Re: Ferrite chokes & saturation
From: Malcolm Watts[SMTP:MALCOLM-at-directorate.wnp.ac.nz]
Sent: Thursday, November 20, 1997 2:33 PM
To: Tesla List
Subject: Re: Ferrite chokes & saturation
> From: Gary Lau 19-Nov-1997 1728[SMTP:lau-at-hdecad.ENET.dec-dot-com]
> Sent: Wednesday, November 19, 1997 3:39 PM
> To: tesla-at-pupman-dot-com
> Cc: lau-at-hdecad.ENET.dec-dot-com
> Subject: Ferrite chokes & saturation
> Many of us employ ferrite core chokes in our xfmr protection networks.
> While the effectiveness (HF attenuation factor) of an L-C low pass filter
> can be fairly easily calculated, I haven't seen any quantitative
> discussion on whether core saturation is occuring. If it is, that pretty
> much wipes out the benefit of an inductor.
> I've looked over the Amidon web site for info on saturating ferrite core
> chokes, but the graphs, beyond being illegable, I just didn't know how to
> apply them.
> 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?
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.
> If it is the HF current that causes saturation, could one then assume
> that if a primary strike occurs, saturation current may then be reached
> and choke effectiveness lost, at the time it is most needed?
> And on the topic of xfmr protection networks, assuming safety gaps
> employed at the xfmr outputs, are series chokes and/or R's integral to
> the safety gap's effectiveness? I'm trying to rationalize why many on
> this list use chokes but no bypass caps, or R's but no chokes. Could
> these series components be viewed as ballast devices, significant only
> if and when the safety gap fires?
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.