JP2004048867A - Multi-output switching power supply device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a multi-output switching power supply device which can surely prevent a damage due to overcurrent. <P>SOLUTION: The multi-output switching power supply device includes resistors R1, R2 respectively connected in series with loads 5, 6, and switches SW1, SW2 respectively connected in parallel with the resistors R1, R2. An end of the resistor R1, which is not connected to the load 5, is connected to an end of the resistor R2, which is not connected to the load 6 via a series circuit of resistors R11 and R22. A total sum of output current of each output system and output current of a secondary side can be detected from a connecting node potential of the resistor R11 and the resistor R22 by on/off switching the switches SW1, SW2. Thus, overcurrent of each output system and overcurent of primary side can be detected, and a damage due to the overcurrent can be surely prevented. <P>COPYRIGHT: (C)2004,JPO

Description

【００３４】
一方、２次側の出力電力の総和Ｐは、以下の（２）式で表すことができる。
Ｐ＝Ｖ１×Ｉ１＋Ｖ２×Ｉ２…（２）
【００３５】
ここで、（１）式及び（２）式よりＰ∝Ｖ０が成り立つための条件は、以下の（３）式となる。
Ｖ１／（Ｒ_２２×Ｒ_１）＝Ｖ２／（Ｒ_１１×Ｒ_２）…（３）
【００３６】
直流出力電圧Ｖ１及びＶ２は設計開始段階での要求値であるから、（３）式を満たすように、抵抗値Ｒ_１、Ｒ_２、Ｒ_１１、及びＲ_２２を設定する。これにより、Ｖ０は２次側の出力電力の総和Ｐに正比例した合成検出電圧となる。また、コンパレータ７の非反転入力端子に印加される基準電圧源８の基準電圧−Ｖ_ＲＥＦ８を取り出し得る最大電力Ｐｍのときの−Ｖ０に一致させる。これにより、コンパレータ７は２次側の出力電力の総和Ｐが最大電力Ｐｍを越えた時点でＨｉｇｈレベルの信号を出力する。すなわち、第１制御状態の場合においては、コンパレータ７はスイッチングトランジスタＱ１の破壊を引き起こす１次側の過電流を検出することができる。
【００３７】
続いて、切替制御回路１１が第２制御状態である場合について説明する。この場合、以下の（４）式が成り立つ。
Ｖ０＝Ｒ_２２×Ｒ_１×Ｉ１／（Ｒ_１１＋Ｒ_２２）…（４）
【００３８】
（４）式より、第２制御状態の場合においては、Ｖ０は電流値Ｉ１に正比例した検出電圧となることがわかる。また、コンパレータ７の非反転入力端子に印加される基準電圧源９の基準電圧−Ｖ_ＲＥＦ９を電流値Ｉ１が許容上限値のときの−Ｖ０に一致させる。これにより、コンパレータ７は電流値Ｉ１が許容上限値を越えた時点でＨｉｇｈレベルの信号を出力する。すなわち、第２制御状態の場合においては、コンパレータ７はダイオードＤ１の破壊や負荷５が設けられる出力系統のＰＣＢのパターン焼け等を引き起こす過電流を検出することができる。
【００３９】
続いて、切替制御回路１１が第３制御状態である場合について説明する。この場合、以下の（５）式が成り立つ。
Ｖ０＝Ｒ_１１×Ｒ_２×Ｉ２／（Ｒ_１１＋Ｒ_２２）…（５）
【００４０】
（５）式より、第３制御状態の場合においては、Ｖ０は電流値Ｉ２に正比例した検出電圧となることがわかる。また、コンパレータ７の非反転入力端子に印加される基準電圧源１０の基準電圧−Ｖ_{ＲＥＦ１０}を電流値Ｉ２が許容上限値のときの−Ｖ０に一致させる。これにより、コンパレータ７は電流値Ｉ２が許容上限値を越えた時点でＨｉｇｈレベルの信号を出力する。すなわち、第３制御状態の場合においては、コンパレータ７はダイオードＤ２の破壊や負荷６が設けられる出力系統のＰＣＢのパターン焼け等を引き起こす過電流を検出することができる。
【００４１】
フリップフロップＦＦ１はコンパレータ７の出力信号が一旦Ｈｉｇｈレベルになると、その後は非反転出力端子から制御回路４に出力する信号をＨｉｇｈレベルに保持する。これにより、制御回路４はフリップフロップＦＦ１からＨｉｇｈレベルの信号を受け取ると、スイッチングトランジスタＱ１のオン期間を短くするか、又はスイッチングトランジスタＱ１の動作周波数を高くするかの制御を行う。これにより、過電流による破壊を防ぐことができる。また、１次側電流の過電流判定及び２次側の各出力系統の電流の過電流判定を一つのコンパレータで行う構成としているので、小型化及び低コスト化を図ることができる。さらに、１次側電流及び２次側の各出力系統の電流をそれぞれ検出しているので、２次側に設けられるダイオードの破壊等を防止するために１次側電流の過電流設定を厳しくする必要がない。したがって、過負荷とならない範囲の限界までスイッチングトランジスタＱ１を効果的に動作させることができる。
