by the detection unit connected to the auxiliary coil. Alternatively, the feedback unit, the constant voltage control unit, or the secondary side control unit connected between the output terminals is controlled based on the current. .

[0008]

According to the above structure, the feedback circuit for adjusting the output voltage, the constant voltage control unit or the secondary side control unit adds the voltage generated in the auxiliary coil to the voltage generated in the secondary coil of the transformer. Will be given a reference voltage. Therefore, even in the case of a low voltage output, a sufficiently high voltage for control can be obtained as a reference voltage, so that the output voltage can be controlled reliably and, for example, from a dummy resistor or the like. The secondary side control unit can be appropriately controlled according to the current that can be detected based on this reference voltage. For example, when the load is high,
Since the dummy resistor can be off-controlled, the circuit load can be reduced.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail below based on the embodiments shown in the drawings. FIG. 1 shows an embodiment of a constant voltage circuit according to the present invention. The constant voltage circuit 10 applies a DC voltage input from a DC power source (not shown) via an input terminal 11 to a primary coil 13a of a transformer 13 via a constant voltage control unit 12, and also a secondary coil of the transformer 13. By connecting the output terminal 14 to the coil 13b and inputting the output voltage generated between the output terminals 14 to the feedback circuit 15, the feedback circuit 15 operates the constant voltage control unit 12 based on the output voltage. A secondary side control unit 16 such as a dummy resistor is connected between the output terminals 14.

The above-mentioned structure is almost the same as that of the constant voltage circuit section of the conventional switching power supply 1 shown in FIG. 7, but in the constant voltage circuit 10 according to the present invention, the transformer 13 is the secondary coil 13b. In addition, an auxiliary coil 13c is further provided on the secondary side, and a reference voltage obtained by adding the voltage generated in the auxiliary coil 13c and the voltage generated in the secondary coil 13b is applied to the voltage / current detection unit 17. As a result, the voltage / current detection unit 17 is configured so that the feedback circuit 15 and the constant voltage control unit 12 are based on the reference voltage.
A control signal is output to the output terminal to adjust the output voltage. Further, the voltage / current detection unit 17 is
By outputting a control signal to the secondary-side control unit 16, the secondary-side control unit 16 is on / off controlled.

According to the constant voltage circuit 10 configured as described above, the control signal output to the feedback circuit 15 and the constant voltage control unit 12 is not limited to the voltage generated in the secondary coil 13b, but also to the auxiliary coil 13c. Since the voltage is generated based on a sufficiently high reference voltage obtained by adding the generated voltage, the feedback circuit 15 and the constant voltage control unit 12 can be surely controlled without the drive voltage becoming insufficient, and the output voltage Will be adjusted accurately.

Further, since the control signal output to the voltage / current detection section 17 is generated based on the current calculated from the reference voltage, the current is higher than the reference value when the constant voltage circuit 10 is under heavy load, for example. When the number increases, the load on the circuit can be reduced by controlling the secondary side control unit 16 to cut off the dummy resistor, for example.

FIG. 2 shows a first embodiment of a switching power supply incorporating a constant voltage circuit according to the present invention. That is, in FIG. 2, the switching power supply 20 receives an AC voltage input from an AC power supply (not shown) via the input terminal 21 by the bridge rectifier 22a and the smoothing capacitor 22b.
Is converted into a direct current by the rectifying / smoothing circuit 22 and is applied to the primary coil 24a of the transformer 24 via the switching transistor 23. By connecting the output terminal 26 via the forward converter circuit 25 composed of, and the output voltage generated between the output terminals 26 is input to the feedback circuit 27 composed of the voltage dividing resistor 27a and the shunt regulator 27b, The feedback circuit 27 controls the oscillation of the switching transistor 23 by the drive control circuit 29 via the photocoupler 28 based on the output voltage.

