Under the above conditions, the input voltage transiently drops to a lower value or 0V, due to the full load of the output, the voltage VBUS of the large output capacitor of the PFC rapidly decreases to a very low value, the VCC of the PFC control IC has a large capacitance, and the VCC current is small , Therefore, the power-down speed of VCC is much lower than that of VBUS, and the power-down speed of VCC is slow. As long as VCC is higher than the UVLO of VCC of the PFC control IC, the PFC control IC is still working.
When the value of VCC is slightly higher than UVLO, the input power supply AC is recharged, and the PFC control IC works directly without the soft-start process. Because the output voltage is relatively low, especially when the power MOSFET is turned on near the peak point of the input sine wave, the PFC inductance and The working peak current of the power MOSFET is very large. If the saturation current margin of the inductor is not enough, or the current sampling resistance of the PFC is too small, the PFC inductor may be saturated, and the power MOSFET may be damaged under the impact of high current.