Design equations for the modified buck converter

AN2928

16/21

First, the inductor core size must be selected, for which it may be helpful to calculate the

minimum area product using application parameters. The minimum required core area

product (AP), where the flux swing is limited by core saturation is:

Equation 35

Equation 36

example, if the input voltage of the modified buck converter is 400 V (such as when a power

factor preregulator is used), then the wires must be well-isolated and the ratio between the

copper and window area is about 0.5, which in the end means that there is 50% of the wire

on the inductor core. The exact calculation using the equation listed here can be found in the

user manual UM0670 (see Section 3: Reference and related materials).

Once the minimum area product is calculated, the designer should then select the right

inductor core and ferrite material according to the higher AP value. The AP value is

calculated from the winding cross section and core cross section. For example, the ETD29

Equation 37

calculated in Equation 35. If the condition derived from Equation 38 is not fulfilled, the

designer must select a bigger inductor core.

Equation 38

A simple way how to calculate the number of inductor turns is shown in Equation 39, since

product datasheets. For example, the ETD29 core with 1 mm gap and N27 ferrite material

has an inductance factor of 124 nH.

AP

MIN

LI

PEAK

I

RM S

⋅⋅

B

MAX

Cl

⋅

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

⎝⎠

⎛⎞

4

3

---

=

Cl

J

MAX

C

R

10

4

–

⋅⋅

=

AP

A

N

A

MIN

⋅

=

AP

MIN

AP

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