Electronic – PWM-driven MOSFET heats up

I think you may be exceeding what is known as the volt-microsecond limit for the transformer. Basically this figure tells you how long you can apply a pulse of a certain voltage before the voltage reverses. Real transformers will saturate and the V.\$\mu s\$ limit tells you, in practical terms how far you can push the transformer.

The waveform initially looks good on the rising edge - it attains a peak of 4 or 5 volts (it looks that way from your pictures) but after about 10 us it goes wrong - this means your transformer is probably rated as having a volt-microsecond product of about 50.

Other problems with the circuit are: -

• You need more gate drive voltage to adequately and efficiently turn the mosfet on
• As you vary mark-space ratio, the peak voltage seen by the gate (from the transformer) will vary considerably - you need to have a clamp circuit on the secondary that keeps the gate drive at a decent level for varying mark-space.
• I don't think that transformer is man-enough to provide the current needed by the gate to efficiently switch the mosfet without incurring heavy losses.
• The transformer drive circuitry also isn't powerful enough to drive a transformer should you have a transformer (and clamp) that would fit the bill efficiency-wise.

Here is a helpful (in parts) article on the subject. And here is a helpful guide on designing transformer circuits for driving mosfets: -

You can probably see in one of the diagrams (fig 3) the problem you are getting.

Your selection of transistor IRF520 if ok. The required gate threshold is 4 volts or less, as seen on data sheet seen here : Data sheet Your Arduino is putting out 5 volts.

The drain load of your device seems ok with being a 12K equivalent and drawing only 1mA at 12 volts is also ok. The IRF510 will turn off even with a high resistance device in the drain circuit.

Your gate resistor is also ok.

Replace your load device with a 12K resistor and test again. You have shown that your transistor will be off when you disconnect the Arduino pin from the gate (resistor pulls gate low).

Since this is a DC circuit, use voltmeter rather than oscilloscope. The problem is in the measuring and possible ground conflicts. Make sure that your 12 volt supply ground has a good connection to the Arduino ground.