At low frequency, MOSFETS are totally able to switch on and off. But when the frequency becomes higher at 450kHz, it seems MOSFETS are not able to block sine wave anymore, no matter if apply or not 5V-signal at OPT_SIG: oscilloscope always shows sine wave on load (R1).
I tried with other parts such as FDP020N06B with very low rds but I get the same result.
I've also found older MOSFETs STP5NB40FP and using them, RMS voltage on load is almost 0, when signal is not applied. I was thinking the high frequency AC to be passing through MOSFET capacitance Cds, but both FDP020N06B (Coss = 3840pF) and IPD60R180P7S (Coss = 19pF) behave the same way.
STP5NB40FP MOSFET has lower continuous drain current and much higher Rds resistance compared to the previous ones. I'd like to choose MOSFET with Rds as low as possible but work as well. So it'll be another characteristic I'm not taking into account. Could you please help me?
I got some screens from my oscilloscope as a clear evidence of that. Blue track is the input sine, yellow track is the signal on the load. You can see the big differences between them. R9 was replaced with a short. SCREEN