Resonant controllers work a little bit differently than PWM does. In PWM, the transfer of power through the converter is controlled by the duty cycle of the switching devices. Duty cycle is the switch-on time divided by the switch cycle time. In pulse width modulation, more power is transferred through the converter when the duty cycle is increased towards the maximum allowable value. When the load lightens, the converter senses that and lowers the duty cycle, delivering less power to the load.