【００４２】
なお、抵抗値Ｒ_１１と抵抗値Ｒ_２２が等しくなるように設定すると、（３）式の条件が以下の（３）’式となるので、抵抗Ｒ１、Ｒ２、Ｒ１１、及びＲ２２の抵抗値を容易に設定できるという利点がある。
Ｖ１／Ｒ_１＝Ｖ２／Ｒ_２…（３）’
【００４３】
次に、本発明に係る多出力スイッチング電源装置の他の構成例を図２に示す。なお、図２において図１と同一の部分には同一の符号を付し、説明を省略する。また、抵抗値Ｒ_１、Ｒ_２、Ｒ_１１、及びＲ_２２の設定も図１の多出力スイッチング電源装置と同一とする。
【００４４】
図２の多出力スイッチング電源装置が図１の多出力スイッチング電源装置と異なる点は、フリップフロップＦＦ１の反転出力端子を新たに設けた電圧制御回路１５に接続し、尚かつ整流回路２、平滑コンデンサＣ０、スイッチングトランジスタＱ１、及びスイッチングトランス３の補助巻線３ｄの接続ノードと制御回路４との間にフォトトランジスタ１４を設けたことである。
【００４５】
電圧制御回路１５は、抵抗Ｒ３〜Ｒ５、発光ダイオード１２、及びシャントレギュレータ１３によって構成される。平滑コンデンサＣ２の両端に、抵抗Ｒ３、発光ダイオード１２、及びシャントレギュレータ１３から成る直列回路と、抵抗Ｒ４及び抵抗Ｒ５から成る直列回路とが並列に接続される。抵抗Ｒ３の一端は平滑コンデンサＣ２の正極性側に接続され、抵抗Ｒ３の他端は発光ダイオード１２のアノードに接続され、発光ダイオード１２のカソードはシャントレギュレータ１３のカソードに接続され、シャントレギュレータ１３のアノードは平滑コンデンサＣ２の負極性側に接続される。また、シャントレギュレータ１３のリファレンス端子が抵抗Ｒ４と抵抗Ｒ５との接続ノードに接続される。さらに、フリップフロップＦＦ１の反転出力端子が発光ダイオード１２とシャントレギュレータ１３との接続ノードに接続される。なお、電圧制御回路１５は最も出力電力の多い出力系統に設けるものとする。すわなち図２の多出力スイッチング電源装置では負荷５の定格電力よりも負荷６の定格電力の方が大きい設定となっている。
【００４６】
そして、フォトトランジスタ１４のコレクタが制御回路４に接続され、フォトトランジスタ１４のエミッタが整流回路２、平滑コンデンサＣ０、スイッチングトランジスタＱ１、及びスイッチングトランス３の補助巻線３ｄの接続ノードに接続される。なお、発光ダイオード１２とフォトトランジスタ１４とがフォトカプラを形成している。
【００４７】
このような構成である図２の多出力スイッチング電源装置の動作について説明する。一次側電流、電流Ｉ１、電流Ｉ２のいずれかが過電流になった後は、フリップフロップＦＦ１の反転出力端子から出力される信号はＬｏｗレベルに保持される。これにより、発光ダイオード１２が強制的にオンになり、それに伴ってフォトトランジスタ１４も強制的にオンになる。
【００４８】
制御回路４は、フォトトランジスタ１４がオンになると動作を停止する。その際、スイッチングトランジスタＱ１がオフの状態を保持し、２次側の全出力が停止状態になる。このため、発光ダイオード１２がオフになり、フォトトランジスタ１４のオンが解除され、制御回路４が動作を開始することになり、スイッチングトランジスタＱ１のオン／オフ制御が再開され、２次側への電力供給が再開される。さらに、過電流の状態のままであれば再度上記動作を繰り返すことになる。すなわち、図２の多出力スイッチング電源装置は断続的な電流制限動作を行う。
【００４９】
電圧制御回路１５は一般によく用いられる既存の電圧制御回路であるので低廉である。したがって、図２の多出力スイッチング電源装置はコストを低減することができる。
【００５０】
次に、本発明に係る多出力スイッチング電源装置の更に他の構成例を図３に示す。図３の多出力スイッチング電源装置は、過電流による破壊を確実に防止することを目的とするのではなく、省電力化回路の動作切り替えを確実にすることを目的としている。なお、図３において図２と同一の部分には同一の符号を付し、説明を省略する。
【００５１】
図３の多出力スイッチング電源装置が図２の多出力スイッチング電源装置と異なる点は、スイッチＳＷ１〜ＳＷ３と、基準電圧源９及び１０と、切替制御回路１１と、フリップフロップＦＦ１とを設けず、ｎ形チャネルＦＥＴであるトランジスタＱ２と、抵抗９と、省電力化回路１６とを新たに設け、コンパレータ７の非反転入力端子を基準電圧源８に直接接続し、コンパレータ７の出力端子をトランジスタＱ２のゲート及び抵抗Ｒ９の一端に接続し、抵抗Ｒ９の他端を平滑コンデンサＣ２の正極性側に接続し、トランジスタＱ２のソースがシャントレギュレータ１３のリファレンス端子に接続され、トランジスタＱ２のドレインが省電力化回路１６に接続されていることである。
【００５２】
省電力化回路１６は、コンデンサＣ３及びＣ４と、ダイオードＤ３と、抵抗Ｒ６〜Ｒ８とによって構成される。抵抗Ｒ６の一端はトランジスタＱ２のドレインに接続される。抵抗Ｒ６の他端はコンデンサＣ３の一端に接続され、コンデンサＣ３の他端は、抵抗Ｒ７を介してダイオードＤ３のカソードに、抵抗Ｒ８を介してダイオードＤ３のアノードに、コンデンサＣ４を介して平滑コンデンサＣ２の負極性側にそれぞれ接続される。また、ダイオードＤ３のカソードが整流ダイオードＤ２のアノードに接続される。
【００５３】
