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AN-9738 Datasheet(PDF) 10 Page - Fairchild Semiconductor |
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AN-9738 Datasheet(HTML) 10 Page - Fairchild Semiconductor |
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10 / 25 page  AN-9738 APPLICATION NOTE © 2011 Fairchild Semiconductor Corporation www.fairchildsemi.com Rev. 1.0.0 • 4/20/11 10 Figure 22. THD Improvement The current that comes from the ZCD pin, when auxiliary voltage is negative, depends on RZCD. The second role of RZCD is also 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 23. Figure 23. 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 24. Figure 24. Internal Sawtooth Wave Slope Variation RZCD also influences control range. Because FL7930B doesn’t detect input voltage, voltage-mode control value is determined by the turn-on time to deliver the 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 (14) and turn-on time adjustment by RZCD guarantees use of the full control range. RZCD for control range optimization is obtained as: ] [ N mA . N V t t s R BOOST AUX MIN , LINE MAX , ON MAX , ON ZCD Ω ⋅ ⋅ ⋅ ⋅ − µ ≥ 469 0 2 28 1 (21) where: tON,MAX is calculated by Equation (14); 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 (20) is normally lower than the value calculated in Equation (21). To guarantee the needed turn on-time for the boost inductor to deliver rated power, the RZCD from Equation (20) is normally not suitable. RZCD should be higher than the result of Equation (21) 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 FL7930B is 1V. However, in some applications, a PFC block is needed to operate normally at light load. To compensate control range correctly, input voltage sensing is necessary, such as with Fairchild’s interleaved PFC controller FAN9612, or special care on sawtooth generator is necessary. To guarantee enough control range at high line, clamping output voltage lower than rated on the minimum input condition can help. (Design Example) Minimum RZCD for clamping capability is calculated as: Ω = − ⋅ = − ≥ k . mA V . mA V . V N N R MAX , LINE BOOST AUX ZCD 9 18 3 65 0 277 2 34 5 3 65 0 2 Minimum RZCD for control range is calculated as: Ω = ⋅ ⋅ ⋅ ⋅ µ − µ µ = ⋅ ⋅ ⋅ ⋅ − µ ≥ k . mA . s . s s N mA . N V t t s R BOOST AUX MIN , LINE MAX , ON MAX , ON ZCD 97 20 55 469 0 5 85 2 9 10 42 28 469 0 2 28 1 A choice close to the value calculated by the control range is recommended. 39kΩ is chosen in this case. |
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