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FAN7930C Datasheet(PDF) 16 Page - Fairchild Semiconductor |
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FAN7930C Datasheet(HTML) 16 Page - Fairchild Semiconductor |
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16 / 22 page  © 2010 Fairchild Semiconductor Corporation www.fairchildsemi.com FAN7930C • Rev. 1.0.0 16 Figure 41. Effect of THD Optimizer By THD optimizer, turn-on time over one AC line period is proportionally changed, depending on input voltage. Near zero cross, lengthened turn-on time improves THD performance. 9. VIN Absent Detection: To save power loss caused by input voltage sensing resistors and to optimize THD, the FAN7930C omits AC input voltage detection. Therefore, no information about AC input is available from the internal controller. In many cases, the VCC of PFC controller is supplied by a independent power source, like standby power. In this scheme, some mismatch may exist. For example, when the electric power is suddenly interrupted during two or three AC line periods; VCC is still live during that time, but output voltage drops because there is no input power source. Consequently, the control loop tries to compensate for the output voltage drop and VCOMP reaches its maximum. This lasts until AC input voltage is live again. When AC input voltage is live again, high VCOMP allows high switching current and more stress is put on the MOSFET and diode. To protect against this, FAN7930C checks if the input AC voltage exists. If input does not exist, soft-start is reset and waits until AC input is live again. Soft-start manages the turn-on time for smooth operation when it detects AC input is applied again and applies less voltage and current stress on startup. 10. Current Sense: The MOSFET current is sensed using an external sensing resistor for over-current protection. If the CS pin voltage is higher than 0.8V, the over-current protection comparator generates a protection signal. An internal RC filter of 40kΩ and 8pF is included to filter switching noise. 11. Gate Driver Output: FAN7930C contains a single totem-pole output stage designed for a direct drive of the power MOSFET. The drive output is capable of up to +500/-800mA peak current with a typical rise and fall time of 50ns with 1nF load. The output voltage is clamped to 13V to protect the MOSFET gate even if the VCC voltage is higher than 13V. VIN t VOUT VAUX MOSFET gate IDS fMIN DMAX VIN Absence Detected NewVCOMP Though VIN is eliminated, operation of controller is normal due to the large bypass capacitor. FAN7930 Rev.00 fMIN DMIN Smooth Soft-Start Figure 42. Operation without VIN Absent Circuit Figure 43. Operation with VIN Absent Circuit |
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