JP2004048820A

JP2004048820A - Insulated power source unit and semiconductor integrated circuit for power source control

Info

Publication number: JP2004048820A
Application number: JP2002199041A
Authority: JP
Inventors: Ryotaro Kudo; 工藤　良太郎; Kenichi Onda; 恩田　謙一; Kyoichi Hosokawa; 細川　恭一; Shinichi Yoshida; 吉田　信一
Original assignee: Renesas Technology Corp
Current assignee: Renesas Technology Corp
Priority date: 2002-07-08
Filing date: 2002-07-08
Publication date: 2004-02-12

Abstract

<P>PROBLEM TO BE SOLVED: To provide a reliable insulated DC-DC converter where there is no such fear that the control is stopped while a MOSFET for synchronous control is kept turned on and a secondary LC circuit causes a resonance, resulting in destruction of the MOSFET for synchronous control. <P>SOLUTION: In the insulated DC-DC converter which has a transformer (T1) for voltage conversion and controls transistors (M1-M4) for driving the primary coil of the transformer and transistors (M5 and M6) for synchronous control with a common circuit (20), the above-mentioned common control circuit is arranged on the secondary side of the transformer, and not primary grounding potential (GND1) but secondary grounding potential (GND2) is given to operate it, and the primary transistors are supplied with control signals via an insulated signal transmission means. <P>COPYRIGHT: (C)2004,JPO

