The customer would like to know why is the source and sink current calculated both use Qgd /tr. He wonders why it isn't Qgd+Qgs / tr.
Please help to check it. Thank you!
Best Regards,
Cherry Zhou
Hi Cherry,
Please see the below answer to your question -
During the MOSFET operation stages, Qgd is the charge being provided by the gate driver (through source/sink current) that results in the MOSFET drain-source voltage changing. This phase is called the 'Miller' region of MOSFET operation - and the rise time (t_rise) for the output voltage (Vds) change depends on this slew rate, which is why the calculation is IDRIVE = Qgd / t_rise.
Qgs is the charge required to get the MOSFET gate from 0V to the miller plateau voltage. At this time, Vds is not changing, because the Miller plateau voltage has not yet been achieved. Therefore, Qgs is not part of the calculation between the slew rate of Vds and the source/sink current.
A great document to refer to for understanding this mechanism would be our app note on Smart Gate Drive (this is the source for the info shared above).
Best Regards,
Andrew
