Current Ripple Mechanism In Digital LLC Converters For Battery Charging Applications

Lithium battery storage and charging issues become hot topics with fast development of electrical vehicles.In addition to charging efficiency,charging current ripple problem has attracted more attention recently.It is noted that the charging current ripple increases the temperature rising of the batteries and hurt the long cycle life and overall performance.This paper mainly focuses on the analytical model of current oscillation and proposes synchronous frequency dither control to reduce the quantization current ripple further.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 the input.With conventional Frequency Modulation(FM),every modulation step is accompanied by quantization error due to the limited resolution of digital controllers.Large current ripple is propagated to the output due to poor frequency modulation and extremely low internal impedance of batteries,which hurts the battery health seriously.The analytical model of output current oscillation is proposed considering the quantization effects and double line frequency input voltage perturbation.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 proposed current oscillation model and effectiveness of synchronous FD control.Less than 9% deviation is found between the derived results by Extended Describing Function(EDF)and the experimental results.One bit dither sequence is implemented on the digital controller STM32F103R8T6.Compared with FM,the discrete PI controller with synchronous FD can reduce the quantization current ripple from 1.1 A to 0.6 A(a reduction of 45%)at 80 V battery voltage,0.85 A to 0.45 A(a reduction of 47%)at 72 V battery voltage and 0.55 A to 0.3 A(a reduction of 45%)at 64 V battery voltage.A current ripple reduction of around 45% is achieved over the entire charging procedure.