JPH03218258A - Switching power supply - Google Patents

Abstract

PURPOSE:To reduce loss at the time of turn OFF by connecting a variable capacitance capacitor in parallel with a switching element in a switching power supply having the switching element connected between one and the other ends of a DC power supply through a transformer or a reactor. CONSTITUTION:During ON interval of a switching element 4, source voltage is applied on the primary winding 3 and a corresponding voltage is induced in the secondary winding 5. When the switching element 4 is turned OFF, collector current of the switching element 4 starts to decrease and a current flows as the energy stored in the primary winding 3 of the transformer 2 is discharged. At this time, a flyback voltage is produced but since a variable capacitance capacitor 13 is connected in parallel with the switching element 4, the flyback voltage is absorbed and the voltage across the switching element 4 rises slowly.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a switching power supply device that includes an inductance or a transformer between a DC power supply and a switching element.

[Conventional technology] A typical conventional switching regulator connects a transformer between a DC power supply and a switching element, provides an output rectifying and smoothing circuit on the secondary side of the transformer, and controls the switching element on and off. It is configured to perform DC-DC conversion.

[Problems to be Solved by the Invention] Incidentally, it is desired to reduce loss in switching elements. The faster the switching speed of the switching element, the less loss there will be, and the slower the switching speed, the more loss will occur. On the other hand, it is already known to connect a capacitor in parallel to a switching element in order to reduce the loss when the switching element is turned off.

This capacitor absorbs the flyback voltage generated in the transformer at turn-off, protects the switching element from excessive voltage, and delays the rise of the voltage to reduce loss. Now, assuming that the switching speed of the switching elements is the same, the larger the capacitance of the parallel capacitor, the greater the voltage delay effect and the smaller the loss. However, if the capacitances of the parallel capacitors are greatly shifted, it becomes difficult to obtain a sufficient reset voltage for the transformer. Also,
The loss at turn-on may increase.

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a switching power supply device that can reduce the loss during turn-off and that can reliably reset a transformer or a reactor.

[Means for Solving the Problems] To achieve the above object, the present invention provides a switching power supply having a switching element connected between one end and the other end of a DC power supply via a transformer or a reactor. The present invention relates to a switching power supply device in which a variable capacitor is connected in parallel to the switching element.

[Operation] In the present invention, when the switching element is turned off, a flyback voltage is generated in the transformer or the reactor. By providing a surge absorption and delay (loss reduction) circuit, a high voltage is prevented from being applied to the switching element during turn-off, and the rise of the voltage is delayed. When such a circuit is constructed with a variable capacitor, a high voltage is not applied to the variable capacitor immediately after turn-off, so the capacitance of the variable capacitor exhibits a large value and the voltage rises. The delay becomes larger and the loss becomes smaller. Thereafter, as the voltage of the variable capacitor increases, the capacitance decreases, allowing the transformer or reactor to obtain the required set voltage.

[Embodiment] Next, a switching regulator according to an embodiment of the present invention will be described with reference to FIGS. 1 and 2. A switching element 4 made of a transistor is connected between one end and the other end of the DC power supply 1 via a primary winding 3 of a transformer 2. A diode 6 is connected to the secondary winding 5 of the transformer 2.
, 7, a reactor 8, and a capacitor 9. A load 11 is connected through an output rectifying and smoothing circuit 10. 12
is a control circuit that controls on/off of the switching cable 4 so as to maintain a constant voltage. Switching cable 4
The variable capacitor 13 connected in parallel with the primary winding 3 absorbs the flyback voltage generated in the primary winding 3 at turn-off, and also delays the rise of the voltage of the switching element 4.

