JPH11235035A - Forward converter with auxiliary power supply circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To realize a highly efficient forward converter. SOLUTION: In a secondary output circuit constituting a forward converter, connecting in parallel with both ends of a rectifier diode 2 a bypass circuit comprising a switching element 4 and a capacitor 5, a controlling voltage of the secondary output circuit is fed to a control circuit 9 via the capacitor 5 to subject alternately a switching element 1 provided in the primary side of the forward converter and the switching element 4 of the bypass circuit to ON-OFF controls. As a result, not only the breakdown voltage of the primary- side switching element 1 can be made allowably low, but also the utilization factor of the transformer of the forward converter can be improved. Also, in the forward converter of a synchronous rectification mode, since the driving waveforms of its switching elements are made rectangular and improved, its high efficiency can be realized.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a switching power supply,
The present invention relates to a forward converter including an auxiliary power supply circuit for increasing the efficiency of a converter.

[0002]

2. Description of the Related Art FIG. 7 shows an equivalent circuit diagram of a forward converter having an auxiliary power supply circuit according to the prior art. The input DC power supply 110 is connected to the primary coil of the transformer 106 via the switch element 101. A rectifier circuit including two diodes 102 and 103 and a smoothing circuit including a parallel capacitor 108 and a reactor 107 are connected to the secondary output circuit, and supply a smoothed DC power to a load 111. A bypass circuit including a diode 104 and a capacitor 105 is connected in parallel to both ends of the rectifying diode 102 to form an auxiliary power supply circuit. A secondary-side control voltage V _CC1 to the control circuit 109 is It is supplied from the capacitor 105.

[0003] Two diodes constituting the rectifier circuit in FIG. 7 are replaced with switch elements (for example, FETs).
FIG. 8 shows an equivalent circuit diagram of a forward converter serving as a synchronous rectifier. Rectifying switch element 102 in FIG.
Is connected in parallel to both ends of the control circuit 10 by a diode 104 and a capacitor 105.
9, the secondary side control voltage V _CC1 is supplied from the capacitor 105.

[0004]

In FIG. 7, when on / off control of the primary side switch element 101 is performed by a drive signal from the control circuit 109, the switch element 101
The voltage V _ds applied to both ends of is shown in FIG. That is, the voltage applied to the primary side of the transformer 106 is clamped by the bypass circuit connected in parallel to both ends of the rectifier diode 102, but _Vds becomes ( _Ei +
V _CC1 ). Therefore, a high breakdown voltage product is required as a switch element, and an expensive and large loss element must be used. Also, since the current i _L flowing through the primary coil of the transformer becomes as shown in FIG. 3B, the BH characteristic curve of the transformer is easily saturated as shown in FIG. Usage efficiency is not good.

In FIG. 8, a primary switch element 101 of a transformer and a rectifying switch element 102 on a secondary side are synchronously turned on and off, and a return switch element 103 is provided.
Is turned on / off in the opposite phase to the two switch elements described above, so that the voltage V
_ds and the voltage V _gs applied to the secondary-side switch elements 102 and 103 are as shown in FIGS. 5A and 5B. As is clear from FIGS. 5A and 5B, V _ds is higher than the input power supply voltage E _i by V _CC1 , and both switch elements 102 and 103 constituting the secondary-side synchronous rectifier are off. Thus, a period occurs in which the internal diode attached to the switch element is turned on, so that the drive waveform by the switch element does not have a rectangular waveform.

[0006]

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned drawbacks of the prior art, and replaces a diode constituting an auxiliary power supply circuit with a switch element and replaces the switch element with a primary element. On / off control is performed in the opposite phase to the switch element on the side and the rectifying switch element to reduce the _Vds of the primary side switch element and improve the transformer utilization efficiency, and configure a synchronous rectifier. Drive waveform of the switching element to be approximated to a rectangular wave.

[0007]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is an equivalent circuit diagram of a forward converter having an auxiliary power supply circuit according to the present invention.
In FIG. 1, a DC power supply 10 and a switch element (for example, FET) 1 are connected to a primary side of a transformer 6. When the switch element 1 is turned on, a current i _L flows through a reactance L equivalent to a primary coil. Flows. A rectifier circuit including diodes 2 and 3 and a smoothing circuit including a reactor 7 and a parallel capacitor 8 are connected to the secondary side of the transformer 6, and supply a smoothed DC power to a load 11. A bypass circuit comprising a switch element 4 and a capacitor 5 is connected in parallel to both ends of the rectifying diode 2 to constitute an auxiliary power supply circuit. The primary side switch element 1 and the switch element of the bypass circuit are provided. A control circuit 9 for alternately turning on / off the control voltage V _CC2 via a capacitor 5 has a control voltage V _CC2.
Is supplied.

