Would there be any problems with putting two P-Channel MOSFETs in series in order to double the maximum operating voltage for drain-source? The body diodes when used this way should act just like ordinary diodes put in series to handle more voltage, correct?


There are a couple of issues that you need to be aware of.

First of all, you need to make sure that they share the blocking voltage equally. Just like with diodes, you need to put high-value resistors in parallel with their drain and source terminals such that the current through the resistors is on the order of 10× the reverse leakage current of the transistors (or diodes). This will equalize the voltage drops to within 10% or so. Also, the speed of the individual devices might vary a bit, so it's also a good idea to put capacitors (typically 1 to 10 nF in line-frequency applications, less at higher frequencies) in parallel with them as well, to tame the transient behavior.

The second big issue is that the gate drive to each transistor needs to be relative to that transistor's source terminal; they can't share a common connection. Depending on the application, you might use transformers, optoisolators or specialized high-side driver ICs to allow each of the gate drive signals to float relative to all the others.

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  • \$\begingroup\$ Thanks Dave, but I'm not totally following you. You said "Just like with diodes, you need high-value resistors in parallel"... I've never in my life seen a need for parallel resistors on series diodes. The diodes are in series so they must automatically share the same current. Did you mistakenly think of parallel mosfets instead of series mosfets? \$\endgroup\$ – JamesHoux May 27 '13 at 23:55
  • \$\begingroup\$ As far as the gate drive to each transistor... yes I see the problem there now that you point it out. It's because the source voltage on each transistor will be different which will entail a different necessary gate voltage when they're off. I'll have to puzzle over this and see if it's worth the trouble. \$\endgroup\$ – JamesHoux May 27 '13 at 23:59
  • \$\begingroup\$ Dave, would you mind looking at my other related question? You mentioned the use of other devices to let signals float. I asked about this particular issue using a single p channel and noone has answered yet: electronics.stackexchange.com/questions/70729/… \$\endgroup\$ – JamesHoux May 28 '13 at 0:04
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    \$\begingroup\$ If you haven't been putting resistors in parallel with your series diodes, then you haven't been doing it right. Different diodes (even if nominally "identical") can have very different amounts of reverse leakage current, which is equivalent to saying that they have very different values of resistance in the reverse direction. The reverse voltage distributes itself across the diodes in proportion to their resistance, and the "better" diodes (lower leakage) may get voltages that exceed their ratings. They'll break down, and the voltage will redistribute itself until the whole string fails. \$\endgroup\$ – Dave Tweed May 28 '13 at 0:08
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    \$\begingroup\$ Now that i reread your explanation, this just seems like a bad idea in general. It will be very complicated. If the first mosfet turns on first, then that will put all the voltage through it to the second mosfet, which could destroy it. Both of them would definitely need to turn on simultaneously or with the right delay. I don't think it's even worth trying to build it and get it to work reliably. \$\endgroup\$ – JamesHoux May 28 '13 at 0:15

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