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17th April 2012, 09:56 AM  #1 
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Join Date: May 2005
Location: Johannesburg

Choke formula  Marty Brown
Hi All
I am presently building a 12V to 330V converter and I have been using a book titled Power Supply Cookbook by Marty Brown as a guide. There is a formula that appears within for calculating the output choke value that I am having a tough time grasping as it seems the result is rather big for such a small smps. The smps operates in pushpull config at 50kHz and is rated for 120mA max. Vin(min) = 11V Vin(max) = 15V Can someone please have a look at my calc. and advise if I misunderstood what was required? Thank you Ashraf 
17th April 2012, 01:55 PM  #2 
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Anybody??

17th April 2012, 02:09 PM  #3 
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Location: Barrio Garay,Almirante Brown, Buenos Aires, Argentina

The inductor at the output only smoots the voltage, and operates as a low pass filter with the out cap. Assuming a delta I, the choke is easy to calculate. Normally it is assumed as a 10% of I load. I also have this book, but it is a bit confusious the math he use. Read any inductor calculation for buck converter type, in fact the inductor operates in the same way as it.
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17th April 2012, 02:39 PM  #4 
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Osvaldo, are you saying that calculating chokes for buck converters are exactly the same as calculating an output choke for a forward converter?

17th April 2012, 02:50 PM  #5  
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Quote:
http://www.ti.com/lit/ml/slup127/slup127.pdf I hope this be usefully.
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17th April 2012, 05:02 PM  #6 
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The formula is dimensionally correct.
The only point open to debate is the 1.4 factor, it represents the level of ripple you tolerate for the current in the choke, but even if you choose a different trade off, the value will remain in the same range. It is a normal value for a high voltage converter: the inductance value increase as the voltage squared.
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18th April 2012, 12:31 AM  #7 
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can you use the common magnetic chokes or distance magnetic chokes for short the wave?
i think we can solve your question regards Jay 
18th April 2012, 10:58 PM  #8 
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Thanks for your responses.
Osvaldo, I read the TI note and I can't seem to find where it explains how the inductance of the choke was calculated. The inductance value of the choke was provided all that the note said was what type of core, winding etc. Elvee, I came across a somewhat differennt formula for calulating the choke from Col. T. McLyman and when I did the calculation I come up with a value of 74.5mH. Now I am more confused as to which value is the correct one. Jay, I don't understand your question. Please will you rephrase. 
19th April 2012, 08:59 AM  #9 
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Can you post your schematic, it will make the discussion easier.
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19th April 2012, 12:02 PM  #10 
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Here is the schematic.

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