METHOD AND APPARATUS FOR IMPROVING LIGHT LOAD EFFICIENCY IN SWITCHING POWER SUPPLIES

摘要

A circuit for transitioning between a discontinuous and a fixed frequency continuous conduction mode (DCM) and (CCM) of a power converter having a driver receiving PWM signals and controlling a switching stage comprises a control switch and a sync switch connected at a common switching node for driving a load. The circuit including a PWM modulator for providing PWM signals; and a mode selector for receiving a duty cycle value, a preset switching period, and a duty cycle at a critical conduction point having a switching frequency equal to the preset switching period and providing an on-time of the control switch and a switching period to the PWM modulator, wherein if the duty cycle value is greater than the duty cycle at a critical conduction point, the PWM modulator will drive will provide the PWM signals to operate the switching stage in the CCM with constant-frequency duty cycle control, and if the duty cycle value is less than duty cycle at a critical conduction point, the PWM modulator will drive will provide the PWM signals to operate the switching stage in the DCM by turning off the sync switch when a load current becomes negative.