Abstract: Based on MEMS and 0.18 um BCD process, an isolated DC-DC converter using 5-kVrms integrated on-chip transformer is proposed. The converter is made up of only two silicon dice, which improves integration and reduces cost. The proposed dc–dc converter delivers up to 0.5 W output power with 29% maximum power efficiency in a compact 7.67 mm*5.8 mm*1 mm SOP-8 package, and its input voltage is 3 V-5.5 V, output voltage is 3.3 V/5 V. In order to control radiated emissions, the random frequency hopping technology is used to control oscillator’s resonant frequency ranging from 145 MHz to 195 MHz through 5-bit hopping. A 25 dBuV reduction of maximum harmonic peak value is achieved.