ferrites - general questions

[j_writer]

I have some questions about ferrite cores. I have given each question/topic its own reference number. I would appreciate if people don't ask me what specific circuit I am designing as there is more than one and I am at a very basic level at this time.

1) Are there generic or standard numbers for the various ferrite grades, such as a DIN or ISO number? For example, Ferroxcube uses numbers like 3C85 or 3C90. Does each manufacturer have its own set of numbers?

2) Similar question for the various core shapes and sizes, are there standards such as DIN or ISO numbers? Or does each manufacturer have its own set of numbers?

I am currently reading some theory on ferrite cores, but what is missing for me is a practical, real world understanding of what happens on the bench. Can anybody tell me what might be the observable effects of the following...

3) Insufficient core size (path area)
4) Wrong material
5) Core saturating (should it ever saturate?)
6) Improper winding technique
7) Improper wire gauge (meaning larger rather than smaller)

For example, in a flyback circuit, what can one tell about the design by looking at waveforms or feeling the core for excessive heat, or other experiential clues? This is not intended to be a complete guide, just some hints would help.

8) What happens if you have an improper number of turns (for example if you use 100:400 turns instead of 50:200 turns? Does this give you a 4x increase in inductance, and what does this do to your circuit?)

Regarding manufacturers core frequency specs, can anybody help me understand...

9) Is the rated frequency normally for sine wave or square wave? Isn't it true that if a transformer will pass all of the harmonics of an undistorted square wave, then the sine wave frequency rating will be much higher?

10) What determines the lower frequency limit? Does the core have an inherent lower limit or is it more a function of the circuit that is driving it?

11) Can ferrites be used in linear as well as switching applications? What are the core selection considerations?

Thank you.

 

[FvM]

You have been refering to the Ferroxcube handbook in a previous post. I think, that most of your question can be answered
from a thorough literature of this or other manufacturers publications, at least generally.

There are also profound text books, e.g. Snelling, Soft Ferrites. You can try to locate at a good engineering college library
or as E-book on the internet.

Some answers from my side:
1) They are manufacturer specific. You should be able to find a cross reference by comparing some corner data.

2) Answered in detail in the Ferroxcube handbook, see chapter Literature and reference publications

3-8) Depends on which of the inductive component's parameters are significant for your circuit. In my opinion, the question
isn't specific to ferrites, it's rather a general circuit design question, like "what's the importance of resistor power rating".
Or ask a baker, how to choose the best flour for his bread... You can easily write complete papers or even books about it.

9) Depends on. Sine for signal, square wave for power applications. Frequency ranges are never sharp for ferrites, so
the difference probably doesn't matter that much. If e.g. core loss is the question of interest, the fundamental would make
the largest part anyway.

10) Ferrites itself have no low frequency limit. A transformer has a low frequency limit either set by saturation flux or L/R time constant.

11) I assume, that the discussion is restricted to soft ferrites. They are mostly used as linear components, also in switching
applications. The optimal core parameters are however partly different for different applications. You'll be aware of,
when either browsing the datasheets or calculating a core for a particular application.

There are however some distinct non-linear applications of soft ferrites, e.g. transductors or special shaped cores, that
achieve a partly saturations. Or fluxgate magnetometers.