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L4984D

Voltage feedforward

8

Voltage feedforward

The power stage gain of PFC pre-regulators varies with the square of the RMS input

voltage. So does the crossover frequency fc of the overall open-loop gain because the gain

has a single pole characteristic. This leads to large trade-offs in the design.

control loop causes large transient current flow during rapid line or load changes that are

limited by the dynamics of the multiplier output. This limit is considered when selecting the

sense resistor to let the full load power pass under minimum line voltage conditions, with

some margin. But a fixed current limit allows excessive power input at high line, whereas a

fixed power limit requires the current limit to vary inversely with the line voltage.

Input voltage feedforward compensates for the gain variation with the line voltage and

allows all of the above-mentioned issues to be minimized. It consists of deriving a voltage

proportional to the input RMS voltage, feeding this voltage into a squarer/divider circuit

(1/V2 corrector) and providing the resulting signal to the multiplier that generates the current

reference for the inner current control loop (see Figure 31).

Figure 31. Voltage feedforward: squarer-divider (1/V2) block diagram and transfer

characteristic

In this way, if the line voltage doubles the amplitude of the multiplier, output is halved and

vice versa, so that the current reference is adapted to the new operating conditions with

(ideally) no need to invoke the slow response of the error amplifier. Additionally, the loop

gain is constant throughout the input voltage range, which improves significantly dynamic

behavior at low line and simplifies loop design.

Actually, deriving a voltage proportional to the RMS line voltage implies a form of integration,

which has its own time constant. If it is too small, the voltage generated is affected by a

considerable amount of ripple at twice the mains frequency that causes distortion of the

current reference (resulting in high THD and poor PF); if it is too large there is a

considerable delay in setting the right amount of feedforward, resulting in excessive

overshoot and undershoot of the pre-regulator output voltage in response to large line

voltage changes. Clearly, a trade-off is required.

The L4984D realizes a new voltage feedforward that, using just two external parts, strongly

minimizes this time constant trade-off issue whichever voltage change occurs on the mains,

AM13248v1

01

2

3

4

0

0.5

1

1.5

2

Vcsx

0.8

Actual

Ideal

5

MULT

3

Rectified mains

"ideal" diode

current

reference

(Vcsx)

9.5V

VFF

E/A output

-

+

1/V

2

MULTIPLIER

L4984D

DETECTOR

MAINS DROP