切換開關模式電源電流感測——第一部分:基本知識

作者: Henry Zhang、Mike Shriver和Kevin B. Scott

電流模式控制由於其高可靠性、迴路補償設計簡單、負載分配功能簡單可靠的特點,而被廣泛用於切換開關模式電源。電流感測訊號是電流模式切換開關模式電源設計的重要組成部分,它用於調節輸出並提供過流保護。圖1顯示了 LTC3855 同步切換開關模式降壓電源的電流感測電路。LTC3855是一款具有周期對周期限流功能的電流模式控制元件。感測電阻RS監測電流。

圖1.切換開關模式電源電流感測電阻(RS)

圖2顯示了兩種情況下電感電流的示波器圖像:第一種情況使用電感電流能夠驅動的負載(紅線),而在第二種情況下,輸出為短路(紫線)。

圖2.LTC3855限流與折返示例,在1.5 V/15 A供電軌上測量。

最初,峰值電感電流由選定的電感值、電源切換開關導通時間、電路的輸入和輸出電壓以及負載電流設置(圖中用"1"表示)。當電路短路時,電感電流迅速上升,直至達到限流點,即RS × IINDUCTOR (IL)等於最大電流感測電壓,以保護元件和下游電路(圖中用"2"表示)。然後,內建的電流折返限制(圖中數位"3")進一步降低了電感電流,以將熱應力降至最低。

電流感測還有其他的作用。在多相電源設計中,利用它能實現精確均流。對於輕負載電源設計,它可以防止電流反向流動,從而提高效率(反向電流指反向流過電感的電流,即從輸出到輸入的電流,這在某些應用中可能不合需求,甚至具破壞性)。另外,當多相應用的負載較小時,電流感測可用來減少所需的相數,從而提高電路效率。對於需要電流源的負載,電流感測可將電源轉換為定電流源,以用於LED驅動、電池充電和驅動鐳射等應用。

在本系列的第二部分"何處放置電流感測電阻"中,我們將說明在電路的哪一個分支中放置電流感測電阻,以及其如何影響操作。

軟體

LTspice

LTspice® 軟體是一款強大、快速、免費的模擬工具、原理圖採集和波形檢視器,其具有增強功能和模型,可改善切換開關穩壓器的模擬。

LTpowerCAD

LTpowerCAD 設計工具是一款完整的電源設計工具程式,可顯著簡化電源設計任務。它引導用戶尋找解決方案,選擇功率級元件,提供詳細效率資訊,顯示快速環路波特圖穩定性和負載瞬變分析,並可將最終設計匯出至LTspice進行模擬。


Authors

henry-jindong-zhang

Henry (Jindong) Zhang

Henry Zhang is an applications engineering manager for power products at Linear Technology. He began his Linear career as an applications engineer in 2001. He became an applications section leader in 2004 and applications engineering manager for power products in 2008. His group supports wide range of products and applications, from small monolithic regulators and power modules, to large kW-level high power, high voltage converters. In addition to supporting power applications and new product developments, his group also develops the LTpowerCAD supply design tool. Henry has broad interests in power management solutions and analog circuits. He has over twenty technical articles, seminars and videos published and 8 power supply patents granted or pending.

Henry graduated from Virginia Polytechnic Institute and State University in Blacksburg, Virginia with his masters and Ph.D. degrees in electrical engineering.

 

mike-shriver

Mike Shriver

Mike Shriver is a Senior Applications Engineer at Analog Devices. He has over 15 years' experience at Linear Technology, now ADI, working in power applications. Prior to Linear Technology, he worked for Artesyn Technologies and Best Power Technology.

Kevin Scott

Kevin Scott

Kevin Scott works as a Product Marketing Manager for the Power Products Group at Analog Devices, where he manages Boost, Buck-Boost and Isolated Converters, LED Drivers and Linear Regulators. He previously worked as a Senior Strategic Marketing Engineer, creating technical training content, training sales engineers and writing numerous website articles about the technical advantages of the company's broad product offering. He has been in the semiconductor industry for 26 years in applications, business management and marketing roles.

Kevin graduated from Stanford University in 1987 with a BS in Electrical Engineering and started his engineering career after a brief stint in the NFL.