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Friday 29 January 2010

500KHz and ferrites

OK hen's teeth time trying to find 3C85 and 3C90 cores. but found a few.... however I recall a conversation I had a while back that you could sometimes use switched mode power supply ferrites. Such as those from PC ATX supplies as they use of high permeability ferrite cores for the switching transformer.

I share the notes I have:
High permeability iron powder cores would be too lossy at 500KHz. Type 77 ferrite is ideal for switched mode supplies operating at 20 - 500 kHz. Although the toroid core is the most efficient shape, the pot-core E-core are often used for ease of winding.

A lot of reading manuals for switch mode supplies and as components in the power supply must of course be suitable for operation at the switching frequency it might safe to assume that T-( )-26 cores are ideal for the storage and filter inductors in supplies operating up to 250kHz. Above 250kHz they see to use T-( )-52 cores or some pot cores (185k), and e-cores (184k).

There sees to be three basic circuit types for smps, and the one chosen will depend upon the application. The most efficient use of the ferrite switching transformer occurs with the push-pull circuit, and as I have some of those and many PC ATX supplies see to be of this type the following information applies.

First a general assumption that smps transformers operating at 20 kHz can have a max. core flux density (Bmax) of 2000 gauss/200 mT. At higher frequencies Bmax must be reduced. A plot of Bmax vs Frequency gives a good idea of the relationship. Excessive temperature rise must be avoided either by using a larger size core and/or wire or reducing the output power. As a guide, the following table shows power capabilities of some different core shapes/sizes at three switching frequencies:

Toroid Cores Switching Freq. (kHz) Pot-Cores Switching Freq. (kHz) E-Cores Switching Freq. (kHz)
20 50 250 20 50 250 20 50 250
FT-240-77 1000 W 1500 W 4000 W

EA-77-625 230 W 350 W 1000 W
FT-140-77 200 W 300 W 850 W PC-3622-77 110 W 190 W 450 W EA-77-500 100 W 210 W 500 W
FT-114A-77 140 W 210 W 600 W PC-2616-77 30 W 50 W 130 W EA-77-375 60 W 100 W 260 W
FT-114-77 70 W 110 W 300 W PC-1811-77 5 W 8 W 20 W EA-77-250 13 W 20 W 55 W
FT-50A-77 5 W 8 W 20 W PC-1107-77 3 W 5 W 12 W EA-77-188 5 W 8 W 20 W

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