AN1621 datasheet(6/14 Pages) STMICROELECTRONICS

6 / 14 page

AN1621 APPLICATION NOTE

6/14

The minimum AP to avoid saturation is:

The AP necessary to limit the core temperature rise not above 30 °C is

Where:

Po = output power

∆B = flux swing

f = switching frequency

η = efficiency

Ke =4 · 10-10

Kh = 4 · 10-5 .

In this application (Po = 312W, η = 90%, f = 200KHz) and considering a maximum ∆B = 200mT for saturation,

the most stringent condition is determined by the formula related to the core temperature raise.

The minimum AP required is 1.97 cm4 . The selected core is ETD39 (AP = 2.2 cm4 ) in 3F3 material.

Assuming total losses of 1% of Po (3W), 2/3 (2W) for the core and 1/3 (1W) for the copper, the specific core

losses are:

In presence of such a power losses, the flux swing can be determined, for 3F3 material, by using the diagram

of material, at 200KHz :

Figure 5. Specific Power Loss as a Function of Frequency Peak Flux Density with

Frequency as a Parameter

or by using the empirical formula

With a specific core losses of 173 mW/cm3 the flux swing must not exceed 130mT

The minimum primary turns is given by:

67.2 P

⋅

∆

⋅

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





1.31

235 P

⋅

η f

⋅

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





1.58

Kh f

Ke f

⋅

()

0.66

Pfe

---------

11.5cm

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

0.173W/cm

50 100 200

∆B(mT)

100

200

500

Ptot

(KW/m3)

D96IN461

Tamb=100°C

3F3

∆

Kh f

Ke f

⋅

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



 0.416

0.173

410

–

200 10

410

–

200 10

⋅

()

⋅⋅

⋅

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





 0.416

130mT

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