AN2782 datasheet(20/39 Pages) STMICROELECTRONICS

20 / 39 page

Designing a fixed off-time PFC

AN2782

20/39

Doc ID 14763 Rev 2

The commercial values selected are RoutH = 1530 MΩ and RoutL = 9.5 kΩ.

Please note that for RoutH a resistor with a suitable voltage rating (>400 V) is needed, or

more resistors in series have to be used.

This pin can also be used as an ON/OFF control input if tied to GND by an open collector or

open drain.

●

Pin 2 (COMP): This pin is the output of the E/A that is fed in one of the two inputs of the

multiplier. A feedback compensation network is placed between this pin and INV [1]. It

has to be designed in with a narrow bandwidth in order to avoid that the system rejects

the output voltage ripple (100 Hz) that would bring high distortion of the input current

waveform. A theoretical criterion to define the compensation network value is to set the

E/A bandwidth (BW) from 20 to 30 Hz.

For a more complex way of compensating the FOT PFC please refer to [1], [2], [3].

A compensated two-pole feedback network for this 400 W FOT PFC has been obtained with

the following values:

to which correspond the following open-loop transfer function and its phase function.

The two bode plot charts are in reference to the PFC operating at the main voltage set point

of 265 Vac and full load. In this condition the crossover frequency is fc = 25 Hz, the phase

margin is 30 ° and the third harmonic distortion is under 3%.

●

Pin 4 (CS): Pin #4 is the inverting input of the current sense comparator. Through this

pin, the L6562A reads the instantaneous inductor current, converted to a proportional

(54)

(55)

159

outH

outL =

159

outH

outL

220

compP =

compS

Figure 12.

Bode plot - open-loop transfer

function

Figure 13.

Bode plot - phase

IFI

Open Loop Transfer Function

-200

-100

100

0.1

100

1000

f [Hz]

Phase F

-200

-150

-100

0.1

100

1000

f [Hz]

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