Abstract: Series-resonant DC-DC converters (SRCs) are core components for power electronic transformers (PETs). However, the energy storage components with large volume, such as capacitors, is one of the reasons that limits power density of PETs. Hence, it is usually desirable to utilize as few passive components as possible. However, the operating characteristics of SRCs change greatly if the ratio of the DC capacitors and the resonant capacitors decreases. To address this issue, this paper comparatively analyzes the operating characteristics of three typical SRCs for the indirect matrix resonant type PET, the half-bridge submodule based SRC, the full-bridge submodule based SRC and the hybrid SRC mixed with half-and full-bridge submodules. The transcendental equations about designing the switching frequency are derived, and the analytic expressions between the switching frequency and the resonant frequency are also derived. Based on the aforementioned analysis, the corresponding switching frequency design methods are proposed. Finally, experimental results verify the correctness of the presented theoretical analysis and the effectiveness of the proposed switching frequency design method.