L6563 - L6563A

Application information

19/39

characteristic

In this way a change of the line voltage will cause an inversely proportional change of the

half sine amplitude at the output of the multiplier (if the line voltage doubles the amplitude of

the multiplier output will be halved and vice versa) so that the current reference is adapted to

the new operating conditions with (ideally) no need for invoking the slow dynamics of the

error amplifier. Additionally, the loop gain will be 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 will be affected by a

considerable amount of ripple at twice the mains frequency that will cause distortion of the

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

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

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

voltage changes. Clearly a trade-off is required.

The device realizes Voltage Feedforward with a technique that makes use of just two

external parts and that limits the feedforward time constant trade-off issue to only one

ground, complete an internal peak-holding circuit that provides a DC voltage equal to the

internal diode and no appreciable overshoot will be visible at the pre-regulator's output; in

be in the hundred ms to achieve an acceptably low steady-state ripple and have low current

distortion; consequently the output voltage can experience a considerable undershoot, like

in systems with no feedforward compensation.

0

1234

0

0.5

1

1.5

2

Vcsx

0.5

Actual

Ideal

5

MULT

3

R5

Rectified mains

R6

"ideal" diode

current

reference

(Vcsx)

9.5V

VFF

E/A output

-

+

1/V

2

MULTIPLIER

L6563

L6563A