AN3065 datasheet(16/33 Pages) STMICROELECTRONICS

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Test results and significant waveforms

AN3065

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Doc ID 16279 Rev 2

ripple at twice the mains frequency that will cause 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 was required.

The L6563S implements an innovative voltage feed-forward which, with a technique that

overcomes this time constant trade-off issue whichever voltage change occurs (both surges

and drops) on the mains. A capacitor CFF (C13) and a resistor RFF (R27 + R32), 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). In this way,

in case of sudden line voltage rise, CFF is rapidly charged through the low impedance of the

internal diode. In 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's output like in systems with no feed-forward compensation.

Figure 19 we find the behavior of the EVL6563S-100W demonstration board in case of an

input voltage surge from 90 to 140 Vac. As shown, 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 where the behavior of a PFC using the L6562A and delivering same

output power is shown. The controller cannot compensate a mains surge and the output

voltage stability is guaranteed by 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. The circuit has the same behavior in case of a mains surge at any input

voltage, and it is also not affected if the input mains surge happens at any point along the

input sine wave.

Figure 18.

L6562A input mains surge 90 Vac to

140 Vac, no VFF input

Figure 19.

EVL6563S-100W TM PFC: input

mains surge 90 Vac to 140 Vac

CH1: Vout

CH2: MULT (pin #3)

CH4: I_AC

CH1: Vout

CH2: MULT (pin #3)

CH3: VFF (pin #5)

CH4: I_AC

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