図３の多出力スイッチング電源装置において、上述した（１）式が成立する。ここで、Ｒ_１≒０とすると、Ｖ０は電流値Ｉ２に正比例した検出電圧となる。そして、コンパレータ７の非反転入力端子に印加される基準電圧源８の基準電圧−Ｖ_ＲＥＦ８を負荷６が待機状態であるときの−Ｖ０に一致させる。
【００５４】
これにより、負荷６が待機状態のときにコンパレータ７がＬｏｗレベルの信号を出力し、それに伴いトランジスタＱ２がオンになる。トランジスタＱ２がオンになると、省電力化回路１６が動作する。一方、負荷６が待機状態でないときにコンパレータ７がＨｉｇｈレベルの信号を出力し、それに伴いトランジスタＱ２がオフになる。トランジスタＱ２がオフになると、省電力化回路１６が動作を停止する。
【００５５】
従来は特願２００１−０９７２８４号明細書に記載されているように軽負荷時の揺らぎ波形を検出し、その検出結果に基づいて待機時か通常動作時かを判定し省電力化回路１６の動作を制御していた。しかしながら、この揺らぎ波形はばらつきが大きく待機時と通常動作時の切替ポイントがずれるという問題があった。この問題点は、特にＷ／Ｗタイプのスイッチング電源装置において顕著であった。
【００５６】
これに対して、図３の多出力スイッチング電源装置では、省電力化回路１６が設けられている出力系統を流れる電流を検出して、その検出結果に基づいて待機時か通常動作時かを判定し省電力化回路１６の動作を制御する。これにより、待機時と通常動作時の切替ポイントがずれることがなくなる。
【００５７】
【発明の効果】
本発明によると、過電流による破壊を確実に防止できる多出力スイッチング電源装置を実現することができる。
【図面の簡単な説明】
【図１】本発明に係る多出力スイッチング電源装置の一構成例を示す図である。
【図２】本発明に係る多出力スイッチング電源装置の他の構成例を示す図である。
【図３】本発明に係る多出力スイッチング電源装置の更に他の構成例を示す図である。
【図４】従来の多出力スイッチング電源装置の一構成例を示す図である。
【符号の説明】
３　　スイッチングトランス
４　　制御回路
５、６　　負荷
７　　コンパレータ
８〜１０　　基準電圧源
１１　　切替制御回路
１２　　発光ダイオード
１３　　シャントレギュレータ
１４　　フォトトランジスタ
Ｃ１，Ｃ２　　平滑コンデンサ
Ｃ３、Ｃ４　　コンデンサ
Ｄ１、Ｄ２　　整流ダイオード
Ｄ３　　ダイオード
ＦＦ１　　フリップフロップ
Ｑ１　　スイッチングトランジスタ
Ｒ１〜Ｒ８、Ｒ１１、Ｒ２２　　抵抗
ＳＷ１〜ＳＷ３　　スイッチ[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a multiple output switching power supply. In particular, it relates to overcurrent protection of a multi-output switching power supply.
[0002]
[Prior art]
FIG. 4 shows an example of the configuration of a conventional multi-output switching power supply. First, the primary side of the multiple output switching power supply of FIG. 4 will be described. An AC power supply 1 such as a commercial power supply is connected to an input side of a rectifier circuit 2 formed by bridge-connecting four diodes. The output side of the rectifier circuit 2 is connected to both ends of the smoothing capacitor C0.
[0003]
Then, a series circuit including a primary winding 3a of the switching transformer 3, a switching transistor Q1 which is an n-type channel FET (Field Effect Transistor), and a current detection resistor R0 is connected to both ends of the smoothing capacitor C0. You. That is, one end of the primary winding 3a of the switching transformer 3 is connected to the positive polarity side of the smoothing capacitor C0, the other end of the primary winding 3a of the switching transformer 3 is connected to the drain of the switching transistor Q1, and the switching transistor Q1 Is connected to the negative polarity side of the smoothing capacitor C0 via the current detecting resistor R0.
