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LTspice simulation of two transistor forward converter is noisy

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Flyback

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Hello,
Our 160W two transistor forward converter simulaton (LTSpice) switches in a noisy and irregular manner when synchronous FETs are used with higher “Qg”.
(the ‘synchronous fets’ are the secondary side fets)
This seems to me that it may be a fault of the simulator(?), because changing the synchronous FETs to an almost similar FET, with a slightly lower Qg parameter, completely solves the problem, and it switchs smoothly. Is this a fault of the simulator?

Using IPB107N20N3 FETs causes noisy switching.
Using BSC900N20NS FETs switches smoothly and properly.

IPB107N20N3 FET datasheet:
http://www.infineon.com/dgdl/Infine...n.pdf?fileId=db3a3043243b5f170124968e7d1f18e7
BSC900N20NS FET datasheet:
http://www.infineon.com/dgdl/Infineon-BSC900N20NS3-DS-v02_02-en.pdf?fileId=db3a30432ad629a6012b144f6b0619db

Please find attached the simulations (LTspice), with the non-noisy, and the noisy switching.

Do you believe this is a simulator fault?
 

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alec_t

Well-Known Member
Most Helpful Member
I'd like to run the sim, but my ancient laptop has a slow processor, limited memory and the hard drive is nearly full! The sim starts, but slows to an interminable crawl. Can you post a cut-down version with just the affected FET section?
 
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