High Frequency Resonant Converters: An Overview on the Magnetic Design and Control Methods

Abstract

Resonant converters, especiallyLLC converters, are deployed in many applications thanks to their reliability and high efficiency. With the introduction of wide band gap (WBG) devices and the soft switching feature of the LLC converter, the switching frequency of the LLC can be pushed to the order of megahertz, to shrink the converter size and increase the power density. However, magnetic design and control challenges stand in the way of doing so. This article addresses the challenges of high-frequency magnetic design for the LLC converter, and presents an overview for the latest solutions on the magnetic design of the LLC transformer. The matrix transformer is suitable for an LLC converter where high output current at high frequency is required. Different integration methods for the matrix transformer are discussed and demonstrations of hardware prototypes are provided. The dynamic behavior of the LLC converter presents another significant challenge against deploying an LLC converter in applications where fast transient response is needed. This article discusses such challenges as well and provides an overview on the latest control techniques, including average current-mode control, charge control, and simplified optimal trajectory control, to improve the dynamic performance of an LLC converter. A 1 megahertz LLC converter with digital control is presented, using simplified optimal trajectory control with improved transient performance over a wide frequency range.