Common Mode Transformer Saturation
D from BC
current for saturation?? Or is that the differential mode current for
saturation?
Of maybe it's the max current through one coil..
Datasheet example:
http://www.cooperbussmann.com/pdf/69eca15b-d981-4cf9-b53a-c878f70dbd24.pdf
Is the differential mode current for saturation much higher than the
common mode current for saturation?
I suppose when a common mode transformer gets a differential current a
field and antifield has to form in the core and I suppose this might
virtually split up the core into something that probably acts like 2
rod cores.
Phil Allison
"D from BC"
** Will he ever learn to read and not make wrong assumptions ??
> Is the datasheet spec for common mode transformers for the common mode
> current for saturation?? Or is that the differential mode current for
> saturation?
> Of maybe it's the max current through one coil..
> Datasheet example:
> http://www.cooperbussmann.com/pdf/69eca15b-d981-4cf9-b53a-c878f70dbd24.pdf
** There is no spec for "saturation" on that page.
The amp ratings are based on temp rise alone.
..... Phil
D from BC
wrote:
ooops.
I didn't check the fine print was at the bottom of the graph..
Yup ..The current spec is based on a heat rise.
And I think that's a rise from the DCR from DC.
And you're right, that sheet has no saturation data.. The inductance
spec is open circuit. There is no inductance listed at Irms max.
Ex.. At 7Arms passing through the transformer the inductance is
...unknown..
But at least I know how warm it'll get if the ambient temp is 85C.
Can I assume the CM transformer gets too hot first (from Idc^2*DCR)
before saturation?
I think the saturation data is not listed cause it's not relevant.
In common mode there's opposing magnetization of the core and I think
that takes more current compared to differential mode.
It's the heat from wire resistance and core temp that set the limits..
Not really the core saturation.
So I am guessing that the inductance at bias (all the way up to
Irmsmax) will be not too far from the open circuit inductance spec.
Am I far off ?? :(
Phil Allison
"D from BC"
"Phil Allison"
>
>>** Will he ever learn to read and not make wrong assumptions ??
>>
>>> Is the datasheet spec for common mode transformers for the common mode
>>> current for saturation?? Or is that the differential mode current for
>>> saturation?
>>> Of maybe it's the max current through one coil..
>>> Datasheet example:
>>> http://www.cooperbussmann.com/pdf/69eca15b-d981-4cf9-b53a-c878f70dbd24.pdf
>>
>>
>> ** There is no spec for "saturation" on that page.
>>
>> The amp ratings are based on temp rise alone.
>>
>
>
> spec is open circuit. There is no inductance listed at Irms max.
> Ex.. At 7Arms passing through the transformer the inductance is
> ...unknown..
> But at least I know how warm it'll get if the ambient temp is 85C.
>
> Can I assume the CM transformer gets too hot first (from Idc^2*DCR)
> before saturation?
** It will never saturate if used as intended.
> I think the saturation data is not listed cause it's not relevant.
> In common mode there's opposing magnetization of the core and I think
> that takes more current compared to differential mode.
** Just a bit.
> It's the heat from wire resistance and core temp that set the limits..
> Not really the core saturation.
> So I am guessing that the inductance at bias (all the way up to
> Irmsmax) will be not too far from the open circuit inductance spec.
>
> Am I far off ?? :(
** If the core gets very hot - like 150C, inductance will fall off a bit.
Curie point temp and all that.
Wimpie
> Is the datasheet spec for common mode transformers for the common mode
> current for saturation?? Or is that the differential mode current for
> saturation?
> Of maybe it's the max current through one coil..
>
> Is the differential mode current for saturation much higher than the
> common mode current for saturation?
> I suppose when a common mode transformer gets a differential current a
> field and antifield has to form in the core and I suppose this might
> virtually split up the core into something that probably acts like 2
> rod cores.
Hello,
For the differential case (so equal current, but opposite direction),
the net field in the core material is close to zero. Therefore current
is limited by ohmic losses in the two windings.
The common mode current (that is the sum of I(1-2) + I(4-3) ) causes a
net magnetic field inside the core. When you look the intended
application, they mention "EMC filters". In most cases common mode
levels generated by equipment are far below the saturation current of
the CM coil. So therefore they do not specify the CM current rating.
Depending on you application, you may run into problems. When the CM
coil is subjected to CM pulses as used in surge immunity testing, the
V*t product may result in high common mode current. Saturation may
also occur when one of the windings carry (part of) DC supply
current. I'm currently working on a power application where this
effect will happen when no measures are taken.
You can measure the CM inductance versus CM current behavior yourself
when you have two same pieces. Put them in series; supply current
through the series circuit (decouple your power supply for the small
signal AC frequency). Both coils now carry equal DC current. You can
use one of the 2 windings (and leave open the unused), or parallel the
two windings.
At the connection of the 2 same-type coils you can inject an AC signal
via a coupling capacitor (and series resistor). This AC current
distributes equally through the inductors. Gradually increase the
current and look to the AC voltage at the interconnection. Above a
certain DC current, the AC signal at the interconnection will drop
because of core saturation.
In most cases you want a high (CMinductance)/(leakage inductance)
ratio. This requires closed magnetic circuits (no air gap) with short
magnetic field paths. This, however, results in low CM current
handling capability.
Best regards,
Wim
PA3DJS
www.tetech.nl
in case of PM, don't forget to remove abc.
Tim Williams
I have a nice 3" high-mu toroid with some 6AWG hookup wire wrapped around
it, for common mode duty. I wound it in the "bunch of turns up the left
side, bunch of turns up the right side in the opposite direction" style, so
it will have quite a lot of leakage inductance. Thing is, if each side
saturates from the ~100A peaks I'll be drawing from it, that will quite
excellently defeat the purpose of having it.
Since the windings oppose, it should indeed act like two rod cores. The
question then becomes, what is the effective airgap, and its corresponding
saturation ampturnsage?
I was going to test it by hooking up a little buck converter to the line end
of the choke, shorting the load end. So I've got maybe 100A drawn through
at piss-all voltage. I've got two ends (the buck converter looks like a
voltage source), so I can play with it in common mode as any old inductor
and watch the B-H curve (well, I-t curve) while varying amps. I'll need to
set up a big row of IRFZ46N's though, there's maybe 200A peak I'd want to
look at. Don't have any way to measure that much DC, either... maybe I
should settle for a smaller scale test...
Of course, if it were wound bifilar, this would be a trivial question. But
that doesn't have much leakage inductance, so you'd definitely need a
differential mode choke for extra filtering. (I should probably have one
anyway, since I'm running a rather low frequency...)
Tim
--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms
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