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I have the following circuit:

click here for datasheet

This is the data sheet for the MOSFET.

Using equations, I worked out that my Rgate should be 2.36 Ω. Rtest was a 470 Ω ceramic block resistor (due to it getting quite hot when turned on).

Connecting the oscillator, I see that I have this ringing effect on the square wave as shown here:

enter image description here

I am just wondering what the cause of this ringing effect is. Maybe you can provide a little detail on what it is and how to fix it. I increased the resistor value to 47 Ω which does lower the effect, but why?

enter image description here

Also, do you have any ideas on how I can calculate the power loss of the MOSFET?

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    \$\begingroup\$ Describe exactly how the oscilloscope probes are connected to the circuit. \$\endgroup\$
    – Elliot Alderson
    Apr 14, 2020 at 14:12
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    \$\begingroup\$ This question is not really answerable without seeing detailed pictures of how your circuit was laid out, how the scope was connected to it, etc. The ringing is due to parasitics and is affected by the physical layout of the circuit. \$\endgroup\$
    – Kuba hasn't forgotten Monica
    Jun 10, 2022 at 19:14
  • \$\begingroup\$ It's interesting that it rings on the falling edge as well; I wonder if common mode is involved as well. All the more reason to see layout, component types, etc. What's policy/procedure anyway, on closing posts like this with an absent author? \$\endgroup\$
    – Tim Williams
    Jun 10, 2022 at 19:58

3 Answers 3

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As others have said, the ringing you see is most likely due to the probing you are using (for example having a crocodile clip to ground and forming a loop). If you are using wire wound resistors then they can also be guilty as they have a relatively high inductance. PCB layout can also be a culprit and if you are using a protoboard then I would look at that first and foremost.

On measuring the MOSFET dissipation I would suggest you record the current with a current probe or a small series resistor on the source of the transistor and the voltage across DS, then integrate voltage times current for one cycle and multiply by frequency. In other words integrate the instantaneous power for one second and you will get the energy dissipated in one second which is numerically equal to the average power:

$$f \int_0^{2\pi} I \times V$$

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The signal rise/fall times are so fast that either you have incorrect scope probing method with long ground clip, or there is too much stray inductance from the PCB wiring. The series resistance slows down signal edges as it forms a RC filter with the FET gate capacitance.

If the probing is correct and the effect is real, then you need a snubber network to dampen the ringing.

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Rtest is probably a wirewound resistor...therefore an inductor. I suspect this is the primary source of the ringing. Oscilloscope probe calibration may be a contributing factor, as well as other long wires connected to the output.

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