A PWM controller compensates a maximum output power of a power converter having a power switch. The PWM controller includes an oscillator for generating a saw signal and a pulse signal, a power limiter coupled to the oscillator for generating a saw-limited signal in response to the saw signal, and a PWM unit coupled to the power limiter and the oscillator to generate a PWM signal for controlling the power switch in response to the saw-limited signal and the pulse signal. The saw-limited signal has a level being flattened during a period of time before an output voltage is generated, and is then transformed to a saw-limited waveform after the period of time.