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Old 11th February 2011, 01:39 AM   #1
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Default Heatsink isolation with two chips

This is for a stereo amplifier with dual LM1875s.

Both chips are supplied from the same power supply, same rails. They're on the same board.

Is it a bad idea to bolt both chips to the same heatsink, without isolation? I understand that the cooling tab is electrically connected to the negative power supply rail.

I could put mica spacers, but that would compromise heat transfer.

Of course, cutting the heatsink in two would also work, but it's easier to make the amplifier more rugged when it's all bolted up together.
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Old 11th February 2011, 07:14 AM   #2
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Use the mica. It's been used successfully for over 50 years.

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Old 11th February 2011, 10:22 AM   #3
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You can bolt the chipamps direct to the heatsink. It then becomes "live". You must ensure it can never be accidentally shorted to any other component.
You may have to add a decoupling capacitor from heatsink to -ve supply and/or to Power Ground.
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Old 11th February 2011, 10:45 AM   #4
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Frankly, I hate live heatsinks. It's far too easy to short something out whilst testing on the bench.
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Old 11th February 2011, 04:36 PM   #5
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It is also inconvenient to touch them.
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Old 11th February 2011, 10:19 PM   #6
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Both chips each have their own dedicated power supply decoupling caps located nearby.

I can relate with the "accidents" relating to a live heatsink. I blew a $220 MP3 by touching its headphone output to a live heatsink...

In this case, I'm not concerned about accidental shorting, since the heatsink will be inside an enclosure.

I was just more wondering about the effects of having power supply current being conducted via the cooling tabs and heatsink rather than the PCB copper tracks.
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Old 12th February 2011, 07:40 PM   #7
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'Live' heatsinks also add possible 'ground loops'. Yes, I know it's the negative rail but the principle is the same. In general it's poor practice unless you have
a really good reason and it's very clear the heatsink is 'live'.

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Old 13th February 2011, 12:23 AM   #8
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Quote:
Originally Posted by pmbrunelle View Post
Both chips each have their own dedicated power supply decoupling caps located nearby.

I can relate with the "accidents" relating to a live heatsink. I blew a $220 MP3 by touching its headphone output to a live heatsink...

In this case, I'm not concerned about accidental shorting, since the heatsink will be inside an enclosure.

I was just more wondering about the effects of having power supply current being conducted via the cooling tabs and heatsink rather than the PCB copper tracks.
The pcb tracks are copper and will carry most of the current anyway, as aluminium has a higher resistance.

In my 6 channel amp, i just used seperate heatsinks, because the lm3875 has the same negative power on its tab.
I prefer much better heat transfer over the live heatsink, as all my heatsinks are inside a box.
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Old 13th February 2011, 06:12 AM   #9
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You mean a massive block of aluminium with several square centimetres of conducting area has less resistance than a 35 µm thick copper track that is a few millimetres wide, if at all?
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Old 13th February 2011, 10:55 AM   #10
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I would imagine that any amp builder would build up the current carrying tracks on the board to hold those currents.

Though your point is very valid.
A large chunk of al would conduct pretty damn well heh.
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