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:


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?

  • 1
    \$\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

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.

  • 1
    \$\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
  • 1
    \$\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

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.