JP4236459B2 - Rectifier circuit and switching power supply provided with the rectifier circuit - Google Patents

Description

【００１５】
なお、数１のＩ_Limitedは制限された電流値、Ｖ_F(LLD)はダイオード１３の電圧、Ｒonは整流スイッチ素子１１のオン抵抗値、Ｒchipは抵抗１２の抵抗値である。
【００１６】
また、整流スイッチ素子１１のボディダイオードに電流に流れないようにするためには、整流スイッチ素子１１のドロップ電圧Ｖ_F(BD)が以下のように設定する。
【００１７】
【数２】

【００１８】
以上のように、設定することにより、整流スイッチ素子１１のボディダイオードには電流は流れなくなる。
【００１９】
続いて、メインスイッチＱ１がターンオフすると、整流スイッチ素子１１がオフするとともに、転流スイッチＱ３がオンし、トランスＬｍの励磁電流が流れる。この際、整流回路１０にはメインスイッチＱ１がオンした場合とは逆方向に電流が流れるが、抵抗１２を設けたこと、並びに、前記した通りに整流スイッチ素子１１のボディダイオードに電流を流さないように設定したことにより、整流スイッチ素子１１のボディダイオードには電流がながれない。一方、ダイオード１３には電流が流れる。この場合、ダイオード１３の逆回復時間が問題になるが、ダイオード１３は逆回復時間が比較的早いもの、例えば、ショットキーバリアダイオードやローロスダイオードを用いることにより、ダイオード１３の逆回復時間の影響が少なくなり、整流回路が高耐圧であっても、トランスＬｍの共振リセットが働く。
【００２０】
以上よりメインスイッチＱ１がオン・オフを繰り返しても、整流スイッチ素子１１のボディダイオードがオンの状態になったり、トランスの共振リセットが行われないということを防ぐことができる。これは、高電圧同期静流回路においても同様の作用をし、整流回路が高耐圧であり、逆回復時間を抑制することができる。
【００２１】
【発明の効果】
本発明によれば、メインスイッチがオンしている場合には、整流回路に設けた整流用のダイオードを設けたことにより、こちらへ電流を流すことにより、整流スイッチ素子のボディダイオードへ流れる電流を抑制し、メインスイッチがターンオフした場合には、抵抗を設けたことにより、整流スイッチ素子のボディダイオードへ流れる電流を抑制することにより、一次−二次を絶縁するトランスの二次側の電圧が高い高電圧同期整流回路においても、整流回路が高耐圧であり、逆回復時間を抑制し、同期整流回路の双方向特性を維持させる効果がある。
【００２２】
また、整流用のダイオードに逆回復時間が比較的早いダイオード、例えば、ショットキーバリアダイオードやローロスダイオードを用いることにより、前記効果が顕著になる。
【図面の簡単な説明】
【図１】 本発明に係る一実施例を示す回路図である。
【図２】 図１図示実施例とは別の実施例を示す回路図である。
【図３】 従来例を示す回路図である。
【符号の説明】
Ｖin 主電源
Ｌｍ トランス
Ｑ１ メインスイッチ
Ｑ２ 整流スイッチ
Ｑ３ 転流スイッチ
Ｌｏ チョーク
Ｃｏ 平滑コンデンサ
１０ 整流回路
１１ 整流スイッチ素子
１２ 抵抗
１３ ダイオード[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a switching power supply used in an electronic device, and more particularly to a rectifier circuit having a rectifying switch element provided in the switching power supply.
[0002]
[Prior art]
FIG. 3 shows a conventional switching power supply. In this power source, the primary and secondary are insulated by a transformer Lm, and a main switch Q1 is provided on the primary side, and a rectifying switch Q2 and a commutation switch Q3 are provided on the secondary side. Further, this conventional example is a high voltage synchronous rectifier circuit. Since the high voltage synchronous rectifier circuit has a high secondary side voltage of the transformer Lm that insulates the primary and secondary, the rectifier switch element Q2 uses a high breakdown voltage FET. Was.
[0003]
There is also a switching power supply in which a diode is connected in parallel to the rectifying switch element and the commutation switch element formed of FETs, and the anode is directed to the source side of each switch element (Patent Literature). 1).
[0004]
[Patent Document 1]
JP-A-5-316725 (page 2-3, FIGS. 1 and 3)
[0005]
[Problems to be solved by the invention]
However, since the on-resistance of the rectifying switch element Q2 is higher than that of the low breakdown voltage FET, a choke current flows through the body diode of the rectifying switch element Q2, and even if the rectifying switch element Q2 is turned off for a while. During this period, the body diode turned on.
[0006]
In addition, the transformer excitation current continues to flow from the drain to the source even though the gate of the rectifying switch element Q2 is low due to the reverse recovery time of the body diode, and the resonance of the transformer is not reset. There was also a problem.
[0007]
The present invention has been made in view of the above problems, and provides a novel rectifier circuit having a short reverse recovery time even at a high voltage, and a switching power supply including the rectifier circuit.
[0008]
[Means to solve the problem]
