Pwm controller having a modulator for saving power and reducing acoustic noise

Abstract

A modulator of a PWM controller is provided for saving power and reducing acoustic noise in the light load and no load conditions. The maximum on-time is kept as a constant and a bias current of the oscillator in the PWM controller is moderated to achieve the off-time modulation. The bias current is a function of the supply voltage and the feedback voltage, which is derived from a voltage feedback loop. A threshold voltage defines the level of the light load. A limit voltage defines the low level of the supply voltage. A bias current synthesizer generates the bias current. Reducing the bias current increases the off-time of the switching period. Once the feedback voltage is decreased lower than the threshold voltage, the bias current is reduced linearly and the off-time of the switching period is increased gradually. When the supply voltage is lower than the limit voltage, the bias current increases and determines a maximum off-time of the switching period. Keeping the maximum on-time as a constant and increasing the switching period by only increasing the off-time prevents magnetic components, such as inductors and transformers, from being saturated. Furthermore, a control circuit disables the oscillator as the PWM frequency may fall into the audio band, therefore the acoustic noise can be greatly reduced in the light load and no load conditions.