AN4350 datasheet(12/34 Pages) STMICROELECTRONICS

12 / 34 page

Circuit description and design guidelines

AN4350

12/34

DocID025181 Rev 1

Equation 19

Replacing Equation 1, Equation 13, and Equation 18 in Equation 17, the RSENSE resistor

results:

Equation 20

The resistors RSENSE, RFB, RDMG, ROS, RCS, RPF determine the output voltage and the

maximum deliverable current setting - suggested accuracy of these parameters is 1%.

2.5

Output filter

The output filter has to be designed to respect the output voltage ripple specification

(

VOUTpk-pk). In this kind of application, the single-stage high power factor, the high

frequency ripple at the switching frequency can be neglected respect to the low frequency

ripple at double line frequency (100/120 Hz).

The output capacitor value can then be estimated using Equation 21:

Equation 21

For this design, three output capacitors of 330

F/64 m/35 V have been selected.

2.6

Voltage control loop compensation

As mentioned in Section 2.3 the voltage control loop must have frequency bandwidth much

lower than the “control current loop” to avoid any interaction between the two loops. The

current loop “works” at the double of line input frequency (100 - 120 Hz), resulting in

suggested voltage loop bandwidth in the range of 5 - 10 Hz (BWCV < 1/10 of current loop

bandwidth).

The small signal system voltage loop gain can be estimated considering the equivalent

schematic in Figure 5.

Equation 22

Gloop_CV(s) = Gp(s)

 Gc(s) = [Gpwm  Gps(s)]  [Gea(s)  Gsh(s)  Gfb]

VAUX

VIN

NAUX

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



RSENSE

---

NAUX

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

RCS

RPF

-----------

IOUT CC

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



VinRMS MIN

 V

inRMS



MIN

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







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

CoutMIN

0.4



--------

IOUT

MAX

fline

V

OUT

PK PK

–



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





0.4



--------

310mA

50Hz 0.75V



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



1005

F

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