When the main switch is turned on, the present invention made to achieve the above object provides a rectifier diode provided in a rectifier circuit, and allows a current to flow through the rectifier switch element. When the main switch is turned off, the current flowing to the body diode of the rectifying switch element is suppressed by providing a resistor when the main switch is turned off. Even in a high voltage synchronous rectifier circuit having a high secondary voltage, the rectifier circuit has a high withstand voltage, can suppress the reverse recovery time, and maintain the bidirectional characteristics of the synchronous rectifier circuit.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments according to the power supply circuit of the present invention will be described below with reference to the accompanying drawings. An embodiment of a rectifier circuit according to the present invention is shown in FIG. 1, and a switching power supply provided with this rectifier circuit is shown in FIG. Vin is a main power source, Lm is a transformer, Q1 is a main switch, Q3 is a commutation switch, Lo is a choke, Co is a smoothing capacitor, 10 is a rectifier circuit, 11 is a rectifier switch element, 12 is a resistor, and 13 is a diode.
[0010]
The switching power supply according to the present embodiment shown in FIG. 2 has a primary-secondary circuit isolated by a transformer Lm, and includes a main switch Q1 on the primary side and a rectifier circuit 10 and a commutation switch Q3 on the secondary side. A rectifier circuit 10 which is a feature of the present invention is shown in FIG.
[0011]
The rectifier circuit 10 includes a rectifier switch element 11 formed of an FET. A resistor 12 is connected to the drain side of the rectifying switch element 11 to form a series circuit. The resistor 12 may be connected to the source side or the source side and the drain side of the rectifying switch element 11. A rectifying diode 13 is connected in parallel to the series circuit and so that the anode faces the source side of the rectifying switch element 11.
[0012]
The rectifier circuit 10 configured as described above is provided in the switching power supply shown in FIG. 2 as follows. The source side of the rectifying switch element 11 and the anode of the diode 13 are connected to the output side minus terminal. That is, the source side of the rectifying switch element 11 and the anode of the diode 13 are connected to the commutation switch Q3, the smoothing capacitor Co, and the load. The resistor 12 and the cathode of the diode 13 are connected to the secondary winding of the transformer Lm.
[0013]
The switching power supply according to the present embodiment is configured as described above and operates as follows. When the main switch Q1 is turned on, a current flows through the choke Lo, and the rectifying switch element 11 is turned on. At this time, a current flows through the rectifying switch element 11 and also flows through a rectifying diode 13 connected in parallel with the series circuit of the rectifying switch element 11 and the resistor 12. As a result, the value of the current flowing through the rectifying switch element 11 is limited. This current value is expressed by the following relational expression.
[0014]
[Expression 1]

[0015]
In Equation 1, I _Limited is a limited current value, V _{F (LLD)} is a voltage of the diode 13, Ron is an on-resistance value of the rectifying switch element 11, and Rchip is a resistance value of the resistor 12.
[0016]
In order to prevent a current from flowing through the body diode of the rectifying switch element 11, the drop voltage V _{F (BD)} of the rectifying switch element 11 is set as follows.
[0017]
[Expression 2]

