In a controller for a power converter, a control terminal can provide a control signal to control a power converter. A cycle of the control signal includes a first time interval and a second time interval. The control circuitry can increase a primary current flowing through a primary winding of transformer circuitry and a secondary current flowing through a secondary winding of the transformer circuitry in the first time interval, and can terminate the increasing of the primary current in the second time interval. The control circuitry can also control the first time interval to be inversely proportional to an input voltage provided to the primary winding.