JPH11164553A - Ac-dc converter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To attain long life, high reliability and low power consumption AC-DC converter by eliminating the need for the use of a primary smoothing capacitor consisting of a large-sized aluminum electrolytic capacitor and reducing a size of the apparatus itself. SOLUTION: An AD-DC converter is provided with first rectifier 2 for rectifying an AC input, voltage detecting means 9 connected in the output side of this rectifier 2, a series circuit of the primary coil of the transformer 3 connected in parallel with the voltage detecting means 9 and a switching means 4 and a second rectifier 5 and capacitor 6 for extracting high-frequency Ac voltage obtained by the switching means 4 from the secondary coil of the transformer 3, and providing an output voltage by rectifying and smoothing such 3, an AC voltage, The on/off ratio of the switching means is determined with the output voltage and a voltage from the voltage detecting means.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an AC-DC converter with a reduced size and improved performance.

[0002]

2. Description of the Related Art FIG. 3 is a circuit diagram of a conventional AC-DC converter, in which a full-wave rectifier circuit, a capacitor input type smoothing circuit, and a ringing choke coil type AC-DC converter are applied. In FIG. 3, 1a, 1b
Is an input terminal for receiving an input AC voltage. 2 is a full-wave rectifier, 3 is a transformer, 4 is a switching transistor as switching means, 5 is a diode, 6 is a secondary smoothing capacitor, 7a and 7b are output terminals, and 8 is a primary smoothing capacitor. Reference numeral 20 denotes a control driving circuit,
1, the error amplifier 22, the photocoupler 23,
It comprises a voltage converter 24, an oscillator 25 and a drive circuit 26.

Next, the operation will be described. Input terminals 1a, 1b
Is rectified and smoothed by the full-wave rectifier 2 and the primary smoothing capacitor 8 so that the DC voltage V
Converted to 1. This DC voltage V1 is converted into a high-frequency AC voltage by the switching transistor 4 and then rectified by the diode 5 and the secondary smoothing capacitor 6.
It is smoothed and output from output terminals 7a and 7b as output voltage V0. This is because during the ON period of the high-frequency switching transistor 4, the power applied to the transformer 3 by the high-frequency AC voltage and the primary winding current I1 of the transformer 3 excited thereby is stored in the core of the transformer 3 as magnetic energy. This is due to discharge from the secondary winding of the transformer 3 during the off period of the switching transistor 4.

The output voltage V0 is compared with a reference voltage source 21 by an error amplifier 22 of the control driving circuit 20. The output of the error amplifier 22 is insulated by the photocoupler 23, converted into a voltage by the current / voltage converter 24, and input to the drive circuit 26 together with the output voltage V2 of the oscillator 25. The drive circuit 26 turns the switching transistor 4 on / off. A drive output voltage V3 driven at the OFF ratio is output. This is shown in FIG.

In FIG. 4, (a) shows the full-wave rectified and smoothed DC voltage V1, (b) shows the primary winding current I1 of the transformer 3, and (c) shows the output voltage V3 of the drive circuit 26. ing. The drive circuit 26 operates so that the on-period of the switching transistor 4 becomes shorter when the output voltage V0 is higher than the reference voltage source 21, and operates so that the on-period becomes longer when the output voltage V0 is lower than the reference voltage source 21. Thus, the output voltage V0 is controlled to be constant.
In the control drive circuit 20, the output voltage feedback control loop including the error amplifier 22, the photocoupler 23, and the current / voltage conversion circuit 24 has a relatively large delay (time) to avoid malfunction such as oscillation due to switching noise. However, if the ripple voltage superimposed on the DC voltage V1 becomes excessive, a large ripple voltage will also be generated in the output voltage V0.

[0006]

Since the conventional AC-DC converter is configured as described above, it is necessary to smooth the input AC voltage of the commercial frequency so that the ripple voltage after rectification becomes sufficiently low. Instead, a large-capacity, high-withstand-voltage primary smoothing capacitor 8 was required, and a large-sized aluminum electrolytic capacitor was used. This not only hinders the miniaturization of the device, but also has the problems that the characteristics of the aluminum electrolytic capacitor significantly limit the life of the device, particularly in a high-temperature environment, and lower the reliability.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-described problems. By performing feed-forward control by voltage detection means in combination with feedback control, a primary smoothing device comprising a large aluminum electrolytic capacitor is provided. It is an object of the present invention to eliminate the use of a capacitor, to reduce the size of a device, and to realize an AC-DC converter with long life, high reliability, and low power consumption.

[0008]

An AC-D according to the present invention is provided.
A C converter, a first rectifier for rectifying the AC input;
A voltage detecting means connected to the output side of the first rectifier; and a transformer detecting means connected in parallel to the voltage detecting means.
A series circuit of a secondary winding and switching means, a second rectifier and a capacitor which take out the high-frequency AC voltage obtained by the switching means from the secondary winding of the transformer, rectify and smooth the output voltage, and A control drive circuit for determining an on / off ratio of the switching means based on an output voltage and a voltage from the voltage detection means.

