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From: Robert Baer
X-Mailer: Mozilla 4.75 [en] (Win98; U)
Subject: Re: Uprating a SMPS
Date: Thu, 12 Sep 2002 08:17:23 GMT
NNTP-Posting-Date: Thu, 12 Sep 2002 01:17:23 PDT
Organization: EarthLink Inc. -- http://www.EarthLink.net
Lyndon Amsdon wrote:
> I recently bought a Maplin stepup power supply. It takes 12v upto
> 60v@4amp. It uses 4 MOSFETs, two for each side of the primary and two
> to clamp any spikes from inductance. On the secondary it's a simple
> rectifier and pi filter. It's controlled by a TL494 SMPS controller.
> The datasheet from Maplin shows how to wrap the transformer, an
> Ferroxcube ETD39.
> My question is this, can I redesign it all and use the biggest ETD59
> to get something like 80v@10amp? I can get better MOSFETs/rectifiers
> but I don't know what to change on the transformer. Primary is made
> up of two 9 turns of 18SWG and secondary is two 26 turns of 20SWG.
> So, it seems the turns ratio is just about right, giving some
> headroom. The voltage is fed back into the TL494 via a resistor
> divider setup for feedback.
> If I keep the same turns ratio, 9 and 26 can I just increase the cross
> sectional area of the enamled wire, wrap it around a larger former and
> make the adjustments to the rest of the diagram? I'm not that
> familiar with wrapping transformers that operate at +50KHz, not any
> sites I could find said anything about it. Maybe in books?
Books seem to not help; like having 3 equations in 15 unknowns.
Seems to be a black art with every case different and subject to
previous sucessful experience.
As long as the ferrite material is the same, one can use a core with
larger cross-section to handle more power and still use the same turns
Core area for a standard transformer goes as the square root of the
power rating; use this as a guide.
Watch out for wire size to handle the larger current; i use the
cross-sectional area measured in square mils as the milliampere rating
for the wire. I find that this gives insignificant winding I*R losses.
You may need to experiment with the number of turns, as the inductance
will change with that larger core.
The number of turns for a standard transformer goes as the inverse of
the square root of the power rating; use this as a guide.
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