I want to use a MOSFET to switch 400V to a circuit. I can switch low-side using an n-channel MOSFET, but it's more convenient to have the circuit with its ground fixed. So I want to switch high-side, but I don't see how I can use a low voltage for the switching (0 - 5V).

I am switching with a 0/5V square pulse and I need Vsg > |Vt| for the switch to be on (i.e. my load is to be connected to Vdd = 400V). With the configuration below, the MOSFET will be on regardless of if I have a HIGH or a LOW on my input pulse.

Does this mean my switching pulse needs to be 0/400V square pulse (a 400V on gate would give Vsg = 0V < |Vt|, turning the FET off)?

My MOSFET: 500V infeneon IPW50R190CE Switching frequencies: 10 MHz

enter image description here

  • \$\begingroup\$ You can use isolated gate drivers. \$\endgroup\$ – Wesley Lee Oct 17 '17 at 18:45
  • \$\begingroup\$ PWM Frequency? Or are you just doing things really slow as On/Off? \$\endgroup\$ – jonk Oct 17 '17 at 18:49
  • \$\begingroup\$ I am aiming for a few tens of MHz for PWM. The load is a driver to make ns pulses and uses the avalanche effect from another transistor. I want to trigger those pulses. \$\endgroup\$ – Denu Oct 17 '17 at 19:13

Mosfet Vgs max is typically +-20V (check the spec-sheet of your chosen device) so you need some intermediate circuit so you only pull the gate below your rail by less than 20V

For low frequency switching applications, something like this is typical.


simulate this circuit – Schematic created using CircuitLab

Set the values of R1 and R2 to voltage divide your supply to set the right Vgs voltage on M1 when Q1 is on. Set R3 so there is enough current in that divider when your logic level is high.

For high voltages you will be using fairly high values for R1 and R2 to keep the current and power levels reasonable. That means the switch will not be very fast. If you want this for a PWM application you will need a more active driver.

  • \$\begingroup\$ Thanks for your answer, do you have any suggestions for active drivers since I'm using a pretty fast PWM (tens of MHz)? Another question, in my circuit the load is floating when the switch is open. In your circuit the R2 is floating when Q1 is off. Is it okay to leave it like this? \$\endgroup\$ – Denu Oct 17 '17 at 19:57
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    \$\begingroup\$ @Denu "tens of MHz" is an entirely new can of worms which completely changes the game. Solutions will be very different. You should have said it from the start. I suggest editing the question or making a new one. \$\endgroup\$ – bobflux Oct 17 '17 at 20:13
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    \$\begingroup\$ @Denu floating R2 is fine... But 10s of MHz at 400V is not. That will be tough to do and not with this circuit. \$\endgroup\$ – Trevor_G Oct 17 '17 at 20:17
  • \$\begingroup\$ This is not possible because ...? Because of the large switching currents required? Are MOSFET gate drivers a good place to start? Also I am using a 500V power MOSFET from infeneon (IPW50R19CE). \$\endgroup\$ – Denu Oct 17 '17 at 21:37
  • \$\begingroup\$ For future reference, what frequencies is this switching circuit good for? Few kHz? \$\endgroup\$ – Denu Oct 17 '17 at 21:39

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