JP2003018839A - Switching power unit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress the power loss at ON of a main switching element by reducing the power loss in the snubber circuit of a switching power unit and reducing the voltage applied to the snubber circuit. SOLUTION: This switching power unit is equipped with a MOS-FET (51) for a snubber which is connected in series to a snubber circuit (8) and besides is turned on only immediately after the MOS-FET (3) is turned off, a differentiating circuit (52) which detects the counter electromotive force generated in the primary winding (2a) of a transformer (2) immediately after the MOS-FET (3) is turned off, an operational amplifier (55) which generates an output signal on high voltage (H) level when the differenting circuit (52) detects the counter electromotive force, and an ON period extension circuit (56) which extends the period of the output signal of the operational amplifier (55) and gives an ON signal VG2 to the gate terminal of the MOS-FET (3).

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention reduces power loss in a switching power supply device, particularly a snubber circuit, and also reduces the voltage applied to the snubber circuit to suppress power loss when the main switching element is on. Belongs to the switching power supply.

[0002]

2. Description of the Related Art FIG. 3 shows a flyback type switching power supply device which has been widely used conventionally.
The switching power supply device shown in FIG. 3 includes a DC power supply (1) composed of a rectifier circuit or a battery (storage battery) connected to an AC power supply, a primary winding (2a) and a secondary winding (2b). The transformer (2) and the MOS-F as the main switching element
A ET (MOS field effect transistor) (3), a rectifying / smoothing circuit (6) having a rectifying diode (4) and a smoothing capacitor (5), and a control circuit (7) for controlling ON / OFF of a MOS-FET (3). ), And a capacitor (9) and a resistor (1
0) and a snubber circuit (8). Transformer (2)
The primary winding (2a) and the MOS-FET (3) are connected to the DC power supply (1) in series. The rectifying / smoothing circuit (6) is connected between the secondary winding (2b) of the transformer (2) and the load (11) and supplies the load (11) with DC power of the voltage V _O. Control circuit (7)
Is the MOS-FE after the reset period of the transformer (2) is completed.
By turning on T (3) and turning off the MOS-FET (3) when the level of the voltage V _O of the load (11) exceeds the level of the reference voltage defining the target value of the output voltage. , The level of the DC output voltage V _O applied to the load (11) is kept constant. The snubber circuit (8) is the primary winding (2
It is connected in parallel with a) and absorbs the surge voltage and current generated when the transformer (2) is reset.

The operation of the switching power supply device shown in FIG. 3 is as follows. As shown in FIG. 4A, at time t _1, a control pulse signal V _G1 of high voltage (H) level is applied from the control circuit (7) to the gate terminal of the MOS-FET (3), and M
When the OS-FET (3) is turned on, a current flows from the DC power supply (1) through the primary winding (2a) of the transformer (2) and the MOS-FET (3), and the energy is supplied to the transformer (2). Is accumulated. As a result, as shown in FIG. 4 (B), the MOS-FET
The voltage V _DS between the drain and source terminals of (3) rapidly drops to 0 [V], and the drain current _ID increases linearly as shown in FIG. 4 (C). At this time, a voltage in the reverse direction is applied from the secondary winding (2b) of the transformer (2) to the rectifying diode (4) forming the rectifying and smoothing circuit (6) to bring it into a non-conducting state. ), The current I _D1 does not flow through the rectifier diode (4), and energy is not transferred from the transformer (2) to the load (11). Further, the voltage V _C1 of the capacitor (9) of the snubber circuit (8) is substantially equal to the power source voltage V _IN of the DC power source (1) and the reverse polarity voltage as shown in FIG. 4 (E).

