Choke formula - Marty Brown
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Archimedes
diyAudio Member

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 push-pull 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
Attached Images
 Choke formula.png (80.8 KB, 261 views) Choke calculation.png (11.9 KB, 257 views)

 17th April 2012, 01:55 PM #2 Archimedes   diyAudio Member     Join Date: May 2005 Location: Johannesburg Anybody??
 17th April 2012, 02:09 PM #3 Osvaldo de Banfield   diyAudio Member     Join Date: Dec 2011 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. __________________ Osvaldo F. Zappacosta. Electronic Engineer UTN FRA from 2001. Argentine Ham Radio LW1DSE since 1987.
 17th April 2012, 02:39 PM #4 Archimedes   diyAudio Member     Join Date: May 2005 Location: Johannesburg Osvaldo, are you saying that calculating chokes for buck converters are exactly the same as calculating an output choke for a forward converter?
Osvaldo de Banfield
diyAudio Member

Join Date: Dec 2011
Location: Barrio Garay,Almirante Brown, Buenos Aires, Argentina
Quote:
 Originally Posted by Archimedes Osvaldo, are you saying that calculating chokes for buck converters are exactly the same as calculating an output choke for a forward converter?
Yes!!!

http://www.ti.com/lit/ml/slup127/slup127.pdf

I hope this be usefully.
__________________
Osvaldo F. Zappacosta. Electronic Engineer UTN FRA from 2001.
Argentine Ham Radio LW1DSE since 1987.

Elvee
diyAudio Member

Join Date: Sep 2006
Quote:
 Originally Posted by Archimedes Anybody??
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.
__________________
. .Circlophone your life !!!! . .
♫♪ My little cheap Circlophoneİ ♫♪

 18th April 2012, 12:31 AM #7 lizhi3amorphous   Banned   Join Date: Apr 2012 Location: China can you use the common magnetic chokes or distance magnetic chokes for short the wave? i think we can solve your question regards Jay
Archimedes
diyAudio Member

Join Date: May 2005
Location: Johannesburg

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.

Attached Images
 Calc 2.png (5.2 KB, 183 views)

 19th April 2012, 08:59 AM #9 Elvee   diyAudio Member     Join Date: Sep 2006 Can you post your schematic, it will make the discussion easier. __________________ . .Circlophone your life !!!! . . ♫♪ My little cheap Circlophoneİ ♫♪
Archimedes
diyAudio Member

Join Date: May 2005
Location: Johannesburg
Here is the schematic.
Attached Images
 PSU Schematic.png (86.5 KB, 175 views)

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