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Old 30th March 2006, 05:47 PM   #11
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mzzj: the values I used were 120 ohm resistor and a .47uF cap, I know thats probably way too high

Oh and Mr. Eva, can the IC put out enough current to do an output of 100mA?
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Old 30th March 2006, 06:27 PM   #12
mzzj is offline mzzj  Finland
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Quote:
Originally posted by ifrythings
by the way anyone know of a good way to dismantal pre-built transformers?
Heat in electrical oven at 150-200 celsius for something like 15 mins. Hot plate of stove can be used also but you need to be more carefull not to heat core too fast as it will cause fractures.
Pry apart with "oven mittens" in hand, sometimes _small_ extra force with chopstick/screwdriver is also required. Works 9 cases out of 10.
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Old 30th March 2006, 07:20 PM   #13
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Thanks for the idea, I shall try that.
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Old 30th March 2006, 08:04 PM   #14
Eva is offline Eva  Spain
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You have to determine how many turns are needed to drive the transformer at your voltage and frequency without saturating it. If you could tell us the cross-sectional area of the center leg of the core, we can made an estimation.

Also, did you try the series 1uF capacitor as I recommended? The transformer will be very likely to saturate and cause high currents to flow without that capacitor, no matter if Rod's schematic doesn't show it.

BTW: I'm not Mr but Ms.
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Old 31st March 2006, 05:11 PM   #15
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Ms. Eva, I'm very sorry about calling you Mr.

The dimentions of the core are on the picture.
Attached Images
File Type: jpg core.jpg (15.9 KB, 96 views)
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Old 31st March 2006, 08:06 PM   #16
Eva is offline Eva  Spain
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Then cross-sectional area is S=6*6=36mm^2=0.000036m^2

Peak flux for your full bridge symmetrical converter would be:

B=V*(0.25/F)/(N*S)

Solving for N:

N=V*(0.25/F)/(B*S)

Taking worst case values...

V=15V (assuming an automotive application)
F=54Khz (you are using a 108Khz clock, aren't you?)
B=100mT (reasonable level for most core materials)

N=16*(0.25/54000)/(0.1*0.000036)=19.3 turns

So your 20 turn figure for the primary was right (as long as you use a 108Khz oscillator).
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Old 31st March 2006, 11:25 PM   #17
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Holly crap, you sure know your stuff...

F=54Khz per switching device or 108 clock is what I'm running so I think those windings will work.

thanks for your input, when I get home I shall try that and post back.

BTW you know of any website that explain how average current monitoring works? I would really like to study it.

Thanks David
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Old 31st March 2006, 11:38 PM   #18
Eva is offline Eva  Spain
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If you mean average current mode control of SMPS, check that paper: http://www.powerdesigners.com/InfoWe...chive/u140.pdf

Remember to add the 1uF non-polar capacitor in series with the primary and to add some load to the secondaries (~25mA).
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Old 1st April 2006, 12:40 AM   #19
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1 question, is that 1uF non-polly for just one of the primary legs or do I put a cap on both of the primarys legs?

will 1uF@50V work?

P.S thanks for the link.
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Old 1st April 2006, 12:49 AM   #20
Eva is offline Eva  Spain
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One capacitor in one leg is enough. Also, it will only have a few dozens milivolts across it, so 1uF 50V is fine.
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