i have some doubts about with this thermal design?
first, the calculations and infos: My load (a motor) uses an average current of 7A, I could not measure the current peak at the beginning of the rotation (we shoot 20A peak).
- Qgc = 100nC
- MOSFET Rds: 850 microOhm (VGS = ~7V) (I over estimated Rds from datasheet)
- Power dissipated by the mosfet: P = I² * R
- P = 400 * 0.000850 = 0.19125 W
Given this power for dissipation, I have a pad (this right over Q1) in the upper layer of approximately 10mmx20mm = 200mm². and in the lower layer of 20mmx20mm = 400mm². Which are connected though vias
I have two doubts:
- Is it possible to use only the upper layer for this thermal dissipation? I would like to manufacture the pcb here in my homemade CNC, and I would not be able to do the "thermal vias" in a homemade way.
- Is it possible to decrease the peak current (in-rush current) that the motor consumes while starting? I imagined that placing a capacitor parallel to Gate / Source would increase the time to charge the gate; this kind of worked, but if the turn-on time increased by 5%, it tripled the turn-off time.