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Can someone verify my thermal calculations?

Andrew J

Maths is not my strong point but I think I've got basic calculation correct.  So: 4 MOSFETs connected in parallel, on a heatsink with a Sil-Pad isolator.  Assume that the MOSFETS are connected correctly and are sharing the load equally.

Thermal Calculation
Max Power 150 W From design limit
Package Dimension 1.648 cm^2 From datasheet for MOSFET, case and tab
Package Dimension 0.255 in^2 Converted
Parallel MOSFETS 4 From design
Power/MOSFET 37.5 W Power shared equally
Max J Temp 175 C From datasheet for MOSFET
Ambient 35 C Inside Case, assumed when running hot
Rjmb 0.5 C/W From datasheet for MOSFET1
Thermal Paste 0 C-in^2/W Sil-Pad doesn't require paste
Isolating Pad 0.3 C-in^2/W Sil-Pad K-6 datasheet
Heatsink 1.4 C/W Natural convection, from datasheet
Junction to case 18.75 C Power per mosfet * Rjmb
Case to Heatsink 2.86875 C Power per mosfet *((paste + pad) * package dimension)
Heatsink 52.5 C Power per mosfet * heatsink
Ambient 35 C
Total 109.11875 C
%Max J Temp 62.35%

MOSFET datasheet: https://4donline.ihs.com/images/VipMasterIC/IC/NEXP/NEXP-S-A0002881467/NEXP-S-A0002882763-1.pdf?hkey=6D3A4C79FDBF58556ACFDE234799DDF0

Sil-Pad K-6 datasheet: https://www.farnell.com/datasheets/90833.pdf 

The "tricky" bit to watch out for is the area calculation for the Sil-Pad to reach the case to heatsink impact, particularly as I'm converting square cm to square inches.  K-6 seems to be pretty good, but I have seen some quite high C/W figures for silicon pads and this calculation is trying to convert from an area-based thermal resistance to a C/W thermal resistance to keep the units the same through the calculation. EDIT: I'm aware the 0.3 figure will be based on pressure applied and the overall area used during manufacturer's testing, unlikely to be TO-220 sized!  However, I do need to include something in the calculation with the intention of seeing how close to maximum allowed I get.  I'll confirm with real life testing of course and I'm also trading that 0.3 value against the heatsink's 1.4 value as I will in fact be using a fan to bring that figure down.

Alse:

1 The MOSFET datasheet table 5/Fig.4 on page 4, covers the Junction to case temperature but it isn't clear.  It states a MAX of 0.5K/W (0.5 C/W) but is this a max allowed or a max it can be - on the basis that it can be lower if the power through the MOSFET is pulsed more quickly?  According to the graph, 0.5 is based on a pulse duration that never exceeds half the cycle (single shot through to theta = 0.5) once that cycle reaches approx. 0.06seconds but these MOSFETs will be used continuously as part of a DC load, in other words theta would be 1 but this is not shown on the graph.  Therefore, 0.5 doesn't seem appropriate and also seems inordinately low for a TO-220 package.

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  • 0

    AJ my feedback is anecdotal. Thermal dynamics was the one course in college I struggled with. Passing it was critical to graduating. I did get my diploma but avoided electronic design. My heatsink choices were always big is better:)

    A system that depends on mechanical means (i.e. fans) to ensure it remains within limits should have a protection circuit to prevent thermal runaway. The transmitters I worked on would back-off power to the finals in order to keep the building from going up in smoke if airflow was compromised. I learned early if you put your toolkit in front of the air intake, you generated alarms. We are talking KV size transmitters.

    It would be unfortunate to build such a system to have it go up in smoke because a fan went unservicable.

  • 0 in reply to colporteur

    To be honest, I mostly 'get' it.  The biggest difficulty I have is translating between the different units that manufacturers use, particularly W/mK.  I do have temperature monitoring in place in the design and can automatically turn it on/off as needed.  For initial testing, I also have a means of adding two additional thermistors so I can check temperatures of components.

  • 0 in reply to Andrew J

    Sound like you have given some thought to the possible eventuality.

    Environmental monitoring was the norm in high-power equipment. I recall vane-switches, not much different than this beast as a  sensor in a complex system to keep 50KV transmitters cool.

    image

    We spent a considerable bit of time doing fan motor and filter maintenance. Few new techs from school had ducting experience. Airflow was critical to keeping the equipment on the air. Banging tin was just part of the job.

    Today's system, just look at the BIOS of a desktop computer, have all kinds of monitoring. Flame detection isn't one of them:) Good luck with your project.

Reply
  • 0 in reply to Andrew J

    Sound like you have given some thought to the possible eventuality.

    Environmental monitoring was the norm in high-power equipment. I recall vane-switches, not much different than this beast as a  sensor in a complex system to keep 50KV transmitters cool.

    image

    We spent a considerable bit of time doing fan motor and filter maintenance. Few new techs from school had ducting experience. Airflow was critical to keeping the equipment on the air. Banging tin was just part of the job.

    Today's system, just look at the BIOS of a desktop computer, have all kinds of monitoring. Flame detection isn't one of them:) Good luck with your project.

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