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FAN7930B Datasheet(PDF) 12 Page - Fairchild Semiconductor |
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FAN7930B Datasheet(HTML) 12 Page - Fairchild Semiconductor |
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12 / 22 page  © 2010 Fairchild Semiconductor Corporation www.fairchildsemi.com FAN7930B • Rev. 1.0.2 12 Since the two OVP conditions are quite different, protection recovering mode is different. Once the first OVP triggers, switching stops immediately and recovers switching when the output voltage is decreased with a hysteresis. When the second OVP triggers, switching can be recovered only when the VCC supply voltage falls below VSTOP and builds up higher than VSTART again and VOVP should be lower than hysteresis. If the second OVP is not used, the OVP pin must be connected to the INV pin or to the ground. Figure 25. Comparison of 1 st and 2nd OVP Recovering Mode 4. Control Range Compensation: On time is controlled by the output voltage compensator with FAN7930B. Due to this when input voltage is high and load is light, control range become narrow compared when input voltage is low. That control range decrease is anti- proportional to the double square of the input voltage. Thus at high line unwanted burst operation easily happens at light load and audible noise may be generated from the boost inductor or inductor at input filter. Different from the other converters, burst operation in PFC block is not needed because PFC block itself is normally disabled during standby mode. To improve this kind of unwanted burst operation at light load, internal control range compensation function is implemented and approximately shows no burst operation until 5% load at high line. 5. 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 26. 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 27. 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. |
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