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From: Hammy on 29 Jul 2010 19:57
On Thu, 29 Jul 2010 08:32:11 -0700, John Larkin
<jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote:

[snip]
>
>The two shiny strips on this amp
>
>ftp://jjlarkin.lmi.net/Amp.jpg
>
>are nickel-plated copper heat spreaders. We buy the copper strip from
>McMaster and machine it then have it plated. The alloy is machinable -
>pure copper is gummy and hard to machine - so the thermal and
>electrical conductivity aren't as good as pure copper, but still
>better than aluminum alloy.
>
>John
>
>
Is that heatsink live? I can't tell if there's an insulator.

If it is live how did you mount it in the enclosure?

What package are the FETS in? They don't look like D2PAK's they look
almost like TO-247's only SMD.

It does look nice though.:-)
From: John Larkin on 29 Jul 2010 20:14
On Thu, 29 Jul 2010 19:57:30 -0400, Hammy <spam(a)spam.com> wrote:

>On Thu, 29 Jul 2010 08:32:11 -0700, John Larkin
><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote:
>
>[snip]
>>
>>The two shiny strips on this amp
>>
>>ftp://jjlarkin.lmi.net/Amp.jpg
>>
>>are nickel-plated copper heat spreaders. We buy the copper strip from
>>McMaster and machine it then have it plated. The alloy is machinable -
>>pure copper is gummy and hard to machine - so the thermal and
>>electrical conductivity aren't as good as pure copper, but still
>>better than aluminum alloy.
>>
>>John
>>
>>
>Is that heatsink live? I can't tell if there's an insulator.

Yes. There are 32 fets, 16 P-channels and 16 N-channels, and all their
drains are hard-clamped to the heatsink.

>
>If it is live how did you mount it in the enclosure?

Two black Delrin blocks on the bottom, visible in the pic, are bolted
to the bottom of the chassis. The holes in the top of the heatsink
engage three plastic pins on the top cover, to add a little stability.

>
>What package are the FETS in? They don't look like D2PAK's they look
>almost like TO-247's only SMD.

They are basically TO-247s without holes, clamp mounted. We just
clipped the leads short and "surface mounted" them.

>
>It does look nice though.:-)

It's a beast. I prefer to not make anything that could break your toe
if you dropped it. The power transformer in this thing weighs more
than one of my employees.

ftp://jjlarkin.lmi.net/PP5.JPG

John

From: Hammy on 29 Jul 2010 20:57
On Thu, 29 Jul 2010 17:14:09 -0700, John Larkin
<jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote:

>On Thu, 29 Jul 2010 19:57:30 -0400, Hammy <spam(a)spam.com> wrote:
>
>>On Thu, 29 Jul 2010 08:32:11 -0700, John Larkin
>><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote:
>>
>>[snip]
>>>
>>>The two shiny strips on this amp
>>>
>>>ftp://jjlarkin.lmi.net/Amp.jpg
>>>
>>>are nickel-plated copper heat spreaders. We buy the copper strip from
>>>McMaster and machine it then have it plated. The alloy is machinable -
>>>pure copper is gummy and hard to machine - so the thermal and
>>>electrical conductivity aren't as good as pure copper, but still
>>>better than aluminum alloy.
>>>
>>>John
>>>
>>>
>>Is that heatsink live? I can't tell if there's an insulator.
>
>Yes. There are 32 fets, 16 P-channels and 16 N-channels, and all their
>drains are hard-clamped to the heatsink.
>
>>
>>If it is live how did you mount it in the enclosure?
>
>Two black Delrin blocks on the bottom, visible in the pic, are bolted
>to the bottom of the chassis. The holes in the top of the heatsink
>engage three plastic pins on the top cover, to add a little stability.

I thought that was just part of the heatsink. My solution probably
won't be so eloquent. I'm probably going to have to use some high temp
automotive type adhesive to mount a non-conductive block of whatever I
can find to the heatsink. Then use that to bolt onto the chassis.

Once I see the heatsink I may come up with a better idea.

They sold seven more of the same heatsink today.

>>What package are the FETS in? They don't look like D2PAK's they look
>>almost like TO-247's only SMD.
>
>They are basically TO-247s without holes, clamp mounted. We just
>clipped the leads short and "surface mounted" them.
>
>>
>>It does look nice though.:-)
>
>It's a beast. I prefer to not make anything that could break your toe
>if you dropped it. The power transformer in this thing weighs more
>than one of my employees.
>
>ftp://jjlarkin.lmi.net/PP5.JPG
>
>John

Yep thats a back breaker alright.
From: John Larkin on 29 Jul 2010 22:55
On Thu, 29 Jul 2010 13:12:53 -0700 (PDT), George Herold
<gherold(a)teachspin.com> wrote:

