gez - OK, and thanks for the drawing!
WGTP - I'm always very impressed by RG's in depth responses. Although I mostly don't understand all of it, I've learned a lot from him (and others - and I really appreciate that). But this one is totally over my head.
R.G. wrote: "But what happens if we flip this thing around the other way and make the source more positive than the drain/gate? It ** should** block any current flow if it operates like a normal diode, which is what we want.
But it doesn't. There is an internal "substrate" diode that's an inevitably part of the making of the MOSFET on silicon that acts like a standard silicon diode with its cathode to the drain and its anode to the source. This does not affect normal drain-positive operation at all, but it lets current flow through unimpeded when the voltage is reversed.
If you use two diode-connected MOSFETs back to back, the substrate diodes are also back to back, and the 0.7V substrate diode drop always conducts before the higher-voltage paralleled MOSFET diode can. You **are** using a standard pair of silicon diodes, just doing it the hard way.
This gets fixed if you put another silicon diode in series with the MOSFET so the current can only flow the "correct" way, through the drain toward the source. The external silicon diode then blocks the reverse voltage, and the opposite-polarity MOSFET can do its MOSFET-diode thing in the reverse direction without being shorted out by the substrate diode of the "off" MOSFET.
So - each diode connected MOSFET is really a silicon diode connected in series with the MOSFET with its cathode connected to the MOSFET gate and drain, and the + input of this mess to the anode of the silicon diode, the output being the source of the MOSFET. Take two of those, connect them up back-to-back, and you get a MOSFET diode clipper."
Can anyone do a translation for dummies like me?
Erik