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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 DC analysis Analysis using oscilloscope

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  • \$\begingroup\$ 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). \$\endgroup\$
    – a concerned citizen
    Jul 29, 2018 at 5:52
  • \$\begingroup\$ @aconcernedcitizen : I have tried with many models, almost same result. I will give a go tho. \$\endgroup\$
    – seccpur
    Jul 29, 2018 at 5:55
  • \$\begingroup\$ 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. \$\endgroup\$
    – Neil_UK
    Jul 29, 2018 at 6:36
  • \$\begingroup\$ @neil_uk: VM3 is pulse train going from 12V - 0V although the plot due multiple signals don't look like one. \$\endgroup\$
    – seccpur
    Jul 29, 2018 at 8:04
  • \$\begingroup\$ Have you tried adding an earth node to the circuit? \$\endgroup\$
    – Andy aka
    Jul 29, 2018 at 10:12

2 Answers 2

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it seems that VM3 is not pulled to ground from the graph, thus the mosfet is not able to turn off. can you plot it seperately?

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  • \$\begingroup\$ Sorry for the late response. The VG1 pulse train, hence VM3, runs from 0v to 12V, but the graph plot looks messed up may be because of the multiple graphs. \$\endgroup\$
    – seccpur
    Jul 30, 2018 at 1:24
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I'm not sure why your mosfet isn't turning on in sim but if your simulating a flyback you should add a diode and cap to your output. Also, the phasing on your transformer is wrong. These things are actually really important to how a flyback operates.

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