JP2000358369A - Switching power supply device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To avoid voltage rise, even if a dummy resistor is eliminated. SOLUTION: A DC voltage from a DC power supply 2 is converted into a switching voltage by the switching of a power FET 6 and supplied to the primary winding of a transformer 4. A switching voltage/induced in the one secondary winding of the transformer 4, is rectified by a diode 8 and a smoothing capacitor 10 and supplied to a load 14. A control circuit 18 controls the FET 6, so as to make the voltage value at a voltage dividing resistor 16b to be a predetermined value. A switching voltage induced in the other secondary winding of the transformer 4 is rectified by a diode 20 and a smoothing capacitor 22 and is supplied to a constant voltage circuit IC 24 which produces and supplies a constant DC voltage to a load 28. A diode 32 is connected between the smoothing capacitor 22 and a smoothing capacitor 10.

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, and more particularly to a switching power supply for suppressing a voltage rise at a light load.

[0002]

2. Description of the Related Art Conventionally, there is a switching power supply as shown in FIG. In this switching power supply device, a switching voltage is generated in the primary winding 4a of the transformer 4 by turning on / off the DC voltage of the DC power supply 2 by the power FET 6.

The switching voltage induced in the secondary winding 4b of the transformer 4 is rectified and smoothed by a diode 8 and a smoothing capacitor 10, and is supplied to a load 14 via output terminals 12a and 12b. The voltage supplied to the load 14 is divided by the voltage dividing resistors 16a and 16b, and the voltage between both ends of the resistor 16b is supplied to the control circuit 18. The control circuit 18 controls the conduction period of the FET 6 so that the input voltage has a predetermined value. As a result, the load 14 is supplied with the voltage controlled by the constant voltage.

[0004] The switching voltage induced in the secondary winding 4 c of the transformer 4 is rectified by the diode 20 and smoothed by the smoothing capacitor 22. The voltage of the smoothing capacitor 22 is supplied to the constant voltage circuit IC 24, and the constant voltage is applied to the output terminals 26a,
The load 28 is supplied to the load 28 via 6b. When a load of a predetermined size is connected to the output terminals 12a, 12b, 26a, 26b as the loads 14, 28, the voltage V1 between the output terminals 12a, 12b is equal to the output terminals 26a, 26b. Is higher than the voltage V3.

When the load 28 is a variable load,
There are times when the load is smaller than a predetermined value, that is, when the load is light. In such a case, the voltage V2 across the smoothing capacitor 22 may be higher than a predetermined value. To suppress this voltage rise, a dummy resistor 30 is connected between both ends of the smoothing capacitor 22.

[0006]

However, in this switching power supply, since the dummy resistor 30 is connected between both ends of the smoothing capacitor 22, power loss occurs due to the dummy resistor 30 and the conversion efficiency is deteriorated. Was invited.
Also, the constant voltage circuit I
The temperature of C24 rises, leading to a decrease in the reliability of IC24. For example, if the dummy resistor 30 is removed in order to reduce the power loss in the dummy resistor 30, a diode 20, a smoothing capacitor 22, and an IC 24 having a higher rated voltage than when the dummy resistor 30 is used are used. This has led to larger components and higher costs.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a switching power supply that does not cause a voltage increase even when a dummy resistor is removed.

[0008]

SUMMARY OF THE INVENTION A switching power supply according to the present invention has a DC power supply. As the DC power supply, for example, a rectified and smoothed commercial AC power supply can be used. Switching means for switching the DC voltage of the DC power supply to convert the DC voltage into a switching voltage is provided. The switching means can be, for example, one semiconductor switching element, or a bridge circuit combining a plurality of semiconductor switching elements can be used. A transformer is provided for supplying the switching voltage to a primary winding. This transformer also has at least two secondary windings. There is provided first DC converting means for converting a switching voltage induced in one secondary winding of the transformer into DC and supplying the switching voltage to a first load. The first direct current conversion means may include, for example, a diode bridge circuit composed of a single rectifying diode or a plurality of diodes, and a smoothing means, for example, a smoothing capacitor. Control means is provided for inputting a signal representing the output voltage of the first DC converting means and controlling the switching means so that the output voltage of the first DC converting means has a predetermined value. The signal representing the output voltage of the first DC conversion means is generated by, for example, the voltage detection means. As the voltage detecting means, for example, a single resistor or a voltage dividing means in which a plurality of resistors are connected in series can be used. Further, the control means can supply a control signal for controlling the conduction period to the switching means, and the control signal has, for example, a constant period, and a portion representing the conduction period of the switching means has a length. It can be a changing PWM signal. Second DC conversion means is provided for converting the switching voltage induced in the other secondary winding of the transformer into DC and supplying the DC to the second load. When the output voltage of the second DC converting means is higher than the output voltage of the first DC converting means, the superimposing means for superimposing the output voltage of the second DC converting means on the output voltage of the first DC converting means, Is provided.

