Multi-mode operation optimizes and improves the power supply efficiency over the entire load range while maintaining good cross regulation in multi-output supplies. To optimize power supply efficiency, four different control modes are implemented. At maximum load, the modulator operates in full frequency PWM mode, as load decreases, the modulator automatically transitions, first to variable frequency PWM mode, then to low frequency PWM mode. At light load, the control operation switches from PWM control to multi-cycle modulation control, and the modulator operates in multi-cycle modulation mode.