Motor controller heatsinking

The controllers for the improved actuators for SMMB have a moderate amount of power to deal with.  During jump maneuvers they can put 60 amps of phase current into the motor, and I’ve applied for very short intervals over 500W of power to a motor.  The FETs on the board are relatively high performance, but there is still a fair amount of heat that has to be dissipated.

When getting started, I knew I would likely have to do something to get heat out of the board and had a two stage plan.  The first was to heatsink the back of the controller board and second, if that wasn’t enough, heat sink the front of the board.

DSC_1529.JPG

The back of the board has copper planes on the bottom-most layer which are connected by many vias to the primary FETs, with the express intent of heatsinking to them.  A heatsink on that side can thermally couple the control board, through the solder mask, to the motor mounting bracket and the motor itself.

moteus_r3_back_annotated

I went with just a simple waterjet cut aluminum plate of appropriate thickness and some kapton tape covering the holes that lacked soldermask on the back of the board.  Here’s a bunch of the first revision back from eMachineShop:

dsc_1571

Installed onto the controller and motor, this looks like:

I tested thermal dissipation of the controller with no back heatsink, with a back heatsink but no thermal paste, and with the heatsink and thermal paste. In each case, I tried to let the temperature reach steady state while wiggling the motor around to apply approximately equal power to all phases.

No Backplate Backplate w/o Paste Backplate w/ Thermal Paste
5A 35C 31C 32C
10A 47C 36C 35C
15A 65C 47C 40C
20A >70C 57C 47C
25A N/A >70C 59C
30A N/A N/A >70C

For these motors, the motors themselves have thermal problems even at 25A continuous, so this means that just the back plate with thermal paste is enough to make the controller not be the limiting factor.  Thus there’s no need to mess with what would be a more complex to design and manufacture front heatsink as well.

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