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I am currently evaluating several power MOSFET’s looking at their switch on and off times.

My testing has employed a few different gate drivers, however the bulk of my testing is with the ST Micro L6491 gate driver IC.

The MOSFET I am hoping to proceed with in my project is the Nexperia PSMNR90-40SSH.

Referencing Vishay document AN608A and the following parameters from the PSMNR90-40SSH datasheet, I calculated the following as the turn on and turn off times.

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Having set up a test circuit using a 24 Ohm gate resistor, the following are the scope images for the total turn on and turn off of this MOSFET. These measurements assume turn on is 10% Vgs to 10% Vds, and turn off is 90% Vgs to 90% Vds. The load in the test circuit is a 3 Ohm thick film resistor.

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Turn-on = 452ns Turn-off = 636ns

I have made about a dozen sets of measurements using various gate resistor values and with three other MOSFETs; all of my measurements are values that far exceed the calculated estimates.

I did find a SPICE model for the PSMNR90-40SSH and modeled the test circuit in NI Multisim. The measurements for turn on and turn off were:

Turn on = 180ns Turn off = 438ns

How do I account for the discrepancy between calculated times and measured values? How are other people estimating accurate switching times? Are there any considerations I should be making that I may have overseen?

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    \$\begingroup\$ Layout parasitics like source and gate inductance for example can have a big impact on switching times. \$\endgroup\$
    – John D
    Jul 16, 2021 at 0:28
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    \$\begingroup\$ You should not be using an x1 probe for this BW.USe a Buffered FET probe or use a 10x passive probe with coil spring and probe tip and gnd clip removed. then calibrate it. Also the layout matters. \$\endgroup\$ Jul 16, 2021 at 1:18
  • \$\begingroup\$ Could you elaborate on why the probe tip matters? I agree layout makes a difference, but by how much? \$\endgroup\$
    – mike
    Jul 16, 2021 at 1:32
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    \$\begingroup\$ It is the ground lead inductance that can cause issues. 8 nH/cm or so with coax capacitance can resonate. Did you use short coax 1:1 then 30 pF /ft \$\endgroup\$ Jul 16, 2021 at 1:39
  • \$\begingroup\$ the long plateau in Turn ON tells me 24 Ohms is less optimal than 5 Ohms yet that's what they used. Ground coupling also adds capacitance but reduces impedance \$\endgroup\$ Jul 16, 2021 at 1:41

1 Answer 1

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Having shortened and used larger gauge wire from driver to gate, and most importantly changing my probing technique (x10, shortened ground path) I was able to get a time that was more in line with my calculated estimate (~160ns with a gate resistor of 24Ohms). Probe ground was way too long; I attached my ground to a more suitable location.

Below is the waveform showing the on time for this MOSFET using 24 Ohms.

oscilloscope screenshot

In summary, layout and proper probing technique makes a huge difference.

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  • \$\begingroup\$ Inductance will be around 10 nH /cm but depending on w/l ratio . \$\endgroup\$ Jul 19, 2021 at 13:37

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