Re: Ferrite chokes & saturation

To: "'Tesla List'" <tesla-at-poodle.pupman-dot-com>
Subject: Re: Ferrite chokes & saturation
From: Tesla List <tesla-at-stic-dot-net>
Date: Fri, 21 Nov 1997 18:36:45 -0600
Approved: tesla-at-stic-dot-net

From: 	Gary Lau  21-Nov-1997 1839[SMTP:lau-at-hdecad.ENET.dec-dot-com]
Sent: 	Friday, November 21, 1997 4:46 PM
To: 	tesla-at-pupman-dot-com
Cc: 	lau-at-hdecad.ENET.dec-dot-com
Subject: 	Re: Ferrite chokes & saturation

>> Does anyone actually know how to determine, either through specs or
>> measurement, if a ferrite core choke is likely to saturate in a Tesla xfmr
>> protection network?  Is it the 60 Hz current or the HF current that is
>> significant, or both?  How would either of these be calculated?

>From: 	Malcolm Watts[SMTP:MALCOLM-at-directorate.wnp.ac.nz]
>Sent: 	Thursday, November 20, 1997 2:33 PM
>Subject: 	Re: Ferrite chokes & saturation

>    It is dependent on peak Ampere-turns applied, initial core 
>permeability and core volume and area.
>    First you have to know or measure the instantaneous peak current 
>you will put through the choke windings. Most manufacturers publish 
>Hanna curves or ballistic induction (B-H) curves for their materials 
>which relate energy storage (0.5LI^2) to peak flux density (Bpk). Bpk 
>is a function of effective core area (flux lines/area). The basic 
>mechanism is something like: for each flux line generated by applied 
>Ampere-turns (N.I) force, there is one less that can "fit" into the 
>core. In effect, the generated flux lines "fill" the core up. You can 
>see that for a maximum number that can be fitted into the core, the 
>process leads to a non-linear reduction in inductance. Typical 200kHz 
>switchmode type ferrites have a figure for Bpk of around 400 - 500 
>mT. The cores start going badly non-linear around 250mT.
>    I have a complete suite of equations I can bring in that will 
>allow you to work out the gapping required to support the flux your 
>windings will generate knowing only the initial permeability, core 
>volume and cross-section diameter or length if you are interested. I 
>use these in designing switchmode supply chokes and transformers.


Thank you Malcolm, you obviously know what your talking about, but I'm
still having trouble.  One of my problems is I bought my torroids at a
swap meet, no specs, vendor name, part number, etc, so I can't tell what
the initial permeability is, I just bought the largest cores I could
find.  When you mention "work out the gapping...", are you suggesting
cutting a gap in the torroids?  My ferrite torroid cores are 3.25" OD,
2.25"ID, 0.50"T, insulated with .04" LDPE & hot melt glue, wound with 51
turns LDPE insulated 22AWG, measuring 14mH.  Could one generalize about
the likelyhood of saturating with 250mA peak 60Hz current (15k/60mA w/
resonant charging est.)?


>I am leaning towards the use of series resistors and bypass caps only
>given the cost and effort of building chokes approaching a Henry or 
>so. Chokes with significant Q can generate enormous spikes across the 
>small stray capacitances present. I will be testing these ideas 
>shortly as I am in a situation where I *have* to use neon transformers
>for a job and they *have* to last.

>Malcolm

Do you know if ferrite core chokes have the same high Q's as air core
units?

Gary Lau
Waltham, MA USA