Maximum gap size for flyback transformer?

What would you say is the maximum gap length for use in the centre leg of the following epcos 49/25/16 core?
Its for a 330W flyback for Class D guitar amplifier supply. (vin = 90-265VAC)

I want to have a 2mm gap in the centre leg (only) , but is this too big?

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.. the overall centre core length is 35.4mm, as you can see from the datasheet.

i have seen 3 mm gap in inductor for welding inverter.
afaik, 330w is such level of power, that is not very suitable for flyback topology...
did you calculate amplitude current of secondary winding?

current is 4A average at output...80v.
Flyback ok because its a class d amplifier load..therefore, average power is low.

3mm gap might be ok, it depends on what percentage of the centre spindle length that that 3mm is...what is the maximum allowed percentage?

There's no thing like an "allowed percentage". But a large air gap will involve increased leakage inductance and possibly additional eddy current losses in the winding. In some designs, the coil former zone around the air gap is kept free of windings to reduce this effects.

Beyond "allowance", when I see a core design with unusual large air gap percentage, I check the design plausibility. The design may be reasonable, though.

thanks, I agree, but ive never found a formal reference saying air gaps in smps transformers mean more leakage.

I was looking at
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...and it says you can reduce leakage by..
1...interleaved winding
2...winding all layers right across the whole spindle length
3...having the layers physically close,

but it says nothing about picking a bigger core (which will inevitably need a smaller gap) to reduce leakage inductance.

All effects are relative, the overall result is a combination of all individual parameters. Of course perfectly interleaved windings won't show much leakage increase related to air gap.

Can it be that the said transformer was designed with an ungapped core halves and "all legs" air gap? https://www.ti.com/lit/ml/slup127/slup127.pdf

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Thinking about the air gap effect on leakage inductance, I must admit that you are probably right. Because leakage inductance is mainly defined by the pure winding geometry, independent of the core. In a first order, it can be calculated or measured for an air core setup.

ive heard that about approximating the leakage by simply measuring the inductance of the thing with an air core, in place of the ferrite core.......but if this is the case, then that means that the lesser the turns (on a given core), the lesser the leakage, which would mean DCM flyback was generally preferable to CCM flyback (because dcm tends to have less turns than ccm), and it would indeed be preferable, because generally, with flybacks, it is the transformer leakage which is the major causation of losses in the flyback smps

On similar lines, we now wish to wind a transformer for a Flyback on a ETD54/28/19 core with 1mm gap (N87 material).
We have a split output rail of +/-40V, 4A. We will thus have two secondary windings, and these will be interleaved between the primary which will be split into three equal layers.
So we have a primary of 30 turns total, and a split secondary of 22 turns total. Each winding is made of three paralleled strands of SWG21 enamelled copper wire.
So we have this interleaved winding structure ,working from the core outwards.....

10 primary turns
11 secondary turns
10 primary turns
11 secondary turns
10 primary turns

We must have UK mains isolation between primary and secondary. (so 3mm margins)
The flyback is 330W peak, but average power is just 40W. (Guitar amplifier power supply)

Do you think we will be able to get a coupling factor of at least 0.994 with this interleaved winding strategy?
We fear that the 1mm gap size is just too wide to get this low leakage inductance?

The windings are each about 2.7mm wide (3 paralleled SWG21 strands) and this is wider than the core gap size so therefore we believe that this will help to reduce leakage inductance?


Epcos ETD54/28/19 core datasheet..(N87 material with 1mm gap)
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FvM said:

Thinking about the air gap effect on leakage inductance, I must admit that you are probably right. Because leakage inductance is mainly defined by the pure winding geometry, independent of the core. In a first order, it can be calculated or measured for an air core setup.

Click to expand...

Right to a first order increasing the gap while keeping everything else about the windings the same will decrease Lm and therefore decrease k but shouldn't really affect leakage. But in a case where you're design has, for example, a restriction on ripple current and frequency, you would have to maintain a constant Lm, and therefore increasing air gap would increase leakage (since you had to add turns). So in that sense one might say "increasing air gap increases leakage," even though the statement isn't really true by itself. I think that's where a lot of misconceptions come from assumptions like this.

sorry but I think increasing air gap would increase leakage, I mean, picture an air gap in a centre post of a ferrite core...now make that gap go bigger and bigger until it is the entire centre post length......you can see that your coupling factor is going to be very poor now.
Essentially I am wanting to get a good k factor so as to get minimal losses in the flyback rcd clamp. I know its the magnitude of the leakage that counts, but due to various issues, its actually a high k factor that is best for getting low loss in the flyback rcd clamp.
I also believe (do you agree?) that if the air gap in the centre post is wider than the width of an individual wire turn, then there will be poor coupling and thus higher leakage...do you agree?
I believe that if one finds that one's gap is wider than the winding wire diameter, then one must use paralleled strands instead, so as to make the "width" of the winding wire wider than the gap width..do you agree?.

Air gap does not affect coupling/leakage L, keep wires away from gap by at least 2mm for a 3mm gap, unless you are using very fine litz (finer than 42AWG).

I think air gap does effect coupling...imagine of you widen the air gap in the centre post of an ETD until it takes out the entire centre post...you are not then going to get good coupling I am sure you agree?.....this is the extreme case, but applies with smaller gaps but just to lesser extent.

The leakage inductance of a transformer is in a first order equal to that of the respective air core transformer. It's the windings geometry that defines it, not the core.

The leakage is affected by the presence of a core only in that having a high mu material near the windings (especially the lead out wires) can add a little to the effective leakage inductance, for a multi sandwich layer construction - the leakage is near identical with or without the cores.