[0004]
Further, the gate of the switching transistor Q1, both ends of the current detection resistor R0, and both ends of the auxiliary winding 3d of the switching transformer 3 are connected to the control circuit 4.
[0005]
Next, the secondary side of the multiple output switching power supply of FIG. 4 will be described. One end of the secondary winding 3b of the switching transformer 3 is connected to the positive side of the smoothing capacitor C1 via the rectifier diode D1, and the other end of the secondary winding 3b of the switching transformer 3 is connected to the negative side of the smoothing capacitor C1. Connected. A load 5 is connected to both ends of the smoothing capacitor C1. The negative side of the smoothing capacitor C1 is at the ground potential.
[0006]
One end of the secondary winding 3c of the switching transformer 3 is connected to the positive side of the smoothing capacitor C2 via the rectifier diode D2, and the other end of the secondary winding 3c of the switching transformer 3 is connected to the negative side of the smoothing capacitor C2. Connected to the side. A load 6 is connected to both ends of the smoothing capacitor C2. The negative side of the smoothing capacitor C2 is at the ground potential.
[0007]
The multi-output switching power supply of FIG. 4 having such a configuration operates as follows. An AC voltage output from the AC power supply 1 is rectified by the rectifier circuit 2 and smoothed by the smoothing capacitor C0 to become a DC input voltage. When the switching transistor Q1 is on, the DC input voltage output from the smoothing capacitor C0 is supplied to the primary winding 3a of the switching transformer 3, and the excitation energy is accumulated in the primary winding 3a of the switching transformer 3. On the other hand, when the switching transistor Q1 is off, the excitation energy stored in the primary winding 3a of the switching transformer 3 is extracted from the secondary windings 3b and 3c of the switching transformer 3. Therefore, the voltages output from the secondary windings 3b and 3c of the switching transformer 3 are rectangular wave AC voltages.
[0008]
The rectangular wave AC voltage output from the secondary winding 3b of the switching transformer 3 is rectified and smoothed by a rectifying and smoothing circuit including a rectifying diode D1 and a smoothing capacitor C1 to become a DC output voltage V1. Then, the DC output voltage V1 is supplied to the load 5. The rectangular AC voltage output from the secondary winding 3c of the switching transformer 3 is rectified and smoothed by a rectifying / smoothing circuit including a rectifier diode D2 and a smoothing capacitor C2 to become a DC output voltage V2. Then, the DC output voltage V2 is supplied to the load 6.
[0009]
The control circuit 4 uses the voltage output from the auxiliary winding 3d of the switching transformer 3 as a power supply voltage, generates a control signal such as a PWM signal, and sends the control signal to the gate of the switching transistor Q1 to control the switching transistor. Control on / off. Further, the control circuit 4 detects the total of the secondary-side output power based on the voltage across the current detection resistor R0, and turns on the switching transistor Q1 when the total of the secondary-side output power exceeds a certain limit value. The period or the operating frequency is controlled to limit the current flowing through the switching transistor Q1. This prevents the switching transistor Q1 from being destroyed due to an overcurrent.
[0010]
[Problems to be solved by the invention]
However, simply detecting the overcurrent from the sum of the output powers on the secondary side, when the difference between the DC output voltage V1 output to the load 5 and the DC output voltage V2 output to the load 6 is large, the rectifier diode D1 Alternatively, an overcurrent may flow through the rectifier diode D2.
[0011]
That is, when the load with the larger DC output voltage value becomes heavier, the total output power on the secondary side increases, the current flowing through the switching transistor Q1 also increases, and the control circuit 4 sets the current detection resistor R0 to Since no overcurrent is detected from the voltage between both ends, no problem occurs. However, when the load with the smaller DC output voltage value becomes heavier, the total output power on the secondary side does not increase so much. No overcurrent is detected from the voltage across the resistor R0. For this reason, there is a possibility that a serious problem such as thermal destruction of the rectifier diode or burnt pattern of the PCB may occur in the output system having the smaller DC output voltage value.
[0012]
Note that the switching power supply disclosed in Japanese Utility Model Laid-Open No. 6-17389 detects overcurrent of the current of each output system, so that the rectifier diode is thermally damaged in the output system with the smaller DC output voltage value. However, there is no possibility that a serious problem such as pattern burning of the PCB or the like may occur, but there is a problem that the switching transistor may be destroyed because the overcurrent of the primary current is not detected. Further, the switching power supply disclosed in Japanese Utility Model Laid-Open No. 6-17389 has a configuration including each comparator for detecting the current of each output system and a comparator for detecting the sum of the secondary-side output currents. However, miniaturization and cost reduction could not be achieved.
[0013]
The present invention has been made in view of the above problems, and has as its object to provide a multi-output switching power supply that can reliably prevent destruction due to overcurrent.
