JP3028656B2 - Switching power supply - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a self-excited switching power supply, and more particularly, to a means for suppressing an overshoot of an output voltage at the time of startup.

[0002]

2. Description of the Related Art A conventional example of this type of switching power supply is shown in FIG. This switching power supply device is an example of a self-excited and flyback type (that is, RCC type), in which a switching transistor 4 as an example of a switching element is connected in series to a primary winding 2a of a transformer 2, and a base of the transformer 2 The output of the winding 2c is fed back to the base of the switching transistor 4 via the oscillation control circuit 6 to oscillate.

In this example, the oscillation control circuit 6 is connected so as to bypass the capacitor 8, the resistor 10 and the base of the switching transistor 4 which are inserted in series between one end of the base winding 2c and the base of the switching transistor 4. The control transistor 12 is provided to control the switching operation of the switching transistor 4. 14 is a starting resistor.
When the switching transistor 4 is turned off, the energy stored in the transformer 2 is extracted from the secondary winding 2b as a DC output (for example, a high voltage output) through the voltage doubler rectifier circuit 16 in this example.

The constant voltage control of the output voltage V ₂ is performed using the constant voltage control circuit 20 as follows. That is, the constant voltage control circuit 20, voltage dividing resistors 2 for dividing the output voltage V ₂
2. Reference voltage source 24, error amplifier 2 for determining the difference between both inputs
When the output voltage V ₂ rises above the reference voltage _VL set by the reference voltage source 24, the output of the error amplifier 26 increases, and this oscillates via the photocoupler 28. The collector current of the control transistor 12 applied to the base of the control transistor 12 in the control circuit 6 increases. As a result, the base current of the switching transistor 4 the switching transistor 4 is shunt acts off direction, increasing the resulting output voltage V ₂ is suppressed.

[0005]

However, in the above switching power supply, the constant voltage control circuit 20 has a certain time constant, and the constant voltage control circuit 20 is activated at the moment of starting (when input voltage is turned on). There is a time delay before the constant voltage control is performed according to 20, during which time there is no control. Therefore, for example, as shown in FIG. 4, there is a problem that a large overshoot in the output voltage V ₂ occurs.

When such a large overshoot occurs, for example, the components constituting the voltage doubler rectifier circuit 16 and the like are adversely affected in terms of withstand voltage, so that not only the reliability of the switching power supply device is lowered but also the load is reduced. Have the same adverse effect.

SUMMARY OF THE INVENTION Accordingly, it is a main object of the present invention to provide a switching power supply device capable of suppressing the output voltage overshoot at startup.

[0008]

To achieve the above object, a switching power supply according to the present invention obtains a difference between an output voltage on the secondary side of the transformer and a reference voltage and outputs a signal corresponding to the difference. An auxiliary constant voltage control circuit that feeds back to an oscillation control circuit and performs constant voltage control in cooperation with the oscillation control circuit, wherein the time constant is smaller than the time constant of the constant voltage control circuit, and the reference voltage is A voltage higher than a reference voltage of the constant voltage control circuit is provided.

[0009]

According to the above configuration, when the switching power supply is started, the auxiliary constant voltage control circuit has a smaller time constant than the original constant voltage control circuit, and thus starts to operate earlier. Even if the original constant voltage control circuit does not operate, the voltage is suppressed from rising above the voltage set by the auxiliary constant voltage control circuit. Thereafter, when the original constant voltage control circuit starts working, the output voltage settles to the voltage set by the constant voltage control circuit because the reference voltage is lower in this circuit. In this way, the overshoot of the output voltage can be kept small.

[0010]

FIG. 1 is a circuit diagram showing a switching power supply according to an embodiment of the present invention. Parts that are the same as or correspond to those in the conventional example of FIG. 3 are denoted by the same reference numerals, and differences from the conventional example will be mainly described below.

In this embodiment, in addition to the above-described constant voltage control circuit 20, an auxiliary constant voltage control circuit 30
Is further provided. That is, the auxiliary constant voltage control circuit 30
Comprises a voltage dividing resistor 32 for dividing the output voltage V ₂ , a reference voltage source 34, and an error amplifier 36 for determining the difference between the two inputs. In this example, the output of the error amplifier 36 is used in the constant voltage control circuit 20. The output from the error amplifier 26 is supplied to the photocoupler 28 together with the output. Of course, instead of this, a photocoupler for the error amplifier 36 may also be provided, and its output may be provided to the base of the control transistor 12 in the oscillation control circuit 6.