Description

【０００１】
【発明の属する技術分野】
本発明は、電圧変換用トランスを備えたスイッチング電源装置さらにはトランスの一次側と二次側の回路が電気的に絶縁された絶縁型ＤＣ−ＤＣコンバータ（直流−直流変換型電源装置）に関し、特に２次側の共振電流の防止に利用して有効な技術に関する。
【０００２】
【従来の技術】
従来、ＤＣ−ＤＣコンバータのひとつに絶縁型ＤＣ−ＤＣコンバータと呼ばれるものがある。絶縁型ＤＣ−ＤＣコンバータは一次側と二次側の絶縁耐圧が数１０００Ｖ以上あることが要求されるため、一次側の回路と二次側の回路にそれぞれ互いに絶縁された別個の基準電位（接地電位）が用いられる。
【０００３】
本発明者等は、負荷変動が比較的大きな電話交換機のようなシステムに好適なスイッチング電源装置を開発するに当たり、図５に示すような絶縁型ＤＣ−ＤＣコンバータについて検討した。
図５の絶縁型ＤＣ−ＤＣコンバータは、ＭＯＳＦＥＴ　Ｍ１〜Ｍ４からなるブリッジ型のスイッチング回路１０でトランスＴ１の１次側コイルを交流駆動し、２次側コイルに誘起される交流電圧をダイオードとして作用する同期制御用ＭＯＳＦＥＴ　Ｍ５，Ｍ６で整流することで、１次側コイルと２次側コイルの巻線比に応じた所望の直流電圧を発生させるようにした電源回路である。本来ダイオードを用いればよい整流素子としてＭＯＳＦＥＴ　Ｍ５，Ｍ６を使用しているのは、ダイオードの順方向電圧による電力損失を減らすことができるためである。ちなみに、ＭＯＳＦＥＴのオン抵抗によるソース・ドレイン間電圧降下はダイオードの順方向電圧よりも小さくすることが可能である。
【０００４】
【発明が解決しようとする課題】
図６は、１次側コイルを交流駆動するスイッチＭＯＳＦＥＴ　Ｍ１〜Ｍ４をオン，オフ制御する信号ＯＵＴ−Ａ〜ＯＵＴ−Ｄと、同期制御用ＭＯＳＦＥＴ　Ｍ５，Ｍ６を制御する信号ＯＵＴ−Ｅ，ＯＵＴ−Ｆのタイミングを示す。図６に示すように、図５のような絶縁型ＤＣ−ＤＣコンバータにおいては、１次側コイルに矢印Ａの方向の電流を流すときには同期制御用ＭＯＳＦＥＴ　Ｍ５を、また１次側コイルに矢印Ａと逆向きに電流を流すときには同期制御用ＭＯＳＦＥＴ　Ｍ６をオフさせないと、２次側コイルが短絡状態となって１次側コイルから２次側コイルへ電力が伝達されないのみならず、ＭＯＳＦＥＴ　Ｍ１，Ｍ４，Ｍ５，Ｍ６またはＭ２，Ｍ３，Ｍ５，Ｍ６に過電流が流れて素子が破壊されてしまうおそれがある。
【０００５】
同期制御用ＭＯＳＦＥＴ　Ｍ５，Ｍ６をオフさせるタイミングは１次側コイルを駆動するＭＯＳＦＥＴ　Ｍ１〜Ｍ４のオン／オフ・タイミングに合わせる必要があるので、同期制御用ＭＯＳＦＥＴ　Ｍ５，Ｍ６を制御する制御信号ＯＵＴ−Ｅ，ＯＵＴ−Ｆは駆動信号ＯＵＴ−Ａ〜ＯＵＴ−Ｄを生成する制御回路２０で生成するのが望ましい。
【０００６】
しかしながら、一次側の基準電位と二次側の基準電位は絶縁されているため、制御回路２０で生成された制御信号ＯＵＴ−Ｅ，ＯＵＴ−Ｆを同期制御用ＭＯＳＦＥＴ　Ｍ５，Ｍ６のゲート端子に伝達するためにはパルス・トランスＴ２が必要である。また、トランスを使用すると制御信号は交流波形として伝達されるため、パルス・トランスＴ２の２次側交流電圧で同期制御用ＭＯＳＦＥＴ　Ｍ５，Ｍ６のゲートを直接駆動することはできず、２次側コイルの電圧を一旦ブーストする必要がある。
そこで、図５の回路では、クランプ用ダイオードＤ１，Ｄ２とブースト用の容量Ｃ２，Ｃ３を有する回路と、ブーストされた信号のレベルを弁別する回路とを設けて、同期制御用ＭＯＳＦＥＴ　Ｍ５，Ｍ６を制御する構成を採用した。
【０００７】
しかしながら、図５の回路においては、例えば１次側の入力電圧ＶＩＮの遮断等により制御回路２０が制御を停止した際に、図７（Ａ），（Ｂ）のように制御信号ＯＵＴ−Ｅ，ＯＵＴ−Ｆの出力がロウレベルに落ちてもブースト用の容量Ｃ２，Ｃ３に電荷が残ってしまう。そして、図５の回路ではこの電荷を引き抜くパスがないため、図７（Ｇ），（Ｈ）のように同期制御用ＭＯＳＦＥＴ　Ｍ５，Ｍ６のゲート電圧ＶＧ５，ＶＧ６がハイレベルに維持されて、Ｍ５，Ｍ６がオンされっぱなしになる。
その結果、出力端子間の平滑容量ＣＳとトランスＴ１の２次側コイルと直列のチョークコイルＬ１，Ｌ２からなるＬＣ回路が共振を起こして、２次側の同期制御用ＭＯＳＦＥＴ　Ｍ５，Ｍ６が破壊されるおそれがあるとともに、共振動作で出力端子間に負の電圧が発生しこの負電圧が印加されることで負荷としての回路やＩＣが破壊されるおそれがあることが分かった。
【０００８】
また、図５の回路においては、例えば軽負荷時等に２次側の同期制御をオフすることで負荷に過大な電圧が印加されないようにする制御が考えられるが、かかる制御のため図７（Ａ），（Ｂ）のように制御信号ＯＵＴ−Ｅ，ＯＵＴ−Ｆを途中でロウレベルに固定したとしても図５の回路ではブースト用の容量Ｃ２，Ｃ３に電荷が残ってしまう。そのため、２次側の同期制御用ＭＯＳＦＥＴ　Ｍ５，Ｍ６のゲート電圧ＶＧ５，ＶＧ６がハイレベルに維持されてＭ５，Ｍ６がオンされっぱなしになって、トランスＴ１の２次側コイルがショートされた状態で１次側コイルが駆動され続けて１次側の駆動用ＭＯＳＦＥＴ　Ｍ１〜Ｍ４の破壊、１次側コイルの焼損といった問題が発生するおそれがある。
【０００９】
さらに、図５の回路においては、電源立上げ時に負荷となる回路やＩＣに急激に大きな電圧が印加されて素子が破壊されるのを防止するため、１次側コイルから２次側コイルに伝達される電力を徐々に増大させるいわゆるソフトスタートと呼ばれる制御が行なわれることがある。このソフトスタート制御では、２次側の同期制御用ＭＯＳＦＥＴ　Ｍ５，Ｍ６はオフさせたまま１次側コイルの駆動を行なうことが考えられる。
【００１０】
かかる方式を採用した電源システムにおいても、ユーザによる誤操作や落雷等の事故の発生で電源の遮断と再投入が短い時間に行なわれたとすると、電源遮断で制御信号ＯＵＴ−Ｅ，ＯＵＴ−Ｆの出力が停止してブースト用の容量Ｃ２，Ｃ３に電荷が残り同期制御用ＭＯＳＦＥＴ　Ｍ５，Ｍ６のゲート電圧がハイレベルに維持されてオンになっている状態で、電源が再投入されて制御回路がソフトスタートを開始すると、トランスＴ１の２次側コイルがショートされた状態で１次側コイルが駆動され続けて１次側の駆動用ＭＯＳＦＥＴ　Ｍ１〜Ｍ４の破壊、１次側コイルの焼損といった問題が発生するおそれがある。
【００１１】
この発明の目的は、同期制御用トランジスタがオンされたまま制御が停止されて２次側のＬＣ回路が共振を起こして同期制御用トランジスタが破壊されるのを回避できる信頼性の高い絶縁型ＤＣ−ＤＣコンバータを提供することにある。
この発明の他の目的は、軽負荷時等に２次側の同期制御をオフすることで負荷に過大な電圧が印加されないようにする制御方式を採用した場合において、同期制御用ＭＯＳＦＥＴがオンされトランスＴ１の２次側コイルがショートされた状態で１次側コイルが駆動され続けて１次側の駆動用トランジスタが破壊されたり１次側コイルが焼損したりするのを回避できる信頼性の高い絶縁型ＤＣ−ＤＣコンバータを提供することにある。
【００１２】
この発明のさらに他の目的は、２次側の同期制御用トランジスタをオフさせたまま１次側コイルの駆動を行なうソフトスタート制御を採用した場合において、電源の瞬断あるいは電源の一時的な低下が発生しても、同期制御用トランジスタがオンされトランスＴ１の２次側コイルがショートされた状態で１次側コイルが駆動され続けて１次側の駆動用トランジスタが破壊されたり１次側コイルが焼損したりするのを回避できる信頼性の高い絶縁型ＤＣ−ＤＣコンバータを提供することにある。
この発明の前記ならびにそのほかの目的と新規な特徴については、本明細書の記述および添付図面から明らかになるであろう。
【００１３】
【課題を解決するための手段】
本願において開示される発明のうち代表的なものの概要を説明すれば、下記のとおりである。
すなわち、本発明は、電圧変換用トランスを有し該トランスの１次側コイルを駆動するトランジスタおよび２次側の同期制御用トランジスタを共通の制御回路で制御する絶縁型ＤＣ−ＤＣコンバータにおいて、上記共通制御回路をトランスの２次側に配置して、１次側基準電位（１次側接地電位）ではなく２次側基準電位（２次側接地電位）を与えて動作させるとともに、１次側駆動トランジスタには絶縁型信号伝達手段を介して制御信号を供給するように構成したものである。
【００１４】
上記した手段によれば、制御回路が２次側に配置されているので、同期制御用トランジスタを制御するための制御信号を絶縁型信号伝達手段を介することなく制御回路から直接与えることができるようになるため、ブースト回路を用いることなく同期整流制御動作停止時に同期制御用トランジスタをオフさせることができ、これにより２次側の整流回路が電圧変換用トランスの２次側コイルと直列に接続されたチョークコイルを有する場合、同期制御用トランジスタがオンされたまま制御が停止されて２次側の整流回路を構成するチョークコイルと平滑容量とからなるＬＣ回路が共振を起こし、同期制御用トランジスタが破壊されるのを防止することができる。
【００１５】
また、軽負荷時等に２次側の同期制御をオフすることで負荷に過大な電圧が印加されないようにする制御方式を採用した電源装置や、２次側の同期制御用ＭＯＳＦＥＴをオフさせたまま１次側コイルの駆動を行なうソフトスタート制御を採用した電源装置で電源の瞬断あるいは電源の一時的な低下が発生した場合に、同期制御用トランジスタをオフしたいときに確実にオフさせることができるため、同期制御用ＭＯＳＦＥＴがオンされトランスの２次側コイルがショートされた状態で１次側コイルが駆動されるのを防止することができる。
【００１６】
【発明の実施の形態】
以下、本発明の好適な実施例を図面に基づいて説明する。
図１は、本発明に係る絶縁型ＤＣ−ＤＣコンバータの第１の実施例を示す。図において、Ｔ１は電圧変換用のトランス、１０はこのトランスＴ１の１次側コイルを交流駆動するスイッチング回路、２０はこのスイッチング回路１０を駆動制御するＩＣ化された制御回路、３０はトランスＴ１の２次側コイルに誘起される交流電圧を整流して直流電圧に変換する全波整流回路、５０は変換された直流電圧の電圧レベルを検出し上記制御回路２０にフィードバックする出力電圧検出回路、ＣＢは整流回路３０で整流された電圧を平滑する平滑容量、ＤＲＶ５，ＤＲＶ６は制御回路からの制御信号ＯＵＴ−Ｅ，ＯＵＴ−Ｆを受けて整流回路３０を構成する同期制御用ＭＯＳＦＥＴ　Ｍ５，Ｍ６を駆動するゲート・ドライバ回路、６０は４８Ｖの入力直流電圧を受けて上記制御回路２０等に必要な１２Ｖのような直流電源電圧Ｖｃｃ１，Ｖｃｃ２を生成して供給するスイッチング・レギュレータからなる補助電源回路である。また、ＲＬは負荷となる回路やＩＣを等価抵抗として表わしたものである。
【００１７】
なお、ゲート・ドライバ回路ＤＲＶ５，ＤＲＶ６を省略して制御回路２０からの制御信号で同期制御用ＭＯＳＦＥＴ　Ｍ５，Ｍ６を直接駆動することもできる。また、同期制御用ＭＯＳＦＥＴ　Ｍ５，Ｍ６を制御回路２０と同一の半導体チップ上に形成することも可能である。
【００１８】
上記スイッチング回路１０は、４８Ｖのような直流電圧が印加される電圧入力端子ＶＩＮと１次側基準電位（接地電位）ＧＮＤ１との間に直列に接続されたＮチャネルＭＯＳＦＥＴ　Ｍ１，Ｍ２およびＭ３，Ｍ４と、これらのスイッチＭＯＳＦＥＴ　Ｍ１〜Ｍ４のゲート端子をそれぞれ駆動するＩＣ化されたゲート・ドライバ回路ＤＲＶ１〜ＤＲＶ４とから構成されている。
【００１９】
制御回路２０は、２次側基準電位ＧＮＤ２が接地電位とされ、図２（Ａ）〜（Ｄ）に示すようなタイミングの制御信号ＯＵＴ−Ａ〜ＯＵＴＤを出力してスイッチＭＯＳＦＥＴ　Ｍ１〜Ｍ４のうちＭ１とＭ４をオン、またはＭ２とＭ３をオンさせることでトランスＴ１の１次側コイルに順方向の電流と逆方向の電流を交互に流して交流駆動する。
【００２０】
また、制御回路２０は、図２に示されているように、トランスＴ１の１次側コイルの駆動ＭＯＳＦＥＴ　Ｍ１〜Ｍ４の制御信号ＯＵＴ−Ａ〜ＯＵＴ−Ｄによるオン・オフ動作に合わせて制御信号ＯＵＴ−Ｅ，ＯＵＴ−Ｆをロウレベルに変化させて、２次側の同期制御用ＭＯＳＦＥＴ　Ｍ５，Ｍ６をオン、オフさせる同期整流制御を行なう。なお、この実施例のＤＣ−ＤＣコンバータでは、１次側コイルの駆動ＭＯＳＦＥＴ
Ｍ１〜Ｍ４の１サイクル期間に対するオン期間の割合は最大５０％である。
【００２１】
また、この実施例では、トランスＴ１の１次側コイルと直列に電流センス用のトランスＴ２が接続され、この電流センス用トランスＴ２の２次側コイルの一端が制御回路２０に、また他端が２次側基準電位ＧＮＤ２に接続され、制御回路２０はトランスＴ２により電流−電圧変換された検出電圧を受けて、１次側コイルの駆動電流の値が所望の範囲に入るように制御信号ＯＵＴ−Ａ〜ＯＵＴ−Ｄの立上り／立下りタイミングとパルス幅を調節する。さらに、制御回路２０は、電流センス用トランスＴ２の２次側コイルの検出電圧に基づいて負荷が非常に軽くなったと判断した場合には、トランスＴ１の２次側の同期制御を停止して同期制御用ＭＯＳＦＥＴ　Ｍ５，Ｍ６をオフさせる制御も行なう。
【００２２】
ゲート・ドライバ回路ＤＲＶ１〜ＤＲＶ４には、制御回路２０から出力される制御信号ＯＵＴ−Ａ〜ＯＵＴ−Ｄがパルス・トランスＴ３，Ｔ４を介して入力されている。より詳細には、パルス・トランスＴ３，Ｔ４は各々２次側コイルが２つずつ設けられており、制御信号ＯＵＴ−Ａ，ＯＵＴ−Ｂがパルス・トランスＴ３の１次側コイルに印加され、制御信号ＯＵＴ−Ｃ，ＯＵＴ−Ｄがパルス・トランスＴ４の１次側コイルに印加されるとともに、パルス・トランスＴ３の２次側コイルＴＬ１，ＴＬ２の一方の端子がゲート・ドライバ回路ＤＲＶ１，ＤＲＶ２の入力端子に接続され、パルス・トランスＴ４の２次側コイルＴＬ３，ＴＬ４の他方の端子がゲート・ドライバ回路ＤＲＶ３，ＤＲＶ４の入力端子に接続されている。
【００２３】
図１において、コイルＴＬ１（ＴＬ３）とＴＬ２（ＴＬ４）に付されている「・」の位置が異なるのは、コイルＴＬ１（ＴＬ３）とＴＬ２（ＴＬ４）とでは巻線の向きが逆であることを意味している。このように、２次側に巻線の向きが逆である２つのコイルＴＬ１，ＴＬ２とＴＬ３，ＴＬ４を設けることにより、同一の制御信号から互いに逆相の信号を生成することができ、これによってスイッチＭＯＳＦＥＴ　Ｍ１とＭ２、またＭ３とＭ４がそれぞれ同時にオン状態またはオフ状態に制御される。
【００２４】
また、上記のようにパルス・トランスＴ３，Ｔ４を介してスイッチＭＯＳＦＥＴ　Ｍ１〜Ｍ４の制御信号を伝達することにより、スイッチング回路１０の基準電位と制御回路２０の基準電位が絶縁されていても正しい制御動作を行なわせることができる。さらに、この実施例では、上記パルス・トランスＴ３，Ｔ４の２次側コイルＴＬ１〜ＴＬ４の他方の端子は、対応するスイッチＭＯＳＦＥＴ　Ｍ１〜Ｍ４のソース端子に接続されている。そのため、スイッチＭＯＳＦＥＴ　Ｍ１〜Ｍ４のゲート電圧のピーク値を、ソース電圧を基準にして設定することができ、各ＭＯＳＦＥＴ　Ｍ１〜Ｍ４のドレイン電流が同一になるように駆動することができる。
【００２５】
一方、ゲート・ドライバ回路ＤＲＶ１〜ＤＲＶ４の接地端子は対応するスイッチＭＯＳＦＥＴ　Ｍ１〜Ｍ４のソース端子に接続されるとともに、ゲート・ドライバ回路ＤＲＶ１〜ＤＲＶ４の電源電圧端子には補助電源回路６０から出力される１２Ｖの電圧がダイオードＤ０を介して供給され、電流の逆流が防止されるようにされている。さらに、ゲート・ドライバ回路ＤＲＶ１〜ＤＲＶ４の電源電圧端子と接地端子との間には容量Ｃ０が接続されており、ＭＯＳＦＥＴ　Ｍ１〜Ｍ４のソース電位が変動してもゲート・ドライバ回路ＤＲＶ１〜ＤＲＶ４の電源電圧端子−接地端子間電圧はほぼ一定に保たれ、逆バイアスの電圧が印加されるのが回避されるようになっている。
【００２６】
また、上記制御回路２０には、抵抗Ｒ１１，Ｒ１２からなる抵抗分割回路で分圧された電圧が入力される端子Ｐ１と、該端子の電圧が所定のレベル以上か以下かを検出する電圧比較回路２１とが設けられており、入力電圧ＶＩＮが所定のレベル以下に下がるとスイッチングＭＯＳＦＥＴ　Ｍ１〜Ｍ４の駆動を停止する。なお、図示しないが、制御回路２０には外部から入力される指令信号ＲＭＴでトランスＴ１の１次側の駆動と２次側の同期制御を停止するリモート制御端子Ｐ２が設けられている。
【００２７】