By turning on the switch element 4 of the bypass circuit after the switch element 1 on the primary side is turned off, the voltage V _ds applied between both terminals of the switch element 1 can be clamped. ) Voltage waveform (E _i
+ V _CC1 ) has a voltage waveform (E _i +
V _CC2 ), and V _ds decreases. That is, since the shaded portion of the voltage waveform in FIG. 3A and the shaded portion shown in FIG. 3C have the same area, V _CC1 > V _CC2 . Also, the current i _L flowing through the primary side coil is changed from that of FIG. 3B to that of FIG. 3A. Since _it1 = _it2 in principle, as shown in FIG. 4B, the area of the hatched portion is equal to the hatched portion of FIG.
By using both the positive and negative characteristic curves, the efficiency of use of the transformer is improved, and the transformer is less likely to be magnetically saturated, so that the shape and size of the transformer can be reduced.

FIG. 2 shows another embodiment of a synchronous rectifier circuit in which two diodes constituting the rectifier circuit in FIG. 1 are replaced by switch elements. A bypass circuit composed of the switch element 4 and the capacitor 5 is connected in parallel to both ends of the rectifying switch element 2, and a control voltage V _CC2 is supplied to the control circuit 9 via the capacitor 5. The primary-side switch element 1 and the secondary-side rectifying switch element 2 are simultaneously turned on and off, and the return-side switch element 3
And the switching element 4 of the bypass circuit are controlled at the same time in the opposite phase to the switching elements 1 and 2. The details of the signal V _gs between the gate and the source for driving the above-mentioned four switch elements are as shown in FIG.

When on / off control of the switch elements 1 to 4 is performed based on the signal waveform shown in FIG. 6, a voltage waveform V _ds applied to both ends of the primary side switch element 1 becomes as shown in FIG. (E _i + V _CC2 ). The voltage waveform when a diode is connected to the bypass circuit is (E _i + V _CC1 ) as shown in FIG. 5A, so that the withstand voltage of the primary-side switch element is (V _CC1 −V _CC2). ) Only lower.

The gate-source voltages V _gs1 and V _gs2 of the switch elements 2 and 3 constituting the synchronous rectifier when a diode is connected to the bypass circuit are as shown in FIG. A period occurs in which both elements are turned off and the internal diode is turned on, and the driving waveform of the switching element is not a rectangular wave, so that the efficiency is low. Switch element 2 when the diode is replaced with a switch element
As shown in FIG. 5 (d), the gate-source voltages of (1) and (3) eliminate the time during which the internal diode is turned on, increase the time during which the synchronous rectification switch element can be used, and achieve high efficiency.

[0012]

As described above, in the forward converter having the auxiliary power supply circuit according to the present invention,
A rectifier diode or a rectifier switch element provided in the secondary output circuit is provided with a bypass circuit composed of a switch element and a capacitor at both ends of the rectifier diode or the rectifier switch element. By doing so, the voltage applied to both ends of the transformer is clamped. As a result, not only can the switching element on the primary side be reduced in withstand voltage, but also the utilization efficiency of the transformer can be increased. Further, in the synchronous rectification type forward converter, the driving waveform of the switch element has a rectangular shape and is improved, so that high efficiency can be realized.

[Brief description of the drawings]

FIG. 1 is an equivalent circuit diagram showing an embodiment of a forward converter including an auxiliary power supply circuit according to the present invention.

FIG. 2 is an equivalent circuit diagram showing another embodiment of the forward converter including the auxiliary power supply circuit according to the present invention.

FIG. 3 is a waveform diagram.

FIG. 4 is a BH characteristic curve of a transformer.

FIG. 5 is a voltage waveform diagram.

FIG. 6 is a signal waveform diagram of a switch element.

FIG. 7 is an equivalent circuit diagram of a forward converter having an auxiliary power supply circuit according to the related art.

FIG. 8 is another equivalent circuit diagram of a forward converter having an auxiliary power supply circuit according to the prior art.

[Procedure amendment]

[Submission date] March 20, 1998

[Procedure amendment 1]

[Document name to be amended] Statement

[Correction target item name] 0004

[Correction method] Change

[Correction contents]

[0004]

In FIG. 7, when on / off control of the primary side switch element 101 is performed by a drive signal from the control circuit 109, the switch element 101
The voltage V _ds applied to both ends of is shown in FIG. That is, the voltage applied to the primary side of the transformer 106 is clamped by the bypass circuit connected in parallel to both ends of the rectifier diode 102, but the number of turns of the transformer 106
Assuming that the ratio is n ₁ : n ₂ , V _ds is (E _i + n ₁ / n _2.
V _CC1 ). Therefore, a high breakdown voltage product is required as a switch element, and an expensive and large loss element must be used. Also, since the current i _L flowing through the primary coil of the transformer becomes as shown in FIG. 3B, the BH characteristic curve of the transformer is easily saturated as shown in FIG. Usage efficiency is not good.

