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Flyback converter with soft switching using auxiliary switch and resonant circuit

  • US 6,005,782 A
  • Filed: 10/16/1998
  • Issued: 12/21/1999
  • Est. Priority Date: 10/16/1998
  • Status: Expired due to Fees
First Claim
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1. A method of operating a flyback converter comprising a main switch, coupled in series with a primary winding of a transformer to supply voltage terminals, an output circuit coupled to a secondary winding of the transformer for providing an output voltage of the converter controlled by a duty cycle of the main switch, and a snubber capacitor for the main switch, comprising the steps of, in each switching cycle of the flyback converter:

  • turning on an auxiliary switch when the main switch is off to provide a resonant circuit comprising the snubber capacitor and a capacitance and an inductance coupled in series with the auxiliary switch, thereby to reduce a voltage of the main switch to substantially zero;

    turning on the main switch while its voltage is substantially zero;

    maintaining a resonant circuit of the capacitance and the inductance via the auxiliary switch and the main switch after the voltage of the main switch is substantially zero, whereby a current of the inductance reverses polarity at a first zero value and reaches a second zero value;

    turning off the auxiliary switch at or after the first zero value and before or at the second zero value of the current of the inductance;

    turning off the main switch at a time determined by the duty cycle; and

    recovering energy from the inductance.

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