AN3112 Datasheet, PDF(5/36 Page) STMicroelectronics – Solution for designing a fixed off-time controlled PFC pre-regulator

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AN3112 Datasheet, PDF (5/36 Pages) STMicroelectronics – Solution for designing a fixed off-time controlled PFC pre-regulator

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AN3112

Operation of an FOT- controlled PFC pre-regulator

In Figure 5 a block diagram of an FOT-controlled PFC pre-regulator is shown. 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 is 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 M switch (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 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 nearly the same architecture as TM control,

the only change is the way the off-time of M is determined. 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.

For the controller, we refer to the L6564 [4]. For a more detailed and complex description of

the fixed off-time technique and in particular the line modulated FOT, please refer to [5].

Doc ID 16820 Rev 3

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