JPH10295077A - Switching power-supply apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a switching power-supply apparatus which reduces a switching loss in the turning-off operation of a switching element even when an input voltage is high by a method wherein, when the charging voltage of a resonance capacitor exceeds a prescribed value in the turning-off operation of the switching element, a stored electric charge is discharged. SOLUTION: When a switching element (FET) 3 is turned on, a voltage across both ends is dropped, and the charging voltage of a resonance capacitor 4 becomes larger than the voltage across the both ends. As a result, a reverse- bias voltage is applied to a diode D1. The reverse-bias voltage is applied as a gate-source voltage with reference to the gate G6 of an FET 6. When it is higher than the threshold value of the FET 6 at a prescribed value, the FET 6 is set continuous, a discharge current from the resonance capacitor 4 flows to the FET 6 and a resistance R2, so as to be superposed on the excitation current of a primary winding 11. Since the discharge current is extremely small and the superposed excitation current is small, the switching current of the FET 3 does not contain a surge current. Consequently, when the FET 3 is turned on, a switching loss is reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a switching power supply having a transformer between a DC power supply and a switching element.

[0002]

2. Description of the Related Art In a switching power supply device such as a DC-DC converter, a transformer and a switching element are connected in series to a DC power supply, and the switching element is turned on and off to generate an alternating current in a secondary winding of the transformer. This is rectified by an output rectifying / smoothing circuit provided on the secondary side to obtain a DC output.

In this type of device, it is known to provide a ringing circuit in which a resonance capacitor is connected in parallel to the primary winding of a transformer, for example, in order to reduce switching loss when the switching element is turned off. . As shown in FIG. 5, assuming that the voltage between both ends of the switching element is V1 and the switching current is I1, this ringing circuit has a damping oscillation (ringing) caused by free resonance of the primary winding of the transformer and the resonance capacitor at turn-off (TOFF). R) is generated, and the voltage generated in the transformer is absorbed by the resonance capacitor to prevent the switching element from being excessively high.
The rising of 1 is delayed to reduce the switching loss.

It is also known to provide a delay circuit for reducing switching loss at the time of turn-on. This delay circuit reduces the switching loss at the time of turn-on by turning on the switching element at the timing of the lowest voltage value of the resonance free oscillation determined by the inductance value of the primary winding and the capacitance of the resonance capacitor.

[0005]

However, in a device having a wide input voltage range, even if a delay circuit for turning on the switching element at the lowest voltage value of the resonance free vibration is provided as described above, even if the input voltage is high, The problem is that, since the minimum voltage value of the resonance free oscillation increases and the charge stored in the resonance capacitor increases, a surge current Is occurs at turn-on (TON) as shown in FIG. was there. Further, noise is generated by the surge current Is, and when an overcurrent detection circuit for protecting the switching element is provided, there is a problem that the surge current Is is erroneously detected as an overcurrent.

The present invention solves the above-mentioned problems and provides a switching power supply device capable of reducing the switching loss at the time of turning on a switching element simply and at low cost even when the input voltage is high. It is intended to be.

[0007]

In order to achieve the above object, a switching power supply according to the present invention comprises: a first switching element connected to a DC power supply via a primary winding of a transformer; And a resonance capacitor connected in parallel with the primary winding of the first switching element, wherein when the first switching element is turned on, the charging voltage of the resonance capacitor is higher than the voltage across the first switching element. A discharge circuit is provided for discharging the charge stored in the resonance capacitor when the value exceeds a predetermined value. According to the above configuration, when the first switching element is turned on, even when the input voltage is high, the accumulated charge of the resonance capacitor is discharged to the discharge circuit, so that a large surge current is not generated in the first switching element.
Switching loss and noise can be reduced.

Preferably, the discharging circuit includes a rectifying element connected between a connection point between the first switching element and the transformer and the resonance capacitor,
And a constant-voltage element connected in series to the
And a control electrode of the second switching element is connected to a connection point between the first resistance and the constant voltage element, and a second resistor connected in series to the switching element is connected in parallel with the second resistance. .

[0009]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows a block diagram of a switching power supply according to one embodiment of the present invention. This device is
The primary winding 11 of the transformer 2 and F
A first switching element 3 such as an ET (field effect transistor) is connected in series. The primary winding 11
Are connected in parallel with each other, and the primary winding 11 and the resonance capacitor 4 constitute a ringing circuit. The FET 3 is PWM-controlled by a control circuit 5 connected to the control electrode (gate) G3. The load 1 is connected to a secondary winding 12 of the transformer 2 via an output rectifying / smoothing circuit 14 including a diode D2 and a capacitor C2.
5 is connected.

Here, the present device is provided with a discharge circuit 8 for discharging the accumulated charge in the resonance capacitor 4 when the charging voltage V2 of the resonance capacitor 4 exceeds a predetermined value larger than the voltage V1 across the FET 3 when the FET 3 is turned on. ing.

The discharge circuit 8 comprises an FET 3 and a transformer 2
A rectifying element D1 such as a diode having a forward direction in the direction of the charging current from the primary winding 11 to the resonance capacitor 4 is connected between the connection point of A resistance R1 and a constant voltage element ZD such as a Zener diode connected in series to the resistance R1, and a second switching element 6 such as an FET and a second resistance (discharge resistance) R2 connected in series to the second switching element 6 are connected in parallel. Be done. The Zener diode ZD is connected to the primary winding 11
Are connected so as to have a forward direction in the direction of the charging current to the resonance capacitor 4, and a control electrode G6 such as a gate of the FET 6 is connected to a connection point between the first resistor R1 and the Zener diode ZD.

