AN2495 datasheet(10/36 Pages) STMICROELECTRONICS

10 / 36 page

Flyback stage design

AN2495

10/36

Doc ID 13127 Rev 5

Figure 5.

Relative core losses versus frequency

From Figure 5, operating at 50 kHz with 220 mT flux excursion, the power dissipation

density is about 300 mW/cm3. Once again, referring to the EPCOS datasheet, the total

volume of ETD 34 is 7.63 cm3, therefore:

Equation 14

Assuming a 95% efficiency for the transformer, only 4 W can be lost on it, of which about 2.3

is lost on the core while the residual 1.7 W is dissipated on the copper. Achieving this

efficiency is detailed in the following Section 3.1.3: Wire size.

3.1.3

Wire size

To chose the right wire size we must know the rms current on both the primary and

secondary sides. Since Ipeak, primary = 1.6 A and I peak, secondary = 16 A

Equation 15

By imposing a 1 W loss on the primary side wire, the maximum series resistance can be

calculated as follows:

From Joule’s law we can calculate the resistance of both the primary and secondary

windings.

Equation 16

co re

0.3

7.6

2.29W

•

rms prima ry

0.65A

and

rm s

o nd ary

sec

6.53A

CU pri

PRM S

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

2.36

⇒

CU sec

SR MS

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

0.016

⇒

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