I need two thermal (electrical) insulating pads for my LM4780 amp (2 chips) and then a few more for the next revision. I've searched here and seen the free sample thread (so far nothing after 2 months) and European suppliers, but there is nothing in USA that I can find.
Anyone have some to sell or know where I can get some? I thought I'd use TO-3 mica pads but after trying for 2 days the heatsink is live (negative DC rail (V-) as per footnote 14 in the data sheet) and I'm pretty sure its not caused by something else.
thanks
Anyone have some to sell or know where I can get some? I thought I'd use TO-3 mica pads but after trying for 2 days the heatsink is live (negative DC rail (V-) as per footnote 14 in the data sheet) and I'm pretty sure its not caused by something else.
thanks
Digikey has a fair selection.
http://www.digikey.com/
Look on pages 789 & 790 of their online catalog.
http://dkc3.digikey.com/PDF/T062/0789-0790.pdf
Fig 7 on page 789 looks about right for the LM4780... but be
aware that the Q3 material on that one is graphite... which IS
electrically conductive.
Looks like fig 5 might be a better alternative... though some
modification might be required.
There is also some bulk sheet materials listed on page 790.
But the extra thickness worries me.
Good luck. Let me know if you find something better.
Best Regards,
Head_spaz
Edit: just saw this on e-bay:
http://cgi.ebay.com/Mica-Sheet-2-25...7608645393QQcategoryZ4660QQrdZ1QQcmdZViewItem
http://www.digikey.com/
Look on pages 789 & 790 of their online catalog.
http://dkc3.digikey.com/PDF/T062/0789-0790.pdf
Fig 7 on page 789 looks about right for the LM4780... but be
aware that the Q3 material on that one is graphite... which IS
electrically conductive.
Looks like fig 5 might be a better alternative... though some
modification might be required.
There is also some bulk sheet materials listed on page 790.
But the extra thickness worries me.
Good luck. Let me know if you find something better.
Best Regards,
Head_spaz
Edit: just saw this on e-bay:
http://cgi.ebay.com/Mica-Sheet-2-25...7608645393QQcategoryZ4660QQrdZ1QQcmdZViewItem
you can use a TO-3 pad, or a pair of ordinary TO-220 pads.
if anyone is interested, there is supplier of mica pads from China which will custom manufacture TO-220 for this format. The pads which would work on the LM4780 or LM4702 -- but I would have to order thousands -- GB anyone????
I should have these manufactured as well, they snug the chip up to the heatsink really well.
if anyone is interested, there is supplier of mica pads from China which will custom manufacture TO-220 for this format. The pads which would work on the LM4780 or LM4702 -- but I would have to order thousands -- GB anyone????
I should have these manufactured as well, they snug the chip up to the heatsink really well.
An externally hosted image should be here but it was not working when we last tested it.
I have a hard time finding stuff on Digikey myself. When you go to the site search "Sil Pad 400".
turning to ebay
Like I said, the TO-3 insulators are not working for me.
How about this Mica Sheet on Ebay?
You could cut it to size and even remove some of the lamination thingies to make it thinner.
Like I said, the TO-3 insulators are not working for me.
How about this Mica Sheet on Ebay?
An externally hosted image should be here but it was not working when we last tested it.
You could cut it to size and even remove some of the lamination thingies to make it thinner.
lgreen,
That mica doesn't really look like the stuff you want... you might be able to cleave off a nice thin piece... but... The good stuff comes from India... dead flat and almost water clear... HARD to find here (phlogopyte mica?).
Call Bergquist 800-347-4572
Tell them you can't find an insulator for this device and you need some plain jane SILPAD 400.
Once they learn that nobody has shelf stock of a precut part, they will send you a sheet. They are very nice people. They have made lots a weird dohickeys for me. Doesn't hurt to tell them you're a consultant and this is a new design... report to us... and that will be your conscience clearing consultation.
If that doesn't work, I'll clean my desk and send you a piece, try the easy way first though!
