AVR-Chat

Kathy Quinlan wrote:
> Hi all,
> 
> I am resurecting an old project atm and I need to build a powersupply, 
> it will take in an average of 120Vdc (from ~100 - 160Vdc) and provide 
> regulated 24V and 5V rails. Total wattage around 60W.
> 
> I chose the uc3842 SMPS controller as the regulator. I am trying to work 
> out the design of the transformer.
> 
> I am using a ferroxcube ETD34/17/11 with the 3C90 cores. I have worked 
> the example in the philips UC3842 appnote ( 
> moon.feld.cvut.cz/hw/philips/acrobat/8014.pdf ) Using the values of the 
> EC35 core with the 3C81 core (it says a 3C8 core but I can find no info 
> and believe the last digit is missing)
> 
> I am getting very strange results in my calculations and as transformer 
> theory is not my area, I need some help lol
> 
> TIA
> 
> Kat.

99% of the energy is stored in the air gap and for a flyback
supply you must have one. Then you must know the core cross-sectional
area, the saturation flux density (0.3 Tesla is usual), length
of the primary pulse, and flyback voltage across the transistor.
 From that, you can determine the turns-per-volt. However, thermal
problems limit how small the transformer can be, and winding
capacitance limits how high the frequency can be.