Next, at time t _{2, the} control circuit (7) switches the MOS
-Control pulse signal V applied to the gate terminal of FET (3)
_{When G1} changes from a high voltage (H) level to a low voltage (L) level as shown in FIG. 4 (A) and the MOS-FET (3) changes from an on state to an off state, as shown in FIG. 4 (B). MOS-F
The voltage V _DS between the drain and source terminals of ET (3) rises rapidly and the drain current _ID becomes almost zero as shown in FIG. 4 (C). This allows the secondary winding of the transformer (2)
Since a forward voltage is applied from (2a) to the rectifying diode (4) of the rectifying and smoothing circuit (6) to make it conductive, the transformer (2)
Energy stored in the rectifying and smoothing circuit from the secondary winding (2b)
It is supplied to the load (11) via (6) and the transformer (2) is reset. Therefore, as shown in FIG. 4D, the current I _D1 flows through the rectifying diode (4) of the rectifying and smoothing circuit (6), and the transformer I
It gradually decreases with the release of stored energy in (2). At this time, the flyback voltage V _FB is generated by the counter electromotive force generated in the primary winding (2a) of the transformer (2), and the surge voltage V _SR and the oscillating wave surge voltage V _SR are generated by the stored energy of the leakage inductance of the transformer (2). Because surge current is generated,
As shown in FIG. 4 (E), the voltage V _C1 of the capacitor (9) of the snubber circuit (8) is the flyback voltage V _FB generated in the primary winding (2a) of the transformer (2) and the surge voltage V _SR is It is equal to the superimposed voltage. The oscillating wave surge voltage V _SR and the surge current are consumed and attenuated by the resistor (10) of the snubber circuit (8), so that the capacitor () of the snubber circuit (8) is discharged at the end of the reset period of the transformer (2). The voltage V _{C1 of} 9) is ₁ of the transformer (2).
It converges on the flyback voltage V _FB of the next winding (2a). on the other hand,
Voltage V _DS between the drain and source terminals of MOS-FET (3)
Is the power supply voltage V _IN of the DC power supply (1) as shown in FIG.
And it is equal to sum voltage of the primary winding (2a) to the superposed voltage of the flyback voltage V _FB and surge voltage V _{S R} generated in the transformer (2), a direct current just before the end of the reset period of the transformer (2) The power supply voltage V _IN of the power supply (1) and the primary winding (2
It converges to the added voltage of the flyback voltage V _FB of a).

At time t _{3, when} the discharge of the stored energy of the transformer (2) is completed and the reset period of the transformer (2) ends, the rectifying diode of the rectifying / smoothing circuit (6) as shown in FIG. 4 (D). The current I _{D1 stops} flowing in (4), and the rectifier diode
(4) becomes non-conductive. At this time, as shown in FIG. 4 (E), the voltage V _C1 of the capacitor (9) of the snubber circuit (8) gradually attenuates and converges to 0 [V] again. At the same time, the voltage V _DS between the drain and source terminals of the MOS-FET (3) gradually converges to the power supply voltage V _IN of the DC power supply (1) while oscillating again as shown in FIG. 4 (B). Then, at time t _{4, the} control pulse signal V _{G1 applied} from the control circuit (7) to the gate terminal of the MOS-FET (3) changes from the low voltage (L) level to the high voltage as shown in FIG. 4 (A). When the level becomes (H) and the MOS-FET (3) changes from the off state to the on state, the DC power supply (1) turns the primary winding (2a) of the transformer (2) and M
A current flows through the OS-FET (3), and energy is stored in the transformer (2). As a result, as shown in FIG. 4B, the voltage V between the drain and source terminals of the MOS-FET (3) is
_DS rapidly drops to 0 [V] and drain current I _D
Rises linearly as shown in FIG. Also, FIG.
As shown in (E), the voltage V _C1 of the capacitor (9) of the snubber circuit (8) becomes substantially equal to the voltage of the opposite polarity to the power supply voltage V _IN of the DC power supply (1).

[0006]

