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FAN9612 Arkusz danych(PDF) 7 Page - Fairchild Semiconductor |
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FAN9612 Arkusz danych(HTML) 7 Page - Fairchild Semiconductor |
7 / 18 page AN-6086 © 2009 Fairchild Semiconductor Corporation www.fairchildsemi.com Rev. 1.0.4 • 4/22/10 7 Figure 11. Input Voltage Sensing The line peak detection circuit is saturated when VIN exceeds 3.7V, as depicted in Figure 11. Therefore, line feedforward does not work for VIN higher than 3.7V, as illustrated in Figure 12. The minimum brownout protection trip point that allows proper line feedforward operation for universal line range is 66VAC (265VAC×0.925/3.7). When the brownout protection threshold is lower than 68VAC for universal range input, line feedforward can be lost for high line and, therefore, the limited output power increases as line voltage increases: .. 2 () 3.7 MAX NO FF MAX IN OUT OUT V PP =⋅ (17) Another effect of VIN sensing saturation is that the phase management threshold as a percentage of nominal output power increases as peak of VIN increases above 3.7V because the line feedforward does not work above 3.7V. Since FAN9611/12 uses peak detection for the line voltage sensing, it is typical to use a small capacitor (CINF) to bypass switching noise. To minimize the effect of sensing delay, the RC time constant between RIN2 and CINF should be smaller than 5% of AC line period. Figure 12. VIN Feedforward Range (Design Example) Setting the brown-out protection threshold at 70VAC and selecting RIN1=2MΩ, RIN2 is obtained as: 1 2 , 6 (2 / 0.925 1) 210 18.9 2 70 / 0.925 1 IN IN LINE UVLO R R V k = − × = =Ω ⋅− Assuming the hysteresis for brownout protection is 3VAC, RIN,HYS is obtained as: .2 ,1 12 3 6 66 3 2 () 2 2 3 18.9 10 (2 10 ) 1.1 2 10 2 10 18.9 10 LINE HYS IN IN HYS IN IN IN VR RR AR R k μ − =− ⋅ + ⋅× = −× ⋅ = Ω ×× + × RIN.HYS can be omitted since the hysteresis of 2.8VAC is obtained without RIN,HYS from Equation 15. CINF is selected as 10nF, which results in RC time constant as: 39 2. ( ) 18.9 10 10 10 189 IN IN HYS INF R RC s τ μ − =+ ⋅ = × ⋅ × = [STEP-4] Determine MOT Pin Resistor The on time of the gate drive signal is proportional to the compensation voltage as shown in Figure 13. The compensation voltage is internally clamped at 4.3V where the maximum on time is obtained. The Maximum On Time (MOT) of the gate drive signal of each channel is programmed by the resistor on the MOT pin as: 12 2 12 , 2 230 10 ( ) 2 IN IN ON MAX MOT IN LINE RR tR RV − + =⋅ × (18) As can be observed in Equation 18, the maximum on time is inversely proportional to the square of line voltage due to the line feed-forward operation. The MOT resistor should be determined by considering the output power since the maximum on time limits the maximum output power during overload condition as: 2 ,, , 2 LINE MAX CH MAX OUT CH ON MAX V PK P t L η ⋅ =⋅ = (19) where PMAX,CH is the limited maximum output power per channel and KMAX is maximum power limiting factor, which is a ratio between the limited maximum output power and nominal output power. Considering the tolerances of inductor, resistor, and controller variation; it is typical to set the limited maximum power as 20~30% higher than the nominal output power (KMAX = 1.2~1.3). |
Podobny numer części - FAN9612 |
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Podobny opis - FAN9612 |
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