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AN4350 Datasheet(PDF) 9 Page - STMicroelectronics |
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AN4350 Datasheet(HTML) 9 Page - STMicroelectronics |
9 / 34 page DocID025181 Rev 1 9/34 AN4350 Circuit description and design guidelines 34 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 NP -------------- LP RFF TD RSENSE EQ ---------------------------------------- 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 OS + ------------------------------------------------------------------------------------------- KVCS CV = = |
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