STCMB1 datasheet(25/59 Pages) STMICROELECTRONICS

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STCMB1

Application information

7.9

PFC: light-load operation

In case of the light-load operation the PFC can work in the burst mode in two ways:

If the output of the error amplifier decreases to 1 V an internal comparator stops the

switching activity, independently by the HB stage. This burst mode is intended to be

used in case of the light-load operation at high mains, in case the burst mode of the HB

resonant stage is not used or has a threshold set at the very low load.

If the HB resonant stage detects a light-load and begins working in the burst mode will

force the PFC working synchronized in the burst mode too. In detail, when the LLC

enters in the idle state of the burst mode, the last PFC pulse is reset, in its natural way,

by TON reaching COMP - 1 V. Conversely, when the LLC restarts switching, at the end

of the idle state, the first PFC pulse is delayed of about 10

s with respect to the first

LLC pulse. Further details in Section 7.20: HB: improved burst mode operation at light-

load on page 37.

Note that the connection of an oscilloscope probes to the COMP pin, when the PFC is in the

deep burst mode, could modify the proper operation of the PFC.

7.10

PFC: inductor current sensing on ground return and PFC

OCP

Figure 12. PFC current sensing on ground

The STCMB1 implements the PFC current sensing on the ground return path, by means of

the ISEN_PFC pin. In this way the signal fed into that pin has the information of the current

level during the entire switching period, differently from other systems monitoring the current

during the PFC MOSFET on-time only.

Monitoring the current during the whole switching period allows safe handling of inductor

saturation conditions: an internal comparator enables the MOSFET to be turned on only if

the inductor has completed its demagnetization, not before. This mechanism prevents the

MOSFET from turning on with incomplete inductor demagnetization, for example following

a mains dip or during an inrush current charging the bulk capacitor. Both these situations

might result in very dangerous condition for the MOSFET because of the very low

impedance of a saturated coil.

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