A novel analysis of high frequency LLC converter for fuel cell systems

This paper presents the analysis and design of isolated dc/dc converter using an unregulated LLC converter for fuel cell applications. The LLC converter operates as an isolated voltage amplifier with a constant voltage gain, and a non isolated converter installed in the input stage regulates the output voltage under a wide variation of fuel cell stack voltage. By separating the functions, the unregulated LLC converter can be operated at an optimal switching condition, and the high-frequency operation of 300 kHz can be accomplished without introducing an excessive switching loss. The prototype converter with a 1-kW design shows an improved efficiency under a 48V input and full load conditions. The circuit models are developed for simulation studies. Closed loop systems are modeled and they are simulated using Matlab Simulink. The results of simulation and hardware are presented. Experimental results are compared with simulation results.