Gyula,
thanks, i thought so, so that is why i asked Jeg to measure his drain - source voltages, because he could have the same problem.
I changed the "Ron" parameter on one IRF840 in the .Model file in the "LTspiceXVII/lib/cmp/M standard.mos" file from 850m to 500m:
.model IRF840 NMOS(Level=3 Gamma=0 Delta=0 Eta=0 Theta=0 Kappa=0.2 Vmax=0 Xj=0 Tox=100n Uo=600 Phi=.6 Rs=6.382m Kp=20.85u W=.68 L=2u Vto=3.879 Rd=.6703 Rds=2.222MEG Cbd=1.415n Pb=.8 Mj=.5 Fc=.5 Cgso=1.625n Cgdo=133.4p Rg=.6038 Is=56.03p N=1 Tt=710n mfg=International_Rectifier Vds=500 Ron=850m Qg=63n)
but that did not change the LTspice simulation outcome, i guess that "Ron" parameter is just an indicator, not a parameter for the RDSon (Drain-source ON resistance).
Anyway, so i have to "match" 2 IRF840 MOSFETs (or any other MOSFET pair) to get it right.
I did try to use a DMM in resistance mode to measure the Ron resistance, but they both measure 0.6 Ohm which is not the specified 850mOhm RDSon.
For info: I measured the "coupling coefficient" (K) of my coils on the gate isolation toroid to be 0.99999, so very close to 1, so to have the LTspice simulation correct.
Itsu
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