Hi
I am about to start a chip amp project using a pair of LM3886TF chips
Am I correct in thinking that LM3886 chips ending in TF have a plastic coating and do not need to be electrically insulated from the heat sink?
Also is it worth using heat conduction paste? And finally should the bolt be Metal on Nylon.
Any help would be greatly appreciated.
Thanks
dh
I am about to start a chip amp project using a pair of LM3886TF chips
Am I correct in thinking that LM3886 chips ending in TF have a plastic coating and do not need to be electrically insulated from the heat sink?
Also is it worth using heat conduction paste? And finally should the bolt be Metal on Nylon.
Any help would be greatly appreciated.
Thanks
dh
Really nice amps Ted. I used a bar just like the second picture for a pair of LM4780s I built. Unfortunately, I didn't put a bolt between them, and the bar flexed and the chips weren't flat on the heatsink. Once I added another bolt in between it was fine. I prefer the bar for non-isolated, especially the larger LM4780 since they have 2 mounting holes. For an isolated LM3886, I think a single bolt in the mounting hole would be easier.
The TF does not need any additional electrical isolation.
If clamped direct to the heatsink then one MUST use a thermal compound to ensure there is no air in the thermal interface.
You can use one of the conformable isolators but all of these increase the thermal resistance and the TF is already partially crippled by the thermal resistance of the plastic encapsulation.
If clamped direct to the heatsink then one MUST use a thermal compound to ensure there is no air in the thermal interface.
You can use one of the conformable isolators but all of these increase the thermal resistance and the TF is already partially crippled by the thermal resistance of the plastic encapsulation.
I use these on the chipamps -- easy to make with a round file -- easier still if you have a mill:
An externally hosted image should be here but it was not working when we last tested it.
kscharf said:
Yes -- I used it in the article -- pulls the device nice and snug to the heatsink -- easily machined with an end-mill and scrap aluminum. I don't have a pic of the one I used on my inverted LM3875 amp.
Here's the mounting methodology on a plate amplifier I found at the town dump:
An externally hosted image should be here but it was not working when we last tested it.
Relocating board components for mounting bar
Hello,
I've pretty much decided on using the described bar method for mounting my lm3886's to the heat sink. However, I have BrianGT's board which I've noticed doesn't allow much room for the mounting bar. My question is can I solder the lm3886 on the topside (normal) and mount all of the capacitors and resistors on the underside? Or maybe just the taller caps? I can't think of any drawback to this option other than the components might need to disperse some heat?
Any thoughts?
Hello,
I've pretty much decided on using the described bar method for mounting my lm3886's to the heat sink. However, I have BrianGT's board which I've noticed doesn't allow much room for the mounting bar. My question is can I solder the lm3886 on the topside (normal) and mount all of the capacitors and resistors on the underside? Or maybe just the taller caps? I can't think of any drawback to this option other than the components might need to disperse some heat?
Any thoughts?
There is really no call for a bar on the TF package. The resistance of the molding over the back of the device is already high enough that high ands distributed force isn't going to get you anywhere important. A screw through the tab with an inch pound or two and a thin but complete film of silicone/zinc oxide paste and you're ready to rock. A little math can easily be used in place of my opinion. Always use some kind of paste and reject the rubber washers for best performance. Plastic screws are okay to load down bars, but they will stretch at normal semiconductor temperatures and possibly cause an unnecessary failure if used directly. Because of the increased resistance to sink on encapsulated packages the problem is actually worsened.
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Shoot! I was too busy reading about the mounting bar that I must have missed the fact that this thread was originally for the TF package. I will be using the un-insulated package with a pad for my amplifiers. I'm pretty much sold on using the mounting bar with the un-insulated chip, so my question still stands on whether I can mount the rest of the circuit to the underside of the board.
I can re-post in the appropriate thread if I need to.
I can re-post in the appropriate thread if I need to.
Andrew E and Jack allude to the TF package as not having a heat dissipation issue, because the TF is electrically isolated.
That is quite misleading.
I believe that the electrical isolation of the TF package makes heat dissipation a big issue. Particularly in trying to keep the chip operating temperatures low enough to get good performance from this protected chip.
A clamp bar, when done properly, is repeatedly shown as giving better heat transfer from chip to sink (Rth c-s).
A bolt can rarely if ever perform as well as a clamp bar in this respect.
The TF needs all the help you can give it to help keep operating temperatures down.
That is quite misleading.
I believe that the electrical isolation of the TF package makes heat dissipation a big issue. Particularly in trying to keep the chip operating temperatures low enough to get good performance from this protected chip.
A clamp bar, when done properly, is repeatedly shown as giving better heat transfer from chip to sink (Rth c-s).
A bolt can rarely if ever perform as well as a clamp bar in this respect.
The TF needs all the help you can give it to help keep operating temperatures down.
if the PCB is double sided with Plated Through Holes (PTH) and the pads on the top side are big enough to effectively solder to, then mounting the components on the bottom side is not an electrical problem.
But be very careful that all components are mounted in the correct holes. Are there any three (or more) pin components? These may all have to be mounted on the top side unless you can bend/re-arrange to pins to fit the wrong side.
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