AN4248 datasheet(9/46 Pages) STMICROELECTRONICS

Part #	AN4248

Description	60 W PSU design details for water purifier system

PDF	46 Pages

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Manufacturer	STMICROELECTRONICS [STMicroelectronics]

Direct Link	http://www.st.com

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9 / 46 page

DocID024214 Rev 1

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AN4248

Mains feed-forward setting

Equation 3

Where Vinmin and Vinmax is the rectified DC voltage at minimum and maximum mains

voltage. VR is the reflected voltage. The parameter Kopt is used to calculate the resistor

divider values connected to VFF pin (RVFF_H and RVFF_L), Which gives the following

values, referring to the schematic (normally same resistor divider network of AC_OK pin can

also be used by splitting RVFF_L into two values):

We have RVFF = 1M, RVFF_L = 510 E;

R4 = R9 = R13 = 330 K, R21 = 5.1 K and R22 = 510E approximately.

Accordingly, we can calculate the sense resistor value connected at the MOSFET source as

per the equations below:

Equation 4

Where Ipkpmax is the peak drain current at Vacmin.

We have following parameters in our design:

–Vinmin = 90ν2 = 127 V

–Vinmax = 265ν2= 375 V

–Ipkp = 2.2 A

–VR = 140 V

Which gives: kopt = 0.0035 and hence Rs = 0.39 Ω. In the schematic R35 = R36 = R29 =

1.2E, R30 = 10E.

AN4248 Datasheet(PDF) 9 Page - STMicroelectronics