[Procedure amendment 2]

[Document name to be amended] Statement

[Correction target item name] 0005

[Correction method] Change

[Correction contents]

In FIG. 8, a primary switch element 101 of a transformer and a rectifying switch element 102 on a secondary side are synchronously turned on and off, and a return switch element 103 is provided.
Is turned on / off in the opposite phase to the two switch elements described above, so that the voltage V
_ds and the voltage V _gs applied to the secondary-side switch elements 102 and 103 are as shown in FIGS. 5A and 5B. FIGS. 5 (a) and as is clear from FIG. 5 (b), V _ds is n ₁ / n ₂ · V _CC1 only higher than the input supply voltage E _i, switching element constituting the secondary-side synchronous rectifier 102 And 103
Are both turned off, and a period occurs in which the internal diode attached to the switch element is turned on, so that the drive waveform by the switch element does not have a rectangular waveform.

[Procedure amendment 3]

[Document name to be amended] Statement

[Correction target item name] 0007

[Correction method] Change

[Correction contents]

[0007]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is an equivalent circuit diagram of a forward converter having an auxiliary power supply circuit according to the present invention.
In FIG. 1, a DC power supply 10 and a switch element (for example, FE) are provided on the primary side of a transformer 6 having a winding ratio of n ₁ : n _2.
T) 1 is connected, and when the switch element 1 is turned on, a current i _L flows through a reactance L equivalent to a primary coil. A rectifier circuit including diodes 2 and 3 and a smoothing circuit including a reactor 7 and a parallel capacitor 8 are connected to the secondary side of the transformer 6, and supply a smoothed DC power to a load 11. Further, a bypass circuit including a switch element 4 and a capacitor 5 is connected in parallel to both ends of the rectifying diode 2 to constitute an auxiliary power supply circuit.
A control voltage V _CC2 is supplied via a capacitor 5 to a control circuit 9 for alternately controlling the switching element 1 on the primary side and the switching element 4 of the bypass circuit to be turned on / off.

[Procedure amendment 4]

[Document name to be amended] Statement

[Correction target item name] 0008

[Correction method] Change

[Correction contents]

By turning on the switch element 4 of the bypass circuit after the switch element 1 on the primary side is turned off, the voltage V _ds applied between both terminals of the switch element 1 can be clamped. ) Voltage waveform (E <sub>i
+ N 1 / n 2 · V</sub> CC1) a voltage waveform _{(E i + n 1 / n} 2 · V CC2 as shown in FIG. 3 (c)) becomes, V _ds is reduced. That is, since the shaded portion of the voltage waveform in FIG. 3A and the shaded portion shown in FIG. 3C have the same area, n ₁ / n
₂ · V _CC1 > n ₁ / n ₂ · V _CC2 . Also, the current i _L flowing through the primary side coil is changed from that of FIG. 3B to that of FIG. 3A. Since _it1 = _it2 in principle, as shown in FIG. 4B, the area of the hatched portion is equal to the hatched portion of FIG. By using both, the efficiency of use of the transformer is improved, and the transformer is less likely to be magnetically saturated, so that the shape and size of the transformer can be reduced.

[Procedure amendment 5]

[Document name to be amended] Statement

[Correction target item name] 0010

[Correction method] Change

[Correction contents]

When on / off control of the switch elements 1 to 4 is performed based on the signal waveform shown in FIG. 6, a voltage waveform V _ds applied to both ends of the primary side switch element 1 becomes as shown in FIG. to become _{(E i + n 1 / n} 2 · V CC2). The voltage waveform when a diode is connected to the bypass circuit is
As shown in FIG. 5A, (E _i + n ₁ / n ₂ · V _CC1 )
Therefore, the withstand voltage of the primary-side switch element is n ₁ / n ₂
× (V _CC1 −V _CC2 ).

[Procedure amendment 6]

[Document name to be amended] Drawing

[Correction target item name] All figures

[Correction method] Change

[Correction contents]

FIG.

FIG. 2

FIG. 3

FIG. 4

FIG. 5

FIG. 6

FIG. 7

FIG. 8

Claims (2)

[Claims] 1. A rectifier diode (2) constituting a secondary output circuit, a bypass circuit comprising a capacitor and a diode is connected in parallel to both ends of a rectifier diode (2), and a control voltage for the secondary output circuit is supplied from the capacitor. In the forward converter having the auxiliary power circuit as described above, a diode constituting the bypass circuit is replaced with a switch element (4), and a switch element (1) provided on a primary side of the forward converter and the switch element (4) A forward converter comprising an auxiliary power supply circuit, wherein the on / off control is performed alternately. 2. A synchronous rectifier circuit comprising two switch elements (2) and (3), and a rectifier switch element (2) in the rectifier circuit is connected in parallel to both ends of a rectifier switch element (2) by a bypass circuit comprising a capacitor and a diode. A forward converter provided with an auxiliary power supply circuit for supplying a control voltage of a secondary output circuit from the capacitor, wherein a diode constituting the bypass circuit is replaced with a switch element (4) and provided on a primary side. The on / off control of the switch element (1) provided on the secondary side and the switch element (2) provided on the secondary side are synchronized and the switch element (3)
And (4) are controlled to be turned on and off in opposite phases.
converter.