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

  • US 6,781,356 B1
  • Filed: 03/24/2003
  • Issued: 08/24/2004
  • Est. Priority Date: 03/24/2003
  • Status: Expired due to Fees
First Claim
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1. A pulse width modulation(PWM) controller having a modulator for saving power and reducing acoustic noise comprising:

  • a bias current synthesizer having four input terminals and two output terminals, wherein, a first output terminal of the bias current synthesizer generates a bias current that determines an off-time of a PWM switching period and a second output terminal of the bias current synthesizer outputs a reference current;

    an oscillator having two input terminals, wherein a first input terminal of the oscillator is connected to the first output terminal of the bias current synthesizer for generating a pulse-signal for PWM switching, an 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 second input terminal of the oscillator is used to turn on/off the oscillator;

    a control circuit having an input terminal and an output terminal, wherein the input terminal of the control circuit is connected to the reference current, which is derived from the second output terminal of the bias current synthesizer, the output terminal of the control circuit is connected to the second input terminal of the oscillator;

    a RS flip-flop for 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 is 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;

    a threshold voltage 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 condition; and

    a supply voltage connected to a fourth input of the 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.

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