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AN1673 APPLICATION NOTE

Using the "Product Area" (AP) criterion, we can get a first approximation of the core area product. The

required AP can be found using the equations Ap1 and Ap2 (we use the same formulas as for the standard

half bridge forward converter).

AP (=AW*Ae) is the winding area (AW, associated to the former) time the magnetic cross section (Ae, as-

sociated to the ferrite).

N.B. For choosing the resonant elements (inductance and capacitor) and its relation with the frequency to

be set, one can have an easy task referring to the convention described in appendix A.

– Start choosing the minimum operative frequency (corresponding to full load) at 65kHz.

[14]

Ap1)

# related to core losses limit

[15]

Ap2)

# related to saturation limit

Where:

K = 0.165 related to the Half Bridge connection;

∆Bmax 0.4T; Pin is the power handled by the transformer.

The needed AP will be the larger of the two resulting values.

Select the ferrite's set EE30 (E30 · 15 · 7) in high frequency (e.g. B2 or F1) material.

Transformer's Winding

To define the winding, first it must be fixed the input DC Bus value.

Since in the discussed design the pre regulator stage (see later in Power Factor section) is requested, let's

start fixing the voltage bus range at 360V to 420V.

The second resonant inductor of the LCL resonant converter can be achieved managing the proper value

of primary inductance of Transformer.

This second inductance allows to properly operating in light load condition holding in the same time the

higher efficiency for the entire load range. In our experience, to take advantage by using the primary in-

ductance as second inductor, a good ratio between its value and the stray inductance (Lmag/Lstray) is in

the range of 3.5 to 7.

The relation between the primary and the stray inductances can be adjusted using the slotted bobbin (that

increases the stray inductance) and introducing an air gap in the transformer's ferrite set (that reduces the

magnetizing inductance).

We choose a two layer slotted former (split the primary and secondary sides). See fig.20.

Figure 20. Transformer

AP1

41.7 Pin

⋅

Kƒ

sw

⋅

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



Kh ƒt

Ke ƒt

2

⋅

+

⋅

()

0.66

⋅

=

AP2

11.1 Pin

⋅

KBmax

∆

ƒ

sw

⋅⋅

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



=

Secondary

Slotted former

Primary

Aux.

N1

Prim.

Aux.

N2a

N2b

T

Sec.

D03IN1460