EVL6563H-100W データシートの表示（PDF） - STMicroelectronics

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EVL6563H-100W

250 W transition-mode PFC pre-regulator with the new L6563H

STMicroelectronics 

AN3118

Test results and significant waveforms

4.3

Voltage feed-forward and brown-out function

The power stage gain of PFC pre-regulators varies with the square of the RMS input

voltage. As does the crossover frequency fc of the overall open-loop gain because the gain

has a single pole characteristic. This leads to large trade-offs in the design. For example,

setting the gain of the error amplifier to get fc = 20 Hz @ 264 Vac means having fc 4 Hz @

88 Vac, resulting in sluggish control dynamics. Additionally, the slow control loop causes

large transient current flow during rapid line or load changes that are limited by the

dynamics of the multiplier output. This limit is considered when selecting the sense resistor

to let the full load power pass under minimum line voltage conditions, with some margin. But

a fixed current limit allows excessive power input at high line, whereas a fixed power limit

requires the current limit to vary inversely with the line voltage.

Voltage feed-forward can compensate for the gain variation with the line voltage and

overcome all of the above mentioned issues. It consists of deriving a voltage proportional to

the input RMS voltage, feeding this voltage into a squarer/divider circuit (1/V2 corrector) and

providing the resulting signal to the multiplier that generates the current reference for the

inner current control loop.

In this way a change of the line voltage will cause an inversely proportional change of the

half-sine amplitude at the output of the multiplier (if the line voltage doubles the amplitude of

the multiplier, output is halved and vice versa) so that the current reference is adapted to the

new operating conditions with (ideally) no need for invoking the slow dynamics of the error

amplifier. Additionally, the loop gain will be constant throughout the input voltage range,

which improves dynamic behavior at low line significantly and simplifies loop design.

Actually, with another PFC embedding the voltage feed-forward, deriving a voltage

proportional to the RMS line voltage implies a form of integration, which has its own 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's output voltage in response to large line voltage changes. Clearly a trade off was

required.

The L6563H realizes an innovative voltage feed-forward which, with a technique that

overcomes this time constant trade off issue whichever voltage change occurs on the mains,

both surges and drops. A CFF (C12) capacitor and a RFF (R27 + R28) resistor, 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 sinewave applied on the 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

pre-regulator's output, like in systems with no feed-forward compensation.

In Figure 21 the behavior of the EVL6563H-250W demonstration board, in case of an input

voltage surge from 90 to 140 Vac, is shown; in the image 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 20; the behavior of a PFC using the L6562A and delivering the same

output power is shown; in 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

stated, its bandwidth is narrow and therefore the output voltage has a significant deviation

Doc ID 16847 Rev 2

15/31

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