In the switching regulator shown in FIG. 1, a power supply voltage is applied to the primary winding 3 during the ON period of the switching element 4, and a corresponding voltage is obtained at the secondary winding 5. When the switching element 4 is turned off, the current Ic of the switching element 4 starts to decrease at t1 as shown in FIG. 2, and a current based on the release of the stored energy in the primary winding 3 of the transformer 2 flows. At this time, a flyback voltage is generated, but since the variable capacitor 13 is connected in parallel to the switching element 4, the voltage is absorbed here, and the voltage VC between both terminals of the switching element 4 gradually rises. . If an ordinary capacitor is connected instead of the variable capacitor 14, the voltage VC will rise as shown by the dotted line in Figure 2, but if the variable capacitor 4 is connected, the voltage VC will rise as shown by the solid line. stand up.

The variable capacitor 13 is composed of a variable capacitance diode, and has a characteristic that the capacitance decreases as the reverse bias voltage increases. Therefore, during a period such as the period t1 to t2, when the voltage of the switching element 4 is relatively low due to the function of the capacitor 13, the variable capacitor 13 has a large capacity, and furthermore, the voltage Vc of the switching element 4 is A noticeable lowering effect occurs. After that,
When the voltage ■C of the switching element 4 increases, the capacitance of the variable capacitor 4 decreases, and becomes almost the same as when only a normal capacitor is used. Therefore, the maximum value of the voltage Vc of the switching element 4 does not differ much between the dotted line and the solid line.

In order to reduce the loss during the turn-off period (1. to t2) of the switching element 4, it is important to reduce the voltage VC of the switching element 4 during this period.

As is clear from the comparison between the dotted line and the solid line of the voltage Vc in FIG. 2, by connecting the variable capacitor 13, the voltage Vc during the turn-off period becomes smaller and the loss becomes smaller. On the other hand, when the voltage of the switching element 4 increases after t2, the capacitance of the variable capacitor 13 decreases, so the voltage suppression effect decreases and the voltage VC becomes relatively high. As a result, it becomes possible to apply a sufficient reset voltage Ij to the transformer 2.

[Modifications] The present invention is not limited to the above-mentioned embodiments, and, for example, the following modifications are possible.

(1) As shown in Figure 3, the variable capacitor 13
A resistor R may be connected in series to form a CR absorber circuit.

(2) As shown in FIG. 4, an ordinary capacitor 21 can be connected to the variable capacitor 13 in the jlfi column.

(3) As shown in FIG. 4, a uJ transformation capacitor 22 may be connected in parallel to the rectifier diode 6 to absorb noise generated during recovery when the diode 6 is turned off.

When the diode 6 is off, the voltage across it is zero, so the capacitance of the variable capacitor becomes large, resulting in a large noise reduction effect.

After that, the capacitance becomes smaller, so the influence on other circuits is reduced. Note that a resistor may be connected in series to the variable capacitor 22 to form a CR circuit.

(4) An example of variable capacitors 13 and 22 other than variable capacitor diodes is a pair of electrodes 3 as shown in FIG.
A variable capacitance multilayer ceramic capacitor (barium titanate ceramic capacitor) having a control electrode 33 and a control electrode 33 can be used.

(5) A snap circuit may be connected in parallel to the primary winding 3.

(6) The present invention can also be applied to a step-up switch regulator in which a reactor is connected between a switching element and a power source.

[Effects of the Invention] As described above, according to the present invention, it is possible to reduce the loss when the switching element is turned off.

[Brief explanation of drawings]

Fig. 1 is a circuit diagram showing a switching regulator according to an embodiment of the present invention, Fig. 2 is a waveform diagram showing the current and voltage of the switching element in Fig. 1, and Figs. 3 and 4 are diagrams of modified examples. A circuit diagram showing a switching regulator. Figure 5 is a principle diagram without showing a town capacitive multilayer ceramic capacitor. 1...power supply, 2...transformer, 3...primary winding,
4... Switching element, 5... Secondary winding, 13.
...Variable capacitor.

Claims (1)

[Claims] [1] A switching power supply device having a switching element connected between one end and the other end of a DC power source via a transformer or reactor, characterized in that a variable capacitor is connected in parallel to the switching element. switching power supply.