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asychronous buck converter need help!!

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nelly1

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I use two same mos diode(G and D connected mosfet which acts like diode) stacked as normal free wheeling diode. The symbol is like the pic1 below because they are insulated mosfet with pwell/dnwell and dnwell/psub junction diodes.

The problem is
1.when the diode is reversed biased, their voltage drop (VDS) are not the same,whatever the mosfet size is large or small..
2.when i add a VDC=Vth between the G and D of the lower mosfet to decrease the forward voltage drop, (when the diode is forward biased and current is small, Vgs=Vth, Vout=Vin-vth), why the reverse voltage drop of the lower mosfet increase?


REALLY NEED SOMEONE HELP!!!!!!!!!

Screenshot.pngScreenshot-2.png
 

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