I have designed a 20W flyback converter, in the open loop operation, when the duty ratio is below 0.5 it works perfectly (hardware not simulation). But when I go above 0.5 , that is in boosting mode, flyback slowly loses it's output voltage. The Vds voltage, also slowly goes down and my DC power supply goes in constant current mode. It looks like my flyback transformer is saturating. I designed and wound transformer myself using area product method, and it has an airgap also. So what can be the possible problem. The circuit is basic flyback circuit without any RC snubber or anything.

  • Since the flyback converter's transformer -which is actually a coupled inductor- operates in the first (i.e. upper left) quadrant of the B-H loop, the converter cannot operate stably with D > 0.5. If you try to keep D > 0.5 then the core of the inductor will never reset thus eventually will saturate after a few cycles. – Rohat Kılıç Nov 18 at 7:12
  • 3
    If the secondary load doesn't dissipate the energy stored in the first part of the switching cycle, then the core will saturate unless the duty cycle is lowered. Show your circuit, detail your transformer, state your input supply voltage etc.. – Andy aka Nov 18 at 10:22

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