JP2002320385A - Switching converter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To effectively prevent a surge generated in the secondary side, of a switching converter providing a synchronous rectifying circuit and an active clamp circuit, when the power supply is turned OFF. SOLUTION: This switching converter comprises the active clamp circuit which receives a DC input voltage with a primary coil of a transformer means via a main switch element, converts an AC voltage generated in a secondary coil of the transformer means to a DC output voltage by smoothing this AC voltage with a synchronous rectifying circuit, and is configured with a series circuit of a reset switch element and a clamping capacitor connected in parallel with the primary coil. In this switching converter, a diode is connected in parallel to the capacitor.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to improvement of characteristics when a power supply is turned off in a switching converter having a synchronous rectifier circuit and an active clamp circuit.

[0002]

2. Description of the Related Art FIG. 4 shows an example of a forward-type switching converter having a synchronous rectifier circuit and an active clamp circuit disclosed in Japanese Patent Application Laid-Open No. 8-331842.

In FIG. 4, a DC input voltage Vi derived from a DC power supply 3 connected between input terminals 1 and 2 is connected to a primary winding N1 of a transformer means 4 and a MOS type FE indicated by a chain line.
This is applied to a series circuit of a main switch element Q1 made of T.

The secondary winding N2 of the transformer means 4 has an MO
Synchronous rectification switch elements Q2, Q3 composed of S-type FETs
The rectified output supplies a DC output voltage V0 to a load 9 connected to output terminals 7 and 8 via a low-pass filter including a choke coil 5 and a capacitor 6.

[0005] N3 and N4 are auxiliary windings formed in the transformer means 4. The AC voltage generated in these auxiliary windings is supplied to the synchronous rectification switch elements Q2, Q2 through appropriate impedance means (resistances and the like) Z1 and Z2. Guided to the gate of Q3, these switch elements are reversibly on / off controlled.

Further, for resetting the transformer means 4, a reset switch element Q4 and a clamp capacitor 5 are provided.
Is connected in parallel with the primary winding N1.

Reference numeral 11 denotes a switching control circuit. When this switching converter is configured as a constant voltage switching power supply, a DC output voltage V0 is derived, a reference voltage proportional to the DC output voltage V0 is compared with its own reference voltage, and an error is generated. A signal S1 for controlling an on / off ratio (duty) of the main switch element Q1 based on the amplified signal is supplied to the gate of Q1.

[0008] The signal S2 is supplied to the gate of the reset switch element Q4 to control the on / off of Q4. FIG.
(A) and (B) show the phase relationship between the signal S1 and the signal S2, which are in opposite phases to each other and have a common off-state period t1 to t2 and t3 to t so that there is no ON period at the same time.
Four.

FIG. 5C is a waveform diagram of the voltage Vb of the primary winding N1, and FIG. 5D is a waveform diagram of the exciting current Ib to the primary winding N1. The reset switch element Q4 is set to t by the signal S2.
During the period in which the on-state voltage is controlled from 2 to t3, the voltage Vc of the clamp capacitor 5 is applied to the primary winding N1 with the opposite polarity,
The exciting current flows in the reverse direction, and the transformer means 4 is reset.

[0010]

The operation when the power supply is stopped in such a configuration will be described with reference to FIG. (A) is a waveform diagram of the current Ir flowing through the secondary winding N2, (B) is a waveform diagram of the voltage Vr generated in the secondary winding N2, and (C) is a drain-source voltage Vds of the main switch element Q1. FIG.

When the power supply stops, the main switch element Q
1 maintains the off state, and the reset switch element Q4 is turned on. Therefore, the charge of the clamp capacitor 10 is discharged via the reset switch element Q4.

This discharge causes the clamp capacitor 10
Of the primary winding N1 and the inductor of the primary winding N1, the clamp capacitor 10 is charged to the polarity opposite to the illustrated polarity, and the main switch element Q
The drain-source voltage Vds of 1 decreases to near 0V.

This change drives the synchronous rectification switch element Q2 ON through the auxiliary winding N3. At this time, the current Ir flowing through the switch element Q2 is in a direction opposite to the steady current direction. This reverse current is due to a resonance phenomenon generated between the capacitor of the clamp capacitor 10 and the inductor of the primary winding N1.

Thereafter, the main switch element Q
When the drain-source voltage Vds of No. 1 is increased again, the switching element Q2 is turned off.
2, a high surge voltage is generated, which may destroy the switching element Q2.

An object of the present invention is to realize a switching converter which has a simple circuit configuration and prevents high stress generated in a synchronous rectification switching element when power is turned off.

