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FAN7930B Datasheet(PDF) 16 Page - ON Semiconductor |
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FAN7930B Datasheet(HTML) 16 Page - ON Semiconductor |
16 / 22 page ![]() www.onsemi.com 16 10. VIN-Absent Detection: To reduce pow er loss caused by input voltage sensing resistors and to optimize THD, the FA N7930B omits AC input voltage detection. Therefore, no infor mation about AC input is available from the internal controller. In many cases, the VCC of PFC controller is supplied by an independent pow er source like standby pow er. In this scheme, some mis match may ex ist. For example, w hen the electric pow er is suddenly interrupted during tw o or three AC line per iods; VCC is still live dur ing that time, but output voltage drops because there is no input pow er source. Consequently, the control loop tries to compensate for the output voltage drop and V COMP reaches its maximum. This lasts until A C input voltage is live again. When AC input voltage is live again, high V COMP allows high sw itching current and more stress is put on the MOSFET and diode. To protect against this, FA N7930B checks if the input A C voltage ex ists. If input does not exist, soft-start is reset and w aits until AC input is live again. Soft-start manages the turn-on time for smooth operation w hen it detects AC input is applied again and applies less voltage and current stress on startup. VIN t VOUT VAUX MOSFET Gate IDS fMIN DMAX High Drain Current! VCOMP Though VIN is eliminated, operation of controller is normal due to the large bypass capacitor. Figure 41. Without VIN-Absent Circuit VIN t VOUT VAUX MOSFET Gate IDS fMIN DMAX Input Voltage Absent Detected NewVCOMP Though VIN is eliminated, operation of controller is normal due to the large bypass capacitor. fMIN DMIN Smooth Soft- Start Figure 42. With VIN-Absent Circuit 11. Current Sense : The MOSFET current is sensed using an external sensing resistor for over-current protection. If the CS pin voltage is higher than 0.8 V, the over-current protection comparator generates a protection signal. An internal RC filter of 40 kΩ and 8 pF is included to filter sw itching noise. 12. Gate Driver Output: FAN7930B contains a single totem-pole output stage designed for a direct dr ive of the pow er MOSFET. The drive output is capable of up to +500 / -800 mA peak current w ith a typical rise and fall time of 50 ns w ith 1 nF load. The output voltage is clamped to 13 V to protect the MOSFET gate even if the VCC voltage is higher than 13 V. |
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