Heat sink design

Thread Starter

tagada47

Joined Nov 13, 2017
14
Hi all,

I'd like to implement some kind of heat sink to cool down the following step-down power converter: <https://www.pololu.com/product/2851>, but I have zero experience with heat sink techniques. The converter board has components on both sides of the PCB, so it's not obvious to me if I can use some kind of sil-pad.

What are my options? Adding a fan is not one of them, and the whole package needs to remain reasonably small.

Thanks in advance for your suggestions.

- Mathieu
 

Thread Starter

tagada47

Joined Nov 13, 2017
14
Kindly explain why you think you need a heat sink.
I plan to use this to continuously to power 144 NeoPixel leds with an average overall DC current of 3 A. The instructions state that "the maximum achievable output current [...] depends on many factors, including the ambient temperature, air flow, heat sinking, and the input and output voltage". The converter will be placed within a closed 40 mm diameter PCB cylinder.

- Mathieu
 

dl324

Joined Mar 30, 2015
9,601
What is the input voltage?

What are the manufacturer recommendations for heatsinking? Only the switch will dissipate any significant power. Can you identify which component that is?
 

Thread Starter

tagada47

Joined Nov 13, 2017
14
What is the input voltage?

What are the manufacturer recommendations for heatsinking? Only the switch will dissipate any significant power. Can you identify which component that is?
The input voltage will be around 7.4 V (2S L-ion pack). So far I could find no specific manufacturer recommendation. I'll look into locating the switch. Should I think about sticking a largish heat sink to the switch itself?

Thanks for helping me out. I realize I'm clueless.

- Mathieu
 

Thread Starter

tagada47

Joined Nov 13, 2017
14
Hello,

As @dl324 said, why do you think you need heatsinks?
Please see my reply to @dl324 above.

If you want to use some, have a look at adhesive heatsinks.
There are 4 chips on the bottom that might use them.
Here is a link to amazon for some examples:
https://www.amazon.com/s/ref=nb_sb_noss?url=search-alias=aps&field-keywords=heatsink+kit+adhesive&rh=i:aps,k:heatsink+kit+adhesive

Bertus
Thanks, I'll look into it. I have doubts about using these on anything other that a flat surface, but both sides of my PCB have components on them. Should I only stick a sink to the switching component?

- Mathieu
 

Thread Starter

tagada47

Joined Nov 13, 2017
14
Alternatively, I could use a similar converter from the same manufacturer, but rated at 9 A (https://www.pololu.com/product/2866) instead of 5 A.

Am I right to suspect that given the same input and the same load, the 9A-rated converter should run significantly cooler than the 5A-rated one?

Thanks again for helping out a noob,

- Mathieu
 

dl324

Joined Mar 30, 2015
9,601
The input voltage will be around 7.4 V
I think you're worrying too much about nothing. The input to output differential is 2.4V. Look up the thermal characteristics of the switching transistor and calculate what the junction temperature will be without a heatsink.
I have doubts about using these on anything other that a flat surface, but both sides of my PCB have components on them. Should I only stick a sink to the switching component?
Only the switching transistor will dissipate any significant power.
 

Audioguru

Joined Dec 20, 2007
11,251
The manufacturer of the step down converter gives no detailed spec's about how much heat. They say the max allowed voltage but probably with no current. They say the max allowed current but probably with the minimum input voltage. They say efficiencies but without saying the currents.

Guessing: Your lithium battery will average 7.4V during a discharge but will be 8.4V when fully charged. 8.4V - 5V= 3.4V x 3A= 10.2W, 15% of 10.2W= 1.53W which is probably too much heat for that small circuit board.
 

Thread Starter

tagada47

Joined Nov 13, 2017
14
The manufacturer of the step down converter gives no detailed spec's about how much heat. They say the max allowed voltage but probably with no current. They say the max allowed current but probably with the minimum input voltage. They say efficiencies but without saying the currents.

Guessing: Your lithium battery will average 7.4V during a discharge but will be 8.4V when fully charged. 8.4V - 5V= 3.4V x 3A= 10.2W, 15% of 10.2W= 1.53W which is probably too much heat for that small circuit board.
Wait, from the plots near the bottom of <https://www.pololu.com/product/2851> it seems that I'd be functioning at ~95% efficiency, so I would expect to be dissipating about 5V x 3A x 0.05 / 0.95 = 0.8W, no? Might be still too much, I have no idea...

- Mathieu
 

Audioguru

Joined Dec 20, 2007
11,251
I didn't see the graph.
5V is across the load, not across the step down circuit. The input is 8.4V when the battery is fully charged and the output is 5V so it has 3.4V across it x 3A= 10.2W x 5%= 0.51W. The manufacturer does not say how much heating it can survive. That is not a good way to sell something.
 
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