AN2485 datasheet(12/29 Pages) STMICROELECTRONICS

12 / 29 page

Test results and significant waveforms

AN2485

12/29

The L6563 achieves Voltage Feedforward with a technique that makes use of just two

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

A capacitor CFF (C18) and a resistor RFF (R26 + R27), both connected to the VFF 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). RFF provides a means to discharge CFF

when the line voltage decreases. In this way, in case of sudden line voltage rise, CFF is

rapidly charged through the low impedance of the internal diode and no appreciable

overshoot is visible at the pre-regulator's output. In case of line voltage drop, CFF is

discharged with the time constant RFF· CFF, which can be in the hundred ms to achieve an

acceptably low steady-state ripple and have low current distortion. The dynamics of the

voltage feed-forward input is limited downwards at 0.5 V, that is, the output of the multiplier

no longer increases if the voltage on the VFF pin is below 0.5 V. This helps to prevent

excessive power flow when the line voltage is lower than the minimum specified value.

The behavior of the EVAL6563-400W demo board in case of an input voltage surge from 90

to 140 Vac is shown in Figure 20. The graph shows that the VFF function provides for the

stability of the output voltage which is not affected by the input voltage surge. In fact, thanks

to the VFF function, the compensation of the input voltage variation is very fast and the

output voltage remains perfectly stable at its nominal value. The opposite is confirmed in

Figure 21. which shows the behavior of a PFC using the L6562 working in FOT and

delivering a similar output power. In case of a mains surge the controller cannot compensate

for it, and the output voltage stability is guaranteed by the feedback loop only. Unfortunately,

as previously stated, its bandwidth is poor therefore the output voltage has a significant

deviation from the nominal value.

The circuit has the same behavior in case of mains surge at any input voltage, and it is not

affected if the input mains surge happens at any point on the input sine wave.

Figure 20.

EVAL6563-400W Input mains surge

from 90 Vac to 140 Vac - full load -

CFF = 470 nF

Figure 21.

L6562 FOT Input mains surge from

90 Vac to 140 Vac - full load - NO

VFF input

CH1: Output voltage

CH2: Input rectified mains voltage

CH4: Input current

CH3: VFF voltage - pin #5

CH1: Output voltage

CH2: Input rectified mains voltage

CH4: Input current

CH3: VFF voltage - pin #5

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