In order to achieve the above object, a feature of the present invention is that a DC input voltage is supplied to a primary winding of a transformer means via a main switch element. The AC voltage generated in the secondary winding of the transformer means is converted to a DC output voltage by smoothing through a synchronous rectifier circuit, and a reset switch element connected in parallel to the primary winding is provided. In a switching converter including an active clamp circuit formed of a series circuit with a clamp capacitor, a diode is connected in parallel to the capacitor.

A second aspect of the present invention is characterized in that a DC input voltage is received by a primary winding of a transformer means via a main switch element, and an AC voltage generated in a secondary winding of the transformer means is synchronously rectified. A switching converter that includes an active clamp circuit configured by a series circuit of a reset switch element and a clamp capacitor connected in parallel with the primary winding while smoothing and converting to a DC output voltage through a circuit; The point is that a Zener diode is connected in parallel to the capacitor.

A feature of the third aspect of the present invention is that the synchronous rectifier circuit includes a pair of switching elements.

According to a fourth aspect of the present invention, a switching control circuit is provided.
The reset switch element is provided with a simultaneous OFF period so as not to have a simultaneous ON period with the main switch element, and is controlled in an opposite phase.

A fifth aspect of the present invention is characterized in that a pair of switching elements forming the synchronous rectifier circuit are turned on and off.
It is controlled by an AC voltage induced in a pair of auxiliary windings formed in the transformer means.

A sixth feature of the present invention is that a pair of switching elements forming the synchronous rectifier circuit are turned on and off.
It is controlled by a signal from the switching control circuit.

According to a seventh aspect of the present invention, the on / off of a pair of switching elements forming the synchronous rectifier circuit is as follows.
It is controlled by an AC voltage induced in the auxiliary winding of the choke coil means forming the smoothing circuit.

A feature of the present invention is that the main switch element, the reset switch element, and the switching element of the synchronous rectifier circuit are realized by MOS FETs.

According to a ninth aspect of the present invention, the transformer means and the choke coil means are integrated.

[0024]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows a specific embodiment in which the present invention is applied to a forward-type switching converter. The same elements as those in the conventional configuration described with reference to FIG. (0000) Dotted block 1
Reference numeral 2 denotes a surge prevention circuit which is a feature of the present invention. The configuration of this circuit is extremely simple, and is realized by a diode 13 connected in parallel to the clamp capacitor 10.

The operation at the time of stopping the power supply in such a configuration will be described with reference to FIG. 2 corresponding to FIG. 6, (A) is a waveform diagram of the current Ir flowing through the secondary winding N2,
(B) is a waveform diagram of the voltage Vr generated in the secondary winding N2,
(C) is a waveform diagram of a drain-source voltage Vds of the main switch element Q1.

When the power supply stops, the main switch element Q
1 maintains the off state, and the reset switch element Q4 is turned on. Therefore, the charge of the clamp capacitor 10 is discharged via the reset switch element Q4.

This discharge causes the clamp capacitor 10
A resonance phenomenon occurs between the first capacitor N1 and the inductor of the primary winding N1, and the drain-source voltage Vds of the main switch element Q1 gradually decreases.

At this time, since the clamp capacitor 10 is clamped by the forward voltage of the diode 13, it is not charged with the polarity opposite to the polarity shown in the figure, the resonance phenomenon does not occur repeatedly, and only one fluctuation occurs. Ends with

Since the drain-source voltage Vds of the main switch element Q1 does not fluctuate, the synchronous rectification switch element Q2 is kept off via the auxiliary winding N3, and the current Ir flowing through the switch element Q2 is zero.

Therefore, a high surge voltage to the switching element Q2 due to the continuation of the resonance, which is generated in the conventional circuit, does not occur essentially, and the stress that destroys the switching element Q2 is effectively prevented.

FIG. 3 shows another embodiment of the surge prevention circuit 12 which is a feature of the present invention. The difference from FIG. 1 lies in the configuration realized by a Zener diode 14 instead of the diode 13. With respect to surge prevention, the role as a diode is the same as in FIG.

When the switching converter has a light load, a burst operation of temporarily turning off the power may be performed to reduce converter loss. At this time, if the reset switch element Q4 located on the upstream side is formed of a bootstrap circuit, it becomes difficult to obtain the driving power, and the reset switch element Q4 may be turned off.

At this time, since the charge of the clamp capacitor 10 rises without being discharged, it may exceed the withstand voltage of the drain-source voltage Vds of the main switch element Q1 and may be destroyed. Incidentally, even when the clamp capacitor 10 is charged to an excessive voltage due to dynamic fluctuation of an external environment such as a commercial power supply or a surge, the voltage may exceed the withstand voltage of the drain-source voltage Vds of the main switch element Q1 and may be destroyed. There is.

