PWM controller having a saw-limiter for output power limit without sensing input voltage

1. Field of the Invention A PWM controller having a saw-limiter for power limit without input voltage sensing. The saw-limiter has an adder, a reference voltage, a scaler and a saw-tooth signal that is generated by the PWM oscillator. The saw-limiter produces a saw-limited voltage. The PWM controller will turn off its output when the current-sense input signal of the PWM controller is higher than the saw-limited voltage. The saw-limited voltage is equal to the reference voltage while a PWM switching period starts. After that, the amplitude of the saw-limited voltage will gradually increase until it reaches its maximum voltage. Subsequently, a saw-tooth like waveform is generated for the saw-limited voltage. The slope of the current-sense input signal is proportional to the line voltage. Therefore, a higher line voltage creates a sharp slope for the current-sense input signal, which will be restricted by a lower saw-limited voltage and produces a shorter PWM signal. The PWM signal will be turned off once the voltage of the current sense input signal is higher than the saw limited voltage. In terms of power limit, using the saw-limiter the power limit will be lower when the line voltage is higher. By properly selecting the value of the scaler an identical output power limit for the low line and high line voltage input can be achieved.