PWM controller having adaptive off-time modulation for power saving

A PWM controller having an adaptive off-time modulator comprising:

• a bias current synthesizer having four inputs and one output to generate a bias current that determines an off-time of a PWM switching period;
• an oscillator having an input connected to the output of the bias current synthesizer for generating a pulse-signal for PWM switching, wherein a maximum on-time of the pulse-signal, is a constant and an off-time of the pulse-signal is increased as the bias current decreases;
• a RS flip-flop generating an on-off signal, wherein the RS flip-flop is set by the pulse-signal and reset by a feedback control;
• an AND-gate having two input terminals for outputting a PWM signal, wherein a first input terminal of the AND-gate is connected to the pulse-signal and a second input terminal of the AND-gate connected to the on-off signal;
• a feedback voltage connected to a first input of the bias current synthesizer, wherein the feedback voltage is derived from the voltage feedback loop of the power supply for controlling the on-time of the PWM signal and regulating the output of the power supply;
• and a threshold voltage is connected to a second input of the bias current synthesizer to determine the level of a light-load condition;
• a limit voltage connected to a third input of the bias current synthesizer to determine the level of a low supply voltage;
• wherein the limit voltage is changed in every PWM switching cycle which affects the bias current to produce a variable PWM switching frequency when the PWM switching frequency is decreased in the light-load and no-load conditions;
• and
• a supply voltage connected to a fourth input of bias current synthesizer, in which the supply voltage is the power supply voltage of the PWM controller;
• wherein the bias current is a function of the feedback voltage, the threshold voltage, the supply voltage, and the limit voltage, such that when the feedback voltage is lower than the threshold voltage, the bias current starts to reduce in response to the decrease of the feedback voltage, and if the supply voltage is lower than the limit voltage, the bias current starts to increase in response to the decrease of the supply voltage.