# Why is the MOSFET in my circuit getting very hot?

I want to drive the inductive coil (26 ohm) at 48V DC. I have a 4N65 N_MOSFET. I'm giving 9V Vgs @ 150Hz switching square signal. For spike protection I used TVS diode having part number 3.0SMCJ170A.

The TVS diode and the MOSFET both heat up. How can I get rid of this?

• Just so you know, 9V does not achieve the rated RDson. The rated RDson is achieved at 10V. If you chose 9V because it was larger than Vgs_th, then don't do that when using the MOSFET as a switch because that is the voltage that the MOSFET just barely starts to conduct at which you do not care about. You care about the voltage it is fully conducting at. Probably not a problem in this case because 9V is so close to 10V, but normally you would drive it with 12V or 15V if the rated RDson is achieved at 10V. Commented Oct 19, 2022 at 4:15
• I tried from 24V to 7V, but not get done. Commented Oct 19, 2022 at 4:18
• @BhushanWaghe Looking at a datasheet I see this. That's up to $2.5\:\Omega$ with $V_{_\text{GS}}=10\:\text{V}$ and $I_{_\text{D}}=2\:\text{A}$ (about where you are at.) That could be as much as $10\:\text{W}$ dissipation while ON. If you aren't doing much to heat-sink it, then of course it is getting hot. Cripes!
– jonk
Commented Oct 19, 2022 at 4:44
• Noted sir, but due to my project fitment issue, i can not place heatsink over the PCB. Can you suggest me another MOSFET which can work fine? Commented Oct 19, 2022 at 7:12
• @BhushanWaghe, don't use a TVS diode. They are for clamping infrequent ESD discharges, not for dissipating high-current spikes so it's understandable that it gets hot or fails. That will then also damage the FET, on top of its other problems. Connect a power diode as a clamp diode across the inductor coil, with the cathode to 48 V and the anode to the FET drain. Zener diodes can also be used and you can easily find lots of detailed explanations on all of this on the internet, as per the answer below. (Please remember to address reply comments to users, using '@username' in your comment.) Commented Oct 19, 2022 at 7:23