Systems and methods are provided for regulating power conversion systems. A system controller includes: a first controller terminal configured to receive a first signal related to an input signal for a primary winding of a power conversation system; and a second controller terminal configured to output a drive signal to a switch to affect a current flowing through the primary winding, the drive signal being associated with a switching period including an on-time period and an off-time period. The switch is closed (e.g., being turned on) in response to the drive signal during the on-time period. The switch is opened (e.g., being turned off) in response to the drive signal during the off-time period. A duty cycle is equal to a duration of the on-time period divided by a duration of the switching period. The system controller is configured to keep a multiplication product of the duty cycle and the duration of the on-time period approximately constant.