さらに、この実施例の制御回路２０には、電源の立上り時に１次側コイルの駆動電流を徐々に増加させて、負荷に急激に大きな電流が供給されるのを回避するソフトスタート回路２２が設けられている。ソフトスタート回路２２は、例えば定電流源とそれにより充電される容量素子とからなる積分回路のような出力電圧を徐々に増加させる回路を備え、電源投入後容量素子の充電電圧が所定のレベルに達するまではスイッチＭＯＳＦＥＴ　Ｍ１〜Ｍ４のオン時間を徐々に長くさせる、つまりトランスＴ１の１次側コイルの電流が次第に増加するような制御を行なう。
【００２８】
また、特に制限されるものでないが、この実施例では、ソフトスタート制御の際には２次側の同期制御用ＭＯＳＦＥＴ　Ｍ５，Ｍ６をオン、オフ制御する信号ＯＵＴ−Ｅ，ＯＵＴ−Ｆをロウレベルに固定させる。かかる制御は、例えばソフトスタートの際にソフトスタート回路２２から出力される信号で制御信号ＯＵＴ−Ｅ，ＯＵＴ−Ｆを出力するバッファ回路を制御して、その出力をロウレベルに固定させるような構成をとることで実現できる。
【００２９】
この実施例においては、全波整流回路３０は、トランスＴ１の２次側コイルの両端子と一方の出力端子ＶＯＵＴ（＋）との間にそれぞれ接続されたチョークコイルＬ１，Ｌ２と、トランスＴ１の２次側コイルの両端子と他方の出力端子ＶＯＵＴ（−）との間にそれぞれ接続された同期制御用ＭＯＳＦＥＴ　Ｍ５，Ｍ６とにより構成されている。
【００３０】
この実施例の全波整流回路３０は、１次側コイルの交流駆動で２次側コイルに誘起された交流電圧により、２次側コイルに図１の上から下へ向って電流が流れるときはチョークコイルＬ２を通して、また２次側コイルに下から上に向って電流が流れるときはチョークコイルＬ１を通してそれぞれ出力端子ＶＯＵＴ（＋）へ電流が流されることにより整流が行なわれ、１次側コイルと２次側コイルの巻線比に応じた直流電圧を発生させる。
【００３１】
チョークコイルＬ１，Ｌ２および同期制御用ＭＯＳＦＥＴ　Ｍ５，Ｍ６をダイオードに置き換えても全波整流回路として機能することができる。ダイオードの代わりに同期制御用ＭＯＳＦＥＴ　Ｍ５，Ｍ６を用いているのは、ダイオードの順方向電圧で損失が生じるのを減らすためである。本発明は、同期制御用ＭＯＳＦＥＴ　Ｍ５またはＭ６のいずれか一方が存在する場合に適用すると有効であり、例えばＭ５がＭＯＳＦＥＴの場合にはＭ６およびＬ１，Ｌ２の代わりにダイオードを使用するようにしてもよい。
【００３２】
出力電圧検出回路５０は、出力端子ＶＯＵＴ（＋），ＶＯＵＴ（−）間に直列に接続された抵抗Ｒ３，Ｒ４からなる抵抗分割回路により構成されており、抵抗Ｒ３，Ｒ４の抵抗比で出力電圧を分割した電圧が制御回路２０に供給され、制御回路２０は内部の電圧比較回路により参照電圧と比較することで所望のレベル以上の出力電圧が発生しているか知ることができるようになっている。
【００３３】
この実施例のＤＣ−ＤＣコンバータにおいては、電源遮断や軽負荷時に制御信号ＯＵＴ−Ｅ，ＯＵＴ−Ｆが共にロウレベルに固定されると、ゲート・ドライバ回路ＤＲＶ５，ＤＲＶ６の出力が共にロウレベルにされて直ちに同期制御用ＭＯＳＦＥＴ　Ｍ５，Ｍ６がオフの状態にされる。その結果、整流回路のチョークコイルＬ１，Ｌ２と平滑容量ＣＢが共振動作を起こすのを回避することができる。
【００３４】
また、軽負荷時等に２次側の同期制御をオフすることで負荷に過大な電圧が印加されないようにする制御方式を採用した場合や２次側の同期制御用ＭＯＳＦＥＴをオフさせたまま１次側コイルの駆動を行なうソフトスタート制御を採用し電源の瞬断あるいは電源の一時的な低下が発生した場合に、制御信号ＯＵＴ−Ｅ，ＯＵＴ−Ｆがロウレベルに変化されると直ちに同期制御用ＭＯＳＦＥＴ　Ｍ５，Ｍ６がオフされるため、同期制御用ＭＯＳＦＥＴがオンされトランスＴ１の２次側コイルがショートされた状態で１次側コイルが駆動され続けて、１次側の駆動用ＭＯＳＦＥＴが破壊されたり１次側コイルが焼損したりするおそれがない。
【００３５】
しかも、図５の回路と比較すると明らかなように、本実施例のＤＣ−ＤＣコンバータは制御信号ＯＵＴ−Ｅ，ＯＵＴ−Ｆを伝達するパルス・トランスや検波された信号のレベルを押し上げるブースト容量などが不用であるため、２次側の回路が簡単になる。また、出力レベルを検出する電圧比較回路や検出したレベルを制御回路２０へフィードバックするためのフォトカプラも不用であるため、部品点数が減少しシステムを安価に構成することができるという利点がある。
【００３６】
図３には、上記補助電源回路６０の構成例が示されている。図３に示されているように、この実施例の補助電源回路６０は、トランスＴ０と、該トランスＴ０の一次側コイルと直列に接続された同期制御用トランジスタＱ０と、該トランジスタＱ０をオン、オフ制御するパルス生成回路ＰＬＧとを有するスイッチング・レギュレータにより構成されている。
【００３７】
通常のスイッチング・レギュレータとの違いは、トランスＴ０はコアを共通にする２個の２次側コイルＴＬ１１，ＴＬ１２を備えており、コイルＴＬ１１の一端はダイオードＤ１１を介して出力端子ＯＵＴ１に、また他端は１次側の基準電圧ＧＮＤ１に接続され、コイルＴＬ１２の一端はダイオードＤ１２を介して出力端子ＯＵＴ２に、また他端は２次側の基準電圧ＧＮＤ２に接続されている点にある。このように、基準電圧が分離して設けられることにより、補助電源回路６０内においても、１次側回路と２次側回路との電気的な絶縁が確保される。
【００３８】
図４には、上記ゲート・ドライバ回路ＤＲＶ５（ＤＲＶ６）の電源系と、制御信号ＯＵＴ−Ｅ，ＯＵＴ−Ｆを出力する制御回路２０側のバッファ回路ＢＦＦの電源系の構成例が示されている。図４に示されているように、制御回路２０には、前記補助電源回路６０から供給される１２Ｖのような電圧Ｖｃｃ２から内部論理回路２３の動作に必要な５Ｖのような内部電源電圧Ｖｃｃｉを生成する内部電源回路２４が設けられており、バッファ回路ＢＦＦはこの内部電源電圧Ｖｃｃｉにより動作する。なお、この内部電源電圧Ｖｃｃｉは参照電圧Ｖｒｅｆとしてチップ外部へ出力される。
【００３９】
上記内部電源回路２４は、１２Ｖの外部電源電圧Ｖｃｃ２が印加される端子と２次側の基準電圧ＧＮＤ２が印加される端子との間に直列に接続されたトランジスタＱ１および抵抗Ｒ２１，Ｒ２２と、抵抗Ｒ２１，Ｒ２２の接続ノードの電位と２．５Ｖのような定電圧とを比較してトランジスタＱ１のベース電圧を制御する差動アンプＡＭＰとから構成されている。一方、外部のゲート・ドライバ回路ＤＲＶ５（ＤＲＶ６）は、基準電圧ＧＮＤ２を接地電位とし５Ｖのような定電圧ＶＢ１を電源電圧として動作するように構成される。
【００４０】
以上本発明者によってなされた発明を実施例に基づき具体的に説明したが、本発明は上記実施例に限定されるものではなく、その要旨を逸脱しない範囲で種々変更可能であることはいうまでもない。例えば上記実施例では、制御回路２０がソフトスタート回路２２を有すると説明したが、ソフトスタート回路を備えていない場合にも本発明を適用することができる。また、スイッチング回路１０を構成するゲート・ドライバ回路ＤＲＶ１〜ＤＲＶ４を省略して、パルス・トランスＴ３，Ｔ４の２次側コイルの誘起電圧で直接スイッチＭＯＳＦＥＴ　Ｍ１〜Ｍ４を駆動するように構成しても良い。
【００４１】
さらに、実施例では、１次側と２次側の絶縁を保ったままそれぞれの側の回路を制御するため信号の伝達にパルス・トランスを用いたが、パルス・トランスの代わりにフォトカプラを用いて１次側と２次側との間の信号の伝達を行なうようにして構成しても良い。
【００４２】
【発明の効果】
本願において開示される発明のうち代表的なものによって得られる効果を簡単に説明すれば下記のとおりである。
すなわち、本発明に従うと、制御回路が２次側に配置されているので、同期制御用トランジスタを制御するための制御信号を絶縁型信号伝達手段を介することなく制御回路から直接与えることができるようになるため、ブースト回路を用いることなく同期整流制御動作停止時に同期制御用トランジスタをオフさせることができ、これにより２次側の整流回路が電圧変換用トランスの２次側コイルと直列に接続されたチョークコイルを有する場合、同期制御用トランジスタがオンされたまま制御が停止されて２次側の整流回路を構成するチョークコイルと平滑容量とからなるＬＣ回路が共振を起こし、同期制御用トランジスタが破壊されるのを防止することができる信頼性の高い絶縁型ＤＣ−ＤＣコンバータを実現することができる。
【００４３】
また、本発明を適用すると、同期制御用トランジスタをオフしたいときに確実にオフさせることができるため、軽負荷時等に２次側の同期制御をオフすることで負荷に過大な電圧が印加されないようにする制御方式を採用した場合や２次側の同期制御用ＭＯＳＦＥＴをオフさせたまま１次側コイルの駆動を行なうソフトスタート制御を採用し電源の瞬断あるいは電源の一時的な低下が発生した場合に、同期制御用ＭＯＳＦＥＴがオンされトランスＴ１の２次側コイルがショートされた状態で１次側コイルが駆動され続けて、１次側の駆動用ＭＯＳＦＥＴが破壊されたり１次側コイルが焼損したりするおそれのない信頼性の高い絶縁型ＤＣ−ＤＣコンバータおよび電源制御用ＩＣを実現できるようになる。
【図面の簡単な説明】
【図１】本発明に係る絶縁型ＤＣ−ＤＣコンバータの一実施例を示す概略構成図である。
【図２】実施例の絶縁型ＤＣ−ＤＣコンバータにおける１次側コイルの駆動制御信号と２次側同期制御信号の電圧波形を示す波形図である。
【図３】実施例の絶縁型ＤＣ−ＤＣコンバータに用いられる補助電源回路の構成例を示す概略構成図である。
【図４】実施例の絶縁型ＤＣ−ＤＣコンバータを構成する制御回路の同期制御信号の出力回路とその電源回路およびゲート・ドライバ回路の電源系の構成を示す回路構成図である。
【図５】本発明に先立って検討した絶縁型ＤＣ−ＤＣコンバータを示す回路構成図である。
【図６】図５の絶縁型ＤＣ−ＤＣコンバータにおける１次側コイルの駆動制御信号と２次側同期制御信号および同期ＭＯＳ駆動回路内の各部の電圧波形を示す波形図である。
【図７】図５の絶縁型ＤＣ−ＤＣコンバータにおける同期制御信号が共にロウレベルに変化された場合のリセット回路と同期ＭＯＳ駆動回路内の各部の電圧波形を示す波形図である。
【符号の説明】
１０　スイッチング回路
２０　制御回路（ＩＣ）
２１　電源レベル検出回路
２２　ソフトスタート回路
３０　整流回路
５０　出力レベル検出回路
Ｔ１　電圧変換用トランス
Ｔ２，Ｔ３　信号伝達用パルス・トランス
ＣＢ　平滑用コンデンサ
Ｌ１，Ｌ２　整流用チョークコイル[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a switching power supply device having a voltage conversion transformer, and further relates to an insulation type DC-DC converter (DC-DC conversion type power supply device) in which the primary and secondary circuits of the transformer are electrically insulated. In particular, the present invention relates to a technique that is effective for preventing a resonance current on the secondary side.
[0002]
[Prior art]
Conventionally, there is a so-called isolated DC-DC converter as one of the DC-DC converters. Since the insulation type DC-DC converter is required to have a withstand voltage of several thousand volts or more on the primary side and the secondary side, separate reference potentials (grounded) are insulated from the primary side circuit and the secondary side circuit, respectively. Potential) is used.
[0003]
The present inventors studied an isolated DC-DC converter as shown in FIG. 5 in developing a switching power supply device suitable for a system such as a telephone exchange having a relatively large load fluctuation.
The insulated DC-DC converter of FIG. 5 drives the primary coil of the transformer T1 with an AC by a bridge-type switching circuit 10 composed of MOSFETs M1 to M4, and acts on the AC voltage induced in the secondary coil as a diode. This is a power supply circuit that generates a desired DC voltage according to the turns ratio between the primary side coil and the secondary side coil by rectifying the synchronous control MOSFETs M5 and M6. The reason why the MOSFETs M5 and M6 are used as the rectifying elements which should originally use a diode is that power loss due to the forward voltage of the diode can be reduced. Incidentally, the voltage drop between the source and the drain due to the ON resistance of the MOSFET can be made smaller than the forward voltage of the diode.
[0004]
[Problems to be solved by the invention]
FIG. 6 shows signals OUT-A to OUT-D for controlling ON / OFF of the switch MOSFETs M1 to M4 for AC driving the primary coil, and signals OUT-E and OUT- for controlling the synchronous control MOSFETs M5 and M6. The timing of F is shown. As shown in FIG. 6, in the insulated DC-DC converter as shown in FIG. 5, when a current in the direction of arrow A flows through the primary coil, the synchronous control MOSFET M5 is used. When a current is passed in the opposite direction, the MOSFET M6 for synchronization control must be turned off, so that the secondary coil is short-circuited and power is not transmitted from the primary coil to the secondary coil. , M5, M6 or M2, M3, M5, M6 may be damaged by overcurrent.
[0005]
