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'[PICLIST] +AFs-EE+AF0-: IGBT Help'
2001\11\08@150320 by Lawrence Lile

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I can't think of a way to use FET's for AC without steering diodes, if you
come up with one post it+ACE-

Now, IR used to make a device that was an LED in combination with a small
solar cell-like voltage generator.  Created total isolation from input to
output, PLUS automatically generated the 10 volts or so required to turn on
a FET+ACE-  As I remember, they were kind of slow, but might be OK for a light
dimmer.

I am frustrated with Triacs, they just eat up too much power and make too
much heat.   A FET could really reduce the heat sink requirements.  But with
a steering diode, how much more efficient can the combo really be?

-- LAwrence

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2001\11\09@135102 by Harold M Hallikainen

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On Thu, 8 Nov 2001 14:01:19 -0600 Lawrence Lile <spam_OUTllileTakeThisOuTspamTOASTMASTER.COM>
writes:
{Quote hidden}

       I agree! It still seems triacs (or dual SCR solid state relay modules)
are hard to beat for phase control. The IR part
(http://rocky.digikey.com/WebLib/int%20rectifier/pvi5050.pdf) is
interesting. It IS slow, and its output current capabilities are severely
limited. I can just imagine it trying to drive an FET gate with a high
Miller effect capacity...
       I DID do an interesting low voltage (12V) dimmer a while back using
FETs. We took one of our standard 24 channel cards (used in the DM1224
and DM2412 at http://www.dovesystems.com) and drove FET gates directly.
The FETs were N channel with the source grounded. The 12V loads were
placed between a full wave rectified 12VAC and the drains of the FETs.
The rectified 12V was not filtered, so we did not have to adjust our
dimming curve (or pay for a filter capacitor). We used a full wave bridge
rectifier, so losses there were a bit high (1.4V), but losses in the FETs
were very low (pretty much due only to gate rise time). It worked well.
However, the loads were not isolated from the rest of the circuitry,
which would be required in a line voltage design, and the lamps ran on DC
(though unfiltered). This would probably not be permitted in a line
voltage unit. In such a case, you'd either have to go with two FETs per
channel with steering diodes and messy drive circuitry, or a bridge
rectifier per channel between the FET and the line to be switched, again
increasing losses due to diode drops.
       So... SCRs and triacs are still hard to beat...

Harold



FCC Rules Online at http://hallikainen.com/FccRules
Lighting control for theatre and television at http://www.dovesystems.com

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2001\11\09@142925 by Bob Blick

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> voltage unit. In such a case, you'd either have to go with two FETs per
> channel with steering diodes and messy drive circuitry, or a bridge
> rectifier per channel between the FET and the line to be switched, again
> increasing losses due to diode drops.

The steering diodes are already in the FETs, just tie the FETs in series
with gates together and sources together.

However the $$ of that is pretty high so I would imagine it would be for
mansion lighting and not theatrical lighting. High voltage FETs with low
on resistance are not cheap and driving them isolated is also spendy.

If the original poster would give details about the application we could
either sympathize or tear him apart :-)

Cheers,

Bob

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