L6563S datasheet(30/42 Pages) STMICROELECTRONICS

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Application information

L6563S

30/42

Doc ID 16116 Rev 3

Figure 41.

Tracking output voltage vs Input voltage characteristic with TBO

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 42) 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 L6563S 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 will be attempted 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 will be considerably increased.

Figure 42.

Effect of boost inductor saturation on the MOSFET current and detection method

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