JPH0851774A - Switching power supply circuit - Google Patents

Abstract

PURPOSE:To provide a switching power supply circuit which can produce power with a desired optimal frequency under light load through simple circuitry at low cost. CONSTITUTION:A DC voltage is fed from the secondary of a transformer 3 to an apparatus 10 under control of a PWM circuit 6 and a switching transistor 7. When a load signal S, representative of light load, is fed from the apparatus 10 to a switching frequency switching circuit 9, the circuit 9 switches the switching frequency of the PWM circuit 6 from a switching frequency for heavy load to a switching frequency for light load.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a switching power supply circuit for supplying a DC voltage to a device by switching means.

[0002]

2. Description of the Related Art Conventional switching power supply circuits of this type include those shown in FIGS. The switching power supply circuit shown in FIG. 3 stores a detection circuit 100 that detects an output voltage and an output current, and a memory that stores a combination of a switching frequency and a switching operation ON width that maximizes efficiency with respect to an output power value. Circuit 101,
From the microcomputer 102 and the microcomputer 102, the output power value is obtained from the output voltage value and the current value detected by the detection circuit 100, and the combination of the highly efficient switching frequency and the switching operation ON width at the output power value is read out. And a control circuit 104 that controls the switching operation of the switching transistor 103 based on the command.

With this configuration, it is possible to finely control the switching frequency and the ON width of the switching operation so that the efficiency is maximized according to the output power value, and thus the efficiency is improved in all load states from light load to heavy load. I am trying to get good output. As a technique similar to this, for example, there is one described in Japanese Patent Laid-Open No. 1-157270.

The switching power supply circuit shown in FIG. 4 continuously operates the switching operation of the switching transistor 111 via the output current detection circuit 110 for detecting the output current and the PWM circuit 113 when the output current becomes smaller than the reference value. Switching circuit 11 for switching from the intermittent operation to the intermittent operation
2 is provided. With this configuration, when the output current is small, the switching operation is switched to the intermittent operation, thereby improving the output efficiency under light load. As a technique similar to this, for example, there is one described in Japanese Utility Model Laid-Open No. 10588/1992.

[0005]

However, in the switching power supply circuit shown in FIG. 3, the microcomputer 10 is used.
2 obtains an output power value from the output voltage value and the current value detected by the detection circuit 100, reads a combination of a highly efficient switching frequency and a switching operation ON width from the output power value from the storage circuit 101, The control circuit 104 controls the switching operation of the switching transistor 103 based on a command from the microcomputer 102 to finely control the switching frequency and the ON width of the switching operation, so that the circuit becomes complicated. Therefore, there is a problem that the circuit cost becomes high.

Further, in the switching power supply circuit shown in FIG. 4, when the output current becomes smaller than the reference value, the switching circuit 112 switches the switching operation of the switching transistor 111 from continuous operation to intermittent operation. Therefore, there is a problem that the switching frequency at light load is limited, and a desired optimum frequency cannot be obtained at light load.

The present invention has been made in view of the above problems, and an object thereof is to provide a switching power supply circuit having a simple and low-cost structure and capable of obtaining a desired optimum frequency at a light load. To do.

[0008]

In order to achieve the above object, a switching power supply circuit of the present invention uses a high-cycle switching frequency for heavy loads and a low-cycle switching frequency for light loads to generate a direct current. A switching means for supplying a voltage to the load side and a switching frequency switching means for switching the switching frequency of the switching means to the switching frequency for the light load based on a load signal indicating a light load state are provided.

According to a second aspect of the switching power supply circuit,
The switching means includes a PWM circuit and a switching transistor that performs a switching operation corresponding to the pulse width of the voltage output from the PWM circuit, and the switching frequency switching means receives the load signal indicating the light load state. At this time, the switching frequency of the PWM circuit is switched to the switching frequency for the light load.

A switching power supply circuit according to a third aspect is
Output current detection means for detecting a direct current corresponding to the direct current voltage is provided, and the switching frequency switching means receives the detection signal from the output current detection means, and when the detection signal indicates a light load state, The switching frequency of the PWM circuit is switched to the switching frequency for the light load.

[0011]

According to the above switching power supply circuit, when the switching means supplies the DC voltage at the switching frequency for heavy loads to the load side, when the load signal indicating the light load state is present, the switching frequency switching means is provided. Thus, the switching frequency of the switching means is switched to the switching frequency for light load.

