AN4461 datasheet(12/70 Pages) STMICROELECTRONICS

12 / 70 page

PFC stage

AN4461

12/70

DocID026130 Rev 2

The conduction time (TON) is the time the MOSFET remains on. TON is updated every

100

µs and based on the PFC output voltage during the previous 200 ms. The PFC

bandwidth is ~5 Hz. The boost MOSFET is turned on when the boost inductor current falls

down to zero. The result of this algorithm is that the inductor current is shaped like a series

of adjacent triangles whose peak's envelope is a half-wave sinusoid in phase with the AC

input line voltage. For geometrical reason, the average current absorbed from the input line

is also sinusoidal.

Figure 5. PFC input current

During light load conditions, a valley skipping mechanism is adopted, allowing a lower

output voltage ripple and lower operating frequencies.

3.2

Protections

The STLUX385A device implements several protections that prevent the component

degradation due to electrical overstresses or overheating. The protections implemented in

the PFC stage are:

Overvoltage protection (OVP): stops the switching activity when the output capacitor

voltage reaches a threshold. The PFC is turned on as soon as the capacitor voltage

returns below the threshold.

Overcurrent protection (OCP): is activated when the inductor peak current reaches

very high values during TON (e.g. due to input overvoltage or output overloading). The

current is read by the RCS shunt resistor and compared with a threshold. The threshold

is dynamic and adjusted depending on the input voltage.

Brownout: When the input voltage is lower than a threshold the PFC stage is disabled.

When the input voltage returns to acceptable levels, the PFC is turned on again. This

prevents overstressing the PFC power components due to operations at very low input

voltages.

Controlled soft start: the protection limits the charging current of the Cpfc capacitor.

This method has two big advantages: a limited inrush current related with the PFC

activation and better control of the output voltage that does not experience

overshooting (and the audible noise associated).

AN4461 Datasheet(PDF) 12 Page - STMicroelectronics