A controller includes an oscillator coupled to a switch to determine a time at which the switch is turned on. The oscillator includes a capacitor, a charging current source and a discharging current source coupled to charge and discharge the capacitor. A time period for the capacitor to charge and discharge between first and second voltage levels is a time period of a switching cycle. A duty cycle mode control circuit is coupled to operate in a first duty control mode in response to a feedback signal being between first and second feedback signal values, operate in a second duty control mode in response to the feedback signal being a between the second and third feedback signal values, and operate in a third duty control mode in response to the feedback signal being between the third and fourth feedback signal values.