Help - Ferrite core loss density chart interpretation? Losses with DC+AC core flux?

[John Woodgate]

(in <[email protected]>) about 'Help - Ferrite
core loss density chart interpretation? Losses with DC+AC core flux?',

deltaT = ( Pmw / Acm^2 ) ^0.833 doesn't even give results that
correspond to their own published example for TSF-7099-41-16-12-0000,
(chart in the powerpoint presentation with the same title).

The exponent selected typically gives rises that are less than half real
measured results.

The sample chart plotted shows a relationship of

deltaT = K ( Pmw / Acm^2 )

where K is 1.2 at low power, increasing to 1.0 at higher power densities
(>4000mW).

Which is close to the general rule of thumb -

1degC/mW/cm^2 +/-20%.

I don't know how they ever worked the exponent into it.

0.833 = 5/6, and there is a '5/6-power (of temperature difference) law
of cooling'. IIRC, it refers to cooling in still air. Newton's Law,
which says that the rate of cooling is proportional to the temperature
difference, applies in free-flowing air.

But if this is the underlying explanation, the exponent in the deltaT
equation should be 1.2, not 0.833, I think.

 

[legg]

(in <[email protected]>) about 'Help - Ferrite
core loss density chart interpretation? Losses with DC+AC core flux?',


0.833 = 5/6, and there is a '5/6-power (of temperature difference) law
of cooling'. IIRC, it refers to cooling in still air. Newton's Law,
which says that the rate of cooling is proportional to the temperature
difference, applies in free-flowing air.

But if this is the underlying explanation, the exponent in the deltaT
equation should be 1.2, not 0.833, I think.

Using an exponent of 1.2 gives rises that are more than double those
plotted in the example.

RL

 

[John Woodgate]

(in <[email protected]>) about 'Help - Ferrite
core loss density chart interpretation? Losses with DC+AC core flux?',

Using an exponent of 1.2 gives rises that are more than double those
plotted in the example.

I didn't suggest that it was realistic; I just suggested where 0.833
came form and that it appeared to be the reciprocal of the value
indicated by the 5/6 power law.

The further explanation may be that the example refers to cooling in
flowing air. Still air conditions would presumably cause a greater
temperature rise.

 

[Don Klipstein]

Using an exponent of 1.2 gives rises that are more than double those
plotted in the example.

Please measure where you can as best as you can should you go so far as
to make some prototype of something.
Rules for prediction of temperature rise are, in a good case,
"one-size-fits-all" and in better cases proponents of such "rules" can
note some significant exceptions.
If you are presented any evidence that your "favorite temperature rise"
rule is unfavorable, I advise to investigate to an extent adequate to
prove or disprove.

Meanwhile, I think that heat transfer as a function of varying "delta T"
should exceed 1 when differing from 1, so I would favor 1.2 over .833.
But if you are presented credible evidence of .833 or whatwever is
unfavorable, you should do the work to either disprove such unfavorable
evidence or else do the work to accomodate it!

- Don Klipstein ([email protected])

 

[legg]

Please measure where you can as best as you can should you go so far as
to make some prototype of something.
Rules for prediction of temperature rise are, in a good case,
"one-size-fits-all" and in better cases proponents of such "rules" can
note some significant exceptions.
If you are presented any evidence that your "favorite temperature rise"
rule is unfavorable, I advise to investigate to an extent adequate to
prove or disprove.

Meanwhile, I think that heat transfer as a function of varying "delta T"
should exceed 1 when differing from 1, so I would favor 1.2 over .833.
But if you are presented credible evidence of .833 or whatwever is
unfavorable, you should do the work to either disprove such unfavorable
evidence or else do the work to accomodate it!

- Don Klipstein ([email protected])

Please review the postings. We were discussing the suitability of an
exponent in a published formula that does not correspond to the
results provided in the same publication.

The published results do tend to follow the simple exponent-free
approximation that is stated in the posting.

A simple multiplying constant of 1.2 would predict rises 20% higher
than a unity constant, and 20% higher than occur in the published data
at the upper limits of the deltaT/power curve for the
TSF-7099-41-16-12-0000 wound core part.

Using either 0.833 or 1.2 as an exponent would predict rises that are
less than half or more than double the published data.

RL