DAP005 datasheet(28/39 Pages) STMICROELECTRONICS

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Application information

DAP005

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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

upslope 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.

To cope with a saturated inductor, the DAP005 is provided with a second comparator on the

current sense pin (CS, pin 4) that stops and latches off the IC if the voltage, normally limited

within 1.1V, exceeds 1.7V. Also the cascaded DC-DC converter can be stopped via the

PWM_LATCH pin that is asserted high. In this way the entire system is stopped and enabled

to restart only after recycling the input power, that is when the Vcc voltages of the DAP005

and the PWM controller go below their respective UVLO thresholds. System safety will be

considerably increased.

6.7

Power management/housekeeping functions

A special feature of this IC is that it facilitates the implementation of the "housekeeping"

circuitry needed to coordinate the operation of the PFC stage to that of the cascaded DC-

DC converter. The functions realized by the housekeeping circuitry ensure that transient

conditions like power-up or power down sequencing or failures of either power stage be

properly handled.

This device provides some pins to do that. As already mentioned, one communication line

between the IC and the PWM controller of the cascaded DC-DC converter is the

PWM_LATCH pin, which is normally open when the PFC works properly and goes high if it

loses control of the output voltage (because of a failure of the control loop) or if the boost

inductor saturates, with the aim of latching off the PWM controller of the cascaded DC-DC

converter as well (Section 6.2: Feedback Failure Protection (FFP) on page 18 for more

details).

Figure 42. Effect of boost inductor saturation on the MOSFET current and detection method

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