The timing for turning off the synchronous control MOSFETs M5 and M6 needs to be adjusted to the on / off timing of the MOSFETs M1 to M4 for driving the primary side coil. Therefore, the control signal OUT- for controlling the synchronous control MOSFETs M5 and M6. E and OUT-F are preferably generated by the control circuit 20 that generates the drive signals OUT-A to OUT-D.
[0006]
However, since the reference potential on the primary side and the reference potential on the secondary side are insulated, the control signals OUT-E and OUT-F generated by the control circuit 20 are transmitted to the gate terminals of the synchronous control MOSFETs M5 and M6. This requires a pulse transformer T2. When a transformer is used, the control signal is transmitted as an AC waveform. Therefore, it is not possible to directly drive the gates of the synchronous control MOSFETs M5 and M6 with the secondary AC voltage of the pulse transformer T2. Voltage needs to be boosted once.
Therefore, in the circuit of FIG. 5, a circuit having diodes D1 and D2 for clamping and capacitors C2 and C3 for boosting and a circuit for discriminating the level of the boosted signal are provided, and the MOSFETs M5 and M6 for synchronous control are provided. The control structure was adopted.
[0007]
However, in the circuit of FIG. 5, when the control circuit 20 stops the control due to, for example, interruption of the input voltage VIN on the primary side, the control signals OUT-E, OUT-E, as shown in FIGS. Even if the output of OUT-F falls to a low level, charges remain in the boost capacitors C2 and C3. Since there is no path for extracting the electric charge in the circuit of FIG. 5, the gate voltages VG5 and VG6 of the synchronous control MOSFETs M5 and M6 are maintained at a high level as shown in FIGS. , M6 are kept on.
As a result, the LC circuit including the smoothing capacitor CS between the output terminals and the choke coils L1 and L2 in series with the secondary coil of the transformer T1 causes resonance, and the secondary synchronous control MOSFETs M5 and M6 are destroyed. It was also found that a negative voltage was generated between the output terminals during the resonance operation, and a circuit or an IC as a load could be broken by the application of the negative voltage.
[0008]
In the circuit shown in FIG. 5, for example, a control may be considered to prevent the application of an excessive voltage to the load by turning off the synchronous control on the secondary side at a light load or the like. Even if the control signals OUT-E and OUT-F are fixed to a low level in the middle as shown in FIGS. 5A and 5B, electric charges remain in the boost capacitors C2 and C3 in the circuit of FIG. Therefore, the gate voltages VG5 and VG6 of the secondary-side synchronous control MOSFETs M5 and M6 are maintained at a high level, M5 and M6 are kept on, and the secondary coil of the transformer T1 is short-circuited. Therefore, there is a possibility that the primary coil continues to be driven, and that the primary drive MOSFETs M1 to M4 are destroyed and that the primary coil is burned.
[0009]
Further, in the circuit of FIG. 5, the power is transmitted from the primary side coil to the secondary side coil in order to prevent a suddenly large voltage from being applied to a circuit or an IC serving as a load when the power is turned on and to break the elements. Control called so-called soft start for gradually increasing the power to be performed may be performed. In this soft start control, it is conceivable that the primary side coil is driven while the secondary side synchronous control MOSFETs M5 and M6 are turned off.
[0010]
Even in a power supply system employing this method, if the power supply is cut off and turned on again in a short time due to an accident such as an erroneous operation by a user or a lightning strike, the output of the control signals OUT-E and OUT-F is performed by the power cutoff. Is stopped, the charge remains in the boost capacitors C2 and C3, and the gate voltages of the synchronous control MOSFETs M5 and M6 are maintained at a high level and are turned on. When the start is started, the primary coil continues to be driven in a state where the secondary coil of the transformer T1 is short-circuited, causing problems such as destruction of the primary-side driving MOSFETs M1 to M4 and burning of the primary coil. There is a possibility that.
[0011]
SUMMARY OF THE INVENTION It is an object of the present invention to provide a highly reliable insulated DC which can prevent the synchronous control transistor from being turned on and the control being stopped to prevent the secondary side LC circuit from resonating and destroying the synchronous control transistor. -To provide a DC converter.
Another object of the present invention is to provide a synchronous control MOSFET which is turned on in a case where a control method for turning off the secondary-side synchronous control during a light load or the like so that an excessive voltage is not applied to the load is adopted. The primary coil is continuously driven in a state in which the secondary coil of the transformer T1 is short-circuited, so that the primary-side driving transistor is prevented from being damaged or the primary coil is burnt. An object of the present invention is to provide an isolated DC-DC converter.
[0012]
Still another object of the present invention is to employ a soft start control for driving a primary coil while keeping a secondary synchronous control transistor turned off, in the case of a momentary interruption of power supply or a temporary decrease in power supply. Occurs, the primary-side coil continues to be driven in a state where the synchronous control transistor is turned on and the secondary-side coil of the transformer T1 is short-circuited, and the primary-side drive transistor is destroyed or the primary-side coil is damaged. It is an object of the present invention to provide a highly reliable isolated DC-DC converter capable of avoiding burnout of a DC / DC converter.
The above and other objects and novel features of the present invention will become apparent from the description of the present specification and the accompanying drawings.
[0013]
[Means for Solving the Problems]
The outline of a representative invention among the inventions disclosed in the present application will be described as follows.
That is, the present invention relates to an insulation type DC-DC converter having a voltage conversion transformer, wherein a transistor for driving a primary coil of the transformer and a secondary synchronization control transistor are controlled by a common control circuit. A common control circuit is arranged on the secondary side of the transformer to operate by applying a secondary reference potential (secondary ground potential) instead of the primary reference potential (primary ground potential) and to operate the primary control circuit. The driving transistor is configured to supply a control signal via an insulating signal transmission unit.
[0014]