>On Jul 28, 12:11�pm, John Larkin
><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote:
>> On Wed, 28 Jul 2010 23:17:53 +1000, Grant <o...(a)grrr.id.au> wrote:
>> >On Wed, 28 Jul 2010 07:48:39 -0400, Hammy <s...(a)spam.com> wrote:
>>
>> >>When mosfets are sharing a heatsink how does this effect the total
>> >>power handling capability of the fets.
>>
>> >>For example if I calculated an allowable 140W dissipation for a single
>> >>fet mounted on a large 150mm x 100mm for a max ambient of 50C and max
>> >>junction of 110C With a fan 1 x 42CFM.
>>
>> >>Would it be possible to get 300 to 400W total dissipation if I
>> >>parallel 2 to 4 FETS on the same heatsink? Or would I have to buy 2 or
>> >>three more of the large heatsink?
>>
>> >>The mosfets are operating in linear mode it's for a variable
>> >>electronic load.
>>
>> >>To avoid thermal runaway in a fet due to Vgs( th) differences between
>> >>devices is it best to use dedicated opamps per fet or large source
>> >>resistors? I've read several papers but thought I'd ask here for
>> >>someone who has maybe done something similar.
>>
>> >I put 8 x TO220 FETs direct (non-insulated) on flat aluminium plate,
>> >3mm thick by 40mm to spread heat onto one side of 80mm square fancooled
>> >heatsink rated 0.3'C/W and could easily handle 400W, with an 8 x 0R33,
>> >50W metal pack resistors on other side of heatsink. �Two parts of the
>> >heatsink mated to form an 80mm by 200mm tunnel that one bolted a fan
>> >to, I used a 90mm fan via adapter.
>>
>> >0R22 source resistor too small to balance FETs better than 100%, so
>> >I selected best match eight from batch of 20, expensive and still
>> >not a good performer for intended use, okay for manual operation.
>>
>> Shifting some of the heat away from silicon and into power resistors
>> is a good deal, when conditions allow, which means serious resistance
>> in the source and/or drain. I like these:
>>
>> ftp://jjlarkin.lmi.net/Welwyn.JPG
>>
>> An opamp per fet is a good way to balance dissipation accurately.
>>
>> John- Hide quoted text -
>>
>> - Show quoted text -
>
>That looks interesting, Can you give me a Welwyn part number. (I was
>on their website but they have a lot of resistors.)
>
>George H.

The one we actually use in production is WDBR2-12RKT, 12 ohms, 200
watts. It looks like the one in my pic. What's slick is that the
springy steel substrate is curled so that it really hugs the heat sink
when you torque it down. They seem to handle high pulsed power very
well. We used them to replace mil-type metal-case wirewounds that were
dying from cycling fatigue.

John







From: Grant on 29 Jul 2010 23:16
On Thu, 29 Jul 2010 20:57:42 -0400, Hammy <spam(a)spam.com> wrote:

>On Thu, 29 Jul 2010 17:14:09 -0700, John Larkin
><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote:
>
>>On Thu, 29 Jul 2010 19:57:30 -0400, Hammy <spam(a)spam.com> wrote:
>>
>>>On Thu, 29 Jul 2010 08:32:11 -0700, John Larkin
>>><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote:
>>>
>>>[snip]
>>>>
>>>>The two shiny strips on this amp
>>>>
>>>>ftp://jjlarkin.lmi.net/Amp.jpg
>>>>
>>>>are nickel-plated copper heat spreaders. We buy the copper strip from
>>>>McMaster and machine it then have it plated. The alloy is machinable -
>>>>pure copper is gummy and hard to machine - so the thermal and
>>>>electrical conductivity aren't as good as pure copper, but still
>>>>better than aluminum alloy.
>>>>
>>>>John
>>>>
>>>>
>>>Is that heatsink live? I can't tell if there's an insulator.
>>
>>Yes. There are 32 fets, 16 P-channels and 16 N-channels, and all their
>>drains are hard-clamped to the heatsink.
>>
>>>
>>>If it is live how did you mount it in the enclosure?
>>
>>Two black Delrin blocks on the bottom, visible in the pic, are bolted
>>to the bottom of the chassis. The holes in the top of the heatsink
>>engage three plastic pins on the top cover, to add a little stability.
>
>I thought that was just part of the heatsink. My solution probably
>won't be so eloquent. I'm probably going to have to use some high temp
>automotive type adhesive to mount a non-conductive block of whatever I
>can find to the heatsink. Then use that to bolt onto the chassis.

Drill & tap heatsink and mount with M5, 3/16", or 1/4" nylon bolts
and spacers? Depends if you doing one off, or production.

Depends how heavy the assembly is. That one I put photos of up uses
1/8" pop rivets and washers to connect PCB-like material as insulator
to aluminium angle, easy for a prototype live heatsink, but it would
break if dropped or pushed too hard.
>
>Once I see the heatsink I may come up with a better idea.
>
>They sold seven more of the same heatsink today.

Must be good then :)
>
>>>What package are the FETS in? They don't look like D2PAK's they look
>>>almost like TO-247's only SMD.
>>
>>They are basically TO-247s without holes, clamp mounted. We just
>>clipped the leads short and "surface mounted" them.
>>
>>>
>>>It does look nice though.:-)
>>
>>It's a beast. I prefer to not make anything that could break your toe
>>if you dropped it. The power transformer in this thing weighs more
>>than one of my employees.
>>
>>ftp://jjlarkin.lmi.net/PP5.JPG
>>
>>John
>
>Yep thats a back breaker alright.



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Prev: Hardware manufacture in prototype quantities
Next: Accounting 23rd ED WARREN Test Bank and solution manual is available for purchase at affordable prices. Contact me at alltestbanks11[at]gmail.com to buy it today. All emails will be answered ASAP.