AN4077 datasheet(16/33 Pages) STMICROELECTRONICS

16 / 33 page

Test results and significant waveforms

AN4077

16/33

Doc ID 022997 Rev 1

time constant. If it is too small, the voltage generated is affected by a considerable amount

of ripple at twice the mains frequency that causes distortion of the current reference

(resulting in high THD and poor PF); if it is too large, there is a considerable delay in setting

the right amount of feed-forward, resulting in excessive overshoot and undershoot of the

pre-regulator output voltage in response to large line voltage changes. Clearly a trade-off

was required. The L6564H realizes an innovative voltage feed-forward which, with a

technique that makes use of just two external parts, overcomes this time constant trade-off

issue whichever voltage change occurs on the mains, both surges and drops. A capacitor

CFF and a resistor RFF, both connected from pin VFF (pin #5) to ground, complete an

internal peak-holding circuit that provides a DC voltage equal to the peak of the rectified

sine wave applied on the MULT pin (pin #3). In this way, in the case of sudden line voltage

rise, CFF is rapidly charged through the low impedance of the internal diode; in the case of

line voltage drop, an internal “mains drop” detector enables a low impedance switch which

suddenly discharges CFF 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 output like

in systems with no feed-forward compensation.

Figure 19 we see the behavior of the EVL6564H-100W demonstration board in the case

of an input voltage surge from 90 to 140 Vac: in the figure it is evident 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 stable at its nominal value. The opposite is

confirmed in

Figure 18: the behavior of a PFC using the L6562A and delivering the same

output power is shown: 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 mentioned, its bandwidth is narrow and therefore the output voltage has a

significant deviation from the nominal value. The circuit has the same behavior in the case of

mains surge at any input voltage, and it is not affected even if the input mains surge

happens at any point of the input sine wave.

Figure 18.

L6562A input mains surge 90 Vac to

140 Vac-no VFF input

Figure 19.

L6564H 100 W TM PFC

demonstration board: input mains

surge 90 Vac to 140 Vac

AN4077 bảng dữ liệu(PDF) 16 Page - STMicroelectronics