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Old 2nd January 2010, 12:14 AM   #1
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Default Idea of heatsink size.

I intend to use a 300v 2 x 22v toroid to power 2 bridged lm4780... What sort of size heatsink would be best to use?

I have a small aluminium box 203 x 152 x 76mm that i would like to fit everything into. So I want as small as possible heatsink.

If you know of any for sale designed for the chip please show link or PM me.

I have spent hours looking on net, but none seem to fit with what I need.
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Old 2nd January 2010, 12:32 AM   #2
tomchr is offline tomchr  United States
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Calculate the max power dissipation, Pdmax, from equation (2) on page 15 of the LM4780 data sheet. Pdmax is the power dissipated in each LM4780.

Then decide on a max heat sink temperature. If you're using an external heat sink where a user might get to touch it, I probably wouldn't go much higher than 60 deg C. If it's an internal heat sink (and you have PLENTY of holes in the chassis for air flow), then 90 C is probably reasonable. Let's call the max temperature, Tmax.

Then calculate the maximum heat sink thermal resistance, theta_sa, as theta_sa = (Tmax-Tambient)/Pdiss, where Tambient is the ambient room temperature (use 25-30 C to be on the safe side), and Pdiss is the amount of power dissipated in the heat sink. If each LM4780 has its own heat sink, Pdiss = Pdmax. If you put both LM4780's on the same heat sink, Pdiss = 2*Pdmax.

Theta_sa is the MAXIMUM thermal resistance, i.e. when you go to buy a heat sink, you'll need one that has a LOWER thermal resistance than Theta_sa.

The thermal resistance is expressed in units of deg C (or K) per Watt. It tells you the temperature rise of that heat sink for one watt of dissipated power given free air flow. Note that smaller heat sinks for BGA's, CPU's, etc. are specified at some level of forced air flow, thus, provide a much lower thermal resistance compared to their size. Unless you want fan cooling, don't get tricked by these heatsinks...

Hope this helps.

~Tom
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Old 2nd January 2010, 01:34 AM   #3
AndrewT is offline AndrewT  Scotland
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once you have decided on Ts max, try to estimate Tjmax for the dual chipamp.

Using Tjmax try to find what National tell you about de-rating the cold Spike limiting values when Tj is greater then 25degC.

BTW,
you can use the 3886 table for recommended minimum size of heatsink.
I then recommend that you double the sink that National specify.
What voltage of power supply are you planning?
What speaker impedance are you going to connect up?
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Last edited by AndrewT; 2nd January 2010 at 01:37 AM.
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Old 2nd January 2010, 03:00 AM   #4
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Wow... I was kinda hoping for a "this one will do it" answer... I'll get the wife on the maths.

I am going to use 300v 2 x 22v toroid and its going into my JVC 8ohm speakers. I think they are rated at 200watt.
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Old 2nd January 2010, 12:40 PM   #5
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Would it be easy to add a couple of PC fans to the outside of the case and use the power supply to run them. I have a few of them laying around.
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Old 22nd April 2010, 02:45 AM   #6
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Everytime i add fans to my system they always seem to add noise to it too. Passive cooling should be sufficient. Have you tried a computer cooler?
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