L6563H
Application information
Figure 44. Tracking output voltage vs Input voltage characteristic with TBO
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6.6
Inductor saturation detection
Boost inductor's hard saturation may be a fatal event for a PFC pre-regulator: the current up-
slope becomes so large (50-100 times steeper, see Figure 45) 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, i.e.
when the output voltage is 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.
To cope with a saturated inductor, the L6563H is provided with a second comparator on the
current sense pin (CS, pin 4) that stops the IC if the voltage, normally limited within 1.1 V,
exceeds 1.7 V. After that, the IC attempts to restart by the internal starter circuitry; the
starter repetition time is twice the nominal value to guarantee lower stress for the inductor
and boost diode. Hence, the system safety is considerably increased.
Figure 45. Effect of boost inductor saturation on the MOSFET current and detection method
Doc ID 16047 Rev 2
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STMICROELECTRONICS [ ST ]
