JPH09308242A - Switching power supply - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a small switching power supply with high power factor and high efficiency by making it possible to set a smoothing capacitor at low voltage even under conditions of a high input voltage or a small load. SOLUTION: In a switching power supply, a smoothing capacitor 32 is connected across output terminals of a full-wave rectifier 21 that is connected to an AC power supply 20. A serial circuit made up of a primary winding 28 of a transformer and a switching element 33 is connected across terminals of the smoothing capacitor 32. A serial circuit made up of an inductor 26, a capacitor 39, and a control winding 31 is connected to a point between the full-wave rectifier 21 and the smoothing capacitor 32. A rectifying smoothing circuit 34 is connected to a secondary winding 29. A load 3 is connected to the output side of the rectifying smoothing circuit 34, while a control circuit 40 for controlling the switching circuit 33 is provided. The control circuit 40 includes a smoothing capacitor voltage detecting unit 53 and a frequency modulation circuit 60 to keep the smoothing capacitor 32 at a prescribed voltage under control.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a switching type DC stabilized power supply device which inputs AC.

[0002]

2. Description of the Related Art FIG. 5 shows a Japanese Patent Application No. Hei.
A configuration example of the switching power supply proposed in No. 177379 is shown. In FIG. 5, 20 is a commercial AC power supply, and 21
Is a full-wave rectifier and comprises diodes 22, 23, 24 and 25. Reference numeral 26 is an inductor, and 27 is a transformer, which is composed of a primary (2) winding 28, a secondary winding 29, and a control winding 31. 32 is a smoothing capacitor, 33 is a switching element, 3
A rectifying / smoothing circuit 4 is composed of a diode 35 and a capacitor 36. 37 is a load, CON is a control circuit, 3
Reference numeral 9 is a capacitor, and 40 is a diode.

In the circuit operation of this switching power supply, the input of the commercial AC power supply 20 is rectified by a full-wave rectifier 21 and smoothed by a smoothing capacitor 32 to a direct current with less ripple, and then the switching element 33 is operated at a frequency higher than the input commercial AC frequency. AC voltage is applied to the primary winding 28 of the transformer 27 by turning it on and off at the output of the transformer 27, and its output is applied to the rectifying and smoothing circuit 34 from the secondary winding 29 of the transformer 27 to rectify and smooth the output voltage of DC. When the switch element 33 is turned on in the basic operation of applying a load to the inductor 2
The current of 6 flows through the control winding 31 of the transformer 27, the primary winding 28, and the switch element 33 via the capacitor 39, so that the capacitor 39 starts to have a voltage, and the voltage is higher than the voltage of the smoothing capacitor 32. Then, the current of the inductor 26 flows into the smoothing capacitor 32 through the diode 40. Therefore, even if the switch element 33 is turned on, the boosting time of the inductor 26 is shortened, which has the effect of shortening the boosting time as the input voltage increases.

While the switch element 33 is off, the current of the inductor 26 flows into the smoothing capacitor 32 via the diode 40, and at the same time, the transformer 27.
The exciting current from the control winding 31 flows through the capacitor 39 and the diode 40, and the capacitor 39 is charged in the direction opposite to the ON period of the switch element 33, and the voltage drops. That is, the boosting time of the inductor 26 becomes shorter as the input voltage becomes higher. Therefore, even if the current of the inductor 26 is a continuous mode in which the current does not return to zero ampere while the switch element 33 is off, the input current is The waveform becomes a waveform substantially corresponding to a sine wave, and the power factor increases.

[0005]

[Problems to be Solved by the Invention]

(3) However, in the circuit configuration of the conventional switching power supply described above, the amount of charge flowing from the control winding 31 is larger than the amount of charge discharged from the smoothing capacitor 32 at the time of a light load, and the voltage of the smoothing capacitor 32 becomes smaller. To rise. For this reason, the smoothing capacitor 32 requires a high withstand voltage capacitor, and since the voltage applied to the switch element 33 also increases, the switch element 33 also requires a high withstand voltage element. Therefore, in general, a switching element having a high breakdown voltage has a larger loss when conducting a current as compared with a switching element having a low breakdown voltage, which causes a problem that the overall efficiency also decreases. The present invention has been made in view of the above points, and prevents the size from increasing by suppressing the voltage of the smoothing capacitor 32 even under the condition of a high input voltage or a light load, and increases the size and loss of the switch element 33. By avoiding the increase, it is possible to provide a compact and highly efficient AC input switching power supply.

