A power converter includes a power stage, a feedback circuit, a ramp generator and a controller. The power stage generates a pulse width modulation signal for forming an output voltage. The controller includes a comparator, a control logic circuit, an on-time generator and an off-time generator. The comparator generates a comparison signal according to a compensation signal from the feedback circuit and a ramp signal from the ramp generator. The control logic circuit generates a first trigger signal according to the comparison signal and an off-time pulse signal. The on-time generator generates a duty signal according to the first trigger signal. The duty signal is transmitted to the power stage for controlling a duty cycle ratio of the pulse width modulation signal. The off-time generator generates the off-time pulse signal according to the duty signal and transmits the off-time pulse signal to the control logic circuit.