I am trying to use MOSFETs to switch AC power. Here is my test circuit:


simulate this circuit – Schematic created using CircuitLab

If I put DC or low frequency ( < 1000khz) across the MOSFETs, the switch turns the MOSFETs on and off perfectly. However, the MOSFETs will not turn off completely when I use high frequencies. I ultimately have to switch 350 khz. If I give it 20V at 350 khz, I get about 10V across the load even if I turn the transistors off by directly shorting the gate to the source.

The high frequency AC seems to be passing through the parasitic capacitance of the MOSFETs and turning them on. Are there MOSFETs, perhaps with low input capacitance, that can switch high frequency power? Is there another way of setting up this circuit that would turn off completely? Or is there another something besides a MOSFET that I need to use? I am trying to avoid relays.

I spent a lot of time searching, and found references to switching 20 khz audio with circuits like this, but I don't see my circuit turning off well even at only 20 khz. I have to switch 150V at 5A and 350 khz on and off once or twice a second.



The drain-source capacitance of the MOSFET can be in the order of 100pF irrespective of what the MOSFET is meant to be doing. With 100pF at 350kHz the impedance is about 4.5 kohm. This means the LEDs still receive some current when you believe the MOSFETs to be "off".

| improve this answer | |
  • \$\begingroup\$ I think the 350khz is passing through the gate capacitance and actually turning the MOSFETs "on". I will be testing parts with lower capacitance, but am doubtful that it will be low enough. However, I must be reinventing the wheel. Other people must have tried to switch frequencies this high, using MOSFETs or other means \$\endgroup\$ – Zosimos Sep 8 '15 at 15:37
  • \$\begingroup\$ No, not according to your question - "I get about 10V across the load even if I turn the transistors off by directly shorting the gate to the source". Reverse transfer capacitance for these devices is quite low. \$\endgroup\$ – Andy aka Sep 8 '15 at 16:36
  • \$\begingroup\$ I played around a little with resistive loads, and I believe you're right. Anyway, I have the lowest capacitance MOSFETs Digikey has on order, but I still think they will pass a lot of 350khz. There must be other people who have switched a signal like this. Is there a better approach to the problem? \$\endgroup\$ – Zosimos Sep 10 '15 at 18:44
  • \$\begingroup\$ Use bipolar transistors is my first thought. How quickly are you switching the AC on and off? \$\endgroup\$ – Andy aka Sep 10 '15 at 18:47
  • \$\begingroup\$ I will only be turning it on for maybe 40ms a couple times a second. Besides the MOSFETs, I came across references to using bipolars, thyristers, and pin diodes for switching AC. At this point, I have put together a circuit with relays. If I get a chance, I will experiment with the other options, and update here. \$\endgroup\$ – Zosimos Sep 14 '15 at 18:46

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.