Second Harmonic Induced Resonant Modes of Fixed-Frequency Operated LLC Converters

The fixed-frequency operated LLC converters can provide a fixed voltage gain with galvanic isolation between the input and output. However, when applied in two-stage AC/DC converters, the equivalent current that flows through the LLC converter, which corresponds to the power flow, contains not only a DC current but also a second harmonic current (SHC) at the double line frequency. This paper reveals that the LLC converter has three different resonant modes with different SHC transfer characteristics. With improper parameter design, the SHC transferred by the LLC converter could be even higher than the SHC from the AC/DC stage, leading to discontinuities of resonant operation in every half AC cycle. This phenomenon increases the DC link ripple voltage and current stress of switching devices, reduces efficiency, and should be avoided in practice. To mathematically model the behavior of LLC converters, an equivalent circuit model considering the unidirectional power transfer characteristic of LLC converters is employed in this paper. The equivalent model can be used to calculate the SHC distribution with different DC link designs and provides better modeling accuracy than typical equivalent models. The analysis is verified by simulations and experimental measurements on a two-stage input-series-output-parallel solid-state transformer.