Frequency Folding for LLC Resonant Converters in EV Charging Applications

Abstract

To realize a wide voltage variation of electric vehicles (EV) battery packs, the conventional LLC converter needs to operate over a wide frequency range, which generally complicates the magnetics design and decreases the efficiency when the operating frequency is far away from the resonant frequency. Responding to the challenge, a frequency folding technique is proposed and explained with an exemplary new LLC converter. The proposed converter utilizes both the inductive region and the capacitive region of the conventional LLC converter through a one-shot operation of the relay during the whole charging process. As the battery voltage increases, the converter switches from the capacitive region to the inductive region. Throughout the charging process, the operating frequency is effectively reduced, and zero-voltage switching is maintained, resulting in a relatively higher overall efficiency. A detailed description on the design procedure of the proposed converter is described. A scaled-down 400-W prototype with 200 V input and 120 V-210 V output range verified the advantages of the proposed concept and measured efficiency of 98% at the maximum output power.