# Calculated vs. Measured MOSFET Switching Times

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.

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.

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?

• Layout parasitics like source and gate inductance for example can have a big impact on switching times. Jul 16, 2021 at 0:28
• 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. Jul 16, 2021 at 1:18
• Could you elaborate on why the probe tip matters? I agree layout makes a difference, but by how much?
– mike
Jul 16, 2021 at 1:32
• 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 Jul 16, 2021 at 1:39
• 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 Jul 16, 2021 at 1:41