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FAN9612 датащи(PDF) 6 Page - Fairchild Semiconductor |
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FAN9612 датащи(HTML) 6 Page - Fairchild Semiconductor |
6 / 18 page AN-6086 © 2009 Fairchild Semiconductor Corporation www.fairchildsemi.com Rev. 1.0.4 • 4/22/10 6 1 OUT AUX ZCD BOOST VN R mA N >⋅ (9) For low-power applications, such as adaptors, the supply voltage (VDD) for FAN9611/12 is supplied by auxiliary winding of inductor, as shown in Figure 10. Since the auxiliary winding voltage varies extensively within half of line cycle, DC blocking capacitor (CB) is used to guarantee stable VDD voltage. When supplying VDD using the auxiliary winding, the turns ratio should be determined as: 2 BOOST OUT AUX Z NV N N V =≅ (10) where VZ is the Zener diode voltage for VDD. The reason for setting the turns ratio such that peak-to-peak voltage of auxiliary winding is around twice of VZ is to guarantee stable VDD supply without under-voltage lockout (UVLO) during startup. AUX OUT BOOST N V N (( )) AUX OUT IN BOOST N VV t N − () AUX IN BOOST N Vt N − Figure 10. VDD Supply Circuit using Auxiliary Winding The average current through the startup resistor should be larger than the startup current (100 μA), determined as: , 22 100 LINE MIN START START V I A R μ π => (11) The startup resistor also should be determined by considering the minimum startup time, which is given as: , 22 ( 100 ) ON DD START LINE MIN START VC t V A R μ π = − (12) where VON is UVLO start voltage for VDD and CDD is the total capacitance of capacitors connected to the VDD pin. Typically 20~50 μF of electrolytic capacitor (C VDD2) is used together with 2~4 μF of bypass capacitor (C VDD1). The power consumption in the startup resistor is obtained as: 2 , LINE MAX LOSS START V P R = (13) (Design Example) Selecting the turns ratio of the inductor (NBOOST/NAUX) as 10, NAUX is obtained as 3 and the value of RZCD is given as: 400 3 40 11 30 OUT AUX ZCD BOOST VN R k mA N mA >⋅ = ⋅ = Ω RZCD is selected as 47kΩ. Assuming that VDD is supplied from auxiliary power supply, VDD supply circuit using the auxiliary winding is not included in this example. [STEP-3] Design VIN Sense Circuit Since the RMS value of AC line voltage is directly proportional to the peak of the line voltage, FAN9611/12 detects the peak value of input voltage to sense the RMS value of the line voltage, which permits a simple voltage divider for line voltage sensing, as shown in Figure 11. The detected line voltage information is used for line under- voltage lockout (UVLO), and line feed-forward for PWM control. When the peak of VIN pin voltage drops below 0.95V, the line UVLO (brownout protection) is triggered and FAN9611/12 stops operating. The RMS value of the line voltage that causes line UVLO is given as: 12 . 2 0.95 ( ) 2 IN IN LINE UVLO AC IN RR VV R + =⋅ (14) The internal switched current source allows UVLO hysteresis on line voltage as: 1 1, 2 , (1) 2 2 IN IN IN HYS IN LINE HYS R RR R VA μ ++ =⋅ ( VAC ) (15) From Equation 15, RIN,HYS with a given hysteresis of line voltage is obtained as: .2 ,1 12 2 () 2 LINE HYS IN IN HYS IN IN IN VR RR AR R μ =− ⋅ + (16) |
Аналогичный номер детали - FAN9612 |
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Аналогичное описание - FAN9612 |
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