AN2794 datasheet(17/39 Pages) STMICROELECTRONICS

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AN2794

Design considerations

Doc ID 14827 Rev 2

17/39

The peak current value across the inductor is:

Equation 52

To select a proper core we must compute the LI2pk value:

Equation 53

Knowing this parameter, from Magnetics’ core chart, a 46.7 mm x 28.7 mm x 12.2 mm Kool

μ toroid, with μ=60 permeability and A

L = 0.086 nH/turn can be selected. The required

number of turns is then:

Equation 54

The resulting magnetizing force (DC bias) is:

Equation 55

The initial value of turns has to be increased by dividing it by 0.8 (as shown in the data

catalog) to take into account the reduction of initial permeability (

e = 39 at full load) at

nominal current value. Then, the adjusted number of turns is:

Equation 56

The wire table shows that at 3 A the AWG20 can be used. With this choice, the maximum

number of turns per layer, for the selected core, is Nlayer= 96 and the resistance per single

layer is rlayer= 0.166Ω. The total winding resistance is then:

Equation 57

and the copper losses are:

Equation 58

The core losses can be evaluated as follows:

•

turns

132

oersteds

turns

165

layer

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