[0014]
[Means for Solving the Problems]
In order to achieve the above object, a multi-output switching power supply according to the present invention comprises a switching transformer having a plurality of secondary windings, and a switching transformer for converting a DC input voltage to an AC voltage by on / off operation. A switching element to be supplied to the primary winding of the switching transformer, a conversion means for converting an AC voltage induced in the secondary winding of the switching transformer into a DC voltage and outputting the DC voltage, and turning on the switching element. Control means for controlling output / off, current detection means for detecting an output current for each output system, power detection means for detecting the sum of output power on the secondary side, detection results of the current detection means and the power detection means And an overcurrent detecting means for detecting an overcurrent from the above, and perform overcurrent protection based on a detection result of the overcurrent detecting means. To.
[0015]
With such a configuration, the overcurrent on the primary side can be detected from the sum of the output power on the secondary side, so that the switching element can be prevented from being destroyed. Further, since the overcurrent of each output system is detected, it is possible to prevent the thermal destruction of the rectifier diode provided in each output system and the burning of the PCB pattern in each output system. Therefore, destruction due to overcurrent can be reliably prevented.
[0016]
Further, the control means may input a detection result of the overcurrent detection means and stop the operation when an overcurrent is detected.
[0017]
In this way, when overcurrent is detected, all outputs on the secondary side can be stopped, so that the safest overcurrent protection can be realized.
[0018]
Further, the control unit inputs a detection result of the overcurrent detection unit, and when an overcurrent is detected, whether to shorten an ON period of the switching element or increase an operating frequency of the switching element. May be controlled.
[0019]
By doing so, if an overcurrent is detected, the current can be reduced on both the primary side and the secondary side, and overcurrent protection can be realized.
[0020]
Further, the current detection means and the power detection means, in each output system, includes a resistor connected in series with a load, and in each output system, a switch that short-circuits the resistance by being closed. Is also good.
[0021]
With such a configuration, it is not necessary for the overcurrent detection means to include a plurality of comparators, so that downsizing and cost reduction can be achieved.
[0022]
Further, the voltage control means for controlling the operation of the control means such that the voltage output from the conversion means becomes a predetermined value in accordance with the voltage output from the conversion means is connected to the output system having the maximum rated output power. The voltage control means may correct the signal output to the control means in order to control the operation of the control means in accordance with the detection result of the overcurrent detection means.
[0023]
In this way, an inexpensive existing voltage control circuit generally used can be used as the voltage control means. Therefore, the cost of the multi-output switching power supply can be reduced.
[0024]
The purpose of the present invention is not to reliably prevent destruction due to overcurrent, but to ensure the operation switching of the power saving circuit. A switching transformer having a line, a switching element for converting a DC input voltage into an AC voltage by an on / off operation and supplying the AC voltage to a primary winding of the switching transformer, and a secondary winding of the switching transformer provided for each output system. A converter for converting an AC voltage induced in the line into a DC voltage and outputting the DC voltage; a control unit for controlling ON / OFF of the switching element; a power saving circuit provided in a predetermined output system; Current detecting means for detecting the output current of the system, and the power saving circuit is configured in accordance with the detection result of the current detecting means. To perform the switching of the work state.
[0025]
With such a configuration, the current flowing through the output system provided with the power saving circuit is detected, and based on the detection result, whether the apparatus is in the standby mode or the normal operation mode is determined, and the operation of the power saving circuit is performed. Control. Thus, the switching point between the standby state and the normal operation does not shift.
[0026]
BEST MODE FOR CARRYING OUT THE INVENTION
An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 shows a configuration example of a multi-output switching power supply device according to the present invention. In FIG. 1, the same parts as those in FIG.
[0027]
First, the configuration of the primary side will be described. The primary side of the multi-output switching power supply device of FIG. 1 has a configuration in which the current detection resistor R0 is removed from the multi-output switching power supply device of FIG.
[0028]
Subsequently, the configuration on the secondary side will be described. The negative side of the capacitor C1 and the load 5 are connected via a resistor R1. Then, the inverting input terminal of the comparator 7 is connected to the connection node between the capacitor C1 and the resistor R1 via the resistor R11. Further, the switch SW1 is connected in parallel to the resistor R1.
[0029]
The negative side of the capacitor C2 and the load 6 are connected via a resistor R2. Then, the inverting input terminal of the comparator 7 is connected to the connection node between the capacitor C2 and the resistor R2 via the resistor R22. Further, the switch SW2 is connected in parallel to the resistor R2.
[0030]
The non-inverting input terminal of the comparator 7 is connected to the negative electrode side of each of the reference voltage sources 8 to 10 via the switch SW3. The positive electrodes of the reference voltage sources 8 to 10 are at the ground potential. The output terminal of the comparator 7 is connected to the set terminal of the flip-flop FF1. The non-inverting output terminal of the flip-flop FF is connected to the control circuit 4 via a photocoupler (not shown).
