Mechanism of current ripple propagation in a digital controlled LLC converter is investigated from three aspects: battery load, quantization error and double line frequency perturbation at input. With conventional Frequency Modulation (FM), every modulation step is accompanied by quantization error due to limited resolution of digital controllers. Large current ripple is propagated to output due to poor frequency modulation and extremely low internal impedance of batteries, which hurts battery health seriously. The synchronous Frequency Dither (FD) is proposed to improve quantization resolution and reduce current ripple further. A detailed design for discrete PI controller with the proposed synchronous FD is presented. A 2 kW Li-Ion battery charger was built to verify the effectiveness of the PI controller with synchronous FD. The quantization current ripple is successfully attenuated by around 45% over entire charging range.
