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FAN7930C Datasheet(PDF) 12 Page - Fairchild Semiconductor |
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FAN7930C Datasheet(HTML) 12 Page - Fairchild Semiconductor |
12 / 22 page ![]() © 2010 Fairchild Semiconductor Corporation www.fairchildsemi.com FAN7930C • Rev. 1.0.0 12 Figure 25. Two Cases of RDY Triggered HIGH Figure 26. Two Cases of RDY Triggered LOW 4. Zero-Current Detection: Zero-current detection (ZCD) generates the turn-on signal of the MOSFET when the boost inductor current reaches zero using an auxiliary winding coupled with the inductor. When the power switch turns on, negative voltage is induced at the auxiliary winding due to the opposite winding direction (see Equation 1) . Positive voltage is induced (see Equation 2) when the power switch turns off. AC IND AUX AUX V T T V ⋅ − = (1) () AC PFCOUT IND AUX AUX V V T T V − ⋅ = (2) where: VAUX is the auxiliary winding voltage; TIND is boost inductor turns; TIND auxiliary winding turns; VAC is input voltage for PFC converter; and VOUT_PFC is output voltage from the PFC converter. Figure 27. Circuit Near ZCD Because auxiliary winding voltage can swing from negative to positive voltage, the internal block in ZCD pin has both positive and negative voltage clamping circuits. When the auxiliary voltage is negative, internal circuit clamps the negative voltage at the ZCD pin around 0.65V by sourcing current to the serial resistor between the ZCD pin and the auxiliary winding. When the auxiliary voltage is higher than 6.5V, current is sinked through a resistor from the auxiliary winding to the ZCD pin. Figure 28. Auxiliary Voltage Depends on MOSFET Switching The auxiliary winding voltage is used to check the boost inductor current zero instance. When boost inductor current becomes zero, there is a resonance between boost inductor and all capacitors at the MOSFET drain pin: including COSS of the MOSFET; an external capacitor at the D-S pin to reduce the voltage rising and falling slope of the MOSFET; a parasitic capacitor at inductor; and so on to improve performance. Resonated voltage is reflected to the auxiliary winding and can be used for detecting zero current of boost inductor and |
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