AN4149 datasheet(4/43 Pages) STMICROELECTRONICS

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CCM PFC using FOT control

AN4149

4/43

DocID023523 Rev 2

CCM PFC using FOT control

Fixed-frequency PWM is not the only alternative when CCM operation is desired. An

additional approach that couples the simplicity and affordability of TM operation with the

high-current capability of CCM operation can be a solution to the problem.

Fixed-frequency PWM modulates both switch ON and OFF times (their sum is constant by

definition), and a given converter operates in either CCM or DCM depending on the input

voltage and the loading conditions. Exactly the same result can be achieved with FOT

approach: a conventional "peak" current mode control, where the ON-time TON of the

external power switch is determined by the peak inductor current reaching the programmed

value and the OFF-time TOFF is determined by a special Fixed-off-time (FOT) modulator in

such a way that the resulting switching period is constant as long as the boost converter is

operated in CCM (i.e. the current in the boost inductor remains greater than zero in a

switching cycle).

In Figure 3 a block diagram of an FOT-controlled CCM PFC pre-regulator is shown. An error

amplifier (VA) compares a portion of the boosted output voltage Vout with a reference VREF

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

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

rectified input voltage VMULT.

The multiplier output (VCSREF) is a rectified sine wave whose amplitude is proportional to

that of VMULT and to VC, and is used as a reference for PWM modulation. The multiplier

output is fed into the inverting input of a PWM comparator that, on the non-inverting input,

receives the voltage VCS from 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 is turned off.

As a result, the multiplier output determines the peak current through the switch and the

inductor, and as, it is a rectified sinusoid, the inductor peak current is also enveloped by a

rectified sinusoid.

When VCSREF and VCS are equal 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 the switch on and starting another switching cycle.

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

For the CCM PFC controller, please refer to Figure 4.

To understand how TOFF needs to be modulated to achieve a fixed switching frequency

independent of the instantaneous line voltage and the load, it is useful to consider the V·s

balance equation for the boost inductor under the assumption of CCM operation:

Equation 1

where Vpk is the peak line voltage, Vout is the regulated output voltage, and θ is the

instantaneous phase angle of the line voltage. Solving for TON we get:

Equation 2

()

−

sin

Vpk

Vout

sin

Vpk

OFF

sin

Vpk

Vout









−

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