LM4780 and cooling

[john_lenfr]

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

 

[Netlist]

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=-

 

[john_lenfr]

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).

 

[Netlist]

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

 

[john_lenfr]

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?)

 

[Russ White]

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.

 

[jackinnj]

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.

 

[slackman]

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...

 

[slackman]

hope this small image shows the idea of heatspreading...

Michael

 

[john_lenfr]

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?

 

[slackman]

The backside of the chip holds a metall area, right? this is for better thermal flow to the heasink. This Area is connected to V-!!

So if you mount this unisolated on the heatsink and the heatsink is mounted directly on the metall case ALL metall areas are connected to V-!!
THIS IS VERY DANGEROUS BECAUSE OF THE HIGH CURRENT PEEKS AND DC-CURRENTS YOU DONT NOTICE IF YOU COME IN CONTACT WITH BUT YOUR BODY WILL DAMAGE!

And: if you have grounded your chassis with the earth-connection there will be a shortcut between earth and V-, so normally the FI should switch power down (in your house/appartement).

Boy, boy - the fundamentals of electricity!

Michael

(Maybe this is the reason why your amp runs hot...)

 

[peranders]

As pointed out the LM chip must be mounted on something that will conduct the heat but I'll adwise also to check your wiring a bit. The speaker cable is very near the 230 mains area. Keep all wiring neat and tidy, that's a good thing and keep all 230 V things apart from the rest.

 

[john_lenfr]

Thanks for your advices.


I'll correct these things quickly!

 

[jackinnj]

The mounting tab of the overture chip is connected to V-

I use an insulated heatsink now -- I fashioned several of these mounting assemblies on my mill -- thy firmly affix the chip to the heatsink:

[IMGHTTPDEAD]http://www.tech-diy.com/LM4780_Bridged/chip_holder3.jpg[/IMGHTTPDEAD]

 

[macboy]

Well, temperature in my room is 30°C.

40°C-> with no signal.

Your amp is definitely, absolutely oscillating.. Without any doubt whatsover. When attached to a heatsink, and with no signal present, the amps should not get warm. The fact that they are getting warm indicates that they are not just sitting there idle. They are doing something. They are oscillating. You need to fix that, not your heatsink. Your heatsink is fine.

 

[BobEllis]

I tend to agree with Macboy. At idle the 4780 typically draws 110 mA, or around 8W with your rails. With that sink you should notice that the chip is warmer than ambient, but barely with no signal.

Put an ammeter in line with the V+ supply (disconnect the v+ lead from the supply, connect one ammeter lead to the wire and the other to the supply where it ws removed) and measure the current - if over 175 mA you have something wrong.

Are you sure that the board is right? Are there any solder bridges? All pins in the proper holes and no legs touching another? Use a magnifying glass to check. Move the signal leads around - try to keep them as far from the output leads as possible.

 

[slackman]

Yes,
this all can be but without isolation of the chip <-> heatsink this circuit is definatly a DANGER!

It also seems to me that the output cables are very close to the back of the chassis. Would be better to solder them at the end of the jacks or to use an additional screw to fix the soldered contact (sorry, don't know the matching words :-/ ).

mic

 

[BobEllis]

slackman,

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

you are right, running those sinks hot would be dangerous. But , john_lenfr took care of it properly. I have live sinks in my lm4780 amp, but they are inside a wooden case, so not a danger.

 

[slackman]

Sorry,
silly me, didn't read acurate enough - sorry again

 

[BobEllis]

I'd much rather have someone question whether I had done things safely than not point out something potentially dangerous. I missed the insulation the first time, too. No apology needed.