In the conventional switching power supply device shown in FIG. 3, when the MOS-FET (3) is turned on, the power supply voltage V _IN of the DC power supply (1) is applied to both ends of the snubber circuit (8). , When the MOS-FET (3) is off, the flyback voltage V _FB generated in the primary winding (2a) of the transformer (2) and the oscillating wave surge voltage V _SR are applied to both ends of the snubber circuit (8). Therefore, the current flows through the snubber circuit (8) both when the MOS-FET (3) is on and when it is off, and the power loss in the resistor (10) in the snubber circuit (8) increases. was there.
The flyback energy of the transformer (2) absorbed by the snubber circuit (8) is the capacitance of the capacitor (9) and the MO.
Since it is equal to the product of the square of the voltage applied to the snubber circuit (8) when the S-FET (3) is off, if the capacitance of the capacitor (9) in the snubber circuit (8) is increased, the MOS- The voltage applied to the snubber circuit (8) can be suppressed to a low level when the FET (3) is turned off, but when the MOS-FET (3) is turned on, the capacitor (9) in the snubber circuit (8) changes the MOS-FET (8). The energy released to 3) becomes large. Therefore, the snubber circuit
Increasing the capacitance of the capacitor (9) in (8) unnecessarily increases the power loss when the MOS-FET (3) is turned on.

Therefore, an object of the present invention is to provide a switching power supply device capable of reducing the power loss in the snubber circuit and reducing the voltage applied to the snubber circuit to suppress the power loss when the main switching element is on. And

[0008]

A switching power supply device according to the present invention comprises a primary winding (2a) of a transformer (2) and a main switching element (3) connected in series to a DC power supply (1).
And a rectifying / smoothing circuit (6) connected to the secondary winding (2b) of the transformer (2) and supplying a DC output (V _O ) to the load (11), and turning on / off the main switching element (3). Control circuit to control (7)
And a snubber circuit (8) that is connected in parallel with the primary winding (2a) and absorbs the surge that occurs when the transformer (2) is reset.
It has and. The control circuit (7) turns on the main switching element (3) after the completion of the reset period of the transformer (2), and when the level of the voltage (V _O ) of the load (11) exceeds the level of the reference voltage. Turn off the main switching device (3). In the switching power supply device of the present invention, the auxiliary switching element (51) that is turned on only immediately after the main switching element (3) is turned off is connected in series with the snubber circuit (8).

When the auxiliary switching element (51) is turned on immediately after the main switching element (3) is turned off, a surge voltage is generated by the counter electromotive force generated in the primary winding (2a) of the transformer (2). Current flows through the snubber circuit (8) and the surge is absorbed. The auxiliary switching element (51) is in the off state except immediately after the main switching element (3) is turned off.
No current flows in the snubber circuit (8). Therefore, the voltage applied to the snubber circuit (8) is only the voltage generated in the primary winding (2a) of the transformer (2) when the main switching element (3) is off. Further, when the main switching element (3) is on, the auxiliary switching element (51) is in the off state, so that energy is not released from the snubber circuit (8) to the main switching element (3). Therefore, it is possible to reduce the power loss in the snubber circuit (8) and reduce the voltage applied to the snubber circuit (8) to suppress the power loss when the main switching element (3) is on.

In one embodiment of the present invention, a snubber circuit
(8) has a capacitor (9) and a resistor (10) connected in series. Since the voltage applied to the capacitor (9) is only the voltage and surge voltage generated in the primary winding (2a) of the transformer (2) when the main switching element (3) is off, the capacitor (9)
May be a small one having a small capacitance. Therefore, the resistor (10) may also be a small one having a small capacitance, which is advantageous in that the snubber circuit (8) can be downsized.

Further, in one embodiment of the present invention, the back electromotive force detection for detecting the back electromotive force generated in the primary winding (2a) of the transformer (2) immediately after the main switching element (3) is turned off. means
(52) and a drive means (55) for applying an ON signal to the control terminal of the auxiliary switching element (51) when the back electromotive force detection means (52) detects the back electromotive force. As a result, the auxiliary switching element (51) can be turned on and current can be passed through the snubber circuit (8) only immediately after the main switching element (3) is turned off. Besides, there is an advantage that the surge generated in the primary winding (2a) of the transformer (2) can be surely absorbed. In practice, the resistance (5
3) and capacitor (54) and main switching element
The counter electromotive force detection means (52) is configured by a differentiation circuit that generates a differential voltage of the reverse polarity voltage (V _FB ) generated in the primary winding (2a) of the transformer (2) when (3) is off, The driving means (55) is composed of an operational amplifier that generates an output signal that turns on the auxiliary switching element (51) when a differential voltage is output from the differentiating circuit.

