# High Vds across MOSFET in saturation mode in Flyback converter during simulation

While simulating a flyback converter, I find that even in saturation mode the MOSFET, in spite of having Rds(on) around 0.74 ohms and the Ids = 5.75mA, is dropping almost the entire drain supply input voltage. The snubber circuit and the sense resister also consumes very less power.

Looks like a newbie query but I am not able to answer, is this high Vds in saturation mode expected, if yes, why? Is it because the power is a pulsating one ? Can a floating gate destroy a MOSFET? Practically I found Vds = 1.4V, does that mean the MOSFET is bad ?

DC Analysis

Transient Analysis

• You may wish to change W and H for the MOS to some more appropiate numbers... or just choose a model for it, say IRF840 (some quick, random example). – a concerned citizen Jul 29 '18 at 5:52
• @aconcernedcitizen : I have tried with many models, almost same result. I will give a go tho. – seccpur Jul 29 '18 at 5:55
• If the VM3 trace is to be believed, the FET is on all the time and you're simply modulating its saturation current. In a flyback, you turn it on, and off. – Neil_UK Jul 29 '18 at 6:36
• @neil_uk: VM3 is pulse train going from 12V - 0V although the plot due multiple signals don't look like one. – seccpur Jul 29 '18 at 8:04
• Have you tried adding an earth node to the circuit? – Andy aka Jul 29 '18 at 10:12