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Old 23rd September 2012, 07:47 PM   #1
treez is offline treez  United Kingdom
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Default Gapping an SMPS transformer core.

I need to put a 1.2mm gap into an SMPS transformer ferrite...(otherwise it would saturate), but this gap size is so big that 3D magnetic theory is needed to calculate the new Reluctance value......(is this true?)

Therefore, how do i calculate the new total core reluctance value with the 1.2mm gap in it.

The core is PQ5050-3C96 and 33 turns and a gap of 1.2mm "should" give it a inductance of 341uH and I(sat) of 9A7.....but will it?

Its and inductor of a boost PFC stage.

PQ5050 datasheet
http://www.ferroxcube.com/prod/assets/pq5050.pdf
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Old 23rd September 2012, 08:07 PM   #2
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by treez View Post
I need to put a 1.2mm gap into an SMPS transformer ferrite...(otherwise it would saturate), but this gap size is so big that 3D magnetic theory is needed to calculate the new Reluctance value
In principle, 3D theory is always needed when you add a significant air gap.

But I am not sure theory is your best option: magnetic reality is difficult to model and theorize, and when you have an approximate value, the best solution is to try calibrated spacers on real pair of cores.
You can start with approximations, like this one: Designing high-current chokes is easy | EDN and refine them with actual tests.

One thing has to be remembered: magnetic components tend to be forgiving, provided you give them enough turns.
Suppose you implement a too small gap: the inductor will saturate at about the same instant (inductance higher, but Isat smaller), but more sharply.
Since the magnetic path doesn't have an homogeneous section, some parts will saturate earlier, creating "emergency air gaps" as needed by the induction created.
This will of course slightly increase the magnetic losses, but in limited parts of the circuit, and the overall effect will be small.

It is better not to abuse of this self compensating mechanism, but a +/-30% tolerance on the inductance value will be transparently absorbed, and anyway Al tolerances are not tight either.
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Old 23rd September 2012, 08:26 PM   #3
treez is offline treez  United Kingdom
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Thanks Elvee,

ill bear that in mind..but also, does anyone know of gaspped ferrite cores bigger than the ETD54/28/19 core by Epcos?

http://www.epcos.com/inf/80/db/fer_07/etd_54_28_19.pdf

...this seems to be the biggest off-the-shelf gapped core in the world.

Surely there is bigger than this?

The advantage of these sold-gapped cores os that they come with an AL value and you can work out your saturation current more exactly than having to use the 3D magnetic theory
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Old 24th September 2012, 02:16 AM   #4
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Ferroxcube has the option of ordering gapped cores also. Their full line catalog shows E cores up to E80/38/20 can be ordered with gaps. They do not offer ETD cores pre gapped.

Ferroxcube - ferrite cores, bobbins & accessories has no direct link to catalogs.
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