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.
Thanks!
Best Answer
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".