A Three-Level LLC Resonant Converter with PFM and PWM Control for Battery Charging Application

In order to meet the need of battery charger, this paper proposes a wide output voltage control by three-level LLC resonant converter (TL-LLC) adopting with the pulse frequency modulation (PFM) and pulse width modulation (PWM). The operating principle of TL-LLC converter with three-level modulation is analyzed at first. However, LLC converter is usually utilized in PFM to adjust the voltage. This modulation method may let the converter operate at high switching frequency to yield lower output voltage, which may induce higher switching loss, core loss and electromagnetic interference. Thus, the PWM control is adopted when operated at higher switching frequency to reach lower output voltage. Moreover, the simulation model by Matlab/Simulink is used to verify the proposed method with constant current (CC) and constant voltage (CV) mode for battery charger. Finally, a digital controlled TL-LLC converter was constructed to verify the effectiveness of the proposed wide voltage range control method under 2.7kW rated power, 800V dc input voltage and 48V~109V de output voltage. The maximum efficiency is 94.6% at 109V and 25A.