[0006]

A switching power supply of the present invention comprises a full-wave rectifier connected to an AC power supply, a smoothing capacitor connected between output terminals of the full-wave rectifier, and a terminal of the smoothing capacitor. A series circuit of a primary winding and a switching element of a connected transformer, a series circuit of an inductor and a capacitor connected between the full-wave rectifier and the smoothing capacitor, and a control winding of the transformer, and the capacitor. A diode provided in parallel with the series circuit of the control winding, the rectifying / smoothing circuit connected to the secondary winding of the transformer and having a load connected to its output terminal, and the rectifying / smoothing circuit. And a control circuit that controls the switch element so that the output voltage of the smoothing capacitor becomes a predetermined voltage, the control circuit detects the voltage of the smoothing capacitor. And the smoothing capacitor voltage detection unit that,
A frequency modulation circuit is provided which changes the ON / OFF cycle of the switch element when the detection voltage of the smoothing capacitor voltage detection section reaches a predetermined voltage or higher.

[0007]

【Example】

(4) FIG. 1 shows an embodiment of the present invention. The same parts as those of the conventional example shown in FIG.
In FIG. 1, reference numeral 40 denotes a control circuit, which is a conventional control circuit CON.
Smoothing capacitor voltage detector 50 and frequency modulation circuit 60
Is added.

As for the operation of the control circuit 40, if the voltage of the smoothing capacitor 32 is below a specified value, the switch element 33 is turned on / off at a constant cycle (frequency), and according to the output voltage of the rectifying / smoothing circuit 34. The width is controlled (PWM) to control the output voltage to a constant value. Next, when the voltage of the smoothing capacitor 32 during a light load becomes equal to or higher than a predetermined value, the voltage detection circuit 50 detects this, and the frequency modulation circuit 60 changes the oscillation frequency of the switch element 32. In this embodiment, by setting the frequency higher than the steady frequency, the ON / OFF cycle of the switch element 33 is advanced to reduce the energy stored in the inductor 26 and limit the voltage rise of the smoothing capacitor 32. Incidentally, FIG. 2 shows the smoothing capacitor 32.
Is a characteristic diagram showing the relationship between the voltage Vcin and the output current I0. FIG. 3 shows the oscillation frequency (f) of the switch element 33 and the output current (I
0) is a characteristic diagram showing the relationship, and as an example, the capacitor 32
When the voltage of 4 reaches 420V or higher, it is prevented from further rising and is maintained at about 430V.

FIG. 4 is a circuit diagram of another embodiment of the present invention, in which a transformer 27 is provided with a tertiary winding 30 and is connected to one end of a smoothing capacitor 32 through a diode 38. An example in which the counter electromotive force generated in the tertiary winding 30 when the element 33 is turned off flows into the smoothing capacitor 32 via the diode 38 and the reset current of the transformer 27 is effectively used is shown.

[0010]

As described above, the voltage of the smoothing capacitor (5) is not changed even under a high input voltage or a light load condition by simply changing the oscillation frequency of the switch element 33 while keeping the conventional main circuit as it is. Since the setting can be made low, the smoothing capacitor 32 can have a low withstand voltage, and the switch element 33 having a low withstand voltage and a small on-resistance can be used. High efficiency can be achieved.

[Brief description of drawings]

FIG. 1 is a circuit diagram of an embodiment of the present invention.

FIG. 2 is a characteristic diagram of an example circuit.

FIG. 3 is a characteristic diagram of an example circuit.

FIG. 4 is a circuit diagram of another embodiment of the present invention.

FIG. 5 Conventional circuit diagram

[Explanation of symbols]

 20 Commercial AC power source 21 Full-wave rectifier 22, 23, 24, 25, 35, 38, 40 Diode 26 Inductor 27 Transformer 28 Primary winding of transformer 29 Secondary winding of transformer 30 Tertiary winding of transformer 31 Transformer Control winding 32 Smoothing capacitor 33 Switch element 34 Rectification smoothing circuit (6) 36, 39 Capacitor 37 Load 40 Control circuit 50 Smoothing capacitor voltage detection unit 60 Frequency modulation circuit

Claims (2)

[Claims] 1. A full-wave rectifier connected to an AC power supply, a smoothing capacitor connected between output terminals of the full-wave rectifier, a primary winding of a transformer connected between terminals of the smoothing capacitor, and a switch element. , A series circuit of the inductor and the capacitor connected between the full-wave rectifier and the smoothing capacitor, and a control winding of the transformer, and a series circuit of the capacitor and the control winding in parallel. A rectifying / smoothing circuit connected to the secondary winding of the transformer and having a load connected to its output terminal, so that the output voltage of the rectifying / smoothing circuit becomes a predetermined voltage. In a switching power supply having a control circuit for controlling the switch element, the control circuit includes a smoothing capacitor voltage detection unit for detecting a voltage of the smoothing capacitor, and the smoothing capacitor. A switching power supply comprising a frequency modulation circuit for varying the ON / OFF cycle of the switch element when the detection voltage of the voltage detector reaches a predetermined voltage or higher. 2. The switching power supply according to claim 1, wherein the transformer is provided with a tertiary winding and is connected to one end of a smoothing capacitor via a diode.