AN4077 datasheet(17/33 Pages) STMICROELECTRONICS

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AN4077

Test results and significant waveforms

Doc ID 022997 Rev 1

17/33

Figure 21 the circuit behavior in the case of mains dip is reported: as previously

described, the internal circuitry has detected the decreasing of the mains voltage and it has

activated the CFF internal fast discharge. As seen, in this case the output voltage changes

but in few mains cycles it returns to the nominal value. The situation is different if the

performance of a controller without the VFF function is still checked. In

Figure 20 the

behavior of a PFC using the L6562A delivering similar output power is reported: in the case

of a mains dip from 140 Vac to 90 Vac the output voltage variation is not very different but the

output voltage requires a longer time to restore the original value. When performing tests

with a wider voltage variation (e.g. 265 Vac to 90 Vac) the output voltage variation of a PFC

without the voltage feed-forward fast discharging is much more emphasized.

The fast discharge system allows to size the external RFF • CFF time constant on the VFF

pin in order to minimize the ripple on the CFF capacitor. The effect of this improvement can

be appreciated comparing the input current shape in a 100 W PFC board with a controller

without fast discharge feature (

Figure 22) in respect to the input current shape in the same

board with the L6564H (

Figure 23).

Figure 23 the input current of the L6564H has a better shape and the 3rd harmonic

current distortion is not noticeable: this demonstrates the benefits of the new voltage feed-

forward circuit integrated in the L6564H.

Figure 20.

L6562A input mains dip 140 Vac to

90 Vac-no VFF input

Figure 21.

L6564H 100 W TM PFC

demonstration board: input mains

dip 140 Vac to 90 Vac

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