Ripple Voltage and Loss Reduction of Single-Phase ISOP SST by Eliminating the Second Harmonic Current in LLC Converters

In two-stage solid-state transformers with input-series output-parallel structure, the LLC converter operating at a fixed switching frequency is a common choice for the isolated DC/DC conversion stage because it can provide a high efficiency and a fixed voltage gain. However, when applied in single-phase AC/DC conversion, the equivalent output current of the AC/DC converters contains a second harmonic current (SHC) at twice the grid frequency. The SHC causes increased ripple voltages and losses in LLC converters and should be minimized. This paper analyzes the propagation mechanism of SHC, especially the influence of both the primary and the secondary side DC link capacitors. A control method to eliminate the SHC is proposed. By adding an additional control loop, the switching frequency and voltage gain of the LLC converter are regulated dynamically to prevent the propagation of SHC. The effectiveness of the proposed control method is verified by simulations.