This paper presents a galvanically isolated power transfer system based on two inductively coupled CMOS oscillators with on-chip isolation transformer. The oscillators share the same current and are loaded by an integrated transformer that performs the combining of the oscillation signals at its secondary winding output. By using a thick inter-metal oxide layer for the integrated transformer, a galvanic isolation rating as high as 5 kV is guaranteed. Thanks to the proposed circuit topology, a 300-mW dc-ac power conversion performance with an excellent 36.5% power efficiency was achieved in a 0.8- μm CMOS technology at 5-V power supply. A complete dc-dc converter was also demonstrated by including a rectifier on a second die. An output power of 200 mW at 8-V output voltage with power efficiency better than 27% was measured.