In the switching power supply 20, the transformer 24, together with the secondary coil 24b, further has an auxiliary coil 24c on the secondary side, diodes 30a and 30b connected to the auxiliary coil 24c, a choke coil 30c and It has a forward converter circuit 30 including a capacitor 30d. The reference voltage obtained by adding the voltage generated in the auxiliary coil 24c and the voltage generated in the secondary coil 24b is the feedback circuit 27.
When the light is applied to the shunt regulator 27b and the light emitting portion 28a of the photo coupler 28, the light emitting portion 28a of the photo coupler 28 is driven.
On the other hand, the output voltage generated between the output terminals 26 is input to the shunt regulator 27b through the voltage dividing resistor 27a, so that the shunt regulator 27b drives and controls the light emitting portion 28a of the photocoupler 28. Has become.

As a result, the light from the light emitting portion 28a enters the light receiving portion 28b of the photocoupler 28, and the drive control circuit 2 is based on the output signal from the light receiving portion 28b.
Reference numeral 9 controls the oscillation of the switching transistor 23 so that the output voltage is maintained at a predetermined voltage.

In this case, even when the voltage of the secondary coil 24b is relatively low, the light emitting section 28 of the photocoupler 28 is used.
Since a voltage obtained by adding the voltage generated in the auxiliary coil 24c to the voltage generated in the secondary coil 24b is applied to a as a drive voltage, the light emitting unit 28a may have a short drive voltage. Without it, it is possible to make the drive emit light reliably.

FIG. 3 shows a switching power supply 2 according to the present invention.
No. 0 second embodiment is shown. In this case, transformer 2
4 is simplified by omitting the diode 30b and the choke coil 30c in the forward converter circuit 30 connected to the auxiliary coil 24c, and the output voltage of the auxiliary coil 24c is rectified only by the diode 24a and the capacitor 30d. Except for the above point, the structure is the same as that of the first embodiment shown in FIG. 2, and the operation thereof is almost the same. In this case, the configuration is simple,
The cost can be reduced, which is particularly effective in the case of a low power switching power supply.

FIG. 4 shows a third embodiment of a switching power supply incorporating the constant voltage circuit according to the present invention. That is, in FIG. 4, the switching power supply 40 receives the AC voltage input from the AC power supply (not shown) through the input terminal 41, and the bridge rectifier 42a and the smoothing capacitor 42b.
Is rectified and smoothed by a rectifying / smoothing circuit 42 composed of, and further applied to a primary coil 44a of a transformer 44 via a switching transistor 43. The output terminal 46 is connected via the forward converter circuit 45 composed of and the output voltage generated between the output terminals 46 is input to the feedback circuit 47, so that the feedback circuit 47 is based on the output voltage. Thus, the drive control circuit 48 controls the oscillation of the switching transistor 43. Further, a dummy resistor 49 is connected between the output terminals 46 so as to protect the circuit when the load is low.

In the switching power supply 40, the transformer 44 includes a secondary coil 44b, a forward converter circuit 50 including an auxiliary coil 44c on the secondary side and a diode 50a and a capacitor 50b connected to the auxiliary coil 44c. And the voltage generated in the auxiliary coil 44c and the secondary coil 44b.
The reference voltage obtained by adding the voltage generated in the above is applied to the collector of the transistor 52 via the resistor 51 and to the base of the transistor 52 via the Zener diode 53, and the collector of the transistor 52 is applied to the dummy resistor 49. It is connected to the base of the switching transistor 54 connected in series.

According to this structure, the output voltage of the secondary coil 44b can be held at a constant voltage by the action of the feedback circuit 47 and the drive control circuit 48, while the voltage generated in the auxiliary coil 44c is Output terminal 46
When the voltage across the resistor 51 exceeds a predetermined voltage by the Zener diode 53 by utilizing the variation in the voltage across the resistor 51, that is, Auxiliary coil 44
When the current flowing through c exceeds a certain value, that is, when the load is high,
A current flows through the Zener diode 53 to turn on the transistor 52, thereby turning off the switching transistor 54, so that the dummy resistor 49 is cut off between the output terminals 46. Thus, under high load,
The circuit load can be reduced by turning off the dummy resistor 49.

FIG. 5 shows a fourth embodiment of the switching power supply according to the present invention. In this case, the configuration is different from that of the embodiment shown in FIG. 4 in that the voltage-duty conversion circuit 55 is inserted between the collector of the transistor 52 and the base of the switching transistor 54. As a result, the switching resistance of the switching transistor 54 is controlled to be turned on and off at a duty ratio according to the voltage, so that the dummy resistor 49 is controlled to be turned on and off, which enables finer control.

