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I'm struggling with the design of a MOSFET based switch for high input voltages.
See schematic of the basic circuit that I've come up with:

schematic

simulate this circuit – Schematic created using CircuitLab

When M3 would be ON, the voltage on it's drain would be approximately 0. This also means that M2 Vgs ~= (-50)V, exceeding the AMR for this MOSFET.

MOSFETs can't sustain this kind of high voltage on Vgs... What could be an appropriate solution for this?

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    \$\begingroup\$ Did you mean to make M3 a P-channel, and M2 an N-channel? \$\endgroup\$ – Phil Frost Aug 14 '13 at 11:09
  • \$\begingroup\$ @Phil Frost: Yes \$\endgroup\$ – dor Aug 15 '13 at 8:47
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It would help if you used an N channel MOSFET symbol for M3 and a P type MOSFET symbol for M2.

To solve your dilemma, if you insert a resistor in series with the drain of M3 (n channel device), you should be able to understand that this forms a mechanism for reducing the gate voltage on M2.

Try a 10k\$\Omega\$ resistor and also make R2 about 10k\$\Omega\$. Now you only get 25V on the gate. If that is too much make R2 smaller like 3k3 - now you'll only get 12.4V on the gate.

And if you need the same circuit to run from a 15V supply, leave R2 as 100k\$\Omega\$ and put a 12V zener across it, cathode to positive rail.

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    \$\begingroup\$ Wouldn't having a resistor in series with the drain of M3 make the OFF to ON transition of M2 slower? Slower transition of M2 with a high load current could ruin M2...? \$\endgroup\$ – dor Aug 14 '13 at 10:17
  • \$\begingroup\$ @dor With 3k3 on the upper transistor's gate and (say) a gate capacitance of 1nF, the CR time will be 3.3usecs and likely not to cause power dissipation problems with M2. If the OP was proposing a PWM circuit then yes, something would need to be done. \$\endgroup\$ – Andy aka Aug 14 '13 at 11:08
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    \$\begingroup\$ @dor how fast do you need it to be? This would be a good requirement to specify in the question. \$\endgroup\$ – Phil Frost Aug 14 '13 at 11:08
  • \$\begingroup\$ @Phil Frost: I don't need it to be fast, but I'm interested in a generic design that suits to high load current. \$\endgroup\$ – dor Aug 15 '13 at 8:48

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