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Old 24th March 2009, 10:26 PM   #1
rhysh is offline rhysh  United Kingdom
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Default Quick LM4780 Heatsink Thermal Rating Q

Hi,

I cannot for the life of me get my head round heatsink thermal ratings.


LM4780
30V +/-
25 deg room temp
8 ohm speaker
balanced bridged operation

Can someone tell me the correct thermal rating i will need?

Thankyou!
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Old 25th March 2009, 09:11 AM   #2
sangram is offline sangram  India
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According to the datasheet calculations, you're looking at about 45-50 watts. This is complicated by the requirement of getting the heat off such a tiny area as the back of a LM4780.

You will need a pretty beefy heatsink. The 4780 itself is 0.8, add 0.5 for the insulator/compound, that's already 1.3. Even a 0.7 C/W takes the total to 2 C/w, which means a temp rise of 100 degrees for the 50 watt heat, which will trip the thermal protection with a 25 degree ambient. Ideally 0.2-0.3 is the range you're looking at, and even then the chips will get hot at worst case dissipation.

On the other hand, the worst case results will not always apply, and you can get away with a slightly smaller sink. The amp should be fine for music without having the thermal protection kick in, though it might not pass a full power square wave test without tripping SPiKE.

Edit: I run a set of 4780 chips in a bridge/parallel setup. The parallel chips help keep the dissipation in the individual chips down, and I run on ~ 2C/W heatsinks for each chip. They run warm but not uncomfortably so.
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Old 25th March 2009, 09:34 AM   #3
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That is understandable, because it is impossible to heatsink that configuration.

Two channels with 30 V bridged into an 8 Ohm load result in a heat dissipation of ~94 W. That limits the junction temperature to ~74 C. The LM4780 has 0,8 K/W. 94 W * 0,8 K/W = 75,2 K, so your heatsink + isolation washers + thermal grease must remain below 74 C - 75,2 K = -1,2 C and still dissipate 94 W.

You stand a chance to heatsink an LM4780 with 4 Ohm on both channels, if you reduce the supply voltage to 22 V. Or you replace the LM4780 with two LM3886T (not TF!), but even then it will be an effort to keep them cool with 4 Ohm load per IC. Or add another LM4780 and convert it into a BPA-200 configuration. Still the same effort to heatsink.
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Old 25th March 2009, 10:39 AM   #4
sangram is offline sangram  India
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Well it will work if he keeps each channel on a 0.3 C/W heatsink (this means about 45 watts per heatsink, not altogether unreasonable given that a lot of the Class A monsters are dissipating 100 watts at idle.

Obviously two channels cannot share a reasonably sized heatsink, it is practically impossible without forced cooling of some kind. But then again there are possibilities to cater to that. A modern Quad-core CPU pushes over a hundred watts of heat out of an area much smaller than the 4780. Inspite of the tightly coupled heatspreader and much lower allowable junction temperatures (under 100 degrees in most cases), the chips manage fine. They only need some reasonable airflow to stay under 60 degrees under peak loads.
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Old 25th March 2009, 10:50 AM   #5
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If needed I can use fans to assist the heatsink, but what would be the smallest heatsink I could get away with running at full load with fan assistance.
Each chip is in a seperate enclosure, as I am running monoblocks. Each chip runs in bridged mode for mono opoeration.
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Old 25th March 2009, 11:10 AM   #6
AndrewT is offline AndrewT  Scotland
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bridged gives double the power into double the impedance.

Using a pair of 30Vdc powered amps is the equivalent to using 4ohm loads.

30Vdc and 4ohm loads gives the maximum plus a bit of ~60W into 4r0.
The bridged pair will give 120W into 8r0.

The dual chip must be designed as two amps, each driving 60W into 4ohms.
That is what is difficult. A misunderstanding of what bridging is doing to your overloaded chipamps.
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Old 25th March 2009, 11:15 AM   #7
rhysh is offline rhysh  United Kingdom
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The amplifier in question is the Sympatico from twisted pear audio.
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Old 25th March 2009, 11:59 AM   #8
sangram is offline sangram  India
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I would use a nice large heatsink and slap a couple of quiet fans on it. I really don't see any issues with anything over 0.3 C/W, or a simple 0.5C/W heatsink and a couple of fans. It is possible to get these chips to work fine with those rails and that load as long as heat is taken away fast enough.

Max power output is *not* = Max dissipation. For most Class AB amps, the worst case dissipation is around 35% of power output, IIRC but I don't have a reference for this. Even the graphs in the chip's datasheets corroborate this, though I don't have an explanation for it myself.

You might want to check with Russ/Brian on this specific question, they will probably point you to something they use. With all due respect, if the 4780 is rated up to 4 ohm loads at 30V rails, it should be able to deal with 8 ohm bridged, as each half of the chip will be looking 'into' a 4 ohm load. It's right at the spec limit for the chip - to be safer 25 volts is what you should be running at but I don't think there'll be an issue as long as the heatsinking is good enough.

@pb - where did the 74 degree limit come from? The LM4780 datasheet rates the bridge amplifier into an 8 ohm load up to 30 volts. I understand the dissipation is very high - around 90 watts as you state and not 45 like I calculated (my error came from using 30V instead of 60V as I should have taken). If you assume 90 watts an 0.8 Tj-c, the case temperature is already at 72 degrees (I assume this is what you meant). Add another 0.5 Tc-a and we get 1.3 C/W Tj-a, with 90 watts of heat this is >100 degrees C. Very very high, but within 150 C limit of the chip. A good CPU air cooler is able to deliver thermal resistance (with a fan) of under 0.2 C/W, which would be able to keep the chip under the limits. A large enough passive sink would also do.

I agree that it is heavily overloaded, but also worst case dissipation is not frequent in its occurrence anyway, and usually not at full power. Plus the chip has protection from temperature (which actually works, I know this the hard way) rise, so though it will run hot it will survive.
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Old 25th March 2009, 12:59 PM   #9
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Why not just lower the voltage a little? According to the Overture Design Guide, +/- 30V into a bridged LM4780 with 8 ohm speakers puts out about 167W of power and needs a heatsink rated at 0.53C/W. If you lowered your supply to +/- 25V, you would have about 115W and would need a heatsink of 1.10C/W. Much more manageable.

Of course the power outputs above depend on the values used to set the gain.
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Old 25th March 2009, 01:03 PM   #10
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m using 22vac transformers, I cannot change these as I alleady have them but what voltage should I be expecting after rectification?
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