L4984D STMicroelectronics, L4984D Datasheet - Page 27

• Manufacturer Part Number: L4984D
• Description: Power Factor Correction ICs Current Mode PFC CCM Line Modulated Bi
• Manufacturer: STMicroelectronics

Specifications of L4984D

• Rohs: yes

L4984
10
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
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 L4984 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
Multiplier
Output
Vcs
method
Inductor not
saturating
T
delay
DI
1.7V
L
Doc ID 023489 Rev 3
t
Multiplier
Output
Vcs
Inductor slightly
saturating
T
DI
delay
L
Figure 34
1.7V
t
Multiplier
Inductor saturation detection
Output
Vcs
) that, during the current
Inductor saturating
hard
T
DI
delay
L
1.7V
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