According to the above-described means, since the control circuit is disposed on the secondary side, a control signal for controlling the synchronous control transistor can be directly supplied from the control circuit without passing through the insulating signal transmission means. Therefore, the synchronous control transistor can be turned off when the synchronous rectification control operation is stopped without using the boost circuit, whereby the secondary rectifier circuit is connected in series with the secondary coil of the voltage conversion transformer. When the synchronous control transistor is turned on, the control is stopped while the synchronous control transistor is turned on, and the LC circuit including the choke coil and the smoothing capacitor forming the rectifier circuit on the secondary side causes resonance, and the synchronous control transistor is turned off. Destruction can be prevented.
[0015]
In addition, a power supply device adopting a control method for preventing an excessive voltage from being applied to the load by turning off the secondary side synchronous control at the time of light load or the like, and turning off the secondary side synchronous control MOSFET. In a power supply device that employs soft-start control that drives the primary coil as it is, it is possible to turn off the synchronous control transistor when it is desired to turn off the synchronous control transistor in the event of an instantaneous power interruption or a temporary drop in the power supply. Therefore, it is possible to prevent the primary coil from being driven when the synchronous control MOSFET is turned on and the secondary coil of the transformer is short-circuited.
[0016]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings.
FIG. 1 shows a first embodiment of an isolated DC-DC converter according to the present invention. In the figure, T1 is a transformer for voltage conversion, 10 is a switching circuit for AC driving the primary coil of the transformer T1, 20 is an integrated control circuit for driving and controlling the switching circuit 10, and 30 is a transformer for the transformer T1. A full-wave rectifier circuit for rectifying the AC voltage induced in the secondary coil and converting it to a DC voltage; 50, an output voltage detection circuit for detecting the voltage level of the converted DC voltage and feeding it back to the control circuit 20; Is a smoothing capacitor for smoothing the voltage rectified by the rectifier circuit 30, and DRV5 and DRV6 drive the synchronous control MOSFETs M5 and M6 constituting the rectifier circuit 30 by receiving the control signals OUT-E and OUT-F from the control circuit. The gate driver circuit 60 receives the input DC voltage of 48 V and receives a DC power supply voltage V such as 12 V necessary for the control circuit 20 and the like. This is an auxiliary power supply circuit including a switching regulator that generates and supplies cc1 and Vcc2. RL represents a circuit or an IC serving as a load as an equivalent resistance.
[0017]
Note that the gate driver circuits DRV5 and DRV6 may be omitted, and the control signals from the control circuit 20 may be used to directly drive the synchronous control MOSFETs M5 and M6. Further, the synchronous control MOSFETs M5 and M6 can be formed on the same semiconductor chip as the control circuit 20.
[0018]
The switching circuit 10 includes N-channel MOSFETs M1, M2 and M3, M4 connected in series between a voltage input terminal VIN to which a DC voltage such as 48 V is applied and a primary-side reference potential (ground potential) GND1. And gate driver circuits DRV1 to DRV4 which are ICs for driving the gate terminals of these switch MOSFETs M1 to M4, respectively.
[0019]
The control circuit 20 sets the secondary-side reference potential GND2 to the ground potential, outputs control signals OUT-A to OUTD at timings shown in FIGS. 2A to 2D, and outputs the control signals OUT-A to OUTD of the switch MOSFETs M1 to M4. When M1 and M4 are turned on or M2 and M3 are turned on, a forward current and a reverse current are alternately supplied to the primary coil of the transformer T1 to perform AC driving.
[0020]
Further, as shown in FIG. 2, the control circuit 20 controls the control signals OUT-A to OUT-D of the drive MOSFETs M1 to M4 of the primary coil of the transformer T1 in accordance with ON / OFF operations. OUT-E and OUT-F are changed to low level to perform synchronous rectification control to turn on and off the secondary-side synchronous control MOSFETs M5 and M6. In the DC-DC converter of this embodiment, the drive MOSFET of the primary coil
The ratio of the ON period to one cycle period of M1 to M4 is 50% at the maximum.
[0021]
In this embodiment, a current sensing transformer T2 is connected in series with the primary coil of the transformer T1, and one end of the secondary coil of the current sensing transformer T2 is connected to the control circuit 20 and the other end is connected to the control circuit 20. Connected to the secondary-side reference potential GND2, the control circuit 20 receives the detection voltage obtained by current-voltage conversion by the transformer T2, and controls the control signal OUT- so that the value of the drive current of the primary coil falls within a desired range. The rise / fall timing and pulse width of A to OUT-D are adjusted. Further, when the control circuit 20 determines that the load has become very light based on the detection voltage of the secondary coil of the current sensing transformer T2, the control circuit 20 stops the secondary side synchronous control of the transformer T1 and performs synchronization. Control for turning off the control MOSFETs M5 and M6 is also performed.
[0022]
Control signals OUT-A to OUT-D output from the control circuit 20 are input to the gate driver circuits DRV1 to DRV4 via pulse transformers T3 and T4. More specifically, each of the pulse transformers T3 and T4 is provided with two secondary coils, and control signals OUT-A and OUT-B are applied to the primary coil of the pulse transformer T3 to control the coil. The signals OUT-C and OUT-D are applied to the primary coil of the pulse transformer T4, and one terminal of the secondary coils TL1 and TL2 of the pulse transformer T3 is connected to the input of the gate driver circuits DRV1 and DRV2. The other terminals of the secondary coils TL3 and TL4 of the pulse transformer T4 are connected to input terminals of the gate driver circuits DRV3 and DRV4.
[0023]
In FIG. 1, the difference between the positions of “•” attached to the coils TL1 (TL3) and TL2 (TL4) is that the coils TL1 (TL3) and TL2 (TL4) have opposite winding directions. Means Thus, by providing two coils TL1, TL2 and TL3, TL4 whose winding directions are opposite to each other on the secondary side, signals having phases opposite to each other can be generated from the same control signal. The switch MOSFETs M1 and M2 and M3 and M4 are simultaneously controlled to be on or off.
[0024]
Further, by transmitting the control signals of the switch MOSFETs M1 to M4 via the pulse transformers T3 and T4 as described above, correct control can be performed even if the reference potential of the switching circuit 10 and the reference potential of the control circuit 20 are insulated. Operation can be performed. Further, in this embodiment, the other terminals of the secondary coils TL1 to TL4 of the pulse transformers T3 and T4 are connected to the source terminals of the corresponding switch MOSFETs M1 to M4. Therefore, the peak values of the gate voltages of the switch MOSFETs M1 to M4 can be set with reference to the source voltage, and the MOSFETs M1 to M4 can be driven so that the drain currents thereof are the same.
