Abstract: Hybrid integrated DC-DC converters are widely used in harsh environments and applications with high-reliability requirements due to their wide operating temperature range and long-term reliability. This paper presents the design of a hybrid integrated DC-DC converter with an input voltage of 28 V and an output of 5 V/20 A, leveraging the excellent high-frequency, low-loss characteristics of gallium nitride (GaN) devices. The design incorporates an active clamp soft-switching topology, hybrid integrated micro-assembly technology, and high-current, low thermal resistance hermetic packaging techniques. The converter operates at a switching frequency of 800 kHz and achieves a peak efficiency of 92%. The paper elaborates in detail on the design methods and technical details of the active clamp power circuit, the control of parasitic parameters and oscillating voltage in the GaN HEMT drive circuit, the optimization of synchronous rectification timing and dead-time, the thick-film hybrid integration process, and heat dissipation design. Through simulations and prototype experiments, it verifies and demonstrates the advantages of GaN HEMTS and hybrid integrated circuits in high power density and high efficiency aspects.