Patent ID: 7755342

Claim:
A circuit for transitioning between a discontinuous conduction mode (DCM) and a fixed frequency continuous conduction mode (CCM) of a power converter, said power converting having a driver receiving PWM signals and controlling a switching stage including a control switch and a sync switch connected at a common switching node for driving a load, the circuit comprising: a PWM modulator for providing PWM signals; and a mode selector for receiving a duty cycle value D, a preset switching period T SET , and a duty cycle at a critical conduction point D MIN having a switching frequency equal to the preset switching period T SET and providing an on-time T ON of the control switch and a switching period T S to the PWM modulator, wherein if the duty cycle value D is greater than the duty cycle at a critical conduction point D MIN , the PWM modulator 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 D is less than duty cycle at a critical conduction point D MIN , the PWM modulator 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.