Further, according to the embodiment of the present invention, since the ON period extending means (56) for extending the output period of the ON signal output from the driving means (55) is provided, the transformer (2) is configured to have the following structure. There is an advantage that the snubber circuit (8) can surely absorb the surge even if the decay time of the surge generated in the secondary winding (2a) is relatively long.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION An embodiment of a switching power supply device according to the present invention will be described below with reference to FIGS. However, in these drawings, the substantially same portions as those in FIGS. 3 and 4 are denoted by the same reference numerals, and the description thereof will be omitted. As shown in FIG. 1, the switching power supply device according to the present embodiment is a snubber MO as an auxiliary switching element that is turned on only immediately after the MOS-FET (3) is turned off.
The S-FET (51) is connected in series with the snubber circuit (8) of the conventional switching power supply device shown in FIG. 3, and the MOS-FET (3) is connected.
The differential circuit (52) as the counter electromotive force detection means for detecting the counter electromotive force generated in the primary winding (2a) of the transformer (2) immediately after the switch is turned off, and the differentiating circuit (52) An operational amplifier (55) as a driving means for generating a high voltage (H) level output signal when detected, and an ON period extension for extending the high voltage (H) level period of the output signal of the operational amplifier (55) The ON period extension circuit (56) as means is added to the conventional switching power supply device shown in FIG. Differentiator (52)
Is configured by connecting a resistor (53) and a capacitor (54) connected in series with the primary winding (2a) of the transformer (2) in parallel,
When the MOS-FET (3) is off, the primary winding (2
The differential voltage V _A of the reverse polarity flyback voltage V _FB generated in a) is output from the connection point A of the resistor (53) and the capacitor (54). The capacitance value of the capacitor (54) of the differentiating circuit (52) is 1/1 of the capacitance value of the capacitor (9) of the snubber circuit (8).
A value of about 0 times is selected, and a resistance value of the resistor (53) of the differentiating circuit (52) is selected to be smaller than 1/10 times the resistance value of the resistor (10) of the snubber circuit (8). In the operational amplifier (55), the inverting input terminal (-) is connected to the connection point A of the resistor (53) and the capacitor (54) forming the differentiating circuit (52), and the non-inverting input terminal (+) is the differentiating circuit ( 52) resistor (53) and transformer (2) primary winding (2a)
Is connected to the connection point B with the differential voltage V _A of the differential circuit (52)
Generates a high voltage (H) level output signal when the voltage exceeds the voltage level at the connection point B. The ON period extension circuit (56)
Output terminal of operational amplifier (55) and snubber MOS-FET (5
The resistor (57) connected between the gate terminal of 1) and the resistor (5
7) and a diode (58) connected in parallel with the snubber MO
It is composed of a capacitor (59) connected between the gate terminal and the source terminal of the S-FET (51). Other configurations are substantially the same as those of the conventional switching power supply device shown in FIG.

In the structure shown in FIG. 1, shown in FIG.
Like time t₁From the control circuit (7) to the MOS-FET (3)
Control pulse signal V of high voltage (H) level at the gate terminal_G1
Is given and the MOS-FET (3) is turned on, the
Current source (1) to primary winding (2a) of transformer (2) and MOS-
A current flows through the FET (3), and energy is transferred to the transformer (2).
Is accumulated. As a result, as shown in FIG.
Voltage V between the drain and source terminals of S-FET (3)_DSIs 0
It rapidly drops to [V] and drain current I _DIs Figure 2
It rises linearly as shown in (C). At this time,
Of the rectifying and smoothing circuit (6) from the secondary winding (2b) of the switch (2)
The reverse voltage is applied to the current diode (4)
As shown in FIG. 2 (D), the rectifier diode
The current I is in (4)_D1Flow from the transformer (2) to the load (11)
Energy transfer is not performed. Also, at this time
Since the MOS-FET (51) for the bus is off,
Voltage V of capacitor (9) of circuit (8)_C1Is shown in Fig. 2 (F)
Sea urchin time t₁When the previous MOS-FET (3) is off
Flyback voltage V generated in the primary winding (2a) of the switch (2)_FB
Is maintained at a voltage substantially equal to.

