A LLC Resonant Converter with Wide Output Voltage Range for USB - Power Delivery

With an exponential rise in the use of smartphones and laptops, a universal adapter to charge both devices is desirable. Typically, the smartphones require power at 5 V (up to 25 W), whereas laptop/PC demand power at 20 V (up to 100 W). The recently launched USB Power Delivery (USB-PD) standard makes this possible. In USB power adapters, half bridge LLC resonant converter is preferred due to soft switching capability and high efficiency. However, conventional LLC resonant converter cannot achieve 1:4 gain (5 V - 20 V). Moreover, converter may operate far from resonant frequency, which increases loss in power devices. To address this challenge, a secondary side reconfigurable structure is proposed in this paper. The proposed circuit maintains the switching frequency relatively close to resonant frequency at both output voltage levels. This ensures soft switching and hence, enhances the conversion efficiency. The reconfiguration does not change the fundamental traits of LLC resonant converter, which facilitates simple design and control. The working principle of proposed circuit is explained with the help of operating modes. The circuit design constraints are also described for obtaining desired performance trade-offs. The proposed converter's operation is verified using circuit simulation in MATLAB/Simulink.