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From: Paul Keinanen on 28 Jul 2010 23:30
On Wed, 28 Jul 2010 09:01:51 -0700, Fred Abse
<excretatauris(a)invalid.invalid> wrote:

>On Wed, 28 Jul 2010 23:17:53 +1000, Grant wrote:
>
>> 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
>
>Assuming that you mean that you bolted, or otherwise clamped 8 transistors
>onto a thin aluminum plate, which you then attached to an aluminum
>heatsink, how's that going to "spread the heat"?
>
>If I understand correctly what you did, that's going to make the
>situation worse than attaching the transistors directly to the heatsink.
>Extra thermal resistance at the interface between the plate and the sink.

Bolting the heat source(s) to a 5-10 mm thick slab of copper the size
of the aluminum heat sink, is quite effective in evenly delivering the
heat into heat sink, since copper heat conductance is 2-3 times higher
than aluminum or aluminum alloys.

From: Grant on 29 Jul 2010 00:49
On Thu, 29 Jul 2010 06:30:59 +0300, Paul Keinanen <keinanen(a)sci.fi> wrote:

>On Wed, 28 Jul 2010 09:01:51 -0700, Fred Abse
><excretatauris(a)invalid.invalid> wrote:
>
>>On Wed, 28 Jul 2010 23:17:53 +1000, Grant wrote:
>>
>>> 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
>>
>>Assuming that you mean that you bolted, or otherwise clamped 8 transistors
>>onto a thin aluminum plate, which you then attached to an aluminum
>>heatsink, how's that going to "spread the heat"?
>>
>>If I understand correctly what you did, that's going to make the
>>situation worse than attaching the transistors directly to the heatsink.
>>Extra thermal resistance at the interface between the plate and the sink.
>
>Bolting the heat source(s) to a 5-10 mm thick slab of copper the size
>of the aluminum heat sink, is quite effective in evenly delivering the
>heat into heat sink, since copper heat conductance is 2-3 times higher
>than aluminum or aluminum alloys.

Yes, I'd like to do that but we couldn't find any busbar copper at the
time. The extra aluminium strip was good enough for the job at the
time, only trying to get about 25W per transistor into the heatsink.

Grant.
From: Jan Panteltje on 29 Jul 2010 08:09
On a sunny day (Thu, 29 Jul 2010 10:22:39 +1000) it happened Grant
<omg(a)grrr.id.au> wrote in <pch156pbrsimuejjogm85t1108um3udpjc(a)4ax.com>:
I put couple photos of my eight transistor linear active load up
>here:
>
> http://grrr.id.au/active-load-linear/
>
>Showing the metal 50W resistors I used, and a view of the transistor
>side, complete with a ridiculous current shunt made from quite a few
>1% 1/2W resistors.
>
>Grant.

For a moment it occured to me that with 8 MOSFETS used as *switches* and 8 resistors
in ratio 1, 2, 4, 8, 16, 32, 64, 128,
you can make 265 load levels, and dissipate next to nothing in the MOSFETS.
Then you need no shunt, as you know what you switched.
Of course the '1' resistor would have to be the biggest one...
From: John Larkin on 29 Jul 2010 11:27
On Thu, 29 Jul 2010 12:09:13 GMT, Jan Panteltje
<pNaonStpealmtje(a)yahoo.com> wrote:

>On a sunny day (Thu, 29 Jul 2010 10:22:39 +1000) it happened Grant
><omg(a)grrr.id.au> wrote in <pch156pbrsimuejjogm85t1108um3udpjc(a)4ax.com>:
>I put couple photos of my eight transistor linear active load up
>>here:
>>
>> http://grrr.id.au/active-load-linear/
>>
>>Showing the metal 50W resistors I used, and a view of the transistor
>>side, complete with a ridiculous current shunt made from quite a few
>>1% 1/2W resistors.
>>
>>Grant.
>
>For a moment it occured to me that with 8 MOSFETS used as *switches* and 8 resistors
>in ratio 1, 2, 4, 8, 16, 32, 64, 128,
>you can make 265 load levels, and dissipate next to nothing in the MOSFETS.
>Then you need no shunt, as you know what you switched.
>Of course the '1' resistor would have to be the biggest one...

That's a nice architecture.

One of my products uses a high-power switching regulator that has a
PWM-switching power fet in series with a resistor into the output
caps... no inductor. It's no more efficient than a linear regulator,
but the power is dissipated in the resistor, not the fet.

This same box has a dual 3-bit, 20 kilowatt DAC, implemented by
switching binary-weighted three-phase transformer secondaries in and
out, 25 volts/LSB.

John



From: John Larkin on 29 Jul 2010 11:32
On Thu, 29 Jul 2010 14:49:19 +1000, Grant <omg(a)grrr.id.au> wrote:

>On Thu, 29 Jul 2010 06:30:59 +0300, Paul Keinanen <keinanen(a)sci.fi> wrote:
>
>>On Wed, 28 Jul 2010 09:01:51 -0700, Fred Abse
>><excretatauris(a)invalid.invalid> wrote:
>>
>>>On Wed, 28 Jul 2010 23:17:53 +1000, Grant wrote:
>>>
>>>> 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
>>>
>>>Assuming that you mean that you bolted, or otherwise clamped 8 transistors
>>>onto a thin aluminum plate, which you then attached to an aluminum
>>>heatsink, how's that going to "spread the heat"?
>>>
>>>If I understand correctly what you did, that's going to make the
>>>situation worse than attaching the transistors directly to the heatsink.
>>>Extra thermal resistance at the interface between the plate and the sink.
>>
>>Bolting the heat source(s) to a 5-10 mm thick slab of copper the size
>>of the aluminum heat sink, is quite effective in evenly delivering the
>>heat into heat sink, since copper heat conductance is 2-3 times higher
>>than aluminum or aluminum alloys.
>
>Yes, I'd like to do that but we couldn't find any busbar copper at the
>time. The extra aluminium strip was good enough for the job at the
>time, only trying to get about 25W per transistor into the heatsink.
>
>Grant.

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



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