AN4149 datasheet(17/43 Pages) STMICROELECTRONICS

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AN4149

Designing a CCM FOT-controlled PFC

inductor peak current at minimum line voltage and rated load, is 27%, as a trade-off

between the inductor size and the stress on the switches.

This choice led to the inductor peak-to-peak ripple current (ILPK) equal to 1.89 A (equation

32).

The value of the inductance L required for the boost inductor can be calculated starting from

the volt-second balance of the inductor. The minimum value of L is then a function of the

duration of the OFF-time, the voltage across the inductor during TOFF, and the inductor

peak-to-peak ripple current.

Equation 46

The latter expression should be calculated at low line and the value found is the minimum

inductance value of the PFC inductor. To find the value of L, first TOFF needs to be

calculated.

In a fixed-off-time control using the L4984D, the off-time (TOFF) is changed proportionally to

the instantaneous line voltage in order to make the switching frequency (fSW) constant. The

switching frequency is determined by a capacitor connected between the TIMER pin and

ground, charged by an accurate internal generator (ITIMER) of 156 μA (typ.), during the OFF-

time, generating a voltage ramp.

When the voltage ramp equals the voltage on the MULT pin, connected through a resistive

divider to the rectified mains to get a sinusoidal voltage reference, the OFF-time of the

power MOSFET is terminated, the gate driver (GD) pin is driven high and the ramp resets at

zero. The timing capacitor CT is then selected with the following formula:

Equation 47

where fsw is the switching frequency and kp the maximum required divider ratio, calculated

considering the maximum value of the multiplier input, that is, the voltage measured on the

MULT pin at maximum mains voltage. According to the datasheet of the L4984D, the linear

operating range is between 0 to 3 V, so the maximum value of the multiplier input

(VMULTmax) is equal to 3 V.

The maximum required divider ratio kp can be now found as:

Equation 48

)

(

)

(

OFF

out

⋅

−

out

TIM ER

⋅

max

265

−

⋅

Vac

VA C

MULT

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