I want to use my alu chassis as the heatsink for 2x lm3886. The chips sit on aluminum that is 1cm thick.
but the chip is getting too hot, the protection kicks in. The chips are the plastic coated type. Will metal backed versions help significantly ? Is the exposed metal the gnd for the chip ?
thanks
but the chip is getting too hot, the protection kicks in. The chips are the plastic coated type. Will metal backed versions help significantly ? Is the exposed metal the gnd for the chip ?
thanks
Use the formula from National Semi's website to calculate the required thermal resistance for the heat sink. For instance, if you have +/- 28VDC on the rails at load, you will need a heat sink with a thermal resistance of approximately 3.9 with the TF package. (The TF package has somewhat worse thermal characteristics than the TA package when you use the latter with a mica insulator.)
You can use this nomograph to calculate the surface area to arrive at the thermal resistance you need.
You can use this nomograph to calculate the surface area to arrive at the thermal resistance you need.
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I agree with the above guy in that you need to double the size of the heatsinks shown in the design examples from National.
Alternately, you could do what I did, and instead of giving up of the large amount of space required, use DC cooling fans, slowed down to reduce noise.
But this is in a car amplifier application. I got surplus fans cheap from eBay and just didn't want to get more heatsinks.
Alternately, you could do what I did, and instead of giving up of the large amount of space required, use DC cooling fans, slowed down to reduce noise.
But this is in a car amplifier application. I got surplus fans cheap from eBay and just didn't want to get more heatsinks.
AndrewT said:
a 10mm slab is about .394 inches, so 12 sq inches (774 sq mm) should do the trick, barring the potential problem to which Andrew refers.
I've found that the National thermal resistance ratings are reliable -- could be that the chip is oscillating -- try putting a 200pF capacitor between the input + and - and see if the thermal issue goes away.
I can't demonstrate that because National choose not to divulge the way Spike protection operates when the chip is at elevated temperatures.
The heatsink data is derived for Tc=150degC, but the performance data is only shown for Tc=25degC. Therein lies the problem.
Basically the datasheet and application notes give sufficient clues to keep the chipamp cool to get good performance from it.
The heatsink data is derived for Tc=150degC, but the performance data is only shown for Tc=25degC. Therein lies the problem.
Basically the datasheet and application notes give sufficient clues to keep the chipamp cool to get good performance from it.
I don't know what kind of load Ted is driving with his LM3886 amps but mine get only slightly warm and they are mounted on a piece of thick aluminium angle with a PC heatsink bolted to one back of the angle.
I suspect he may have oscillation or something else causing his chips to get so hot!
I suspect he may have oscillation or something else causing his chips to get so hot!
MartyM said:
I tested this baby's mate (the LM4780) quite extensively for an AX article -- you can run the thing at full power for hours (I ran it for 12 hours) on the heat sink which derives from the Nat Semi design tool.
What you do need to know (by experimenting) is the thermal impedance value of the heat sink in your application. I believe that I used a Dale 10 ohm, 50 watt transistor and digital thermometer. The resistor was firmly affixed to the heat sink (it screws on), with thermal grease, and cooked at 10W for about 2 hours. log the change in temperature -- and le voila -- you have the thermal impedance in situ.
If you have an amplifier that's sitting on a wire rack a few inches off the ground it will have a much different heat dissipation profile than an amplifier sitting on a hardwood floor or carpet.
I can get an LM4780 to thermally cycle fast enough to make some really nasty stuff happen.
"for hours (I ran it for 12 hours) on the heat sink which derives from the Nat Semi design tool."
Oh, just one second-is that in the PA100 configuration? Perhaps I've been comparing apples to oranges all along.
I had to add fan cooling (although full speed was not required) for my PA100s which drive 4 Ohm loads as intended.
But then again my goal was to reduce overall temperature more than is technically required as my heatsinks are actually also serve as structural units for the chassis ("walls"), and the heat transfers to other misc. components.
Perhaps my first statements weren't defined well as I left out the context-sorry.
Oh, just one second-is that in the PA100 configuration? Perhaps I've been comparing apples to oranges all along.
