AN4350 datasheet(11/34 Pages) STMICROELECTRONICS

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DocID025181 Rev 1

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AN4350

Circuit description and design guidelines

Using the calculated value of NAUX, NP, ROS and RCS the RPF resistor results:

Equation 17

Figure 4. HPF connections

The voltage loop changes the voltage on the CS pin to keep the output voltage regulated.

Respect to the standard circuit (no high power factor implementation trough RPF and ROS),

the contributions proportional to VIN trough the RPF resistor and the offset trough the ROS

resistor give of course a modification of the voltage across the RSENSE resistor generating

a different gain between the RSENSE and CS pin.

The VOS offset voltage is basically selected to have no input current when the input voltage

is close to zero at the minimum line condition: as a consequence the voltage across the

RSENSE resistor is basically equal to the voltage generated by the RPF resistor.

The RSENSE can be estimated imposing the equality between the two ratios, in the worst-

case condition (minimum line and maximum deliverable current IOUTCC):

Equation 18

where VAUX is the voltage present at the auxiliary winding when the MOSFET is ON:

RPF



265V



76.8k

 19T



 15 1k 125T





–

0.75 76.8k

 125T





–

125T

0.75 76.8k

 15V 1k







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

59k



VCS

RSENSE EQ

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

RCS

VAUX

RPF

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

-------









RSENSE

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

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