Hello all,

I built a simple circuit to drive a nichrome wire heating element. Unfortunately, the FET is overheating withing a few minutes of power being applied. When I scope the drain, I find it several volts above ground with an AC waveform on it. The gate signal looks good to me. I control the FET with PWM but for the moment I am driving it with 100% on time. Power is supplied from a variac into a 1000VA isolation transformer to the circuit. Currently running around 20-30VAC. I have used colors in coordination to the scope screen shot to show where I am probing; blue is the gate, green is the drain, purple is the high side.

Schematic:



(http://nicksmachine.org/img/4hv/scope2.png)

Scope waveforms:


http://nicksmachine.org/img/4hv/DSC_0002.JPG

Any ideas on what I am doing wrong? I added a snubber to the FET between the drain and source terminals, 2nF cap in series with 20ohm resistor. This is not shown on the schematic.


Mods Note:
Please upload the schematics and image files to the forum, the files had been uploaded.

 

What is the resistance of the heating element?

 

7.1 ohms, measured at room temp.

 

At 20V that would be about 3A. From the FET datasheet Rds is about 0.57Ω @3A and 10V gate-source voltage. That means you should expect about 1.7V on the drain and I read your scope correctly it reads about 2V - well within all those 'about's. It follows that there will be about 5W disipated in the FET. What is the thermal resistance of its heatsink?

 

Appears that you need a MOSFET with a lower Rds(on).

 

I am using this heatsink: https://www.digikey.com/products/en?keywords=ra-t2x-51e

With no fan so 3.5 C/W but I might add a fan to reduce thermal resistance to 1 C/W. I use thermal paste, no sil-pad so it is not electrically isolated.

I did order some new FETs (R6030KNZ1C9) which have RDS on of 0.130ohm max which is significantly less. Should have them tomorrow.

I measured the current FETs case at 156C after just a few minutes and shut it off.

 

What is the reference for the 12V+ rail? Is it GND, or HV- ? Looks to me as though the 12V supply should be floating.