I had to add fan cooling (although full speed was not required) for my PA100s which drive 4 Ohm loads as intended.
But then again my goal was to reduce overall temperature more than is technically required as my heatsinks are actually also serve as structural units for the chassis ("walls"), and the heat transfers to other misc. components.
Perhaps my first statements weren't defined well as I left out the context-sorry.
I would expect a chipamp fitted with the heatsink that National recommend to operate very close to, if not exactly, as National specify.
That means that the National information on chip temperature will apply for that size of heatsink under those operating conditions.
Yes, I would expect it to perform flawlessly all day at full power into the specified test load.
Now, what did the data sheet say?
Theta c-s =0.2Cdegrees/W (metal backed 3886, not TF, direct to heatsink with thermal compound)
Tc varies from 90degC to 138degC.
Tj=150degC.
These conditions will allow the chip to operate to resistive loaded specification.
Now, consider what other parts of the chip are also at or near 150degC.
How well will they perform?
How will they cope with a reactive load?
How will Spike operate at this elevated temperature?
What current limits will Spike start cutting in at?
Note that these example operating temperatures are just short of over temperature shut down.
Now look at the general specification of the chipamp.
Almost every parameter is specified for Tc=25degC.
I think I would like the Tc=25degC parameters (or better) for my audio performance. I would not waste my time using the degraded parameters that would apply to Tc=~120degC.
Keep the chips cool
Adopt a heatsink about double the dissipation capacity that National recommend as the minimum.
That means that the National information on chip temperature will apply for that size of heatsink under those operating conditions.
Yes, I would expect it to perform flawlessly all day at full power into the specified test load.
Now, what did the data sheet say?
Theta c-s =0.2Cdegrees/W (metal backed 3886, not TF, direct to heatsink with thermal compound)
Tc varies from 90degC to 138degC.
Tj=150degC.
These conditions will allow the chip to operate to resistive loaded specification.
Now, consider what other parts of the chip are also at or near 150degC.
How well will they perform?
How will they cope with a reactive load?
How will Spike operate at this elevated temperature?
What current limits will Spike start cutting in at?
Note that these example operating temperatures are just short of over temperature shut down.
Now look at the general specification of the chipamp.
Almost every parameter is specified for Tc=25degC.
I think I would like the Tc=25degC parameters (or better) for my audio performance. I would not waste my time using the degraded parameters that would apply to Tc=~120degC.
Keep the chips cool
Adopt a heatsink about double the dissipation capacity that National recommend as the minimum.
Many commercial amplifiers have far too small heatsinks for their maximum power ratings. They work inspite of that, because few people use their amps for hours on end at full load.
The Overture Design Guide from www.national.com indicates that you need at least 1,92 K/W of heatsink per IC @25°C ambient temperature. If you mount both ICs on the same heatsink, go for less than half that number. That is already a big heatsink, something like Fischerelektronik SK85/100SA (100x160x40mm). Depending on your listening customs, and on the airflow inside and around your amplifier you need smaller or even much bigger heatsinks.
A weak point may also lie in bad heat conductance among the four sides and the corner posts of your box. You could try to use thermal grease not only between IC and heatsink, but also between all components of your box.
With 30V at the rails and 6 Ohm load you are already past the maximum power dissipation the IC can handle without a fan. According to AN-1192 the isolated version can only dissipate 30W without fan. The Overture Design Guide calculates Pd=31,9 W for your configuration.
The Overture Design Guide from www.national.com indicates that you need at least 1,92 K/W of heatsink per IC @25°C ambient temperature. If you mount both ICs on the same heatsink, go for less than half that number. That is already a big heatsink, something like Fischerelektronik SK85/100SA (100x160x40mm). Depending on your listening customs, and on the airflow inside and around your amplifier you need smaller or even much bigger heatsinks.
A weak point may also lie in bad heat conductance among the four sides and the corner posts of your box. You could try to use thermal grease not only between IC and heatsink, but also between all components of your box.
With 30V at the rails and 6 Ohm load you are already past the maximum power dissipation the IC can handle without a fan. According to AN-1192 the isolated version can only dissipate 30W without fan. The Overture Design Guide calculates Pd=31,9 W for your configuration.
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