FIG. 6 shows a fifth embodiment of the switching power supply according to the present invention. In this case, the output voltage of the forward converter circuit 30 is different from that of the embodiment shown in FIG. 3 in that a Zener diode 31 and a photocoupler 32 are connected in series. .

As a result, the output voltage of the secondary coil 24b can be held at a constant voltage by the action of the feedback circuit 27 and the drive control circuit 29.
Since the voltage generated in the auxiliary coil 24c fluctuates so as to increase as the load on the output terminal 26 increases, when the voltage applied to the Zener diode 31 exceeds a predetermined value, that is, the output terminal 26. When the so-called high load in which the current flowing through the circuit exceeds a certain value, a current flows from the Zener diode 31 to drive the light emitting section 32a of the photocoupler 32. As a result, the light from the light emitting portion 32a enters the light receiving portion 32b, and the light receiving portion 32b
The drive control circuit 29 turns off the switching transistor 23 based on the output signal from b, and the power supply is turned off to protect the circuit.

[0024]

As described above, according to the present invention,
The feedback circuit for adjusting the output voltage, the constant voltage control unit or the secondary side control unit is given a reference voltage obtained by adding the voltage generated in the auxiliary coil to the voltage generated in the secondary coil of the transformer. Therefore, even in the case of a low voltage output, a sufficiently high voltage for control can be obtained as a reference voltage, so that the output voltage can be controlled reliably and, for example, a dummy resistor or the like. The secondary-side control section consisting of is controlled appropriately according to the current that can be detected based on this reference voltage.For example, when the load is high, the dummy resistor is turned off to reduce the circuit load. Can be mitigated. Thus, according to the present invention, there is provided a constant voltage circuit in which the output voltage is surely controlled even in the case of a low voltage output, and the dummy resistor does not become a burden on the circuit when the load is high. .

[Brief description of drawings]

FIG. 1 is a block diagram showing a schematic configuration of a constant voltage circuit according to the present invention.

FIG. 2 is a circuit diagram showing a concrete first embodiment of the constant voltage circuit according to the present invention.

FIG. 3 is a circuit diagram showing a concrete second embodiment of the constant voltage circuit according to the present invention.

FIG. 4 is a circuit diagram showing a specific third embodiment of the constant voltage circuit according to the present invention.

FIG. 5 is a circuit diagram showing a concrete fourth embodiment of the constant voltage circuit according to the present invention.

FIG. 6 is a circuit diagram showing a specific fifth embodiment of the constant voltage circuit according to the present invention.

FIG. 7 is a circuit diagram showing an example of a conventional switching power supply.

[Explanation of symbols]

 10 constant voltage circuit 11 input terminal 12 constant voltage control unit 13 transformer 14 output terminal 15 feedback unit 16 secondary side control unit 17 detection unit 20 switching power supply 21 input terminal 22 rectification smoothing circuit 23 switching transistor 24 transformer 25 forward converter circuit 26 output Terminal 27 Feedback circuit 28 Photocoupler 29 Drive control circuit 30 Forward converter circuit 31 Zener diode 32 Diode 40 Switching power supply 41 Input terminal 42 Rectification smoothing circuit 43 Switching transistor 44 Transformer 45 Forward converter circuit 46 Output terminal 47 Feedback circuit 48 Drive control circuit 49 Dummy resistor 50 Forward converter circuit 51 Resistor 52 Transistor 53 Zener diode 54 Switch Tsu quenching transistor 55 voltage - Duty converting circuit

Claims (1)

[Claims] 1. An input voltage is applied to a primary coil of a transformer through a constant voltage controller connected in series, and an output terminal is connected to a secondary coil of the transformer. In the constant voltage circuit, the output voltage generated between the output terminals is input to the constant voltage control section by the feedback section so that the output voltage can be kept constant. A coil is provided, and by detecting the voltage or current of the auxiliary coil by a detection unit connected to the auxiliary coil, the feedback unit, the constant voltage control unit, or the output terminal based on the detected voltage or current. A constant voltage circuit characterized in that a secondary side control unit connected between them is controlled.