AN2623 datasheet(12/25 Pages) STMICROELECTRONICS

12 / 25 page

Design circuit

AN2623

12/25

Considering that for the L5991 the maximum allowed voltage value at the current sense (IS,

pin n°13) is 1 V, it is possible to determine the value of the current sense resistor (R9 in

Figure 4):

Equation 32

where I1totpeak is the peak value of i1tot(t) .

The maximum turn ratio k between primary and reset winding, as known in technical

literature, in order to achieve the complete demagnetization of the transformer is the

following:

Equation 33

Choosing

Equation 34

the necessary number of turns for reset winding is NR=41.The maximum reverse voltage

and average current of reset diode are given by (IAVE-1°magn is the magnetizing average

current of the primary side):

Equation 35

Equation 36

The Bipolar ultrafast diode STTH110 was chosen. Concerning the MOSFET the maximum

drain voltage is:

Equation 37

with VdropReset as the voltage drop in the reset diode. The max rms drain current is:

Equation 38

so the Zener-protected SuperMESH Power Mosfet STW12NK90Z was chosen. Calculating

the estimated total MOSFET power losses, it is easy to concludethat a substantial heatsink

(around Rth≤5 °C/W) is necessary.

1totpeak

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

0.23

Ω

max

–

max

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

max

⇒

-------

0.96

REV

–

DCmax

806 V

⋅

AVE

–

AVE

° magn

–

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

0.11 A

drainMax

dcMax

dropReset

–

()

---

⎝⎠

⎛⎞ V

dcMax

838V

⋅

drainRMS

tot1RMS

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