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FAN7930MX Datasheet(PDF) 12 Page - Fairchild Semiconductor |
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FAN7930MX Datasheet(HTML) 12 Page - Fairchild Semiconductor |
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12 / 22 page  © 2010 Fairchild Semiconductor Corporation www.fairchildsemi.com FAN7930 • Rev. 1.0.1 12 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) and 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 and TAUX are boost inductor turns and auxiliary winding turns respectively, VAC is input voltage for PFC converter and VOUT_PFC is output voltage from the PFC converter. Figure 25. Circuit Near ZCD Because auxiliary winding voltage can swing from negative voltage 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 26. Auxiliary Voltage Depends on MOSFET Switching To check the boost inductor current zero instance, auxiliary winding voltage is used. When boost inductor current becomes zero, there is a resonance between boost inductor and all capacitors at 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 as detecting zero current of boost inductor and valley position of MOSFET voltage stress. For valley detection, a minor delay by the resistor and capacitor is needed. A capacitor increases the noise immunity at the ZCD pin. If ZCD voltage is higher than 1.5V, an internal ZCD comparator output becomes HIGH and LOW when the ZCD goes below 1.4V. At the falling edge of comparator output, internal logic turns on the MOSFET. Figure 27. Auxiliary Voltage Threshold When no ZCD signal is available, the PFC controller cannot turn on MOSFET, so the controller checks every switching off time and forces MOSFET turn on when the off time is longer than 150μs. It is called restart timer. Restart timer triggers MOSFET turn on at startup and may be used at the input voltage zero cross period. s 150 μ Figure 28. Restart Timer at Startup |
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