A partial-voltage buck regulator controls a high-voltage bus to a full-bridge circuit such that overall switching losses are reduced and distributed. Low-voltage buck regulator devices are PWM-switched when high output voltage is needed, and high-voltage bridge devices are PWM-switched when low output voltage is needed. A variable input power supply, which may be operated in a high power factor mode, can adjust the input bus voltages to achieve optimum performance for a given magnetic resonance imaging sequence.