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AN3142

Operation of an FOT-controlled PFC preregulator

Doc ID 17005 Rev 3

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Operation of an FOT-controlled PFC preregulator

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

(VA) compares a portion of the preregulator'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 is a rectified sinusoid,

VCSREF, which has an amplitude 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 switch M (typically a MOSFET) and the L

inductor 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 the M and the L inductor. As

VCSREF is a rectified sinusoid, the inductor peak current is also enveloped by a rectified

sinusoid. The line current Iin is 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 in which TOFF has elapsed, sets the PWM latch, therefore 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 almost the same

architecture as TM control, just the way the off-time of M is determined also 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 refer to the L6563S [

4], which is suitable for

power applications of a few hundred watts 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 [

5].

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