Constant Resonant Current Limiting Strategy for LLC Converter Without Current Sensing

There are great current shocks in LLC resonant converter during startup and over load condition. Conventional open loop current limiting method suffers from limited effect and complicated design process, while closed loop current limiting method requires extra current sensing. This paper investigates the relationship between resonant current, output voltage, and switching frequency based on state plane analysis, then a constant resonant current limiting strategy without current sensing is proposed, which adjusts switching frequency corresponding to the output voltage. The proposed strategy can realize constant peak value of resonant current during startup, shock load stepping, and over load condition, achieving optimal control of the primary current stress. Together with conventional output voltage regulating strategy, regulation of output voltage under normal load condition is also guaranteed. The maximum available output voltage under current limiting condition is estimated, and simple parameter design method of current limiting strategy is provided.