# High Side N Channel MOSFET Switching

I have a project where I need to be able to switch a 24v solenoid with a 28v discrete signal. I would like to use high side switching where 28v turns on the solenoid and 0v turns off the solenoid.

From my research, it seems like one possible way of doing this would be to use a N channel MOSFET. Because my signal voltage is higher than the supply voltage, my Vgs is already at 4v without any additional circuitry. Are there any issues with this method? It seems like it is not recommended to use an N channel MOSFET as a high side switch without additional circuitry typically, but it seems like in this situation it could work well.

• Why do you want to switch the high side? Low side is much simpler and you'd probably need to use resistors to lower Vgs anyway as 28V is quite high for most MOSFETs Commented May 24 at 15:15
• Don't forget you'll need to use a logic-level FET. 4 V isn't enough to turn on most power FETs. Commented May 24 at 18:35
• Please provide more details like a schematic and/or model numbers of the parts. Commented May 24 at 18:49

switch a 24v solenoid with a 28v discrete signal

First thing I'd ask is what's the tolerance on that 28V? Is it 26-30V, or 27.9-28.1? If gate voltage is too low, the FET may not turn on fully and instead dissipate a lot of heat and smoke... so if the 28V is an internal voltage in your design and you know the tolerance then OK. But if it's an external input, it would be risky.

Then you need to protect the gate against overvoltage (ESD...) both positive and negative, something like that:

IMO if the input signal is external it would be worth it to add a bit of complexity (ie, a 5 cent BJT and a few resistors) to switch a PMOS instead. Or a LM339 if you need accurate voltage thresholds.

One possible other way would be a simple relay with enough voltage rating, 30 Volts may be fine.

Another way may be a Zener diode with around 3.9 Volt reverse working voltage. Depending on the current consumption of your solenoid, you could calculate the power dissipation in the diode. Zener diodes with a rating of 1 or 1.5 Watt are widely available.

Either way, you always should add a flyback diode in parallel to the solenoid.

• Another possibility for simple load switching is a photovoltaic opto FET driver: vishay.com/docs/83469/vom1271.pdf Commented May 24 at 18:58