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From: Jim Thompson on 28 Jul 2010 19:45
On Thu, 29 Jul 2010 09:27:26 +1000, Grant <omg(a)grrr.id.au> wrote:

[snip]
>
>On top of all that, I work with materials easy to find, I'd really
>enjoy being able to mill out a heatsink from chosen material as I
>recall Jim T. doing for his fan cooled power resistor.
>
[snip]
>
>Grant.

My fan-cooled heat-sink was simply a conventional finned shape, except
I had 2X 6-foot lengths of it. Someone traded it to me for an
alternator. (When I did alternator regulator designs I had Ford, GM,
American Motors and Chrysler alternators in piles in my garage ;-)

I milled the edges (for a bolt-together fit) so that I had 4-way
inward-facing fins with a square outline that fit a muffin fan on the
end exactly.

Total length was about a foot.

If I still have some of that extrusion I'll post a photo.

...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

Spice is like a sports car...
Performance only as good as the person behind the wheel.
From: Grant on 28 Jul 2010 20:22
On Wed, 28 Jul 2010 09:11:11 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote:

>On Wed, 28 Jul 2010 23:17:53 +1000, Grant <omg(a)grrr.id.au> wrote:
>
>>On Wed, 28 Jul 2010 07:48:39 -0400, Hammy <spam(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

Nice, they're not cheap though.

I'm using cheaper ceramic air cooled resistor banks for a hybrid power
DAC I'm putting together.
>
>An opamp per fet is a good way to balance dissipation accurately.

Yes, but I think something better than LM324s :) I didn't have much
joy with them, but the friend I built it for gave me a handful of
logic MOSFETs, they were difficult to get going in linear mode :(

At least with the construction methods I was using back then.

I had a very nice 300W ultrasonic oscillator at one point, didn't
manage to stabilise it for the range of power converters I was
trying it out with.


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.
From: Hammy on 28 Jul 2010 20:58
On Thu, 29 Jul 2010 10:22:39 +1000, Grant <omg(a)grrr.id.au> wrote:

[snip]
>>
>>An opamp per fet is a good way to balance dissipation accurately.
>
>Yes, but I think something better than LM324s :) I didn't have much
>joy with them, but the friend I built it for gave me a handful of
>logic MOSFETs, they were difficult to get going in linear mode :(

I have some dual LM7322MA use 1 for each FQA24N50. I still might use a
buffer though 4.5nF input capacitance fets so it wouldnt hurt.;-) I
want to be able to do load steps. Probably use a NJT4031 and NJT4030P
push-pull NPN-PNP. Those are 3A bipolars with a minumum gain of 100.
Should be able to pop those FETS around quickly.

>At least with the construction methods I was using back then.
>
>I had a very nice 300W ultrasonic oscillator at one point, didn't
>manage to stabilise it for the range of power converters I was
>trying it out with.
>
>
>I put couple photos of my eight transistor linear active load up
>here:
>
> http://grrr.id.au/active-load-linear/

That is quite the beast. :-)

I want to keep the size down maybe shoebox size or there is no point
in me doing it. That way I can just put it on the bench vs lightbulbs
and fifty wires running all over the place when I want to test
something.

>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.

I was just figureing on two resistors per leg for shunts. 1 resistor
3.3 ohm for upto 1.5A 1W with another resistor 0.1ohm 2W UPTO 10A.
Those current values would be divded by 4 so power dissapation is
resonable.A simple toggle for switching ranges.
>Grant.
From: John Larkin on 28 Jul 2010 21:17
On Wed, 28 Jul 2010 17:41:00 -0400, Hammy <spam(a)spam.com> wrote:

>On Wed, 28 Jul 2010 13:55:05 -0700, John Larkin
><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote:
>
>[SNIP]
>>
>>I've got good results from the configuration you describe, putting the
>>fan in the middle of the heat sink, blowing directly down into the
>>fins, with air squirting out both ends. This is called "impingement
>>cooling" I think. The fets in the middle have a less air velocity but
>>better lateral spreading effect, and the fets on the ends get a lot of
>>air velocity, so it works out pretty well to distribute the fets
>>fairly evenly, just avoiding the very ends of the heatsink.
>>
>>I have this crackpot theory that the back pressure caused by the air
>>scraping the fins should reduce the fan air volume to about half of
>>the fan's zero-back-pressure flow spec. Sort of an impedance matching
>>thing.
>>
>>The other important thing is to keep Tc-s low by having the heatsink
>>be really flat, and not use insulators.
>
>I'm not going to be using insulators. I might use two fans mounted
>directly overtop.

If I've learned anything about air flow, it's that I don't know much
about it. Air is peverse and counter-intuitive. When we do an
enclosure design, we use air flow meters and incense sticks to tell
how much air is flowing and even in what direction it's flowing. I've
seen card cages with a big fan tray just below the boards, blowing a
hurricane of air up, and found a few card slots where the air was
leisurely moving *down*. We make wood and cardboard mockups and push
them around util we measure stuff that will work.

So, make provision for moving those fans around, and measuring
temperatures, and do whatever works.

John

From: John Larkin on 28 Jul 2010 22:11
On Thu, 29 Jul 2010 10:22:39 +1000, Grant <omg(a)grrr.id.au> wrote:

>On Wed, 28 Jul 2010 09:11:11 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote:
>
>>On Wed, 28 Jul 2010 23:17:53 +1000, Grant <omg(a)grrr.id.au> wrote:
>>
>>>On Wed, 28 Jul 2010 07:48:39 -0400, Hammy <spam(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
>
>Nice, they're not cheap though.
>
>I'm using cheaper ceramic air cooled resistor banks for a hybrid power
>DAC I'm putting together.
>>
>>An opamp per fet is a good way to balance dissipation accurately.
>
>Yes, but I think something better than LM324s :) I didn't have much
>joy with them, but the friend I built it for gave me a handful of
>logic MOSFETs, they were difficult to get going in linear mode :(

Most mosfets get very unhappy at higher voltages and moderate
currents, with power dissipation well below their ratings. You really
have to read the SOAR curves, and then maybe test them, for linear
use.

ftp://jjlarkin.lmi.net/ExFets.jpg

John


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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.