This article presented a high-efficiency push-pull resonant converter solution for auxiliary power supply in 70-kV isolated applications. This power supply consists of a two-stage structure with a resonant push-pull inverter and a buck regulator. A series resonance is introduced in the inverter with its operation and parameter design to achieve high-efficiency voltage isolation. A buck converter with feedforward control is presented with the small-signal model and controller design to regulate the output voltage. Besides, the transformer is another challenge as a crucial part of the voltage isolation. Specifically, the optimization design of toroidal core and sector windings is presented, including the analysis of isolation, loss analysis, and leakage inductance. This article presents comprehensive converter and transformer models, describing the proposed power supply's design process. Furthermore, a case study is demonstrated under 70-kV isolation with 36-84 V input and 48-V output. A prototype of a 200-W, 1.4-L isolated power supply has been built to demonstrate the design model's validity and process. Results convincingly indicate that the proposed converter has the highest efficiency of 93.1% and can maintain the 70-kV isolation voltage with good regulation performance.