AN2454 datasheet(7/24 Pages) STMICROELECTRONICS

7 / 24 page

AN2454

Flyback stage

7/24

Once the turn ratio is calculated, the system must be stabilized to ensure that the converter

operates in discontinuous mode. Equation 3 guarantees that the energy on the primary coil

will be completely transferred to the secondary coil before the next cycle occurs.

Equation 3

A safety margin of 20% is recommended to guarantee the complete demagnetization of the

primary side (see Equation 4).

Equation 4

where Tonmax is the maximum power-on time, Treset the time needed to demagnetize the

transformer inductance, and TS the switching time.

Combining Equation 3 and Equation 4 , Tonmax, may be calculated using Equation 5:

Equation 5

Once output power has been set to 2 W and the desired efficiency to 60%, the operating

switching frequency must be chosen. To do this, a value of 50 kHz should be selected. It is

then necessary to calculate the primary inductance (LP) of the transformer. Using Equation

6, input power (PIN) may be calculated to give an approximate value which does not account

for losses due to the power switch, the input bridge and the rectified network.

Equation 6

Using Equation 7, LP may be calculated as follows:

Equation 7

Peak current, (IP) on the primary side may be calculated using Equation 8.

Equation 8

It is also important to determine the maximum primary current, Irms(primary), (see Equation 9)

and maximum secondary current, Irms(secondary), (see Equation 10) to obtain correct

dimensions for the wire size of the primary windings.

dcminTonmax

------- V

F diode

()T

reset

flTreset

onmax

reset

–

0.8T

onmax

fl0.8TS

dcmin

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

1.66P

OUT

---

PIP

•

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

---V

onmax

PTS

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

dcmin

onmax

3.33T

SPOUT

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

11mH

≈

dcminTonmax

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

110mA

≈

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