[0018]
As described above, by setting, no current flows through the body diode of the rectifying switch element 11.
[0019]
Subsequently, when the main switch Q1 is turned off, the rectifying switch element 11 is turned off, the commutation switch Q3 is turned on, and the exciting current of the transformer Lm flows. At this time, a current flows through the rectifier circuit 10 in a direction opposite to that when the main switch Q1 is turned on. However, the resistor 12 is provided and no current flows through the body diode of the rectifier switch element 11 as described above. With this setting, no current flows through the body diode of the rectifying switch element 11. On the other hand, a current flows through the diode 13. In this case, the reverse recovery time of the diode 13 becomes a problem, but the diode 13 has a relatively fast reverse recovery time, for example, the influence of the reverse recovery time of the diode 13 by using a Schottky barrier diode or a low loss diode. Even if the rectifier circuit has a high breakdown voltage, resonance reset of the transformer Lm works.
[0020]
As described above, even when the main switch Q1 is repeatedly turned on and off, it is possible to prevent the body diode of the rectifying switch element 11 from being turned on or the resonance reset of the transformer from being performed. This also works in a high voltage synchronous static current circuit, the rectifier circuit has a high breakdown voltage, and the reverse recovery time can be suppressed.
[0021]
【The invention's effect】
According to the present invention, when the main switch is turned on, by providing the rectifying diode provided in the rectifier circuit, the current flowing to the body diode of the rectifying switch element When the main switch is turned off, the voltage on the secondary side of the transformer that insulates the primary-secondary is high by suppressing the current flowing to the body diode of the rectifying switch element by providing a resistor. Also in the high voltage synchronous rectifier circuit, the rectifier circuit has a high withstand voltage, and has an effect of suppressing the reverse recovery time and maintaining the bidirectional characteristics of the synchronous rectifier circuit.
[0022]
Further, the use of a diode having a relatively fast reverse recovery time, such as a Schottky barrier diode or a low-loss diode, as the rectifying diode makes the above-described effect remarkable.
[Brief description of the drawings]
FIG. 1 is a circuit diagram showing an embodiment according to the present invention.
FIG. 2 is a circuit diagram showing an embodiment different from the embodiment shown in FIG.
FIG. 3 is a circuit diagram showing a conventional example.
[Explanation of symbols]
Vin Main power supply Lm Transformer Q1 Main switch Q2 Rectifier switch Q3 Commutation switch Lo Choke Co Smoothing capacitor 10 Rectifier circuit 11 Rectifier switch element 12 Resistor 13 Diode

Claims (7)

A resistor is connected to the source side or the drain side or the source side and the drain side of the rectifying switch element formed of an FET to form a series circuit, in parallel with the series circuit, and to the source side of the rectifying switch element. A rectifier circuit, wherein a rectifier diode having a relatively fast reverse recovery time is connected so that the anode faces. 2. The rectifier circuit according to claim 1 , wherein the rectifier diode is a Schottky barrier diode. 2. The rectifier circuit according to claim 1 , wherein the rectifier diode is a low-loss diode. In the switching power supply with rectifier circuit according to any one of claims 1 to 3, using the rectifier circuit as a rectifier switch, connecting the anode of the source and the diode of the rectifying switching element on the output side of the switching power supply A switching power supply, wherein a drain side of the rectifying switch element and a cathode of the diode are connected to an input side of the switching power supply. 5. A switching power supply comprising the rectifier circuit according to claim 4 , wherein a source side of the rectifying switch element and an anode of the diode are connected to an output negative terminal of the switching power supply. 6. A switching power supply comprising the rectifier circuit according to claim 4 or 5 , wherein a primary and a secondary are insulated by a transformer. 7. A switching power supply comprising the rectifier circuit according to claim 6 , wherein a commutation switch is provided on the secondary side, and the commutation switch is connected to a source side of the rectification switch element and an anode of the diode. Switching power supply.

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