Owen Duffy wrote:
Chuck wrote in
:


Owen, you may find this of interest:

http://standards.ieee.org/reading/ieee/ept/trans.pdf
trans.pdf (application/pdf Object)

Sevick's briefly discusses
voltage-dependency of ferrite losses.

Chuck

Thanks Chuck,

My original quote was from Sevick, and the following paragraph from your
reference illustrates the predominantly dielectric loss theme:

"Probably the most difficult (and controversial) specification to
establish for these devices is the power rating. The loss mechanism is
completely different from that of the conventional transformer, which is
current-dependent (hysteresis, wire, and eddy-current losses). With these
broadband devices, which mainly use ferrite cores or beads, the losses
are voltage-dependent (a dielectric-type loss). Therefore, higher-
impedance devices or devices subjected to mismatched loads of higher-
impedances, have larger voltage gradients along their transmission lines,
and hence, more loss."

Other works that I have read describe the loss mechanisms as quite
complex; magnetic loss described by a complex mu value that is
temperature, frequency and flux dependent, resistive loss in the core
material, dielectric loss in the core material, and resistive loss in the
conductors. Sevick seems to say that only one of these is relevant, or
that loss can be simplfied to a single equivalent loss, the dielectric
loss.

I guess it is appealing to equate loss that increases with frequency to
an equivalent dielectric effect, but the loss is flux dependent and in a
non-linear way, so it doesn't seem to fit well with a simple dielectric
equivalence.

Owen

Owen, I didn't see any reference in
Sevick's fourth edition to the
voltage-dependency of core losses.

Instead, the following quote seems to
characterize his philosophy in the book:

"With transmission lines, the flux is
effectively canceled out in the core and
extremely high efficiencies are possible
over large portions of the
passband--losses of only 0.02 to 0.04 dB
with certain core materials.

Therefore, the power ratings of
transmission line transformers are
determined more by the ability of the
transmission lines to handle the
voltages and currents than by the size
and conventional properties of the core."

He is unambiguous in arguing that there
is a significant lack of understanding
of the differences between conventional
transformers and transmission line
transformers.

This edition has a copyright date of
2001 whereas the IEEE paper is dated 1993.

Hope that helps.

Chuck






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