[0031]
Further, the switching control circuit 11 is connected to respective control terminals of the switches SW1, SW2 and SW3.
[0032]
The operation of the multi-output switching power supply of FIG. 1 having such a configuration will be described. The switching control circuit 11 turns off the switches SW1 and SW2, closes the contact point of the switch SW3 connected to the reference voltage source 8, and turns off the switch SW1 and turns on the switch SW2. 9 and a third control state in which the switch SW1 is turned on and the switch SW2 is turned off and the contact of the switch SW3 connected to the reference voltage source 10 is closed. Are switched in chronological order.
[0033]
First, the case where the switching control circuit 11 is in the first control state will be described. Assuming that the ground potential is zero and the potential of the inverting input terminal of the comparator 7 is -V0, the following equation (1) holds. Here, R ₁ , R ₂ , R ₁₁ , and R ₂₂ are resistance values of the resistors R ₁ , R ₂ , R ₁₁ , and R ₂₂ , respectively, I 1 is a current value flowing through the resistor R 1, and I 2 is a current value flowing through the resistor R 2. .
(Equation 1)

[0034]
On the other hand, the total sum P of the output power on the secondary side can be expressed by the following equation (2).
P = V1 × I1 + V2 × I2 (2)
[0035]
Here, from the expressions (1) and (2), the condition for P∝V0 to be satisfied is the following expression (3).
_{V1 / (R 22 × R 1} ) = V2 / (R 11 × R 2) ... (3)
[0036]
Since the DC output voltages V1 and V2 are required values at the stage of starting the design, the resistance values R ₁ , R ₂ , R ₁₁ and R ₂₂ are set so as to satisfy the expression (3). As a result, V0 becomes a combined detection voltage that is directly proportional to the sum P of the output power on the secondary side. Further, the reference voltage −V _REF8 of the reference voltage source 8 applied to the non-inverting input terminal of the comparator 7 is made _equal to _−V0 at the time of the maximum power Pm at which the reference power can be extracted. Thus, the comparator 7 outputs a High-level signal when the total output P of the secondary side exceeds the maximum power Pm. That is, in the case of the first control state, the comparator 7 can detect an overcurrent on the primary side that causes the destruction of the switching transistor Q1.
[0037]
Subsequently, a case where the switching control circuit 11 is in the second control state will be described. In this case, the following equation (4) holds.
V0 = R ₂₂ × R ₁ × I1 / (R ₁₁ + R ₂₂ ) (4)
[0038]
From equation (4), it can be seen that in the case of the second control state, V0 is a detection voltage directly proportional to the current value I1. Further, the reference voltage -V _REF9 of the reference voltage source 9 applied to the non-inverting input terminal of the comparator 7 is made _equal to _-V0 when the current value I1 is at the allowable upper limit. Thus, the comparator 7 outputs a High-level signal when the current value I1 exceeds the allowable upper limit. That is, in the case of the second control state, the comparator 7 can detect an overcurrent that causes destruction of the diode D1 and burns of the pattern of the PCB of the output system provided with the load 5.
[0039]
Subsequently, a case where the switching control circuit 11 is in the third control state will be described. In this case, the following equation (5) holds.
V0 = R ₁₁ × R ₂ × I2 / (R ₁₁ + R ₂₂ ) (5)
[0040]
From equation (5), it can be seen that in the case of the third control state, V0 is a detection voltage that is directly proportional to the current value I2. Further, the reference voltage -V _REF10 of the reference voltage source 10 applied to the non-inverting input terminal of the comparator 7 is made to match -V0 when the current value I2 is at the allowable upper limit. Thus, the comparator 7 outputs a High level signal when the current value I2 exceeds the allowable upper limit. That is, in the case of the third control state, the comparator 7 can detect an overcurrent that causes destruction of the diode D2 and burns of the pattern of the PCB of the output system in which the load 6 is provided.
[0041]
Once the output signal of the comparator 7 goes high, the flip-flop FF1 keeps the signal output from the non-inverting output terminal to the control circuit 4 high. Thus, when the control circuit 4 receives the High level signal from the flip-flop FF1, it controls whether the ON period of the switching transistor Q1 is shortened or the operating frequency of the switching transistor Q1 is increased. Thus, destruction due to overcurrent can be prevented. Further, since the overcurrent determination of the primary current and the overcurrent determination of the current of each output system of the secondary side are performed by one comparator, downsizing and cost reduction can be achieved. Furthermore, since the primary side current and the current of each output system of the secondary side are detected respectively, the overcurrent setting of the primary side current is made strict to prevent the destruction of the diode provided on the secondary side. No need. Therefore, the switching transistor Q1 can be operated effectively up to the limit of the range where no overload occurs.
[0042]
Incidentally, when the resistance value _{R 11} the resistance value _{R 22} is set equal, since (3) of the condition is the following (3) 'formula, resistors R1, R2, R11, and the resistance value of R22 There is an advantage that it can be easily set.
