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Discontinuous quasi-resonant forward converter

  • US 7,570,497 B2
  • Filed: 06/08/2006
  • Issued: 08/04/2009
  • Est. Priority Date: 05/26/2006
  • Status: Expired due to Fees
First Claim
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1. A discontinuous resonant forward converter for converting an input dc voltage to an output dc voltage, the converter comprising:

  • first and second dc inputs;

    a transformer having primary and secondary windings with matched polarities;

    a controllable switch for switching power from said dc inputs through said primary winding of said transformer, said controllable switch and said primary winding of said transformer being coupled in series between said first and second dc voltage inputs;

    first and second dc voltage outputs;

    a rectifier coupled to said secondary winding of said transformer, said rectifier and said secondary winding of said transformer being coupled in series between said first and second dc voltage outputs;

    a smoothing capacitor having a first connection coupled to receive dc power from said rectifier at a first connection node, said first connection node being coupled to said first dc voltage output, said smoothing capacitor having a second connection coupled to said second dc voltage output;

    a controller having an output coupled to said controllable switch and being configured to control said switch such that a voltage waveform on said secondary winding has a first portion during which said switch is on and current flows into said first connection node to provide a secondary loading current, and second portion during which said switch and said rectifier are both off;

    wherein substantially no current flows into said first connection node during said second portion of said voltage waveform;

    a resonant circuit to demagnetize the transformer during the second portion, the resonant circuit including magnetising inductance (Lmag) and leakage inductance (Lleak) of said transformer representing a level of energy stored in said transformer, a resonant characteristic of said resonant circuit varies in response to changes in energy stored in said transformer, which in turn is dependent on said secondary loading current just before turn off of said switch; and

    wherein said controller is configured to receive at least one input signal associated with energy stored in said transformer and control an on pulse width of said switch dependent on energy stored in said transformer such that said on pulse width is adjusted to maintain resonance with varying said secondary loading current.

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