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.