A switching power converter includes an input for receiving an input voltage, an output for providing an output voltage, a controller, and a power circuit coupled between the input and the output and including at least one switch. The controller is configured to determine a duty cycle for the switch to regulate the output voltage and control the switch with a PWM drive signal having an on-time and an off-time. The PWM drive signal has a constant frequency and a duty cycle equal to the determined duty cycle when the determined duty cycle is greater than or equal to a minimum duty cycle. The PWM drive signal has a variable frequency and a duty cycle equal to the minimum duty cycle when the determined duty cycle is less than the minimum duty cycle.