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Flyback converter AL value vs core loss and air gap

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ivan_mateo

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Hi everyone,

In flyback converter, how I can find equations or relation between air gap and core loss? When I increase air gap of exp N97, the reluctance will increase due to in core l(path in core) will increase. And AL value will less. The formula is L=N'2xAL, but I can not change L value because I calculated 320uH from power and eff. As know if I change N, the copper will be change. There is a confusion about that. How can I figure out this problem? What is the relation between AL(it can be air gap) between core loss?
 

The relation is primarily between flux delta, frequency and core losses. For a given core, flux is roughly proportional to ampere turns. For a first oder estimation, I would expect that losses increase at least with square of peak current.
 

you need to know the peak current on the pri side you can safely switch, you say you know the desired Lpri = 320uH, for a given core this information lets you calculate the turns and air gap, based on a suitable choice for Bmax, at 100kHz, Bmax <= 120mT for a cool running design.

L.I^2 = B.H.Vgap = (B^2/uo).Ae. Lg <- this gives you Lg for a given Bpk and Ae of a chosen core.

For a given Vin min, V/Lpri = di/dt this sets the max ON time, Vflyback/Lpri = di/dt gives the max off time ( so don't make the freq faster than the two combined )

assuming full DCM or CrCM control ( best for RFI noise and diode heating )

When you know Lg, AL = Ae/Lg . Uo ( nH/T^2) which gives you N for a known Lpri

happy designing ...
 

Easy peasy said:
you need to know the peak current on the pri side you can safely switch, you say you know the desired Lpri = 320uH, for a given core this information lets you calculate the turns and air gap, based on a suitable choice for Bmax, at 100kHz, Bmax <= 120mT for a cool running design.

L.I^2 = B.H.Vgap = (B^2/uo).Ae. Lg <- this gives you Lg for a given Bpk and Ae of a chosen core.

For a given Vin min, V/Lpri = di/dt this sets the max ON time, Vflyback/Lpri = di/dt gives the max off time ( so don't make the freq faster than the two combined )

assuming full DCM or CrCM control ( best for RFI noise and diode heating )

When you know Lg, AL = Ae/Lg . Uo ( nH/T^2) which gives you N for a known Lpri

happy designing ...
Click to expand...

When I calculated N for primary, it will give me about copper losses. But the problem is what is the relation between core losses and AL value or air gap?
 

core losses are related to Bpk and freq, not the gap, a large gap ( > 1mm ) will start to cause losses in wire wound close to it and give you hot spotting and extra wire losses...
 

Hi,
The difference in core volume due to air gap is negligible as the gap is usually very small compared to the length of the centerpost (and even smaller compared to the entire length of the core).

Consequently, change in core loss due to this factor is negligible too.

Remember: Pcore = Pv*Vcore.
 

Dear akanimo - you can add a gap without changing the dimensions of the core - this is often done for flyback transformers that are gapped right through.

Generally speaking if you increase the gap - and this then corresponds to a smaller Bpk ( not always the case ) then the core losses may decrease due to the lower Bpk, however in a flyback, the L is proportional to the gap, so if the gap goes up by 50% the L will fall to 67% of previous, this may result in higher peak & rms currents in the primary and higher transformer losses overall ...
 

Generally speaking if you increase the gap - and this then corresponds to a smaller Bpk ( not always the case ) then the core losses may decrease due to the lower Bpk, however in a flyback, the L is proportional to the gap, so if the gap goes up by 50% the L will fall to 67% of previous, this may result in higher peak & rms currents in the primary and higher transformer losses overall ...
Click to expand...

It depends on which parameter is kept constant when changing the gap. If you keep I²L, on-time, core flux and losses will reduce. If you keep the on-time, losses will be almost constant, but copper losses surely increase.

I made an error in my previous post #2 by ignoring the variation in effective permeability, please apologize for the confusion.
 

And from EPCOS, I found this datasheet. For calculation air gap there is some formulas.

Ekran Alıntısı.PNG
 

Ivan, you need to provide the L value you have come up with and the turns and the input volts min, and the output power wanted - then the members of this forum can help you - and oh yes DCM flyback, or CCM, or a mix ... ?
 

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