In the above configuration, when the FET 3 is turned on, a current flows through the primary winding 11 of the transformer 2 to store energy. At this time, the polarities of the two windings 11 and 12 of the transformer 2 are as shown in FIG. FET
When the diode 3 is turned off, the diode D2 conducts, and the energy of the secondary winding 12 is released to the output rectifying / smoothing circuit 14,
DC output is obtained. That is, the flyback method is used. The control circuit 5 controls the FET 3 based on the output voltage of the output rectifying / smoothing circuit 14 detected by the output voltage detecting means (not shown) so that the FET 3 is kept on and off.

When the input voltage from the DC power supply 1 is high, the device operates as follows when the FET 3 is turned on and off. FIG. 2 shows operation waveforms of the voltage V1 across the FET 3 and the switching current I1. Also,
TON when FET3 is turned on and TOF when turned off
Indicated by F.

First, the FET 3 shown in FIG.
N), the voltage V1 across the FET 3 drops (α in FIG. 2), and the charging voltage V2 of the resonance capacitor 4 becomes
, The voltage on the cathode side becomes higher than the voltage on the anode side of the diode D1, and a reverse bias voltage is applied to the diode D1. When the input voltage is high, the electric charge stored in the resonance capacitor 4 increases and the reverse bias voltage increases. This reverse bias voltage is added as a gate-source voltage to the gate G6 of the FET 6, and when the gate-source voltage is higher than a threshold value (threshold voltage) of the FET 6 set to a predetermined value, the FET 6 becomes conductive. Then, the discharge current I2 from the resonance capacitor 4 flows between the drain and source of the FET 6 and the second resistor R2. The threshold is regulated by the Zener diode ZD. The current value of the discharge current I2 is determined by the Zener voltage of the Zener diode ZD, the second
Is determined by the resistance value of the resistor R2 and the threshold value of the FET6.

At this time, the discharge current I flowing through the discharge circuit 8
2 is superimposed on the exciting current which is the initial current of the primary winding 11. During the period in which the accumulated charge in the resonance capacitor 4 is discharged, the discharge may be completed during the turn-on period of the FET 3, so that the value of the discharge current I2 is extremely small.
Since the superimposed excitation current is also small, as indicated by β in FIG.
The switching current I1 flowing through the FET 3 has a waveform that does not include the surge current Is as shown by a broken line in the related art.

As described above, since a large surge current does not occur in the FET 3 when the FET (first switching element) 3 is turned on, the switching loss caused by the overlap between the voltage V1 across the FET 3 and the switching current I1 is reduced. . Further, even when the switching speed at the time of turn-on is reduced, the switching loss does not increase and the slope of the voltage V1 applied to the FET 3 becomes gentle, so that the voltage harmonic noise is reduced. Further, since there is no need to provide a delay circuit for turning on the FET 3 at the timing of the lowest voltage value of the resonance free oscillation of the primary winding 11 and the resonance capacitor 4 as in the related art, it is possible to achieve simple and low cost.

In FIG. 2, when the FET 3 is turned off (T
OFF), the voltage V1 across the FET 3 rises slowly due to the resonance free oscillation of the primary winding 11 of the transformer 2 and the resonance capacitor 4 because the resonance capacitor 4 is connected via the diode D1. Therefore, switching loss at the time of turn-off is reduced.

As shown in FIG. 3, a snubber circuit 16 connected in parallel with the primary winding 11 of the transformer 2 may be added to the circuit of FIG. This snubber circuit 16
The diode D3 and a parallel circuit including a resistor R3 and a capacitor C3 are connected in series. Further, as shown in FIG. 4, a resistor R4 may be added to the circuit of FIG. 1 to form a snubber circuit in which the resonance capacitor 4 and the resistor R4 are connected in series. Thereby, FET3
When the device is turned off, the switching loss can be further reduced.

In the present invention, an FET is used for the first switching element, but a bipolar transistor or the like may be used.

[0020]

According to the present invention, when the first switching element is turned on, even if the input voltage is high, the accumulated charge in the resonance capacitor is discharged to the discharge circuit, so that a large surge current flows through the first switching element. Is not generated, so that switching loss and noise can be reduced.

[Brief description of the drawings]

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

FIG. 2 is a diagram showing the operation of the above device.

FIG. 3 is a configuration diagram illustrating a switching power supply device according to another embodiment.

FIG. 4 is a configuration diagram showing a switching power supply device according to another embodiment.

FIG. 5 is a diagram showing an operation of a conventional switching power supply device.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... DC power supply, 2 ... Transformer, 3 ... First switching element, 4 ... Resonant capacitor, 6 ... Second switching element, 8 ... Discharge circuit, D1 ... Rectifier element, R1 ... First resistor, R2 ... 2 resistance, ZD: constant voltage element.

Claims (2)

[Claims] 1. A switching power supply device comprising: a first switching element connected to a DC power supply via a primary winding of a transformer; and a resonance capacitor connected in parallel to the primary winding of the transformer. A switching circuit provided with a discharge circuit for discharging the accumulated charge of the resonance capacitor when the charging voltage of the resonance capacitor exceeds a predetermined value larger than a voltage across the first switching element when the first switching element is turned on. Power supply. 2. The discharging circuit according to claim 1, wherein a rectifying element is connected between a connection point of the first switching element and the transformer and the resonance capacitor, and the first resistor and the first resistor are connected to each other. A constant voltage element connected in series to
A second switching element and a second resistor connected in series to the second switching element are connected in parallel, and a control electrode of the second switching element is connected to a connection point between the first resistor and the constant voltage element. Switching power supply.