According to the switching power supply circuit of the second aspect, when the switching frequency switching means receives the load signal indicating the light load state, the switching frequency of the PWM circuit is switched to the switching frequency for the light load.

According to the switching power supply circuit of the third aspect, when the switching frequency switching means receives the detection signal indicating the light load state from the output current detecting means, the switching frequency of the PWM circuit is changed to the switching frequency for the light load. Switch.

[0014]

Embodiments of the present invention will be described below with reference to the drawings. 1 is a block diagram showing a switching power supply circuit according to a first embodiment of the present invention. In the figure,
Reference numeral 1 denotes an AC power supply, and an output side of the AC power supply 1 has a smoothing capacitor 2 for converting the AC voltage from the AC power supply 1 into a DC voltage.
Are connected in parallel. Further, a primary winding is connected to the transformer 3 in parallel with the smoothing capacitor 2. A rectifier 4 that rectifies the output voltage of the secondary winding and a smoothing capacitor 5 that smoothes the output voltage from the rectifier 4 are connected to the output side of the secondary winding of the transformer 3.

Reference numeral 6 is a PWM circuit, and this PWM circuit 6
For example, a switching frequency for heavy load of 100 Hz and a switching frequency for light load of 25 Hz, for example, are set. The switching frequency for heavy load of 100 Hz is the fundamental frequency and is used when the device 10 is in the heavy load state. Further, the switching frequency for light load is set according to the type of the device 10. In this example, it is set to 25 Hz as described above. This PW
The base of the switching transistor 7 is connected to the output side of the M circuit 6 to form switching means. The collector of the switching transistor 7 is connected to the primary winding of the transformer 3, and the emitter is connected to the AC power supply 1.

That is, by applying a voltage having a long pulse width generated by the PWM circuit 6 to the base of the switching transistor 7 and lengthening the ON time of the switching transistor 7, the DC voltage applied to the primary side of the transformer 3 is changed. By increasing the frequency, the voltage output to the secondary side can be increased. On the contrary, the PWM circuit 6
By applying the voltage with a short pulse width generated in step 1 to the base of the switching transistor 7,
The voltage output to the secondary side of the transformer 3 can be reduced by shortening the ON time of.

Reference numeral 8 is an output voltage feedback circuit,
It is connected in parallel to the smoothing capacitor 5. The output side of the output voltage feedback circuit 8 is connected to the PWM circuit 6. As a result, the output voltage of the transformer 3 is fed back to the PWM circuit 6 via the output voltage feedback circuit 8, the PWM circuit 6 changes the pulse width based on this input voltage, and a stable power supply voltage is supplied to the device 10. It is supposed to be supplied.

A switching frequency switching circuit 9 as switching frequency switching means is provided in such a switching power supply circuit. The switching frequency switching circuit 9 has an output side connected to the PWM circuit 6 and an input side connected to the device 10.
The switching frequency switching circuit 9 has a function of controlling the PWM circuit 6 based on the load signal S from the device 10. Specifically, when the device 10 is in a heavy load state, the load signal S indicating "1" is input to the switching frequency switching circuit 9, and when the device 10 is in a light load state,
The load signal S indicating “0” is input to the switching frequency switching circuit 9. Then, when the load signal indicating “0” is input to the switching frequency switching circuit 9, the switching signal C for switching to the switching frequency for the light load is output from the switching frequency switching circuit 9 to the PWM circuit 6. On the contrary, when the load signal indicating “1” is input to the switching frequency switching circuit 9, the switching signal C for switching to the switching frequency for heavy load is output from the switching frequency switching circuit 9 to the PWM circuit 6.

In order to improve the efficiency of the switching power supply circuit, it is necessary to reduce the switching frequency of the switching transistor 7 to reduce the switching loss of the switching transistor 7. However, if the switching frequency is lowered when the device 10 is in a heavy load state, the switching power supply circuit becomes large. Therefore, in this embodiment, the switching frequency for heavy load is set to be high when the device 10 is in a heavy load state (for example, 100 Hz), and the switching power supply circuit is downsized.