In this switching power supply, for example, the second load is a light load, and the voltage of the second DC converting means is:
When the voltage is higher than the voltage of the first DC converter, the voltage of the second DC converter is superimposed on the voltage of the first DC converter.
As a result, the control means controls the switching means such that the first DC voltage, on which the voltage of the second DC means is superimposed, is actually a predetermined voltage. I do. Therefore, even if a dummy resistor is not provided on the output side of the second DC converting means, it is possible to suppress an increase in the output voltage of the second DC converting means, and to use a diode used for the second DC converting means. It is not necessary to use a capacitor having a large rating for the capacitor or the smoothing capacitor, a small diode or a smoothing capacitor can be used, and it is possible to prevent an increase in the size and cost of the entire switching power supply device.

Also, the output side of the second direct current conversion means and the second
Even if, for example, a constant voltage circuit IC is provided as a constant voltage adjusting means between the load and the load, since no dummy resistor is used, large heat generation does not occur, and the reliability of the constant voltage adjusting means is reduced. Performance can be improved.

The superimposing means may be a diode having an anode connected to the second DC converting means and a cathode connected to the first DC converting means. In this case, only one diode needs to be provided, so that it is not necessary to increase the cost to obtain the above effects. As the superimposing means, for example, a normally open switching element is provided between the output side of the second DC converting means and the output side of the first DC converting means, and the output voltage of the first DC converting means and And a comparison means for comparing the output voltage of the second DC conversion means with the output voltage of the second DC conversion means. When the output voltage of the second DC conversion means is higher than the output voltage of the first DC conversion means, the comparison means The device may be configured to close.

[0012]

FIG. 1 shows a switching power supply according to an embodiment of the present invention. This switching power supply has a DC power supply 2. As the DC power supply 2, for example, a commercial AC power supply that is converted into DC by rectifying means, for example, a rectifying circuit and smoothing means, for example, a smoothing capacitor, can be used.

A primary winding 4a of the transformer 4 and a switching means, for example, a semiconductor switching element, more specifically, a drain-source conductive path of the power FET 6, are connected between both ends of the DC power supply 2. Instead of the power FET 6, a bipolar transistor or IGBT can be used. Further, instead of a single semiconductor switching element, an inverter circuit constituted by a plurality of semiconductor switching elements can be used.

The transformer 4 has two secondary windings 4b and 4c. DC conversion means is connected between both ends of the secondary winding 4b. This DC conversion means includes, for example, a rectifying diode 8 having an anode connected to one end of the secondary winding 4b.
And the cathode of the rectifying diode 8 and the secondary winding 4b.
, And a smoothing capacitor 10 connected between the other end of the power supply. The voltage between both ends of the smoothing capacitor 10 is supplied to the load 14 via the output terminals 12a and 12b. Instead of a single rectifying diode, a rectifying bridge circuit including a plurality of rectifying diodes can be used. In addition, a smoothing reactor may be used instead of or in addition to the smoothing capacitor 10.

The voltage across the load 14 is detected by voltage detecting means, for example, a voltage divider composed of a series circuit of resistors 16a and 16b. That is, the voltage between both ends of the resistor 16 b is proportional to the voltage between both ends of the load 14, and is supplied to the control circuit 18. The control circuit 18 generates a control signal so that the input voltage across the resistor 16b becomes equal to a preset voltage value, and transmits the control signal to the control terminal of the semiconductor switching element, that is, the gate of the power FET 6. To supply. The control signal is, for example, a PWM control signal. For example, while the control signal is at the H level, the power FET 6 is turned on. Thus, the power FET 6
The voltage between both ends of the load 14 is controlled by adjusting the conduction period of the load 14. This is the voltage of the control system that is controlled at a constant voltage.

The other secondary winding 4c of the transformer 4 is also provided with a DC converter. This DC conversion means is also used for the secondary winding 4c.
Rectifier diode 20 having one end connected to the anode
And a smoothing capacitor 22 connected between the cathode of the diode 20 and the other end of the secondary winding 4c. The rectifying diode 8 and the smoothing capacitor 10
The modification described in relation to (1) can also be applied to the rectifying diode 20 and the smoothing capacitor 22.

The voltage across the smoothing capacitor 22 is
A voltage of a non-control system which is not directly controlled by the control circuit 18 and is a constant voltage adjusting means such as a constant voltage circuit I.
It has been input to C24. The output voltage of the constant voltage circuit IC24 is converted to a constant voltage, and output terminals 26a and 26b
To the load 28.

The voltage V1 between the output terminals 12a and 12b is
The control circuit 18 and the constant voltage circuit IC 24 are selected so that the voltage becomes higher than the voltage V3 between the output terminals 26a and 26b.

For example, when the load 28 is light, the voltage V2 across the smoothing capacitor 22 is higher than the voltage when the load 28 is a normal load. This increased voltage V2
Is higher than the voltage V1 between the output terminals 12a and 12b, a superimposing circuit for superimposing the increased voltage V2 on the voltage V1 is provided. As this superposition circuit, a unidirectional conduction element, for example, a diode 32 is provided. That is, the anode of the diode 32 is connected to the connection point between the smoothing capacitor 22 and the cathode of the rectifying diode 20, and the cathode of the diode 32 is connected to the connection point between the cathode of the rectifying diode 8 and the smoothing capacitor 10. Have been. The output terminal 12b and the output terminal 26b
And are connected.