😉
That mica doesn't really look like the stuff you want... you might be able to cleave off a nice thin piece... but... The good stuff comes from India... dead flat and almost water clear... HARD to find here (phlogopyte mica?).
Call Bergquist 800-347-4572
Tell them you can't find an insulator for this device and you need some plain jane SILPAD 400.
Once they learn that nobody has shelf stock of a precut part, they will send you a sheet. They are very nice people. They have made lots a weird dohickeys for me. Doesn't hurt to tell them you're a consultant and this is a new design... report to us... and that will be your conscience clearing consultation.
If that doesn't work, I'll clean my desk and send you a piece, try the easy way first though!
😉
poobah said:
Bergquist people are so nice they'll call a day after the catalogue and sheets slide in the mail box, AND invite themselves over.
lgreen,
I got mine from Peter at audiosector.com. He will precut them and make the appropriate size holes for your chips as well. His prices are reasonable.
Dominick
I got mine from Peter at audiosector.com. He will precut them and make the appropriate size holes for your chips as well. His prices are reasonable.
Dominick
explosion!
Well, Bergquist is not working out, so I tried some other pads. I carefully delaminated and cut those mica pads from ebay, and put them on. Result- 1 chip ok, other chip exploded!
Damage to Pad/Heatsink The silver divit in the insulator is actually not a hole, its just shiny for some reason.
Blowup Chip 2 Note vaporized pins! It looks like V- went out the mute resistor! But the mute resistor was not touching anything (pretty sure on this).
I am confused; I carefully measured resistance of V- to sink (open circ) and sink-to-ground (open cir), when powered up after 1 sec there was lightning and sparking. not too fun.
Having removed the chip-
- anyone got those Bergquist sheets?
- can I put another chip in the board or is the board likely bad now?
- what passive components on the board could have been damaged?
Well, Bergquist is not working out, so I tried some other pads. I carefully delaminated and cut those mica pads from ebay, and put them on. Result- 1 chip ok, other chip exploded!
Damage to Pad/Heatsink The silver divit in the insulator is actually not a hole, its just shiny for some reason.
Blowup Chip 2 Note vaporized pins! It looks like V- went out the mute resistor! But the mute resistor was not touching anything (pretty sure on this).
I am confused; I carefully measured resistance of V- to sink (open circ) and sink-to-ground (open cir), when powered up after 1 sec there was lightning and sparking. not too fun.
Having removed the chip-
- anyone got those Bergquist sheets?
- can I put another chip in the board or is the board likely bad now?
- what passive components on the board could have been damaged?
Poof
Looks like you used graphite thermal paste, which is very conductive. So the isolation pad did nothing, but give a good layer path to the heat sink.
Looks like you used graphite thermal paste, which is very conductive. So the isolation pad did nothing, but give a good layer path to the heat sink.
From their website:
A very lame description of its electrical conductivity... How did you wind up with this stuff?
Bummer
A very lame description of its electrical conductivity... How did you wind up with this stuff?
Bummer
Sorry dude,
You may have a bad chip.
If you look at the formulation for this, it is silver balls and ester oil.
A thin film and high pressure IS exactly your situation.
A far as conductivity is concerned... this is matter of applied voltage. What may be negligibly conductive at 3.3 Volts may be a different story at the 60 Volts or so you have from V+ to V-
Use the normal stuff next time. The thermal efficacy of the grease/compound becomes moot once an insulator is employed.
And, if your heat sink is isolated, why bother with the insulator at all?
🙂
You may have a bad chip.
If you look at the formulation for this, it is silver balls and ester oil.
A thin film and high pressure IS exactly your situation.
A far as conductivity is concerned... this is matter of applied voltage. What may be negligibly conductive at 3.3 Volts may be a different story at the 60 Volts or so you have from V+ to V-
Use the normal stuff next time. The thermal efficacy of the grease/compound becomes moot once an insulator is employed.
And, if your heat sink is isolated, why bother with the insulator at all?
🙂
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