Next, the operation of this embodiment will be described.
When the output side of the switching power supply circuit and the input side of the switching frequency switching circuit 9 are connected to the device 10 and driven,
The AC voltage from the AC power supply 1 is converted into a DC voltage by the smoothing capacitor 2 and applied to the primary winding of the transformer 3.
At this time, if the device 10 is in a heavy load state, the heavy load signal S indicating “1” is continuously input from the device 10 to the switching frequency switching circuit 9, and the PWM circuit 6 operates at the switching frequency for heavy load. Then, the switching transistor 7 is controlled by the voltage pulse width control of the PWM circuit 6.
Is switched, and a high-frequency DC voltage is applied to the primary side of the transformer 3. This output voltage is rectified by the rectifier 4 and smoothed by the smoothing capacitor 5, and then the device 1
Applied to zero.

When the device 10 is in a light load state,
The light load signal S indicating “0” is input from the device 10 to the switching frequency switching circuit 9, and the switching frequency C is output from the switching frequency switching circuit 9 to the PWM circuit 6 to switch to the switching frequency for light load. This allows
The PWM circuit 6 operates at a low cycle switching frequency for light loads, and the efficiency of the output voltage is improved.

Thus, according to this embodiment, the device 10
Since the switching frequency is set to be high in the heavy load state, the switching power supply circuit can be downsized with a simple structure, and as a result, the cost of the switching power supply circuit can be reduced. it can. Also,
Since the switching frequency for the light load can be set according to the type of the device 10, it is possible to obtain the optimum switching frequency at the time of the light load according to the type of the device.

FIG. 2 is a block diagram showing a switching power supply circuit according to the second embodiment of the present invention. The same elements as those in FIG. 1 will be described with the same reference numerals. In this embodiment, the output current detection circuit 11 as the output current detection means is used.
Is different from the first embodiment described above.

The output current detection circuit 11 is a circuit that detects the output current of the secondary winding of the transformer 3 and inputs the load signal S indicating the detected current to the switching frequency switching circuit 9. As a result, when the device 10 is in a light load state, a small current is output from the transformer 3, so that the load signal S indicating a small current is input from the current detection circuit 11 to the switching frequency switching circuit 9 and the switching frequency switching circuit. 9
A switching signal C for switching to the switching frequency for light load is output from the PWM circuit 6, and the PWM circuit 6 operates at a low cycle switching frequency. The rest of the configuration, functions and effects are similar to those of the first embodiment, so the description thereof is omitted.

[0025]

As described above, according to the switching power supply circuit of the present invention, a high cycle switching frequency for heavy loads is used, and this switching frequency is switched to a low cycle switching frequency for light loads when the load is light. Therefore, the switching power supply circuit can be downsized with a simple structure, and as a result, the cost of the switching power supply circuit can be reduced. Further, since the switching frequency for light load can be freely set, there is an effect that an optimum switching frequency can be obtained when the device has a light load.

[Brief description of drawings]

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

FIG. 2 is a block diagram showing a switching power supply circuit according to a second embodiment of the present invention.

FIG. 3 is a block diagram showing a switching power supply circuit according to a conventional example.

FIG. 4 is a block diagram showing a switching power supply circuit according to another conventional example.

[Explanation of symbols]

 1 AC power supply 3 Transformer 6 PWM circuit 7 Switching transistor 8 Output voltage feedback circuit 9 Switching frequency switching circuit 10 Device

Claims (3)

[Claims] 1. A switching means for supplying a DC voltage to a load side using either a high-cycle switching frequency for heavy loads or a low-cycle switching frequency for light loads, and a load signal indicating a light load state. A switching frequency switching means for switching the switching frequency of the switching means to the switching frequency for the light load based on the switching power supply circuit. 2. The switching means comprises a PWM circuit and a switching transistor that performs a switching operation corresponding to the pulse width of the voltage output from the PWM circuit, and the switching frequency switching means sets the light load state to the light load state. The switching power supply circuit according to claim 1, wherein the switching frequency of the PWM circuit is switched to the switching frequency for the light load when receiving the load signal shown. 3. An output current detecting means for detecting a DC current corresponding to the DC voltage is provided, and the switching frequency switching means receives a detection signal from the output current detecting means, and the detection signal indicates a light load state. 2. The switching power supply circuit according to claim 1, wherein the switching frequency of the PWM circuit is switched to the switching frequency for the light load when indicating.