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Old 29th November 2006, 01:23 PM   #1
Niila is offline Niila  Finland
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Default LM4780 Heat problem

Hello!

Im building a LM4780 stereo amplifier as my schoolwork. Im using one chip for two channels with 36V rails. The problem is that it produces too much heat. I could only try it for like 5 seconds before it was too hot for my fingers. Of course i used heatsink.

I used signal generator for testing.. I have double checked everything and there should not be any problems. I used the schematic found in the datasheet.

Heres some pictures:

Picture 1

Picture 2

Picture 3

Thank you!
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Old 29th November 2006, 02:00 PM   #2
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Default Heat

What size heat sink?
Do you have a scope? check for oscillation.
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Old 30th November 2006, 08:51 AM   #3
Niila is offline Niila  Finland
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Without input the amp doesnt oscillate. Heatsink is well big enough.
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Old 30th November 2006, 09:23 AM   #4
Niila is offline Niila  Finland
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Its just that it gets too hot in like 5 to 10 seconds when input signal is connected.

Even if i touch the input wires with my fingers, the amp takes 200mA current and gets hot.

The sine signal in scope looks very "fat", but clean. I will take some pictures tomorrow.
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Old 30th November 2006, 10:50 AM   #5
Nordic is offline Nordic  South Africa
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Use a kitchen towel to wipe that flux off the solder with some acetone. I'm pretty sure I see some solder droplets crossing rails or bridgeing them close enough to cause problems with capacitive coupling. like the 6th pin from the left on the botton row in the second picture also the 3rd from left in that row, looks bent suspiciously close to the ground plane.
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Old 1st December 2006, 07:57 AM   #6
space is offline space  Norway
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If the signal looks "fat", isn't that an indication of oscillation?

space
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Old 1st December 2006, 08:02 AM   #7
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Yes, and it may well be beyond the resolution of your scope. What happens with the input shorted to earth?
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Old 1st December 2006, 09:17 AM   #8
es44 is offline es44  Denmark
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To me it looks like Nordic has a point. I don't like the looks of those solderings either. Some heavy solder droppings scattered all around, and i'm not sure if my eyes are fooling me, but to me it looks like a dry solder at picture 2, second soldering from beneath at the far right side.
Resolder, and clean up, that may do it.

Also a good magnifying glass could be handy.

Good luck
Ebbe
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Old 1st December 2006, 11:29 AM   #9
Niila is offline Niila  Finland
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There is no solder droppings really.. its just some dirt sticked in to pcb. Some of the joints arent so good because the pads are too small. There should not be any flux left on pcb.

Anyway i will try to solder it once more. Thanks for the tips..

I used electronic microscope to solder the chip in the first time

Heres a screenshot..
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Old 1st December 2006, 12:33 PM   #10
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Whoa, that looks like major oscillation to me! What's your gain?

Rune
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