The operation of the auxiliary constant voltage control circuit 30 is basically the same as the operation of the constant voltage control circuit 20. That is, when the output voltage V ₂ rises above the reference voltage V _H set by the reference voltage source 34, the output of the error amplifier 36 increases,
This is applied to the base of the control transistor 12 in the oscillation control circuit 6 via the photocoupler 28, and the collector current of the control transistor 12 increases. As a result, the base current of the switching transistor 4 is shunted and the switching transistor 4 works in the off direction,
Increase of the resulting output voltage V ₂ is suppressed.

In this embodiment, however, the time constant of the auxiliary constant voltage control circuit 30 is smaller than the original time constant of the constant voltage control circuit 20 (this is, for example, the error amplifier 36).
This can be realized by making the time constant of the surrounding circuit smaller than the time constant of the circuit around the error amplifier 26), and the reference voltage V _H set by the reference voltage source 34 is set to the reference voltage source 24.
Is set higher than the reference voltage _VL set in the above.

According to the above configuration, when the switching power supply device is started, the auxiliary constant voltage control circuit 30 has a smaller time constant than the original constant voltage control circuit 20 and thus starts to operate earlier. Therefore, even if the original constant voltage control circuit 20 does not operate, the output voltage V ₂ rises above the voltage set by the auxiliary constant voltage control circuit 30 as shown by a solid line in FIG. 2, for example. Is suppressed. Thereafter, when the original constant voltage control circuit 20 starts to operate, the output voltage V ₂ is settled to the voltage set by the constant voltage control circuit 20 because the reference voltage of this circuit 20 is lower. That is, in this embodiment, the overshoot of the portion shown by the broken line in FIG. 2 is cut by the auxiliary constant voltage control circuit 30. In this way, the overshoot of the output voltage V ₂ is kept small.

In addition, according to the above, even if a part of the original constant voltage control circuit 20 that is earlier than the photocoupler 28 is broken for some reason, the output voltage V ₂ is reduced by the function of the auxiliary constant voltage control circuit 30. because it is held at a voltage that is set at the same circuit 30 can output voltage V ₂ is prevented from excessively rising. If a photocoupler is provided on the auxiliary constant voltage control circuit 30 side as well as on the constant voltage control circuit 20 side, the above protection can be performed even if the entire constant voltage control circuit 20 is broken.

The present invention is not limited to the flyback system as in the above example, but can be applied to a forward system.

[0017]

As described above, according to the present invention, the overshoot of the output voltage at the time of starting can be suppressed small by the function of the auxiliary constant voltage control circuit as described above. Further, even if the original constant voltage control circuit is broken for some reason, it is possible to prevent the output voltage from excessively rising by the function of the auxiliary constant voltage control circuit. As a result, an excessive increase in output voltage is eliminated, so that components and loads of the switching power supply can be protected, and reliability and safety are improved.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing a switching power supply device according to an embodiment of the present invention.

FIG. 2 is a diagram showing an example of a rising characteristic of an output voltage in the device of FIG.

FIG. 3 is a circuit diagram illustrating an example of a conventional switching power supply device.

4 is a diagram showing an example of a rising characteristic of an output voltage in the device of FIG.

[Explanation of symbols]

 2 Transformer 2a Primary winding 2b Secondary winding 2c Base winding 4 Switching transistor 6 Oscillation control circuit 20 Constant voltage control circuit 30 Auxiliary constant voltage control circuit

Claims (1)

(57) [Claims] 1. A method in which a switching element is connected in series to a primary winding of a transformer, and an output of a base winding of the transformer is fed back to a control electrode of the switching element to oscillate. An oscillation control circuit for controlling a switching operation; and obtaining a difference between an output voltage on the secondary side of the transformer and a reference voltage and feeding back a signal corresponding to the difference to the oscillation control circuit to cooperate with the oscillation control circuit. And a constant voltage control circuit for performing constant voltage control, wherein a difference between an output voltage on the secondary side of the transformer and a reference voltage is obtained, and a signal corresponding to the difference is fed back to the oscillation control circuit. An auxiliary constant voltage control circuit that performs constant voltage control in cooperation with the oscillation control circuit, the time constant of which is smaller than the time constant of the constant voltage control circuit, and A switching power supply device, wherein the reference voltage is higher than the reference voltage of the constant voltage control circuit.