AN2782 datasheet(5/39 Pages) STMICROELECTRONICS

5 / 39 page

AN2782

Operation of an FOT-controlled PFC pre-regulator

Doc ID 14763 Rev 2

5/39

Operation of an FOT-controlled PFC pre-regulator

Figure 5 shows a block diagram of an FOT-controlled PFC pre-regulator. An error amplifier

(VA) compares a portion of the pre-regulator's output voltage Vout with a reference VREF

and generates an error signal VC proportional to their difference. VC, a DC voltage by

hypothesis, is fed into an input of the multiplier block and multiplied by a portion of the

rectified input voltage VMULT. At the output of the multiplier, there will be a rectified sinusoid,

VCSREF, whose amplitude is proportional to that of VMULT and to VC, which represents the

sinusoidal reference for PWM modulation. VCSREF is fed into the inverting input of a

comparator that, on the non-inverting input, receives the voltage VCS on the sense resistor

Rsense, proportional to the current flowing through the switch M (typically a MOSFET) and

the inductor L during the ON-time of M. When the two voltages are equal, the comparator

resets the PWM latch and M, supposedly already ON, is switched off.

Figure 5.

Block diagram of an FOT-controlled PFC pre-regulator

As a result, VCSREF determines the peak current through M and the inductor L. As VCSREF is

a rectified sinusoid, the inductor peak current is enveloped by a rectified sinusoid as well.

The line current Iin will be the average inductor current that is the low-frequency component

of the inductor current resulting from the low-pass filtering operated by the EMI filter.

The PWM latch output Q going high activates the timer that, after a predetermined time

TOFF has elapsed, sets the PWM latch, thus turning M on and starting another switching

cycle. If TOFF is such that the inductor current does not fall to zero, the system operates in

CCM. It is apparent that FOT control requires nearly the same architecture as TM control,

just the way the off-time of M is determined changes. It is not a difficult task to modify

externally the operation of the standard TM PFC controller so that the off-time of M is fixed.

As a controller we will refer to the L6562A [4], which is suitable for a few hundred watts

power applications because of its gate drive capability and its high noise immunity.

For a more detailed and complex description of the fixed off-time technique and in particular

the line modulated FOT, please refer to [7].

AN2782 Datasheet(PDF) 5 Page - STMicroelectronics