Power Mosfet datasheet test circuit Rgext value- switching times

[jumbodas]

Nearly every vendor has similar test circuit for switching times. It uses an external gate resistor. Switching time strongly depends on this part.
Does anyone knows how suppler decided value of this part? I mean precisely how it is determined the right value for test circuit. I mean what is the clear criteria to use a particular value.
I understand too low will cause ringing, too high will increase switching times but what is the exact criteria to select a specific value?

If you know the answer, please do share this is your opinion or this is what vendor reported. Thankyou.

 

[KlausST]

Hi

sadly I don´t know the answer regarding math and formulas.

It´s always a compromise and the designer needs to decide which way to go.
What plays a role:
* mosfet
* switching times
* driver circuit
* PCB layout
* schematic
* application requirements

***
For example: on a bad layout you have increased stray inductance, thus the increased risk fro ringing. You need to compensate the bad layout with increased series resistance to .. and reduced switching speed .. and increased switching loss.

Klaus

 

[ahsrabrifat]

You may find these helpful:

 

[dick_freebird]

Every power MOSFET has a drain dV/dt which can trigger bipolar snapback conduction (bad). The minimal gate resistor to prevent this given the drive source and Cdg particulars, may be what you see (larger does not benefit the spec).