MOSFET在DC／DC變換器中的主要效率損失

　　硅MOSFET已成為DC／DC轉(zhuǎn)換器設計中的一個關鍵部件，它提供了實現(xiàn)高效率、脈寬調(diào)制控制策略所需的高速開關和電流處理能力。朝著更高的效率驅(qū)動將更強烈要求MOSFET，特別是在有約束的尺寸設計，減少空間的數(shù)量，可以在散熱器和其他冷卻援助。

　　這種趨勢正在推動使用降低RDS（on）的MOSFET，以及通過良好的電荷存儲特性提供低開關損耗。本文將討論典型的DC / DC轉(zhuǎn)換器應用中MOSFET的一些關鍵參數(shù)。

　　金屬氧化物半導體場效應晶體管（MOSFET）已成為功率變換電路的關鍵部件。與雙極電路相比，它的低電荷存儲使其成為開關型DC/DC變換器中處理大功率功能的首選元件。

　　典型的DC / DC轉(zhuǎn)換器采用兩個MOSFET。一種是控制或高側(cè)FET，如圖1所示為Q1。另一種是同步或低端FET，在圖1中被標記為Q2。低端FET的名稱來自晶體管將電源軌連接到地上的事實。相反，高側(cè)FET連接輸入和輸出電源軌。

　　The high- and low-side FETs are synchronized， which gives the switching converter its commonly used name of synchronous rectifier. Only one switch can be open at any one time and the controller will generally organize switching events to prevent overlap in switching between the two transistors. One transistor will be switched off before the other is allowed to turn on. This behavior prevents losses caused by cross-conduction.

　　As well as the nominal maximum power that the DC/DC converter can deliver， the two MOSFETs need to handle voltage spikes caused by parasitic inductances and a current level equal to the maximum output current， plus 50 percent of the ripple current. Losses from the two transistors are important. Not only will static losses from the on-resistance through the transistor cause lower efficiency， but so will losses incurred during switching. The switching losses are especially important in DC/DC architectures that employ hard switching; this is where the transistor is exposed to non-zero voltage and current during switching events.