V1 / R ₁ = V2 / R ₂ (3) ′
[0043]
Next, another configuration example of the multiple output switching power supply device according to the present invention is shown in FIG. In FIG. 2, the same parts as those in FIG. The settings of the resistance values R ₁ , R ₂ , R ₁₁ , and R ₂₂ are the same as those of the multiple output switching power supply device of FIG.
[0044]
The difference between the multi-output switching power supply of FIG. 2 and the multi-output switching power supply of FIG. 1 is that the inverted output terminal of the flip-flop FF1 is connected to a newly provided voltage control circuit 15, the rectifier circuit 2, and the smoothing capacitor. A phototransistor 14 is provided between the control circuit 4 and a connection node between C0, the switching transistor Q1, and the auxiliary winding 3d of the switching transformer 3.
[0045]
The voltage control circuit 15 includes the resistors R3 to R5, the light emitting diode 12, and the shunt regulator 13. A series circuit including a resistor R3, a light emitting diode 12, and a shunt regulator 13 and a series circuit including a resistor R4 and a resistor R5 are connected in parallel to both ends of the smoothing capacitor C2. One end of the resistor R3 is connected to the positive polarity side of the smoothing capacitor C2, the other end of the resistor R3 is connected to the anode of the light emitting diode 12, the cathode of the light emitting diode 12 is connected to the cathode of the shunt regulator 13, The anode is connected to the negative polarity side of the smoothing capacitor C2. Further, the reference terminal of the shunt regulator 13 is connected to a connection node between the resistors R4 and R5. Further, the inverted output terminal of the flip-flop FF1 is connected to a connection node between the light emitting diode 12 and the shunt regulator 13. Note that the voltage control circuit 15 is provided in an output system having the largest output power. That is, in the multi-output switching power supply of FIG. 2, the rated power of the load 6 is set to be larger than the rated power of the load 5.
[0046]
The collector of the phototransistor 14 is connected to the control circuit 4, and the emitter of the phototransistor 14 is connected to a connection node of the rectifier circuit 2, the smoothing capacitor C <b> 0, the switching transistor Q <b> 1, and the auxiliary winding 3 d of the switching transformer 3. The light emitting diode 12 and the phototransistor 14 form a photocoupler.
[0047]
The operation of the multi-output switching power supply of FIG. 2 having such a configuration will be described. After any one of the primary current, the current I1, and the current I2 becomes an overcurrent, the signal output from the inverting output terminal of the flip-flop FF1 is held at the Low level. As a result, the light emitting diode 12 is forcibly turned on, and accordingly, the phototransistor 14 is also forcibly turned on.
[0048]
The control circuit 4 stops operating when the phototransistor 14 is turned on. At that time, the switching transistor Q1 is kept off, and all outputs on the secondary side are stopped. For this reason, the light emitting diode 12 is turned off, the phototransistor 14 is turned off, the control circuit 4 starts operating, the on / off control of the switching transistor Q1 is resumed, and the power to the secondary side is reduced. Supply is resumed. Further, if the overcurrent state remains, the above operation is repeated again. That is, the multi-output switching power supply device of FIG. 2 performs an intermittent current limiting operation.
[0049]
The voltage control circuit 15 is inexpensive because it is an existing voltage control circuit that is commonly used. Therefore, the multi-output switching power supply of FIG. 2 can reduce the cost.
[0050]
Next, still another configuration example of the multiple output switching power supply device according to the present invention is shown in FIG. The multi-output switching power supply device of FIG. 3 is not intended to reliably prevent destruction due to overcurrent, but is intended to ensure that the operation of the power saving circuit is switched. Note that, in FIG. 3, the same parts as those in FIG.
[0051]
3 is different from the multi-output switching power supply of FIG. 2 in that switches SW1 to SW3, reference voltage sources 9 and 10, a switching control circuit 11, and a flip-flop FF1 are not provided. A transistor Q2, which is an n-channel FET, a resistor 9, and a power saving circuit 16 are newly provided, a non-inverting input terminal of the comparator 7 is directly connected to the reference voltage source 8, and an output terminal of the comparator 7 is connected to the transistor Q2. The other end of the resistor R9 is connected to the positive side of the smoothing capacitor C2, the source of the transistor Q2 is connected to the reference terminal of the shunt regulator 13, and the drain of the transistor Q2 is That is, it is connected to the conversion circuit 16.
[0052]
The power saving circuit 16 includes capacitors C3 and C4, a diode D3, and resistors R6 to R8. One end of the resistor R6 is connected to the drain of the transistor Q2. The other end of the resistor R6 is connected to one end of the capacitor C3. The other end of the capacitor C3 is connected to the cathode of the diode D3 via the resistor R7, to the anode of the diode D3 via the resistor R8, and to the smoothing capacitor via the capacitor C4. C2 is connected to the negative polarity side. Further, the cathode of the diode D3 is connected to the anode of the rectifier diode D2.
