Datasheet

AN2994 Test results and significant waveforms
Doc ID 15796 Rev 2 15/38
To overcome this issue, the new L6563S integrates an innovative circuitry that allows getting
a fast transient response regardless of the voltage change that occurs on the mains, both
surges and drops. Therefore, if there is a sudden rise in the line voltage, C
FF
is rapidly
charged through the low impedance of the internal diode and no appreciable overshoot is
visible at the pre-regulator's output. If there is a drop in the line voltage, an internal "mains
drop" detector enables a low impedance switch that suddenly discharges C
FF
, therefore
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 pre-regulator's output like in systems with no
feed-forward compensation.
Figure 23 shows the behavior of the EVL6563S-400W demonstration board in the case of
an input voltage surge from 90 to 140 Vac: in the graphic, 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 24: it shows the behavior of a PFC using the L6562 operating in FOT
and delivering a similar output power. In the case of a mains surge, the controller cannot
compensate it and the output voltage stability is guaranteed by the feedback loop only.
Unfortunately, as previously said, its bandwidth is narrow and thus the output voltage has a
significant deviation from the nominal value. The circuit has the same behavior in the case of
a mains surge at any input voltage, and is also not affected if the input mains surge happens
at any point of the input sine wave.
Figure 21. EVL6563S-400W input current
shape at 100 Vac, 60 Hz,
C
FF
= 470 nF, R
FF
= 390 kΩ
Figure 22. EVL6563S-400W input current
shape at 100 Vac, 60 Hz,
C
FF
= 1.5 µF, R
FF
= 390 kΩ
THD [%]: 3.39% - 3RD harmonic: 0.126 A THD [%]: 2.75% - 3RD harmonic: 0.109 A
CH3: V
FF
voltage - pin #5 CH3: V
FF
voltage - pin #5
CH4: Input current CH4: Input current