L4984D datasheet(27/35 Pages) STMICROELECTRONICS

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L4984D

Inductor saturation detection

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

upslope becomes so large (50-100 times steeper, see Figure 34) 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, even a well-designed boost inductor may occasionally saturate when the boost

stage recovers after a missing line cycle. This happens when the restart occurs at an

unfavorable line voltage phase, i.e. when the output voltage is lower than the rectified input

voltage as this reappears. As a result, in the boost inductor the inrush current coming from

the bridge rectifier and going to the output capacitor adds up to the switched current.

Furthermore, there is little or no voltage available for demagnetization.

To cope with a saturated inductor, the L4984D is provided with a second comparator on the

current sense pin (CS, pin 4) that stops the IC if the voltage, normally limited within 0.88 V,

exceeds 1.7 V. After that, the IC is restarted by the internal starter circuitry; the starter

repetition time is low enough (300

μs typ.) to guarantee low stress for the inductor, the

Power MOSFET and the boost diode.

Figure 34. Effect of boost inductor saturation on MOSFET current and detection

method

AM13249v1

Tdelay

DIL

Multiplier

Output

Vcs

1.7V

Tdelay

Inductor not

saturating

Inductor slightly

saturating

Inductor saturating

hard

Multiplier

Output

Multiplier

Output

1.7V

DIL

Vcs

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