[0053]
In the multi-output switching power supply device of FIG. 3, the above-described expression (1) is satisfied. Here, if R ₁ ≒ 0, V0 is a detection voltage directly proportional to the current value I2. Then, the reference voltage −V _REF8 of the reference voltage source 8 applied to the non-inverting input terminal of the comparator 7 is made _equal to _−V0 when the load 6 is in the standby state.
[0054]
Thus, when the load 6 is in the standby state, the comparator 7 outputs a low-level signal, and accordingly, the transistor Q2 is turned on. When the transistor Q2 is turned on, the power saving circuit 16 operates. On the other hand, when the load 6 is not in the standby state, the comparator 7 outputs a high-level signal, and accordingly, the transistor Q2 is turned off. When the transistor Q2 is turned off, the power saving circuit 16 stops operating.
[0055]
Conventionally, as described in the specification of Japanese Patent Application No. 2001-097284, a fluctuation waveform at the time of light load is detected, and based on the detection result, it is determined whether the standby state or the normal operation is performed, and the operation of the power saving circuit 16 is performed. Had control. However, this fluctuation waveform has a large variation, and there is a problem that the switching point between the standby state and the normal operation is shifted. This problem was particularly noticeable in a W / W type switching power supply.
[0056]
On the other hand, in the multi-output switching power supply device of FIG. 3, the current flowing through the output system in which the power saving circuit 16 is provided is detected, and based on the detection result, it is determined whether the standby mode or the normal operation mode is performed. Then, the operation of the power saving circuit 16 is controlled. Thus, the switching point between the standby state and the normal operation does not shift.
[0057]
【The invention's effect】
ADVANTAGE OF THE INVENTION According to this invention, the multiple output switching power supply device which can prevent destruction by an overcurrent reliably can be implement | achieved.
[Brief description of the drawings]
FIG. 1 is a diagram showing a configuration example of a multiple output switching power supply device according to the present invention.
FIG. 2 is a diagram showing another configuration example of the multiple output switching power supply device according to the present invention.
FIG. 3 is a diagram showing still another configuration example of the multiple output switching power supply device according to the present invention.
FIG. 4 is a diagram illustrating a configuration example of a conventional multi-output switching power supply device.
[Explanation of symbols]
REFERENCE SIGNS LIST 3 switching transformer 4 control circuit 5, 6 load 7 comparator 8 to 10 reference voltage source 11 switching control circuit 12 light emitting diode 13 shunt regulator 14 phototransistor C1, C2 smoothing capacitor C3, C4 capacitor D1, D2 rectifying diode D3 diode FF1 flip-flop Q1 Switching transistors R1 to R8, R11, R22 Resistors SW1 to SW3 Switches

Claims (6)

A switching transformer having a plurality of secondary windings, a switching element for converting a DC input voltage into an AC voltage by an on / off operation and supplying the AC voltage to a primary winding of the switching transformer, and a switching element provided for each output system. A multi-output switching power supply device comprising: a conversion unit that converts an AC voltage induced in a secondary winding of a transformer into a DC voltage and outputs the DC voltage; and a control unit that controls ON / OFF of the switching element.
Current detection means for detecting an output current for each output system; power detection means for detecting the sum of secondary-side output powers; and overcurrent detection for detecting overcurrent from the detection results of the current detection means and the power detection means Detection means,
A multi-output switching power supply device, wherein overcurrent protection is performed based on a detection result of the overcurrent detection means. 2. The multiple output switching power supply device according to claim 1, wherein the control unit inputs a detection result of the overcurrent detection unit, and stops operation when an overcurrent is detected. 3. The control means inputs a detection result of the overcurrent detection means, and controls whether to shorten an on-period of the switching element or increase an operating frequency of the switching element when an overcurrent is detected. The multi-output switching power supply device according to claim 1, which performs the following. The said current detection means and the said power detection means are provided with the resistance connected in series with the load in each output system, and the switch which short-circuits the said resistance by being closed in each output system. A multi-output switching power supply device according to any one of the above. Voltage control means for controlling the operation of the control means such that the voltage output from the conversion means becomes a predetermined value according to the voltage output from the conversion means is provided in the output system where the rated output power is the maximum,
The multi-output switching according to any one of claims 1 to 4, wherein the voltage control means corrects a signal output to the control means for controlling an operation of the control means in accordance with a detection result of the overcurrent detection means. Power supply. A switching transformer having a plurality of secondary windings, a switching element for converting a DC input voltage into an AC voltage by an on / off operation and supplying the AC voltage to a primary winding of the switching transformer, and a switching element provided for each output system. A multi-output switching power supply device comprising: a conversion unit that converts an AC voltage induced in a secondary winding of a transformer into a DC voltage and outputs the DC voltage; and a control unit that controls ON / OFF of the switching element.
A power saving circuit provided in a predetermined output system, current detection means for detecting an output current of the predetermined output system,
The multi-output switching power supply device, wherein the power saving circuit switches an operation state according to a detection result of the current detection means.

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