Due to the connection of the Zener diode 14, the voltage of the clamp capacitor 10 does not rise above the Zener voltage Vz. Therefore, the drain-source voltage Vds of the main switch element Q1 does not exceed Vi + Vz and is protected from destruction.

In the embodiment described above, the on / off of the pair of switching elements Q2 and Q3 forming the synchronous rectification circuit is controlled by the AC voltage induced by the pair of auxiliary windings formed in the transformer means. The on / off of these may be controlled by a signal from the switching control circuit 11, or may be controlled by an AC voltage induced in an auxiliary winding formed in the choke coil means 5 forming a smoothing circuit. good.

Further, the present invention can be effectively applied to a switching converter having a configuration in which the transformer means 4 and the choke coil means 5 are integrated. Further, the topology of the switching converter is illustrated as a forward type, but the present invention can be effectively applied to a switching converter having another topology such as a flyback type.

[0037]

As described above, according to the present invention,
In a switching converter having a synchronous rectifier circuit and an active clamp circuit, a switching converter that effectively prevents a surge generated on the secondary side when power is turned off can be realized with a simple circuit configuration.

Further, according to the present invention, when a burst operation for temporarily turning off the power supply is performed to reduce converter loss, or when a dynamic fluctuation or surge of an external environment such as a commercial power supply causes an influence of a surge, the clamp capacitor 10 may be used. The main switching element Q
The drain-source voltage Vds of No. 1 can be maintained at Vi + Vz or lower, and is protected from destruction.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing an embodiment in which the present invention is applied to a forward-type switching converter having a synchronous rectifier circuit and an active clamp circuit.

FIG. 2 is a signal waveform diagram of each unit when the power is turned off in FIG.

FIG. 3 is a circuit configuration diagram of a main part showing another embodiment of the present invention.

FIG. 4 is a configuration diagram of a conventional forward-type switching converter including a synchronous rectifier circuit and an active clamp circuit.

FIG. 5 is a signal waveform diagram of each section for explaining the operation of FIG. 5;

6 is a signal waveform diagram of each unit when the power is turned off in FIG.

[Explanation of symbols]

 1, 2 input terminal 3 DC power supply 4 transformer means 5 choke coil 6 capacitor 7, 8 output terminal 9 load 10 clamp capacitor 11 switching control circuit 12 surge prevention circuit 13 diode Q1 main switch element Q2, Q3 synchronous rectification switch element Q4 reset Switch element N1 Primary winding N2 Secondary winding N3 Auxiliary winding N4 Auxiliary winding

Claims (9)

[Claims] 1. A DC input voltage is received by a primary winding of a transformer means via a main switch element, and an AC voltage generated on a secondary winding of the transformer means is smoothed via a synchronous rectifier circuit. A switching converter for converting to a DC output voltage and having an active clamp circuit configured by a series circuit of a reset switch element and a clamp capacitor connected in parallel to the primary winding, wherein a diode is connected in parallel to the capacitor. A switching converter characterized by the following. 2. A DC input voltage is received by a primary winding of a transformer means via a main switch element, and an AC voltage generated on a secondary winding of the transformer means is smoothed via a synchronous rectifier circuit. A switching converter that converts an output voltage into a DC output voltage and includes an active clamp circuit including a series circuit of a reset switch element and a clamp capacitor connected in parallel to the primary winding, wherein a zener diode is connected in parallel to the capacitor. A switching converter characterized by being connected. 3. The switching converter according to claim 1, wherein said synchronous rectifier circuit comprises a pair of switching elements. 4. A switching control circuit, wherein the switching control circuit controls the reset switch element to have a simultaneous OFF period so as not to have a simultaneous ON period with the main switch element, and to be controlled to an opposite phase. The switching converter according to any one of claims 1 to 3, wherein 5. An on / off control of a pair of switching elements forming said synchronous rectifier circuit is controlled by an AC voltage induced in a pair of auxiliary windings formed in said transformer means. The switching converter as described. 6. The switching converter according to claim 3, wherein on / off of a pair of switching elements forming the synchronous rectification circuit is controlled by a signal of the switching control circuit. 7. An on / off control of a pair of switching elements forming the synchronous rectifier circuit is controlled by an AC voltage induced in an auxiliary winding of a choke coil means forming the smoothing circuit. 3. The switching converter according to 3. 8. The switching converter according to claim 1, wherein the main switch element, the reset switch element, and the switching element of the synchronous rectifier circuit are realized by MOS-type FETs. 9. The system according to claim 1, wherein said transformer means and said choke coil means are integrated.
9. The switching converter according to any one of claims 1 to 8.