DFS04M datasheet(13/42 Pages) STMICROELECTRONICS

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AN1262 APPLICATION NOTE

this best-fit equation refers to natural convection cooling.

6) Calculate the actual flux swing, the actual core losses and the allowed copper losses. The flux swing will be

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:

PCu = Ptot - PFe [W]

(10)

7) Design windings. The goal is to find the right wire size so that copper losses are within the limit stated by

(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 (

ρ100 = 2.303·10-6 Ω·cm) and the average length-per-turn (4) of the bobbin associated to

the selected core:

(12)

(13)

A wire table (like the sample one shown in table 11) will be looked up and a wire with a copper area (ApCu,

AsCu) equal or greater than the minimum above calculated will be selected. Anyway, to minimize skin effect,

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 (

∅ 0.57 mm, ACu =

0.2573 mm2). If ApCu is larger, a number (Nwp, Nws) of such (or smaller) wires will be paralleled so as to

achieve the desired total area:

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

∆

Lp lp

⋅

Np A

⋅

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

[]

⋅

∆

]

⋅⋅

⋅

2Ip

RMS

⋅

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

[

Ω]

2Is

RMS

⋅

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

[

Ω]

Cumin

100

Np L

⋅⋅

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

[cm

2 ]

Cu m in

100

Ns L

⋅⋅

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

[]

Nwp

Cumin

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

Nws

Cumin

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

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