 |
Electronic Components Datasheet Search |
|
|
Electronic Components Datasheet Search |
|
AN-9731 Datasheet(PDF) 9 Page - Fairchild Semiconductor |
|
||||||||||||||||||||||||||||||
AN-9731 Datasheet(HTML) 9 Page - Fairchild Semiconductor |
|
|
9 / 22 page  AN-9731 © 2011 Fairchild Semiconductor Corporation www.fairchildsemi.com Rev. 1.0.0 • 3/24/11 9 Figure 19. ZCD Current and Sawtooth Generator When AC input voltage is almost zero, no negative current is generated from inside; but sourcing current, when input voltage is high, is used to raise the sawtooth generator slope and turn-on time is shorter. As a result, turn-on time, when AC voltage is zero, is longer compared to AC voltage, in peaks shown as in Figure 20. Figure 20. THD Improvement The current that comes from the ZCD pin, when auxiliary voltage is negative, depends on RZCD. The second role of RZCD is related to improving the Total Harmonic Distortion (THD). The third role of RZCD is making the maximum turn-on time adjustment. Depending on sourcing current from the ZCD pin, the maximum on-time varies as in Figure 21. Figure 21. Maximum On-Time Variation vs. IZCD With the aid of IZCD, an internal sawtooth generator slope is changed and turn-on time varies as shown in Figure 22. Figure 22. Internal Sawtooth Wave Slope Variation RZCD also influences control range. Because FL7930 doesn’t detect input voltage, voltage-mode control value is determined by the turn-on time to deliver needed current to boost output voltage. When input voltage increases, control voltage decreases rapidly. For example, if input voltage doubles, control voltage drops to one quarter. Making control voltage maximum when input voltage is low and at full load is necessary to use the whole control range for the rest of the input voltage conditions. Matching maximum turn-on time needed at low line is calculated in Equation (7) and turn-on time adjustment by RZCD guarantees use of the full control range. RZCD for control range optimization is obtained as: N mA 469 . 0 N V 2 t t s 28 R BOOST AUX MIN , LINE MAX , ON 1 MAX , ON ZCD ⋅ ⋅ ⋅ ⋅ − μ ≥ (14) where: tON,MAX is calculated by Equation (7); tON,MAX1 is maximum on-time programming 1; NBOOST is the winding turns of boost inductor; and NAUX is the auxiliary winding turns. RZCD calculated by Equation (13) is normally lower than the value calculated in Equation (14). To guarantee the needed turn on-time for the boost inductor to deliver rated power, the resulting RZCD from Equation (13) is normally not suitable. RZCD should be higher than the result of Equation (14) when output voltage drops as a result of low line voltage. When input voltage is high and load is light, not much input current is needed and control voltage of VCOMP touches switching stop level, such as if FL7930 is 1V. However, in some applications, a PFC block is needed to operate normally at light load. To compensate control |
Similar Part No. - AN-9731 |
|
Similar Description - AN-9731 |
|
|
|
Link URL |
| Privacy Policy |
| ALLDATASHEET.NET |
| Does ALLDATASHEET help your business so far? [ DONATE ] |
About Alldatasheet | Advertisement | Contact us | Privacy Policy | Link Exchange | Manufacturer List All Rights Reserved©Alldatasheet.com |
| Russian : Alldatasheetru.com | Korean : Alldatasheet.co.kr | Spanish : Alldatasheet.es | French : Alldatasheet.fr | Italian : Alldatasheetit.com Portuguese : Alldatasheetpt.com | Polish : Alldatasheet.pl | Vietnamese : Alldatasheet.vn Indian : Alldatasheet.in | Mexican : Alldatasheet.com.mx | British : Alldatasheet.co.uk | New Zealand : Alldatasheet.co.nz |
|
Family Site : ic2ic.com |
icmetro.com |
English ▼
English
Chinese
German
Japanese
Russian
Korean
Spanish
French
Italian
Portuguese
Polish
Vietnamese
Indian
Mexican
British
New Zealand