With this configuration, when the voltage V2 becomes higher than the voltage V1 as described above, the diode 32 conducts, and the voltage drop across the diode 32 from the voltage V2 (from 0.4 to 4.0). 1V) is supplied to the resistors 16a and 16b, and the current flowing to the load 14 increases. At this time, the voltage of the resistor 16b increases. The control circuit 18 controls the conduction period of the power FET 6 so that the increased voltage becomes equal to a predetermined voltage, that is, the voltage V2 becomes higher than the voltage drop in the diode 32. That is, the control circuit 18 controls the power FET 6 so that the voltages V2 and V1 become substantially equal. Note that when the loads 14 and 28 are in a state of a predetermined value, that is, in a state of a rated load, V2 is smaller than V1, the diode 32 is non-conductive, and the control circuit 18 determines that the value of V2 is Regardless,
The FET 6 is controlled according to the value of V1.

FIG. 3 shows the relationship between the load current flowing through the load 28 and the voltage V2 when the diode 32 is removed in the switching power supply device of FIG. The voltage V2 is equal to the voltage V1
The value is much larger than that of the rectifier diode 20 and the smoothing capacitor 22.

FIG. 4 shows the relationship between the load current flowing through the load 28 and the voltage V2 in the conventional switching power supply (using the dummy resistor 30) shown in FIG. As is clear from the comparison with FIG. 3, the voltage V2 at the time of light load is lower than that in the case where the dummy resistor 30 is not provided, but the power loss due to the dummy resistor 30 occurs, and the conversion efficiency is reduced. As a result, the temperature of the constant voltage circuit IC 24 rises due to the heat generated by the dummy resistor 30 and I
This leads to a decrease in the reliability of C24.

FIG. 5 shows the relationship between the load current flowing through the load 28 of the switching power supply device of FIG. 1 and the voltage V2. Even when the load current is small, even when the dummy resistor 30 is not used, The voltage V2 is almost equal to V1, and it is not necessary to use a diode, a smoothing capacitor 22, and a constant voltage circuit IC24 having a high rating. Further, since the dummy resistor 30 is not used, no large heat is generated, and the reliability of the constant voltage circuit IC 24 is not reduced. Moreover, the only device used to make V2 approximately equal to V1 is the diode 32 alone.

In the switching power supply of the above-described embodiment, the constant voltage circuit IC 24 is provided. However, this is a case where the constant voltage circuit IC 24 is removed, and the switching power supply is not connected to the load 28 and is not loaded. In such a case, the diode 32 needs to be provided because the voltage of V2 rises from the rated load. That is, the present invention can be implemented even when the constant voltage circuit IC 24 is removed. Further, in the switching power supply device of the above-described embodiment, the transformer has two secondary windings, but may have three or more secondary windings.

[0025]

As described above, according to the present invention, by providing the superimposing circuit, it is possible to suppress an increase in voltage at a light load without providing a dummy resistor.

[Brief description of the drawings]

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

FIG. 2 is a block diagram of a conventional switching power supply device.

3 is a diagram illustrating a relationship between a load current flowing through a load and a voltage of a smoothing capacitor when a dummy resistor is removed in the switching power supply device of FIG. 2;

4 is a diagram showing a relationship between a load current and a voltage of a smoothing capacitor 22 in the switching power supply device of FIG.

5 is a diagram showing a relationship between a load current and a voltage of a smoothing capacitor 22 in the switching power supply device of FIG.

[Explanation of symbols]

 2 DC power supply 4 Transformer 6 Power FET (switching means) 8 20 Rectifier diode (DC converting means) 10 22 Smoothing capacitor (DC converting means) 18 Control circuit (control means) 32 Diode (superimposing means)

Claims (3)

[Claims] A DC power supply; switching means for switching a DC voltage of the DC power supply to convert the DC voltage into a switching voltage; and a transformer having the switching voltage supplied to a primary winding and having at least two secondary windings. A DC converter for converting a switching voltage induced in one of the secondary windings into DC and supplying the DC voltage to a first load, and an output voltage of the first DC converter being set to a predetermined value. A control means for controlling the switching means by inputting a signal representing an output voltage of the first DC overvoltage, and converting the switching voltage induced in the other secondary winding into a DC, A second DC converting means for supplying to the load; and when the output voltage of the second DC converting means is higher than the output voltage of the first DC converting means, the output voltage of the second DC converting means is changed to the first DC voltage. The output voltage of the A switching power supply device comprising: 2. The switching power supply device according to claim 1, wherein a constant voltage adjusting means is provided between an output side of the second DC converting means and the second load. 3. The switching power supply device according to claim 1, wherein the superimposing means is a diode having an anode connected to the second DC conversion means and a cathode connected to the first DC conversion means. .