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Old 16th January 2003, 08:42 AM   #1
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Default SMPS Transformer core types

Quick question...
In high frequency Switch mode power supplies (50khz about squarewave) is a ferite core better than an iron powder core?
THe ones I have access to are a 44mm outside diamter iron powder and a 34mm outside diameter ferite core.
Which of these will conduct the most power or which is more suitable to the frequency?
Thanks
fr0st
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Old 16th January 2003, 10:00 AM   #2
JBL is offline JBL  Canada
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I greaty think that the ferrite core will be better.

iron powder is nopt suited for this application.
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Old 16th January 2003, 10:28 AM   #3
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Check www.epcos.com or www.ferroxcube.com and search for Ferrite cores.

Before you know how much power a core can handle you must first know the following:
- Core type
- Core size
- Core material
- Converter topology
- Operating frequency
- Max. ambient temperature
- Max. temperature rise resulting in max. allowable losses
- Insulation distance
- Kreepage distance
- Max. winding area
- Winding structure
- Amount of wiring layers due to the proximity effect
- Maximum allowable 'B' in mT to avoid core saturation.

Good luck!
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Old 17th January 2003, 01:48 PM   #4
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I found a lot of info on the Epcos site, but the reference of all references on magnetic components (manufacturer that is) is www.mag-inc.com

I still ended up with Epcos cores due to the excellent SMPS demo board paper.

I have not looked much at frequencies as low as 50KHz, but my guess is that Iron cores are likely not optimall even at that frequency.

Petter
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Old 17th January 2003, 04:40 PM   #5
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Iorn powder has a high loss at high frequency. Iorn powder cores with a low mu value are suited for really low frequency applications such as line filters and audio filters. Ferrites are for higher frequency applications, but there are many other things to consider before just "picking a core material". After a ferrite is chosen, the actual type of ferrite must then be selected. The power ferrites are P, F, W from Magnetics Inc. (called other things by other manufacturers). The operating frequency chosen is based on the load demands and physical constraints and temperature rise. Pick up the Abraham Pressman book " Switching Power Supply Design" and pick up the Keith Billings book " Switching Power Supply Design Handbook".
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Old 17th January 2003, 05:15 PM   #6
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Default take a look at Int'l Rectifier link below

at www.irf.com
http://www.irf.com/technical-info/appnotes/an-1024.pdf


also -- the National Semi "Simple Switcher" software specifies the optimal transformer to use -- you can work backwards from the Nat Semi spec to the materials you have on hand. By the way, the software also specifies the capacitor ESR and the like and there are even PCB layouts on the nat semi site.

Linear tech has switcher-cad on their site. I have it loaded on my machine, but haven't really used it.

Lastly, if you know any ham radio ops down there -- the two most recent issues of <em><b>QEX</em></b>, discuss SMPS -- in fact, the most recent issue discusses choice of core materials, transformer design, etc. and all the E&M you forgot after you left college!

When I first started building SMPS I just used cores from ripped apart computer supplies -- you need an impedance meter to characterize the core -- and it's not a matter that more inductance is better -- but you can do a lot with these junkers and they will handle a lot of current.
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Old 17th January 2003, 08:50 PM   #7
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Hi,

Designing transformers for SM-PSU’s is not an easy task if you want to get most out of it. There is more than the core alone. It starts with the topology, forward or flyback for instance make a huge difference concerning the transformer. Also factors like skin effect and proximity effect are factors to deal with. And yes some metal powder cores like the ones from Magnetics are excellent up to 100 kHz in some applications. Look around at this site, there is a load of useful information.

There is an excellent book about designing SM-PSU’s Switching Power Supply Design from Abraham I. Pressman:

Click the image to open in full size.

It handles near everything you want to know about this subject and almost the best book about IMHO. But yes it is a bit expensive.

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