Potting a 510W LED driver PCB to improve thermal performance?

[treez]

Hello,
We have a 510W switch mode LED driver PCB (three 170W channels) and we cannot mount it on a heatsink a there’s no room in the enclosure for a heatsink.
There’s also no room for fans in the enclosure.
Also, we cannot have vents in the enclosure sides.
In order to try and make the thermal situation for this SMT PCB as best as possible, we believe that we should pot it?…because then at least the hot components will better interface to the air inside the enclosure.
We believe the (60mm by 70mm) PCB can be coated in rubbery, low thermal impedance potting compound…..rather like the way a lollipop is kind of grown over a lollipop stick.
What do you think of this idea?
(obviously we will do everything we can with adding as much thermal spreading copper to the PCB also)

 

[FvM]

It's mainly the PCB surface, insulating air layer, inner and outer enclusure surface that defines the achievable heat dissipation.

The only relevant improvement in case of a hermetic enclusure would be direct PCB to enclosure metal contact or ultimately complete potting. The external enclosure still sets a power limit.

You should talk about expected power losses rather than output power.

 

[D.A.(Tony)Stewart]

I would need more details on layout, schematic and hotspots, to advise.

What is the total worst case dissipation per device , the junction temp. Rise desired & actual ?

There may be better FET choices to reduce losses, but little room to improve Rth.

 

[treez]

Regarding your suggestion, along similar lines....I reckon lob a slab of 'gloop' Potting compound against the enclosure metal wall and ram the entire PCB into it. ...and it'll get stuck there, conducting its heat to the enclosure walls..?

 

[D.A.(Tony)Stewart]

That would increase temp rise unless you filled it with oil. If you want help pls answer my questions.

 

[treez]

we chose the lowest rdson fet around with also low qg...it was in one of those DFN5X6 packages......The diode dissipation was about 450mW, and the fet and inductor were less. (of course its three channel, so there are three diodes, three fets, three inductors.

 

[treez]

The total losses in this three channel LED driver when driving the maximum power led load is as follows..

AON6284 FET conduction loss = 0.23W.
AON6284 FET switching loss = 0.19W (Switching frequency = 200KHz)
SBRT25U80 diode conduction loss = 0.105W
MSS1210-333 Coilcraft inductor loss (total) = 0.438W (by coilcraft web loss calculator)
R(sense), hi-side, 1812 = 0.310W
R(sense), lo-side, 2//1206’s = 0.172W
…Obviously the above get multiplied by three for the total loss’s to the components of this SMPS….therefore , the total component loss in this SMPS is just 4.335W
…That’s efficiency of around 99%…sounds unrealistic, but that’s the figures.

 

[D.A.(Tony)Stewart]

So where is the thermal performance issue?

 

[Orson Cart]

If you are to pot the circuits - use a compound with the HIGHEST thermal conductivity that you can afford - this will spread the heat from the "hot" parts and lower temps, the extra surface area provided by the potting compound gives the ability to shift the heat to air.