Application information

L6563 - L6563A

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6.6

Inductor saturation detection (L6563 only)

Boost inductor's hard saturation may be a fatal event for a PFC pre-regulator: the current

upslope becomes so large (50-100 times steeper, see Figure 40) that during the current

sense propagation delay the current may reach abnormally high values. The voltage drop

caused by this abnormal current on the sense resistor reduces the gate-to-source voltage,

so that the MOSFET may work in the active region and dissipate a huge amount of power,

which leads to a catastrophic failure after few switching cycles.

However, in some applications such as ac-dc adapters, where the PFC pre-regulator is

turned off at light load for energy saving reasons, even a well-designed boost inductor may

occasionally slightly saturate when the PFC stage is restarted because of a larger load

demand. This happens when the restart occurs at an unfavorable line voltage phase, so that

the output voltage may drop significantly below the rectified peak voltage. As a result, in the

boost inductor the inrush current coming from the bridge rectifier adds up to the switched

current and, furthermore, there is little or no voltage available for demagnetization.

Figure 38. 80W, wide-range-mains PFC pre-regulator with tracking boost function active

Figure 39. Tracking boost and voltage feedforward blocks