AN4350 datasheet(9/34 Pages) STMICROELECTRONICS

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AN4350

Circuit description and design guidelines

Referring to the HVLED815PF datasheet, the average output voltage can be easily

programmed trough the RFB resistor, using Equation 9:

Equation 9

where NAUX and NSEC are the auxiliary and secondary turn numbers respectively and VREF

is the internal reference voltage (2.51 V typ.).

The RDMG resistor is designed to keep the line feed-forward and it can still be calculated as

shown in the HVLED815PF datasheet, considering the equivalent RSENSE_EQ resistor

defined in Equation 1:

Equation 10

where RFF and TD - the internal feed-forward resistor and the MOSFET turns OFF delay

time respectively. For more details see section 4.7: “Voltage feed-forward block” in the

HVLED815PF datasheet.

Using the calculated RDMG resistor, the RFB resistor can be calculated using Equation 9.

Equation 11

2.4

High power factor implementation

Referring to Figure 4, the RPF resistor (R6 on the application schematic) gives a contribution

proportional to the input voltage on the CS pin: as a consequence the input current is

proportional to the input voltage during the line period, implementing a high power factor

correction.

In particular, the contribution proportional to the input voltage is generated using the

auxiliary winding - diode in series to the RPF resistor is needed to avoid any injection on the

CS pin when the auxiliary winding is positive.

As mentioned in Section 2.3, through the ROS resistor (the R14 resistor on the schematic

between the CS pin and VCC pin) a positive offset on the CS pin is added, in order to keep

a good THD. This offset can be designed using Equation 12:

Equation 12

where VCSCV is the voltage on the CS pin that is imposed by the voltage control loop and

K is a constant that by experience is selected in the range of 2/3 - 3/4.

RFB

RDMG

VREF

NAUX

NSEC

-------------- VOUT





 V

REF

–

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



RDMG

NAUX

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

LP RFF



TD RSENSE



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



19T

125T

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

 1.4mH 45



100ns 1.08





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

82k



RFB

82k



2.51V

19T

33T

---------- 25V





 2.51V

–

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



17.3k



VOS

VCC

RCS RSENSE

 

RPF





RCS RSENSE





RPF

 R

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

KVCS



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