epcos e-cores as audio Transformers

[jkeny]

This might be an idiotic question & I don't know enough to answer this myself but I thought some experts from here could.

I have access to a number of epcos e cores + bobbins & I wondered if they could be used (with suitable winding) as audio input and/or output tranfsormers?

Their intended application ,from what I can tell, is as SMPS inductors? Quote from application note:

The preferred materials for manufacture of E cores are the SIFERRIT materials N27, N87, T46 and N30. - N27 is recommended for power applications in the frequency range up to about 100 kHz

The cores I have are N27 Ferrite material and form a square 43mm*43MM
http://ie.farnell.com/3110205/electrical-circuit-protection/product.us0?sku=EPCOS-B66325GX127.

 

[guitar_joe]

not with those materials.

 

[jkeny]

Help me to understand why, though! Does it not allow the signal in the audio band to pass through unhindered?? What effect does the core material have on transformers? All very basic questions but I don't know the answers

 

[guitar_joe]

ok simply...

the material used in those cores is designed to run at high frequency. >50,000hz at a rough guess, and up to 100,000hz.

the material in audio transformers is designed to run at low frequency ~20hz to 20,000hz.

generally speaking grain oriented silicon steel is the preferred material for audio tx's.

as you can see transformers that operate on low frequencies are much bigger.

 

[jkeny]

Thanks for coming back guitar_joe,
I understand the intended operating point is far higher than the audio band but I wondered what happened the signal below this point - above it, i presume the signal is progressively blocked but below it what happens?

 

[smoking-amp]

The ferrite cores will still work for audio frequency. But they have about 1/4 the maximum flux density of M6 steel, so you will need something about 4 times bigger core area than the usual output xfmr. for the same Watts. (4x area will be about 8X volume or weight)

Since the typical ferrites are intended for high freq. operation, they are all much smaller than typical steel audio xfmrs. So, a no go fit, unless you can get some gov't surplus stuff for particle accelerators. Even then, the large required size will compromise the HF response due to the excessive distributed capacitance from the longer windings.

There are some schemes around though to use the ferrite at the high freq. to transform carrier modulated audio. Search on "Berning".

Don

 

[jkeny]

Thanks Smoking-amp,
Very informative post, as usual - I forgot to say that I wanted to use them as input not output trafos! From what you say, this might work?

Will look up"Berning"

 

[smoking-amp]

Hmmm, for input xfmr use, these might be a problem. They (ferrites) have higher coercive force than permalloy material (ie, high hysteresis distortion). Need to compare the specific material info with the material you would normally use. If its driven from a low impedance source then probably OK.

N27 material has Hc of 23 amp/m and initial permeability ui= 2000

M6 grain oriented material has Hc of 7.95 amp/m and ui of 1500

Amorphous iron alloys HC of .48 to 3.2 amp/m and ui approx. 2000
(varies greatly with field strength, not usually spec'd)

4-79 Permalloy: Hc 2 amp/m ui of 30,000 to 60,000

Supermalloy: Hc 0.16 amp/m ui of 100,000+

Obviously, Supermalloy is the winner by far.

 

[jkeny]

Thanks smoking-amp,
Looks like I will stick with using a toroidal PS trafo as input for the moment. I suppose the bobbins could be used and suitable laminations found?