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### Network # Flyback Transformator Design, core cross sectional area?

J

#### julius

Jan 1, 1970
0
Hi,
im in the process of building a flyback transformator that can handle 125W @ 80khz.

Thats what i got so far:
EE core with airgap
N27 as core material
f = 80khz

im a little bit confused how to calculate the airgap for the core,

(0.4*3.14*Lpri*Ipk)10^8
lgap =
Ac*B2max

if thats the correct equation i dont know where to get the value for Ac (core cross-sectional area)?

T

#### Terry Given

Jan 1, 1970
0
julius said:
Hi,
im in the process of building a flyback transformator that can handle 125W @ 80khz.

Thats what i got so far:
EE core with airgap
N27 as core material
f = 80khz

im a little bit confused how to calculate the airgap for the core,

(0.4*3.14*Lpri*Ipk)10^8
lgap =
Ac*B2max

if thats the correct equation i dont know where to get the value for Ac
(core cross-sectional area)?

Life will become much easier if you ignore idiotic units and stick to MKS.
The equation for inductance then becomes:

L = Uo*Ue*N^2*Ae/le

ungapped core:
==============
Uo = 4*pi*10^-7 = permeability of free space
Ue = effective permeability of material - around 3000 for ungapped N27, 1
for air
Ae = effective cross-sectional area of flux path. for an EE core, it is the
area of the centre leg (thru which all flux passes) in m^2
le = effective path length - given in the datasheet for an EE core in m
L = inductance, H

the use of "effective" parameters basically converts any core into an
"equivalent" toroid. Ae is easy, but le is trickier as you need to look at
the actual flux path, so the manufacturer just tells you this in the
datasheet.

gapped core:
============
by the time your air gap is above about 0.2mm, high-perm ferrite material
behaviour is swamped by the air gap - almost all of the energy is
concentrated in the gap. The inductance then approximates that of an
air-core coil with Ue = 1, Ae = original centre-leg Ae, and le = gap length,
so

Lgapped = Uo*N^2*Ae/lgap where lgap is in m, Lgapped in H

ie

lgap = Uo*N^2*Ae/Ldesired where lgap is in m, Ldesired in H

normally you pick a core that allows you to stick in sufficient copper
(beware skin/proximity effect - you have been warned . Then choose a
number of turns that prevents core saturation due to volt-seconds. then gap
it to get the right inductance (its OK to fiddle the gap by hand too - its
just easy to calculate once you know how).

With a CCM flyback you also have to consider "DC" flux, not just magnetising
flux, and this makes a big difference.

cheers
Terry

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