Canonical Model and Design Methodology for LLC DC/DC Converter With Constant Current Operation Capability Under Shorted Load

Different from the pulse width modulation dc/dc converters, short-circuit operation is one of the most unpredictable and challenging aspects in an LLC dc/dc converter. In order to reveal how the switching frequency affects the resonant voltage and current stress under shorted load, as well as associated output current, short-circuit operation of the LLC converter with and without diode clamping is analyzed, respectively, in this paper. A critical switching frequency, i.e., 1.5 × natural resonant frequency, is identified for the first time, which is the frequency boundary of the effectiveness of clamping diodes. Based on the derived mathematic model, canonical current gain curves and a generic voltage/current stress portrait ( Q-k map) under shorted load are proposed for the operation prediction. A new design methodology is further proposed for the design of the LLC dc/dc converter with constant current regulation capability under shorted load. The theoretical analysis and application of the proposed model are verified thoroughly by simulation and hardware experiment.