Hello,

I read the Microchip's app note in the attached files, making me curious about the gate current equation. (I understand the idea of charging cap but this look too simple). So, I made a simple switching simulation below.

EDIT: [The input is 12-0V pulse having 10ns of rise and fall time.] I measured the current waveform through R1: I(R1).
To see if the equation highlighted in the app note is valid or not, I calculated the average value of I(R1) waveform by copying the waveform to Excel, finding the average value over the rise time (0.0014ms). So, I got Ig(avg) = 10.9mA. I plugged this in the equation Ig = Qg/t_transition. We know from the IRFZ44N datasheet that at Vgs = 12V, Qg = 45nC. So, the equation yields t_transition = 4.1284us, which is not equal to the rise time from simulation (0.0014ms).

So, did I get the equation wrong?

Thank You

 

Hello,

I read the Microchip's app note in the attached files, making me curious about the gate current equation. (I understand the idea of charging cap but this look too simple). So, I made a simple switching simulation below. The input is 5-0V pulse having 10ns of rise and fall time. I measured the current waveform through R1: I(R1).
To see if the equation highlighted in the app note is valid or not, I calculated the average value of I(R1) waveform by copying the waveform to Excel, finding the average value over the rise time (0.0014ms). So, I got Ig(avg) = 10.9mA. I plugged this in the equation Ig = Qg/t_transition. We know from the IRFZ44N datasheet that at Vgs = 12V, Qg = 45nC. So, the equation yields t_transition = 4.1284us, which is not equal to the rise time from simulation (0.0014ms).

So, did I get the equation wrong?

Thank You

I've never been able to get it to work either. I could come close by breaking the current up into the 3 phases of turn on, but never to match several FETs.