AN4163 Datasheet, PDF (15/33 Pages) STMicroelectronics – This application note describes the demonstration board EVL4984-350W

AN4163
Test results and significant waveforms
amount of ripple at twice the mains frequency causing 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 is required. For reference, in Figure 23 and Figure 24 the comparison of the input current
shape and the measurement of the THD and 3^rd harmonic amplitude for different C_FF values
taken from a similar board using the former L4984D are shown.
Figure 23. EVL4984-350W input current shape Figure 24. EVL4984-350W input current shape
at 100 V_ac - 60 Hz - C_FF = 470 nF, R_FF = 390 k
at 100 V_ac - 60 Hz - C_FF = 1 µF, R_FF = 1 M
THD [%]: 3.6 % - 3^RD harmonic: 0.07 A
CH3: VFF - pin #5
CH4: input current
AM13410v1
THD [%]: 2.8 % - 3^RD harmonic: 0.057 A
CH3: VFF - pin #5
CH4: input current
AM13411v1
To overcome this issue the new L4984D has integrated an innovative circuitry which allows
getting a fast transient response for whichever voltage change occurs on the mains, both
surges and drops. Thus, in case of sudden line voltage rise, C_FF will be rapidly charged
through the low impedance of the internal diode and no appreciable overshoot will be visible
at the pre-regulator's output. 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 pre-regulator’s output like in systems with no feed-forward compensation.
In Figure 25 the behavior of the EVL4984-350W demonstration board in case of an input
voltage surge from 90 to 140 V_ac has been analyzed. As shown the V_FF function provides
for the stability of the output voltage which is not affected by the input voltage surge. 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, as opposed to Figure 26, which shows
the behavior of a PFC using the L6562 working in FOT and delivering 400 W in case of
a mains surge. The controller cannot compensate it and the output voltage stability depends
on 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.
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