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Old 20th June 2005, 08:49 PM   #1
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Default LM4780 and cooling

I'm frenchh so sorry for my poor english.

I have a problem with the LM4780?


I made it but temperature is very hot. I mesured 40°C to 51°C on the heatsink with my multimeter!


Because it is my first realisation, I don't know if it is normal?


Power:+-38V

Do you need for more informations?


At the same time i post some photos:

Back

Front

Side

Inside
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Old 20th June 2005, 08:58 PM   #2
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Hi John and welcome

I fixed your links, if you happen to correct them on your site let me know and I'll redo the job.

40 to 51° above ambient. Of course it's hot in France these days.
What is the room temp?
Is that with no signal or at normal volume?
I see that your heatsinks look fairly heavy so perhaps the amp is oscillating.
Do you have a scope at hand?

-=Hugo=-
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Old 20th June 2005, 09:42 PM   #3
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Well, temperature in my room is 30°C.

40°C-> with no signal.
51°C-> high volume, and the Spike sytem start .

Maybe this could help you:

Loads are 6 ohm 50W speakers, so not realy adapted for a 60W amplifier.


Sorry but I don't have a scope with me (unfortunately for me).
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Old 20th June 2005, 09:52 PM   #4
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Without looking at the datasheet, 10°C above ambient looks fairly normal to me for a 60W amp and 21°C at full power also. A lot depends on the idle current of the chip, and the heat sink properties.
Only a scope could tell you if the amp oscillates.
Also, a little fan can do wonders these days.

/Hugo
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Old 20th June 2005, 10:36 PM   #5
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Ok .

Thanks for your answers.


How can I measure/obtain the "idle current of the chip"? And what it is?

"Only a scope could tell you if the amp oscillates." -> How I measure if I had a scope? (Between groung and output?)
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Old 20th June 2005, 10:51 PM   #6
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Are you running the LM4780 in parallel? If so make sure both side have their resistors very closely matched, otherwise your amp will run hot because the two halves are fighting each other. I had that problem, and resistor matching fixed it.

I also have a LM4780 amp that powers my mains, and for added safety just to be sure I never hit the SPIKE protection I added a 12V fan, which is powered by about 4V so it runs very slow, it keeps the chips very cool, and is silient.

I hope that helps.
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Old 21st June 2005, 03:08 AM   #7
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You can calculate the thermal resistance (C/W) from National Semi's website -- go to the overture design tools -- for +/- 38VDC the heat sink may be a bit undersized.

i would also direct the switch leads to the power supply away from the input connectors -- very easy to couple a.c. into the input -- route at right angle away from the input RCA jacks.
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Old 21st June 2005, 06:27 AM   #8
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Hi John,
in my opinion the heatsink for two (you used two chips each in parallel-mode, right?) chips is toooooo small! also i think this heatsink isn't good for cooling without a fan: the rips are too close for passive cooling...

In my 4780er i used a heatsink with about 0.8 K/W thermal resistence. Because auf the dimensions of such a heatsink you have to spread the active area of the chip by adding an additional heatspreder. I put a copperplate with about 4 or 5 times the chip area and 6mm thickness between chip and heatsink so i increased the active area which transferes the heat from chip to the heatsink.

BTW: don't forget to mount the chip isolated on the heatspreader/ heatsink...

Michael
PS:I place a photo of my solution if i'm back at home...
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Old 21st June 2005, 07:00 AM   #9
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hope this small image shows the idea of heatspreading...

Michael
Attached Images
File Type: jpg heatsink.jpg (48.8 KB, 658 views)
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Old 21st June 2005, 08:11 AM   #10
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No, I don't use the chip in any parallel/bridge mode.

I use it in the basic configuration: 2*60W. I utilized the datasheet of National Semi to make my pcb (figure 7 in the datasheet ).

I mounted one package on the heatsink. The heatsink results of a 2*100W old amplifier (maybe a STK**** , but I don't know).

I made this amplifier for a very low price. I baugh only resistors and capa (exept the 2*6800µf).

For the package I utilized a mika isolation and thermal paste.

Considering your remarks, it is normal that heats much.

To Slakman: you wrote "BTW: don't forget to mount the chip isolated on the heatspreader/ heatsink..." . What is the consequence of no?
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