Monday 8 February 2021

QRP Labs QCX Mini - Part 2: Adding Cooling

The QCX Mini! An amazingly small and versatile mini CW - only transceiver for amateur radio. Read all about it here:

QRP Labs Kits website

The kit is really nice out of the box, but there's always room for modifications. Some are necessary, some are just nice-to-haves or just plain silly (I'll leave it up to you to categorize this particular mod).

Cooling the RF finals

The QCX Mini is advertised as a 5-Watt transceiver and this goal can definitely be achieved on a 12 Volt power supply, especially if you follow Hans Summer's video tutorial here:

Hans Summers: Tuning up your QCX+

A bit of tweaking yielded just over 5 Watt @12 Volts. However, increasing the power supply voltage to 14.4 Volts (standard output for a LiFePo4 battery pack) then saw an increase to over 7 Watt! The RF power amplifier did start to run a bit hot though. And as usual, that's when my OCD decided to take over. What if there was a way to improve the cooling?

Let's dig in shall we?

The RF power amp is designed around three BS170 MOSFETS, held down by an M3 bolt and washer. First I decided to just add a second (larger) washer on top, and add some heat sink paste to improve heat conduction. Like this:

While this will probably help a bit, it didn't look like a worthwhile improvement. Hmm, I wonder if there are cooling solutions small enough to fit on top of the transistors?
But of course there are!

BGA heat sinks can be as small as 14x14x6mm, almost (but not quite) small enough to fit in the QCX Mini. A couple of the fins will need to be removed or filed down, and it needs a 3mm center hole.
I also reshaped the bottom somewhat to improve thermal contact with the transistors:

That's better! Add some new thermal paste and refit, using the original bolt and a smaller washer:

And yes, it really does fit in the very limited space available under the display board:

Will it make a difference?

This whole exercise was meant only to satisfy my curiosity really. I don't know if it will make a difference in the long run; I don't even know if cooling these transistors is even necessary or effective. But at least I've given them all the care and attention I was able to think up. I will still keep a number of BS170 in stock though, just in case...

Any comments on this article? Need more details? Please drop me a line below!

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