Next, time t₂Control circuit (7) to MOS
-Control pulse signal V applied to the gate terminal of FET (3)
_G1Is low from high voltage (H) level as shown in FIG.
The voltage (L) level is reached and the MOS-FET (3) is in the ON state.
When it is turned off from the MOS-F, as shown in FIG.
Voltage V between the drain and source terminals of ET (3)_DSIs rapidly
Drain current I_DAs shown in Fig. 2 (C)
Is almost zero. This allows the secondary winding of the transformer (2)
Forward direction from (2a) to rectifying diode (4) of rectifying and smoothing circuit (6)
Voltage is applied and it becomes conductive, so transformer (2)
Energy stored in the rectifying and smoothing circuit from the secondary winding (2b)
It is supplied to the load (11) via (6) and the transformer (2) is reset.
To be Therefore, as shown in FIG.
The current I flows through the rectifier diode (4) of the path (6)._D1Flows, transformer
It gradually decreases with the release of stored energy in (2). This
In the case of, the back electromotive force generated in the primary winding (2a) of the transformer (2)
Flyback voltage V due to force_FBOccurs and
Vibration due to the stored energy of the leakage inductance of the space (2)
Wavy surge voltage V_SRAnd surge current is generated
Spare at the connection point A of the resistor (53) and the capacitor (54) of the path (52).
Iku-shaped differential voltage V _AOccurs. Differentiation of the differentiation circuit (52)
Voltage V_AAs a result, as shown in FIG. 2 (E), the operational amplifier (55)
Output signal of high voltage (H) level is generated from the
Capacitor (59) via diode (58) in extension circuit (56)
Is rapidly charged to a high voltage (H) level.

Thus, at time t₂For snubber MOS-
FET (51) changes from OFF state to ON state.
Voltage V of capacitor (59) of extension circuit (56)_G2Is shown in Fig. 2 (E)
As shown in, the resistance (57) gradually decreases, so
Time t₂After that, the voltage V of the capacitor (59)_G2Is for snubber
ON until it drops below the threshold level of OS-FET (51)
Hold the state. Therefore, as shown in FIG.
Voltage V of capacitor (9) of circuit (8)_C1Is the transformer (1)
Flyback voltage V generated in the secondary winding (2a)_FBVibration wavy
Surge voltage V_SRBecomes equal to the superimposed voltage. here
Then, the voltage V of the capacitor (9) of the snubber circuit (8)_C1Is time t
₂The flyback power of the primary winding (2a) of the transformer (2) has been used for a long time.
Pressure V_FBSince the voltage is kept almost equal to
Compared to the case of Fig. 3, the change of the voltage applied to the
The surge voltage becomes smaller than that shown in Fig. 4 (E).
V_SRThe peak value of is suppressed. Also, vibration wave surge
Voltage V_SRAnd surge current is snubber MOS-FET (51)
Consumed by the resistor (10) of the snubber circuit (8) during the on period of
Because of the attenuation, the voltage V of the capacitor (9) of the snubber circuit (8)
_C1Is the flyback voltage V of the primary winding (2a) of the transformer (2)
_FBConverge to. On the other hand, the drain-source of the MOS-FET (3)
Voltage between source terminals V_DSIs a DC power source as shown in Fig. 2 (B).
Source (1) power supply voltage V_INAnd the primary winding (2a) of the transformer (2)
Generated flyback voltage V_FBAnd surge voltage V_SRSuperposition of
Voltage is equal to the sum of the voltage and the reset voltage of the transformer (2).
Power supply voltage V of the DC power supply (1) near the end of the_INAnd tiger
Flyback voltage V of the primary winding (2a) of the sensor (2)_FBAddition of
It converges to the calculation voltage.

