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AN-9732 Datasheet(PDF) 8 Page - Fairchild Semiconductor |
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AN-9732 Datasheet(HTML) 8 Page - Fairchild Semiconductor |
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8 / 22 page  AN-9732 APPLICATION NOTE © 2011 Fairchild Semiconductor Corporation www.fairchildsemi.com Rev. 1.0.0 • 3/23/11 8 field. Because most IC pins have several pF parasitic capacitance, CZCD can be eliminated when RZCD is higher than 30k . However, a small capacitor would be helpful when auxiliary winding suffers from operating noise. The PFC control loop has two conflicting goals: output voltage regulation and making the input current shape the same as input voltage. If the control loop reacts to regulate output voltage smoothly, as shown in Figure 16, control voltage varies widely with the input voltage variation. Input current acts to the control loop and sinusoidal input current shape cannot be attained. This is the reason control response of most PFC topologies is very slow and turn-on time over AC period is kept constant. This is also the reason output voltage ripple is made by input and output power relationship, not by control-loop performance. VIN & VOUT t VCONTROL IACIN Figure 16. Input Current Deterioration by Fast Control If on-time is controlled constantly over one AC period, inductor current peak follows AC input voltage shape and achieves good power factor. Off-time is basically inductor current reset time due to the boundary mode and is determined by the input and output voltage difference. When input voltage is at its peak, the voltage difference between input and output voltage is small, and long turn-off time is necessary. When input voltage is near zero, turn-off time is short, as shown in Figure 17 and Figure 18. Though inductor current drops to zero, there is a minor delay, explained above. The delay can be assumed as fixed when AC is at line peak and zero. Near AC line peak, the inductor current decreasing slope is slow and inductor current slope is also slow during the ZCD delay. The amount of negative current is not much higher than the inductor current peak. Near the AC line zero, inductor current decreasing slope is very high and the amount of negative current is higher than positive inductor current peak because input voltage is almost zero. Figure 17. Inductor Current at AC Voltage Peak Figure 18. Inductor Current at AC Voltage Zero Negative inductor current creates zero current distortion and degrades the power factor. Improve this by extending turn- on time at the AC line input near the zero cross. Negative auxiliary winding voltage, when MOSFET is turned on, is linearly proportional to the input voltage. Sourcing current generated by the internal negative clamping circuit is also proportional to sinusoidal input voltage. That current is detected internally and added to the internal sawtooth generator, as shown in Figure 19. |
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