Aluminium bar heatsink for prototype 4kW SMPS

[cupoftea]

Hi,
Doing a 4kW SMPS.....4 interleaved PFC stages....then a 4kW LLC converter.
For the first prototype it needs to be cheap......
Will it be OK to heatsink the four TO247 LLC FETS to the 4cm by 1cm aluminium bar below, and just have a fan blow along it?
(using sufficient insulation tabs etc)

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Will drill holes in it so can screw the tabs to the heatsink

 

[andre_luis]

The thicker the width near to the switching device, the greater the ability to spread short-term thermal gradients, so I would avoid thin bars.

 

[cupoftea]

Thanks yes, i think i will choose 5cm tall, by 3cm thick..should be minimum acceptable?

 

[dick_freebird]

Seems pretty short of surface area, if you're looking to do any
continuous duty max load testing.

There's not one decent finned heat sink in any of the junk boxes?
Not like they go bad....

 

[Easy peasy]

look up thermal resistance to air of some common heatsink sections - catalogs show this - then compare to your bar, a section that has 0.5 degC/W of temp rise for 10 CFM ( cubic feet per minute), can absorb 40 watts with a 20 deg C rise - I doubt your bar is anywhere near this efficacious ...

 

[cupoftea]

Thanks, the E2A-T247-38E looks a good size, but poor performance.

https://www.farnell.com/datasheets/2602810.pdf

...for some reason, they only chart it up to 5W

Each TO247 PFC FET carries 10.6A RMS, and each TO247 LLC FET carries 8A rms.

The 650V LLC FETs are SiC and the cheap ones are unfortunately 180mR Rdson(max)
The 650V PFC FETs are standard silicon so even the cheap ones are sub 100mR.

If the FETs run up to 180degC for a while then i'm not too bothered as long as they dont blow...this is a cheap-as-poss prototype. I remember working on a downhole cascaded 300W buck, and they had SiC FETs operating in a 180degC ambient, (so they said)

 

[Easy peasy]

oh dear - what did you say your air flow was ... ?

 

[cupoftea]

..thanks, hoping to use these cheap 24V fans

https://uk.farnell.com/multicomp/mc002694/axial-fan-40mm-24vdc-8-9cfm-25/dp/2885240

...just 9CFM

Thanks for the CFM vs C/W data.....for this cheap prototype, we are certainly willing to tolerate semiconductor junction temps up to 140degC....possibly more....its times like this when i wish id documented a bit more some of the thermal destruction testing that ive done in the past..........but time was never allotted for documenting.....had to move on......woops.

 

[cupoftea]

Do you know what the Tj woudl be for a TO247 dissipating 15W, and attached to the E2A T247 38E heatsink?....and with a 10CFM fan in the enclosure?

E2A T247 38E heatsink

Sink Ex | Ohmite Mfg Co

www.ohmite.com

...it says its 14degC/W but thats with no blown air