At time t _{3, when} the discharge of the stored energy of the transformer (2) is completed and the reset period of the transformer (2) ends, the rectifying diode of the rectifying / smoothing circuit (6) as shown in FIG. 2 (D). The current I _{D1 stops} flowing in (4), and the rectifier diode
(4) becomes non-conductive. At this time, snubber MOS-F
Since the ET (51) is in the off state, the capacitor (9) of the snubber circuit (8) is not discharged, and the voltage V _C1 of the capacitor (9) of the snubber circuit (8) as shown in FIG. Holds the level before time t ₃ . At the same time, MOS-FET
The voltage V _DS between the drain and source terminals of (3) gradually converges to the power supply voltage V _IN of the DC power supply (1) while undergoing damping oscillation as shown in FIG. 2 (B). Then, at time t _{4, the} control circuit (7)
2A, the control pulse signal V _{G1 applied} to the gate terminal of the MOS-FET (3) changes from a low voltage (L) level to a high voltage (H) level as shown in FIG. When is turned from the off state to the on state, current flows from the DC power supply (1) through the primary winding (2a) of the transformer (2) and the MOS-FET (3), and energy is accumulated in the transformer (2). It As a result, as shown in FIG. 2B, the voltage V _DS between the drain and source terminals of the MOS-FET (3) rapidly drops to 0 [V], and the drain current _ID becomes as shown in FIG. 2C. It rises linearly as shown. In addition, snubber MOS-FET (51)
2 is in the off state, the voltage V _C1 of the capacitor (9) of the snubber circuit (8) holds the level before time t ₄ as shown in FIG. 2 (F).

In this embodiment, when the snubber MOS-FET (51) is turned on immediately after the MOS-FET (3) is turned off, the reverse winding generated in the primary winding (2a) of the transformer (2). The flyback voltage V _FB is generated by the electromotive force, and the stored energy of the leakage inductance of the transformer (2) generates an oscillating wave-shaped surge voltage V _SR and a surge current, which causes a current to flow through the snubber circuit (8), resulting in a surge voltage V _SR and surge current are absorbed. Since the snubber MOS-FET (51) is in the off state except immediately after the MOS-FET (3) is turned off, no current flows in the snubber circuit (8). Therefore, the voltage applied to the snubber circuit (8) is only the flyback voltage V _FB generated in the primary winding (2a) of the transformer (2) when the MOS-FET (3) is off. Further, when the MOS-FET (3) is on, the snubber MOS-FET (51) is in the off state, so that energy is not released from the snubber circuit (8) to the MOS-FET (3). Therefore, the snubber circuit (8) resistance (1
Since the power loss in 0) is only during the on period of the snubber MOS-FET (51) immediately after the MOS-FET (3) is turned off, the power loss in the snubber circuit (8) can be reduced. Also,
Since the voltage applied to the snubber circuit (8) is only the flyback voltage V _FB generated in the primary winding (2a) of the transformer (2) when the MOS-FET (3) is off, the snubber circuit (8) The voltage applied to can be reduced. When the MOS-FET (3) is on, the snubber MOS-FET (51) is off and energy is not released from the snubber circuit (8) to the MOS-FET (3). It is possible to suppress the power loss when (3) is turned on. Further, since the voltage applied to the snubber circuit (8) is low, the capacitor (9) that constitutes the snubber circuit (8) may be a small one with a small electrostatic capacitance,
Since the resistor (10) can also be a small one with a small capacity,
There is an advantage that the snubber circuit (8) can be downsized. Also, MO
Immediately after the S-FET (3) turns off, the primary winding of the transformer (2)
The counter electromotive force generated in (2a) is detected by the differentiating circuit (52), and when the differentiating circuit (52) detects the counter electromotive force, the operational amplifier (55)
From the ON period extension circuit (56) to the snubber MOS-F
Since the ON signal V _G2 is applied to the gate terminal of ET (51),
The snubber circuit turns on the snubber MOS-FET (51) only for a short period immediately after the MOS-FET (3) is turned off.
A current can be applied to (8). Therefore, MOS-FE
There is an advantage that the surge voltage V _SR and the surge current generated in the primary winding (2a) of the transformer (2) immediately after T (3) is turned off can be surely absorbed. Further, since the on period of the snubber MOS-FET (51) is extended by the on period extension circuit (56), the surge voltage V _SR and the surge current generated in the primary winding (2a) of the transformer (2) are attenuated. The snubber circuit (8) has an advantage that the surge voltage V _SR and the surge current can be reliably absorbed even when the time is relatively long.

