Datasheet

Test results and significant waveforms AN3065
16/33 Doc ID 16279 Rev 2
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 feed-forward, resulting in excessive overshoot and undershoot of
the pre-regulator's output voltage in response to large line voltage changes. Clearly a trade-
off was required.
The L6563S implements an innovative voltage feed-forward which, with a technique that
overcomes this time constant trade-off issue whichever voltage change occurs (both surges
and drops) on the mains. A capacitor C
FF
(C13) and a resistor R
FF
(R27 + R32), both
connected to the V
FF
(pin #5), complete an internal peak-holding circuit that provides a DC
voltage equal to the peak of the rectified sine wave applied on pin MULT (pin #3). In this way,
in case of sudden line voltage rise, C
FF
is rapidly charged through the low impedance of the
internal diode. In case of line voltage drop, an internal "mains drop" detector enables a low
impedance switch which suddenly discharges C
FF
avoiding a long settling time before
reaching the new voltage level. Consequently, an acceptably low steady-state ripple and low
current distortion can be achieved without any considerable undershoot or overshoot on the
preregulator's output like in systems with no feed-forward compensation.
In Figure 19 we find the behavior of the EVL6563S-100W demonstration board in case of an
input voltage surge from 90 to 140 Vac. As shown, it is evident that the V
FF
function
provides for the stability of the output voltage which is not affected by the input voltage
surge. In fact, thanks to the V
FF
function, the compensation of the input voltage variation is
very fast and the output voltage remains stable at its nominal value. The opposite is
confirmed in Figure 18 where the behavior of a PFC using the L6562A and delivering same
output power is shown. The controller cannot compensate a mains surge and the output
voltage stability is guaranteed by the feedback loop only. Unfortunately, as previously stated,
its bandwidth is narrow and thus the output voltage has a significant deviation from the
nominal value. The circuit has the same behavior in case of a mains surge at any input
voltage, and it is also not affected if the input mains surge happens at any point along the
input sine wave.
Figure 18. L6562A input mains surge 90 Vac to
140 Vac, no V
FF
input
Figure 19. EVL6563S-100W TM PFC: input
mains surge 90 Vac to 140 Vac
CH1: Vout
CH2: MULT (pin #3)
CH4: I_AC
CH1: Vout
CH2: MULT (pin #3)
CH3: V
FF
(pin #5)
CH4: I_AC