PWM controller
having a modulator for saving power and reducing acoustic noise
英文摘要
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.
