Interleaved three-level LLC resonant converter with fixed-frequency PWM control

This paper proposes to utilize the fixed-frequency Pulse-Width Modulation (PWM) control strategy to replace the traditional phase-shift modulation for realizing an interleaved three-level LLC resonant converter. Result obtained from this research is suitable for application in areas where high voltage, high power, and high stability DC-DC conversion is required. The proposed converter consists of two modules of three-level LLC resonant converter, in which the switching frequency is kept constant and phase angle between modules is 90° shifted. Based on the basic resonance circuit theory, the principle of PWM operation in an environment of half bridge resonant conversion is addressed first, in which the Fourier series analysis is applied and pulse-width factor is defined. The PWM pattern appropriate for half bridge resonance applications is discussed. In addition, the design algorithm based on the worst case design condition is considered for control margin reservation. Finally an interleaved LLC resonant converter of 50 kHz and 1.2kW, with input voltage 520V and output voltage 48V, is implemented to verify the theory and design, in which MOSFETs are used as power switch and Digital Signal Processor TMS320F28035 is the control platform. As a result, the experiments show that the resonant converter can be successfully controlled by the fixed-frequency PWM method. The output voltage ripple is improved by 50%. The maximum converter efficiency measured is 98% and is 94.9% at rated condition.