[0025]
On the other hand, the ground terminals of the gate driver circuits DRV1 to DRV4 are connected to the source terminals of the corresponding switch MOSFETs M1 to M4, and output from the auxiliary power supply circuit 60 to the power supply voltage terminals of the gate driver circuits DRV1 to DRV4. A voltage of 12 V is supplied through the diode D0 so as to prevent a reverse current from flowing. Further, a capacitor C0 is connected between the power supply voltage terminals of the gate driver circuits DRV1 to DRV4 and the ground terminal, so that the power supply of the gate driver circuits DRV1 to DRV4 does not change even if the source potentials of the MOSFETs M1 to M4 fluctuate. The voltage between the voltage terminal and the ground terminal is kept substantially constant, so that application of a reverse bias voltage is avoided.
[0026]
The control circuit 20 includes a terminal P1 to which the voltage divided by the resistance dividing circuit including the resistors R11 and R12 is input, and a voltage comparison circuit for detecting whether the voltage of the terminal is equal to or higher than a predetermined level. When the input voltage VIN falls below a predetermined level, the driving of the switching MOSFETs M1 to M4 is stopped. Although not shown, the control circuit 20 is provided with a remote control terminal P2 for stopping the primary side drive and the secondary side synchronous control of the transformer T1 by a command signal RMT input from the outside.
[0027]
Further, the control circuit 20 of this embodiment is provided with a soft start circuit 22 for gradually increasing the drive current of the primary side coil at the time of rising of the power supply so as to avoid suddenly supplying a large current to the load. Has been. The soft start circuit 22 includes a circuit for gradually increasing the output voltage, such as an integration circuit including a constant current source and a capacitor charged by the soft current source. Until it reaches, the on-time of the switch MOSFETs M1 to M4 is gradually increased, that is, control is performed so that the current of the primary coil of the transformer T1 gradually increases.
[0028]
Although not particularly limited, in this embodiment, during the soft start control, the signals OUT-E and OUT-F for turning on and off the secondary-side synchronous control MOSFETs M5 and M6 are set to low level. Fix it. This control is performed, for example, by controlling a buffer circuit that outputs control signals OUT-E and OUT-F with a signal output from the soft start circuit 22 at the time of soft start, and fixing the output to a low level. It can be realized by taking.
[0029]
In this embodiment, the full-wave rectifier circuit 30 includes choke coils L1 and L2 connected between both terminals of the secondary coil of the transformer T1 and one output terminal VOUT (+), respectively. Synchronous control MOSFETs M5 and M6 connected between both terminals of the secondary coil and the other output terminal VOUT (-), respectively.
[0030]
The full-wave rectifier circuit 30 of this embodiment is configured such that when an AC voltage is induced in the secondary coil by the AC drive of the primary coil, a current flows through the secondary coil from top to bottom in FIG. When a current flows from the bottom to the top through the choke coil L2 and the secondary side coil, rectification is performed by flowing the current to the output terminal VOUT (+) through the choke coil L1, and the rectification is performed. A DC voltage corresponding to the winding ratio of the secondary coil is generated.
[0031]
Even if the choke coils L1 and L2 and the synchronous control MOSFETs M5 and M6 are replaced with diodes, they can function as a full-wave rectifier circuit. The reason why the synchronous control MOSFETs M5 and M6 are used instead of the diode is to reduce the occurrence of loss in the forward voltage of the diode. The present invention is effective when applied to the case where one of the synchronous control MOSFETs M5 and M6 is present. For example, when M5 is a MOSFET, a diode may be used instead of M6 and L1 and L2. Good.
[0032]
The output voltage detection circuit 50 is configured by a resistance dividing circuit including resistors R3 and R4 connected in series between output terminals VOUT (+) and VOUT (-), and the output voltage is determined by the resistance ratio of the resistors R3 and R4. Is supplied to the control circuit 20, and the control circuit 20 can determine whether an output voltage equal to or higher than a desired level is generated by comparing the reference voltage with an internal voltage comparison circuit. .
[0033]
In the DC-DC converter of this embodiment, when the control signals OUT-E and OUT-F are both fixed to low level during power cutoff or light load, the outputs of the gate driver circuits DRV5 and DRV6 are both set to low level. Immediately, the synchronous control MOSFETs M5 and M6 are turned off. As a result, it is possible to prevent the choke coils L1 and L2 of the rectifier circuit and the smoothing capacitor CB from performing a resonance operation.
[0034]
Further, when a control method for turning off the secondary-side synchronous control at the time of light load or the like to prevent an excessive voltage from being applied to the load is employed, or when the secondary-side synchronous control MOSFET is kept off, Soft start control for driving the next side coil is employed, and in the case where the power supply is momentarily cut off or the power supply temporarily drops, the synchronous control is performed immediately when the control signals OUT-E and OUT-F are changed to the low level. Since the MOSFETs M5 and M6 are turned off, the synchronous control MOSFET is turned on and the primary coil continues to be driven while the secondary coil of the transformer T1 is short-circuited, and the primary drive MOSFET is destroyed. Or the primary coil is not burned.
[0035]
Further, as is apparent from comparison with the circuit of FIG. 5, the DC-DC converter of this embodiment has a pulse transformer for transmitting the control signals OUT-E and OUT-F, a boost capacitance for boosting the level of the detected signal, and the like. Is unnecessary, the circuit on the secondary side is simplified. Further, since a voltage comparison circuit for detecting the output level and a photocoupler for feeding back the detected level to the control circuit 20 are not required, there is an advantage that the number of parts is reduced and the system can be configured at low cost.
[0036]
FIG. 3 shows a configuration example of the auxiliary power supply circuit 60. As shown in FIG. 3, the auxiliary power supply circuit 60 of this embodiment includes a transformer T0, a synchronization control transistor Q0 connected in series with a primary coil of the transformer T0, and a transistor Q0 turned on. And a switching regulator having a pulse generation circuit PLG that performs off control.
