Zen v5 power supply heatsink

[PedroPO]

Hi guys!

I almost finished my zen V5 and I'm dealing with heat problems with Q3 and Q4 of the power supply. With a small heatsink It get over 60ºC in 10 min or so.

How does one know what is dissipated power of the fets?

I have 24V regulated and the amp is biased at 1.5A.

[moe29]

Pedro! good to see you. glad to see you're still around.

I'm not sure how you can figure the dissipation of those FETS,
but it seems like you may need a bigger heatsink for them, or if
you can't do that, put a small fan on it.

...or let it sit for an hour and see what your max temp is. If it's
around 60C maybe just let it be?

m.

[PedroPO]

Yes I'm still here, just a little to bussy to do the good stuff

My sinks are very small (3x5 cm) and if the temperature arrives at 60ºC in just 10 minutes I wonder what will happen in a hour or so

if I use a clamp meter I could measure the current flowing trough the fet, but I'll need to take it from the board were it is right now.

I'll send a pic later with the amp as is.

[PedroPO]

here is the promised picture.

See the size of the sinks in the psu.

they are small, but how bigger do they need to be?

[joensd]

Power dissipation depends on your "input voltage" of the regulator measured after the coils (so at C5).
(Measured voltage minus regulated voltage) multiplied with bias.
I think the regulator is well described in the ZenV3 article.

[moe29]

Pedro, I wouldn't be surprised if those fets weren't dissipating
around 20 watts each - maybe more.

When i was using a regulated power supply on my SOZ i mounted
the FETS on the main heat sinks. Zen version 4 does the same thing.

[joensd]

Even if you leave out the "zener string" the fets dissipate around 4V*bias and so 6W.
It´s hard to judge the height of your sinks but they look fairly small even for 6W.
Put them on the main sink like moe29 suggested.

[PedroPO]

can't do it. the psu will be on a different case.

I'll use big heatsinks in the psu

[joensd]

I'll use big heatsinks in the psu

Keep in mind that exterior heatsinks where the air can flow through the fins from the bottom (in comparison with your PCB-mounted heatsinks) will be much more effective.
Depending on dissipation they probably don´t have to be SO big.
To get it cheaper you could also regulate a bit higher, say V+ is 28V. So you "move" some power dissipation from the PS-regulator to the amp. Regulation should be sufficient if you still have at least 4V margin to the voltage after the inductors.

[azira]

Originally posted by PedroPO
Hi guys!

I almost finished my zen V5 and I'm dealing with heat problems with Q3 and Q4 of the power supply. With a small heatsink It get over 60ºC in 10 min or so.

How does one know what is dissipated power of the fets?

I have 24V regulated and the amp is biased at 1.5A.

Measure the voltage from drain to source of your FETS. Multiply by current through your fets.

Power = V x I

--
Danny