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AN1262 APPLICATION NOTE

this best-fit equation refers to natural convection cooling.

given by:

(8)

and the corresponding core losses can be calculated with the formulae in table 9:

(9)

The allowed copper losses will obviously be:

(10)

(10). At this moment, losses due to skin and proximity effect will not be accounted for. The construction tech-

nique of the transformer will be such that these effects will be minimized.

Copper losses will be equally apportioned to the primary and the secondary winding (the power handled by

the auxiliary one is negligible). Therefore the maximum primary and secondary winding resistance will be re-

spectively:

;

(11)

The primary and secondary conductor copper cross-section area will be obtained considering the resistivity

of copper at 100°C (

the selected core:

(12)

(13)

the selected wire diameter should not exceed 2·

δ, where δ is the skin depth of copper (about 0.3 mm at 65

kHz and 100°C). In practice, the maximum wire size for minimum skin effect is AWG23 (

achieve the desired total area:

where the results will be rounded up to the next larger integer.

B

∆

Lp lp

pk

⋅

Np A

e

⋅

---------------------- 10

4

T

[]

⋅

=

P

Fe

V

e

kB

p

∆

f

sw

q

[W

]

⋅⋅

⋅

=

Rp

P

Cu

2Ip

RMS

2

⋅

------------------------

[

Ω]

=

Rs

P

Cu

2Is

RMS

2

⋅

------------------------

[

Ω]

=

Ap

Cumin

ρ

100

Np L

t

⋅⋅

Rp

---------------------------------

[cm

=

As

Cu m in

ρ

100

Ns L

t

⋅⋅

Rs

---------------------------------

cm

2

[]

=

Nwp

Ap

Cumin

Ap

Cu

-----------------------

=

Nws

As

Cumin

As

Cu

----------------------

=