[0037]
The difference from a normal switching regulator is that the transformer T0 has two secondary coils TL11 and TL12 having a common core, and one end of the coil TL11 is connected to an output terminal OUT1 via a diode D11 and to another output terminal OUT1. The terminal is connected to the primary-side reference voltage GND1, one end of the coil TL12 is connected to the output terminal OUT2 via the diode D12, and the other end is connected to the secondary-side reference voltage GND2. By providing the reference voltages separately in this manner, electrical insulation between the primary side circuit and the secondary side circuit is ensured even in the auxiliary power supply circuit 60.
[0038]
FIG. 4 shows a configuration example of the power supply system of the gate driver circuits DRV5 (DRV6) and the power supply system of the buffer circuit BFF of the control circuit 20 that outputs the control signals OUT-E and OUT-F. . As shown in FIG. 4, the control circuit 20 receives an internal power supply voltage Vcci such as 5 V necessary for the operation of the internal logic circuit 23 from a voltage Vcc2 such as 12 V supplied from the auxiliary power supply circuit 60. An internal power supply circuit 24 for generating is provided, and the buffer circuit BFF operates with the internal power supply voltage Vcci. The internal power supply voltage Vcci is output to the outside of the chip as a reference voltage Vref.
[0039]
The internal power supply circuit 24 includes a transistor Q1 and resistors R21 and R22 connected in series between a terminal to which a 12V external power supply voltage Vcc2 is applied and a terminal to which a secondary-side reference voltage GND2 is applied; The differential amplifier AMP controls the base voltage of the transistor Q1 by comparing the potential of the connection node between R21 and R22 with a constant voltage such as 2.5V. On the other hand, the external gate driver circuit DRV5 (DRV6) is configured to operate using the reference voltage GND2 as the ground potential and the constant voltage VB1 such as 5V as the power supply voltage.
[0040]
Although the invention made by the inventor has been specifically described based on the embodiments, the present invention is not limited to the above-described embodiments, and various changes can be made without departing from the spirit of the invention. Nor. For example, in the above embodiment, the control circuit 20 has been described as including the soft start circuit 22, but the present invention can be applied to a case where the control circuit 20 does not include the soft start circuit. Further, the gate driver circuits DRV1 to DRV4 constituting the switching circuit 10 may be omitted, and the switch MOSFETs M1 to M4 may be directly driven by the induced voltages of the secondary coils of the pulse transformers T3 and T4. good.
[0041]
Furthermore, in the embodiment, a pulse transformer was used for signal transmission to control the circuits on each side while maintaining insulation between the primary side and the secondary side, but a photocoupler was used instead of the pulse transformer. The signal may be transmitted between the primary side and the secondary side.
[0042]
【The invention's effect】
The following is a brief description of an effect obtained by a representative one of the inventions disclosed in the present application.
That is, according to the present invention, since the control circuit is arranged on the secondary side, a control signal for controlling the synchronous control transistor can be directly supplied from the control circuit without passing through the insulating signal transmission means. Therefore, the synchronous control transistor can be turned off when the synchronous rectification control operation is stopped without using the boost circuit, whereby the secondary rectifier circuit is connected in series with the secondary coil of the voltage conversion transformer. When the synchronous control transistor is turned on, the control is stopped while the synchronous control transistor is turned on, and the LC circuit including the choke coil and the smoothing capacitor forming the rectifier circuit on the secondary side causes resonance, and the synchronous control transistor is turned off. A highly reliable isolated DC-DC converter that can be prevented from being broken can be realized.
[0043]
In addition, when the present invention is applied, since the synchronous control transistor can be reliably turned off when it is desired to turn off, an excessive voltage is not applied to the load by turning off the secondary-side synchronous control at a light load or the like. When the control method is adopted, the soft start control that drives the primary coil while the secondary synchronous control MOSFET is turned off is adopted, and instantaneous interruption of power supply or temporary decrease of power supply occurs. In this case, the primary control coil continues to be driven in a state where the synchronous control MOSFET is turned on and the secondary coil of the transformer T1 is short-circuited, and the primary drive MOSFET is destroyed or the primary coil is disconnected. It is possible to realize a highly reliable isolated DC-DC converter and a power supply control IC that are not likely to be burned.
[Brief description of the drawings]
FIG. 1 is a schematic configuration diagram showing one embodiment of an insulation type DC-DC converter according to the present invention.
FIG. 2 is a waveform chart showing voltage waveforms of a drive control signal of a primary coil and a secondary synchronization control signal in the insulated DC-DC converter of the embodiment.
FIG. 3 is a schematic configuration diagram illustrating a configuration example of an auxiliary power supply circuit used in the insulated DC-DC converter according to the embodiment;
FIG. 4 is a circuit configuration diagram showing a configuration of an output circuit of a synchronous control signal of a control circuit constituting the isolated DC-DC converter of the embodiment, a power supply circuit thereof, and a power supply system of a gate driver circuit.
FIG. 5 is a circuit diagram showing an insulation type DC-DC converter studied prior to the present invention.
6 is a waveform diagram showing a drive control signal of a primary coil, a secondary synchronization control signal, and a voltage waveform of each part in a synchronous MOS drive circuit in the isolated DC-DC converter of FIG. 5;
FIG. 7 is a waveform diagram showing voltage waveforms of various parts in the reset circuit and the synchronous MOS drive circuit when both the synchronous control signals in the insulated DC-DC converter of FIG. 5 are changed to low level.
[Explanation of symbols]
10 Switching circuit 20 Control circuit (IC)
Reference Signs List 21 power supply level detection circuit 22 soft start circuit 30 rectification circuit 50 output level detection circuit T1 voltage conversion transformer T2, T3 signal transmission pulse transformer CB smoothing capacitors L1, L2 rectification choke coil

