AN1889 datasheet(10/33 Pages) STMICROELECTRONICS

10 / 33 page

Application description

AN1889

10/33

Equation 3

The second step is to ensure that the converter operates in discontinuous mode, the below

formula will guarantee that the energy on the primary coil will be completely transferred to

the secondary before the next cycle occurs:

Equation 4

The next formula, giving a safe margin, guarantees the complete demagnetization of the

primary side.

Equation 5

Where Tonmax is the maximum on time, Treset is the time needed to demagnetize the

transformer inductance and TS is the switching time.

Combining Equation 4 and Equation 5 Tonmax results being:

Equation 6

The next step is to calculate the peak current. Fixed the output power to 48 W and the

desiderated efficiency (80% in this case), we have also the freedom to choose the switching

frequency. At this purpose, 50 kHz has been selected. The first step is to calculate the

primary inductance. By using an approximated formula that does not take into account the

losses on the power switch, on the input bridge and on the rectified network, we have:

Equation 7

hence

Equation 8

From here now we can calculate the peak current on primary

Equation 9

It is important also to determine the maximum value of Irms primary current to fix the number

of the primary turns as shown in the next section.

-------

dcmax

–

Spike

–

min

arg

–

Fdiode

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

1700

750

–

200

–

250

–

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

dcmin

onmax

•

------- V

F diode

()T

reset

•

onmax

reset

0.8T

onmax

fl0.8TS

dcmin

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

10.66

µs

≅

1.25P

OUT

---

pIp

•

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

---V

dcmin

onmax

•

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

dcmin

onmax

•

2.5T

SPOUT

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

2.95mH

dcmin

onmax

•

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

0.9A

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