DFS04M datasheet(12/42 Pages) STMICROELECTRONICS

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AN1262 APPLICATION NOTE

12/42

(6)

In this equation

∆T is the hot-spot temperature rise (located in the core center leg, where heat can be re-

moved more difficultly), defined as

∆T = Tmax - Tamb. For reliability reasons Tmax is usually limited at 100°C

where, by the way, ferrites usually feature minimum losses. Ku is the window utilization factor, that is the por-

tion of the total core window area occupied by the windings, which can be estimated equal to 0.4 for margin

wound construction and to 0.7 for triple insulated wire construction.

The smallest core with an AP greater then APmin will be chosen from the catalog data (the core list of table

10 can be used as a reference). If there is a core with an AP < APmin but very close to, it might be worthwhile

trying to design with this smaller core before trying the larger one.

2) Calculate the required minimum number of primary turns of the primary winding. It will be given by:

3) Define primary and secondary windings' turns number. In the case of single-output under consideration, the

secondary winding turns number Ns will be simply:

that is, the result of the division will be rounded up to the next larger integer. The actual primary turns will then

be calculated, rounding the result to the closest integer.

Np = [Ns · n + 0.5].

It can be convenient to round to the next even number when interleaved winding technique is to be used for

transformer construction, so as to split the primary in two equal halves.

4) Calculate the air gap length. The gap length (lg) needed to get the desired inductance Lp will be calculated

with the following empirical formula:

(7)

If the calculated value is not available as a standard part, if possible, the primary turns number can be adjust-

ed a little bit to get an off-the-shelf part.

The air gap should be located on the core center leg only, to minimize radiated fields. In prototyping, center

leg grinding to get nonstandard gap values can be avoided by keeping the two half-cores apart by about half

the calculated value with spacers.

5) Calculate transformer total losses. The allowed total transformer losses (Ptot) can be calculated by dividing

the hot-spot temperature rise

∆T by the thermal resistance of the wound core Rth(core):

If the manufacturer does not provide thermal data, Rth(core) can be estimated. It has been shown [1] that there

is a good correlation between core's area product and thermal resistance, regardless of its shape:

Rth(core) ≈ 23 · AP

-0.37

[°C/W];

min

RMS

⋅

---

∆

max

⋅⋅

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





 1.316

[]

⋅

min

Lp 0.7

⋅

max

⋅

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

⋅

--------

----------

---------

⋅







------

[]

tot

∆

th c ore

()

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

[]

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