Looking at the PCB layout, the fan hub will take up a good amount of the heatsink area. I assume there will be very little airflow underneath it so I was wondering whether this will be an issue.
Do you think I should find a different place to mount it, or maybe leave a gap between the heatsink and the fan? I'm also unsure about whether to mount the fan blowing up or down.
Edit: The rectangle in the center is the heatsink (the inner part is the footprint on the PCB, the outer part is the size at the top of the heatsink since the fins spread apart), the smaller circle is the fan hub, the bigger circle is the fan, the big rounded square is the fan housing.
I swapped the 120mm fan for an 80mm one, which is the smallest size for which the screw holes will fit around the heatsink.
Some extra info:
- I'm designing this as a prototype to be used without a case around it. If I ever end up putting this in a case, it will have enough airflow not to affect performance too much.
- MOSFET's max channel temp is 150°C
- MOSFET's channel-case Rth is 0.462°C/W
As I understand it:
This means that the max case temperature is 150°C - (80W/6 * 0.462°C/W) = 143.84°C
At 25°C ambient, the heatsink would need to have a Rth of (143.84°C-25°C) / 80W = 1.48°C/W or less.
The heatsink's Rth ranges from 1.3°C/W at 1m/s airflow to 0.6°C/W at 4m/s. The fan has an airflow of 1.55m³/min which equates to 4.04m/s over an area of 8cm*8cm.
I suspected that I shouldn't be expecting an actual Rth of 0.6°C/W with the fan mounted above the heatsink, but I was wondering how bad it would actually be. By 'an issue' I meant that the MOSFET's channel temperature would get close to or exceed the maximum of 150°C.
I mounted the fan on top of the heatsink in the first place since that seemed like the most obvious solution and because that takes up the least amount of PCB area. I'm not sure how I would mount it vertically.