The embodiment of the present invention is not limited to the above embodiment, and various modifications can be made. For example, in the above embodiment, the present invention is applied to the snubber circuit (8) in which the capacitor (9) and the resistor (10) are connected in series.
Even when the present invention is applied to a snubber circuit including a capacitor and a resistor and having another configuration or a snubber circuit configured by an element other than the capacitor and the resistor (such as a choke coil), substantially the same effect as that of the above embodiment is obtained. can get. In addition, the capacitor (9) and the resistor (1
The connection order of 0) and the snubber MOS-FET (51) may be exchanged. Further, in the above embodiment, the mode in which the MOS-FET is used as the switching element is shown, but a bipolar transistor, an IGBT (insulated gate type bipolar transistor), a J-FET (junction type field effect transistor), a thyristor or the like is also a switching element. Can be used as.

[0020]

According to the present invention, the power loss in the snubber circuit can be reduced, and the voltage applied to the snubber circuit can be reduced to suppress the power loss when the main switching element is on, so that the power consumption is small. It is possible to use a small snubber circuit and an inexpensive main switching element having a low breakdown voltage. Therefore, it is possible to greatly contribute to downsizing, loss reduction, and manufacturing cost reduction of the switching power supply device.

[Brief description of drawings]

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

FIG. 2 is a waveform diagram showing the voltage and current of each part of FIG.

FIG. 3 is an electric circuit diagram showing a conventional switching power supply device.

FIG. 4 is a waveform diagram showing the voltage and current of each part of FIG.

[Explanation of symbols]

(1) ・ ・ DC power supply, (2) ・ ・ Transformer, (2a) ・ ・ Primary winding, (2b) ・ ・ Secondary winding, (3) ・ ・ MOS-FET
(Main switching element), (4)
(5) ・ ・ Smoothing capacitor, (6) ・ ・ Rectifying and smoothing circuit,
(7) ・ ・ Control circuit, (8) ・ ・ Snubber circuit, (9) ・ ・ Capacitor, (10) ・ ・ Resistance, (11) ・ ・ Load, (51) ・
・ Snubber MOS-FET (auxiliary switching element),
(52) ・ ・ Differentiation circuit (back electromotive force detection means), (53) ・ ・
Resistance, (54) ・ ・ Capacitor, (55) ・ ・ Operational amplifier (driving means), (56) ・ On period extension circuit (on period extension means), (57) ・ ・ Resistance, (58) ・ ・ Diode ,
(59)

Claims (5)

[Claims] 1. A primary winding and a main switching element of a transformer connected in series to a DC power source, and a rectifying / smoothing circuit connected to a secondary winding of the transformer and supplying a DC output to a load. A control circuit for controlling ON / OFF of the main switching element, and a snubber circuit connected in parallel with the primary winding and absorbing a surge generated when the transformer is reset are provided. A switching power supply device that turns on the main switching element after a reset period ends and turns off the main switching element when the voltage level of the load exceeds a reference voltage level, wherein the main switching element is off. A switch characterized in that an auxiliary switching element which is turned on only immediately after being connected is connected in series with the snubber circuit. Grayed power supply. 2. The switching power supply device according to claim 1, wherein the snubber circuit has a capacitor and a resistor connected in series. 3. A counter electromotive force detecting means for detecting a counter electromotive force generated in the primary winding of the transformer immediately after the main switching element is turned off, and the counter electromotive force detecting means detects the counter electromotive force. 2. A drive means for applying an ON signal to the control terminal of the auxiliary switching element when the operation is performed.
Alternatively, the switching power supply device according to item 2. 4. The counter electromotive force detection means is composed of a differential circuit having a resistor and a capacitor and generating a differential voltage of a reverse polarity voltage generated in the primary winding of the transformer when the main switching element is off. 4. The switching power supply device according to claim 3, wherein the driving means is constituted by an operational amplifier that generates an output signal that turns on the auxiliary switching element when the differential voltage is output from the differentiating circuit. 5. The switching power supply device according to claim 3, further comprising an ON period extension unit that extends an output period of the ON signal output from the drive unit.