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From: Fred Bartoli on 29 Jul 2010 14:32
John Larkin a �crit :
> 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.
>
He he...

and are multiplying DACs too, but won't like a DC reference...

--
Thanks,
Fred.
From: George Herold on 29 Jul 2010 16:12
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.
From: Grant on 29 Jul 2010 16:15
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 where I'm heading with a new design, but top 3 bits are
thermometer code to ease the large resistor switching problem.

7/8 is thermometer code, to 63/64 with weighted resistors, remaining
1/64 (minus a smidge smaller than a whisker, got to be precise here :)
by power opamps fed by dual 8 bit DAC. 16 bit ADC for feedback.

PIC in there too ;) There's an 8 way resistor bank switched in with
P-channel FETs to add high/low range. 63 x 47R 17W plus 24 x 6R8 17W,
plus a couple LM675T Opamps. Hmm, and a fan, or two, and . . .


Nine teeth left, dentures next month, ouch... Posting before coffee.

Grant.
From: Grant on 29 Jul 2010 16:22
On Thu, 29 Jul 2010 08:27:39 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote:

>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.

But there's a win somewhere? Less noise?
>
>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.

Lovely, glad I'm not going there :) Getting close to a kW is enough
for me this year. Plus have the same thing detect a mV/minute drift.

Aim high, see what can be achieved.

Grant.
From: Grant on 29 Jul 2010 16:49
On Thu, 29 Jul 2010 08:32:11 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote:

>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.

Looks good!

>
>John
>
>
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