Claims

Transformer for voltage conversion,
A switching circuit that switches an electric current that flows through a primary coil of the transformer to perform AC driving;
A synchronous control transistor connected between one terminal of a secondary coil of the transformer and a reference potential terminal and controlled to be turned on and off in synchronization with a switching operation of the switching circuit; A rectifier circuit that rectifies a current flowing through the secondary coil and outputs a DC voltage by controlling the ratio of the ON period to be 50% or more;
A capacitor for smoothing the voltage rectified by the rectifier circuit;
A control circuit for generating a first control signal for controlling the switching circuit and a second control signal for controlling on / off of the synchronous control transistor;
Wherein the first control signal generated by the control circuit is supplied to the switching circuit via an insulation type signal transmission means, and a reference potential of a circuit on a primary side of the voltage conversion transformer is provided. And a reference potential of a secondary-side circuit is insulated, and the control circuit operates using the secondary-side reference potential as an operation reference.

Transformer for voltage conversion,
A switching circuit that switches an electric current that flows through a primary coil of the transformer to perform AC driving;
A choke coil connected in series with the secondary coil of the transformer and connected between one terminal of the secondary coil and a reference potential terminal are turned on and off in synchronization with the switching operation of the switching circuit. A full-wave rectifier circuit including a synchronous control transistor and rectifying a current flowing through the secondary coil to output a DC voltage;
A capacitive element for smoothing the voltage rectified by the full-wave rectifier circuit;
A control circuit for generating a first control signal for controlling the switching circuit and a second control signal for controlling on / off of the synchronous control transistor;
Wherein the first control signal generated by the control circuit is supplied to the switching circuit via an insulation type signal transmission means, and a reference potential of a circuit on a primary side of the voltage conversion transformer is provided. And a reference potential of a secondary-side circuit is insulated, and the control circuit operates using the secondary-side reference potential as an operation reference.

A first MOS transistor connected between a power supply voltage terminal and a first terminal of a primary coil of the transformer; and a switching circuit connected between a first terminal of the primary coil of the transformer and a reference potential terminal. A second MOS transistor, a third MOS transistor connected between the power supply voltage terminal and a second terminal of the primary coil of the transformer, and a second terminal of the primary coil of the transformer and a reference potential terminal. A fourth MOS transistor connected between the first MOS transistor and the fourth MOS transistor; a period during which the first MOS transistor and the fourth MOS transistor are simultaneously turned on; a period during which the second and third MOS transistors are simultaneously turned on; 3. The insulated electrode according to claim 1, wherein a primary-side reference potential is applied. Apparatus.

4. A driver circuit that receives a control signal output from the control circuit and turns on and off the corresponding transistor in response to a gate terminal of the first to fourth MOS transistors, respectively. 4. The insulated power supply device according to claim 1.

An auxiliary power supply circuit that generates a power supply voltage required for the operation of the control circuit and the driver circuit based on a voltage applied to the power supply voltage terminal, wherein the auxiliary power supply circuit includes a power supply required for an operation of the driver circuit. A first voltage conversion circuit that generates a voltage; and a second voltage conversion circuit that generates a power supply voltage required for operation of the control circuit, wherein the first voltage conversion circuit operates based on a primary-side reference potential. 5. The insulated power supply device according to claim 4, wherein the second voltage conversion circuit operates based on a secondary-side reference potential.

A driver circuit for turning on and off the synchronous control transistor in response to the second control signal, the driver circuit being configured to operate based on a primary-side reference potential. Item 6. The insulated power supply device according to any one of Items 1 to 5.

7. The insulated power supply device according to claim 1, wherein said insulated signal transmission means is a pulse transformer.

The control circuit determines a light load state based on a magnitude of a current flowing through a primary coil of the transformer, and in the light load state, stops outputting a control signal for controlling the synchronous control transistor. The insulated power supply device according to any one of claims 1 to 7, configured to output a signal for controlling a switching circuit.

The control circuit outputs a signal for controlling the switching circuit so as to gradually increase the current flowing through the primary coil of the transformer when the power is turned on while stopping the output of the control signal for controlling the synchronization control transistor. The insulated power supply device according to any one of claims 1 to 8, wherein the insulated power supply device is configured to perform the following operations.

The insulated power supply device according to any one of claims 1 to 9, wherein the rectifier circuit includes a choke coil connected in series with a secondary coil of the transformer.

Transformer for voltage conversion,
A switching circuit that switches an electric current that flows through a primary coil of the transformer to perform AC driving;
A choke coil connected in series with the secondary coil of the transformer and connected between one terminal of the secondary coil and a reference potential terminal are turned on and off in synchronization with the switching operation of the switching circuit. A full-wave rectifier circuit including a synchronous control transistor and rectifying a current flowing through the secondary coil to output a DC voltage;
A capacitive element for smoothing the voltage rectified by the full-wave rectifier circuit;
A semiconductor integrated circuit used for a power supply device including:
A control circuit for generating a first control signal for controlling the switching circuit and a second control signal for controlling ON / OFF of the synchronous control transistor, and the synchronous control transistor are formed on the same chip. Characteristic semiconductor integrated circuit for power supply control.

Synchronous control transistors are provided between one terminal of a secondary coil of the transformer and a reference potential terminal and between the other terminal of the secondary coil and a reference potential terminal, respectively. The power supply control semiconductor integrated circuit according to claim 11, wherein the power transistors are alternately turned off by a control signal from the control circuit.

13. The semiconductor integrated circuit for power control according to claim 11, wherein the synchronization control transistor is controlled so that a ratio of an ON period to one cycle period is 50% or more.