AN2870 datasheet(10/41 Pages) STMICROELECTRONICS

10 / 41 page

Device blocks description

AN2870

10/41

3.2

PFC section

3.2.1

Error amplifier

The error amplifier (E/A, Figure 4) is used to close the output voltage control loop. Its non

inverting input is connected to a precise voltage reference (2.52 V), the inverting input and

the output are externally available (pin 10 –INV; pin 9 – COMP).

The compensation network, placed between pins INV and COMP, is needed to reject the

mains ripple.

The E/A output dynamic is internally clamped: it can swing between 2.25 V and 4.2 V in

order to speed up the recovery after the E/A saturates low due to an over-voltage (static

OVP) or saturates high because of an over-current.

Figure 4.

PFC section block

3.2.2

Multiplier block and THD optimizer

The multiplier (Figure 4) gives the sinusoidal voltage reference to the current sense in order

to absorb from the mains a sinusoidal current. This current will be function of both input

voltage and load current then this block has two inputs: the first one (Pin MULT – 8) takes a

partition of the instantaneous rectified line voltage and the second one (Pin COMP – 9) is

the output of the E/A.

An internal voltage clamp (1 V) sets the maximum allowed voltage of the multiplier output,

then it act as PFC current limiter.

When the rectified input voltage reaches 0 V the boost inductor cannot store enough energy

to discharge the input capacitor: this event increases the THD. In order to avoid this

additional distortion, a THD optimizer block is placed between the output of the multiplier

and the current sense comparator.

The characteristic curves of the multiplier block are reported in Figure 14.

AN2870 Datasheet(PDF) 10 Page - STMicroelectronics