In the above configuration, the control drive circuit corrects the oscillator output voltage based on the error amplifier that compares the output voltage with the reference voltage and the output of the error amplifier and the output of the voltage detection means, and applies the corrected output voltage to the switching means. And a drive circuit.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment 1 Hereinafter, Embodiment 1 of the present invention will be described with reference to FIG. In FIG. 1, reference numerals 1a and 1b denote input terminals for receiving an input AC voltage. 2 is a full-wave rectifier (first rectifier), 3 is a transformer,
4 is a switching transistor as switching means, 5 is a diode (second rectifier), 6 is a secondary smoothing capacitor, 7a and 7b are output terminals, 9 is a voltage detector,
The primary winding of the transformer 3 and the series circuit of the switching transistor 4 are connected in parallel. The voltage detector 9 can be realized by a voltage dividing circuit using a resistor or the like. Reference numeral 20 denotes a control drive circuit, which includes a reference voltage source 21, an error amplifier 22, a photocoupler 23, a current / voltage converter 24, and an oscillator 25.
And the drive circuit 26. The output of the error amplifier 22, the output of the voltage detector 9, and the output of the oscillator 25 are applied to the drive circuit 26, and the output of the drive circuit 26 drives the switching transistor 4. doing.

Next, the operation will be described. Input terminals 1a, 1b
Is input to the input AC voltage and full-wave rectified by the full-wave rectifier 2 to be converted to a full-wave rectified voltage V1. This full-wave rectified voltage V1 is converted into a high-frequency AC voltage by the switching transistor 4, then rectified and smoothed by the diode 5 and the secondary smoothing capacitor 6, and output voltage V0
Are output from the output terminals 7a and 7b. This is because during the ON period of the high-frequency switching transistor 4, the power applied to the transformer 3 by the high-frequency AC voltage and the primary winding current I1 of the transformer 3 excited thereby is stored in the core of the transformer 3 as magnetic energy. This is due to discharge from the secondary winding of the transformer 3 during the off period of the switching transistor 4.

The output voltage V0 is compared with a reference voltage source 21 by an error amplifier 22 of the control drive circuit 20. The output of the error amplifier 22 is insulated by a photocoupler 23, converted in voltage by a current / voltage converter 24,
Is added to At the same time, the full-wave rectified voltage V1 is input to the drive circuit 26 as the output of the voltage detector 9 together with the output voltage V2 of the oscillator 25. The drive circuit 26 outputs a drive output voltage V3 for driving the switching transistor 4 at a predetermined on / off ratio.
The value is controlled by the output of. This is shown in FIG.

In FIG. 2, (a) shows a full-wave rectified voltage V
1, (b) shows the primary winding current I1 of the transformer 3, and (c) shows the output voltage V3 of the drive circuit 26, respectively. The drive circuit 26 operates so that the on-period of the switching transistor 4 becomes shorter when the output voltage V0 is higher than the reference voltage source 21, and operates so that the on-period becomes longer when the output voltage V0 is lower than the reference voltage source 21. . At the same time, when the magnitude of the output of the voltage detector 9, that is, the full-wave rectified voltage V1 is high, the ON period of the switching transistor 4 functions to be shorter, and when the full-wave rectified voltage V1 is low, the ON period becomes longer. Work to be longer.

By these actions, the drive circuit 26 controls the on / off ratio of the switching transistor so that the output voltage V0 becomes constant. In this circuit, an error amplifier 22, a photocoupler 23, a current / voltage conversion circuit 24
The output voltage feedback control loop constituted by the above has a relatively large delay (time constant) in order to avoid a malfunction such as oscillation due to switching noise or the like. Since feed-forward control for controlling the on / off ratio of the transistor 4 is performed, even when a large ripple voltage is superimposed on the input voltage such as a full-wave rectified voltage, the superimposition of the ripple voltage on the output voltage V0 is reduced. Can be. Thus, the primary smoothing capacitor 8 required for the conventional AC-DC converter shown in FIG. 3 can be omitted.

[0015]

As described above, according to the present invention, the on / off ratio of the switching means is determined by using the feed-forward control circuit of the voltage detection means together with the feedback control circuit. Responsiveness to input ripples is improved while ensuring the stability of the circuit, and even if switching is performed without smoothing the full-wave rectified voltage, ripple superposition on the output voltage can be reduced. As a result, a large aluminum electrolytic capacitor is not required, the device can be reduced in size, low reliability and short life due to the characteristics of the aluminum electrolytic capacitor can be avoided, and the reliability and long life of the device can be improved. By using the voltage detecting means, low power consumption can be achieved.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing an AC-DC converter according to Embodiment 1 of the present invention.

FIG. 2 is a waveform chart illustrating an operation of the first embodiment.

FIG. 3 is a circuit configuration diagram showing a conventional AC-DC converter.

FIG. 4 is a waveform diagram illustrating the operation of a conventional AC-DC converter.

[Explanation of symbols]

1a, 1b input terminals, 2 full-wave rectifiers, 3 transformers, 4 switching transistors, 5 diodes,
6 Secondary smoothing capacitor, 7a, 7b output terminal, 9
Voltage detector, 20 control drive circuit, 21 reference voltage source,
22 error amplifier, 23 photocoupler, 24 current /
Voltage converter, 25 oscillator, 26 drive circuit.

Claims (2)

[Claims] 1. A first rectifier for rectifying an AC input, a voltage detecting means connected to an output side of the first rectifier, a primary winding and a switching means of a transformer connected in parallel to the voltage detecting means. And a high-frequency AC voltage obtained by the switching means is taken out from the secondary winding of the transformer, rectified and smoothed to obtain an output voltage.
An AC-DC converter comprising: a rectifier and a capacitor, and a control drive circuit that determines an on / off ratio of the switching means based on the output voltage and a voltage from the voltage detection means. 2. A control drive circuit comprising: an error amplifier that compares an output voltage with a reference voltage; and a drive circuit that corrects an oscillator output voltage based on an output of the error amplifier and an output of a voltage detection means and applies the corrected output voltage to a switching means. The AC-DC converter according to claim 1, further comprising: