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From: "Michael Denner"
Subject: Re: Uprating a SMPS
Date: Fri, 13 Sep 2002 18:30:16 +0100
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You will need to recalculate the number of turns if you use a bigger core
such as the etd 59 the formula is simple N= V/(2pie*f*B*A) where N is the
number of turns on the primary, F switching frequency, B is the flux density
and A is the core area taken from the ETD59 data sheet. Then work out the
new secondary turns from the primary turns/voltage. When you upgrade the
fets you may also need to upgrade the driver circuits too since bigger fets
have higher gate capacitances which have to be charged and discharged check
out the application notes on the IR website. To increase power o/p you could
simply up the frequency recalculate the number of turns and keep the same
transformer core. You'd still have to uprate the fets, drivers and rectifier
"Robert Baer" wrote in message
> Lyndon Amsdon wrote:
> > Hi,
> > 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?
> > Thankyou
> 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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