Application information
L6563H
Figure 39. Voltage feedforward: squarer-divider (1/V2) block diagram and transfer characteristic
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In this way a change of the line voltage causes 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 is constant throughout the input voltage range, which
improves significantly dynamic behavior at low line and simplifies loop design.
Actually, 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 feedforward, 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 a NEW voltage feed forward that, with a technique that makes use of
just two external parts, strongly minimizes this time constant trade-off issue whichever
voltage change occurs on the mains, both surges and drops. A capacitor C and a resistor
FF
RFF, both connected from the pin VFF (#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 pin
MULT (#3). In this way, in case of sudden line voltage rise, C is rapidly charged through
FF
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
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 feedforward
compensation.
The twice-mains-frequency (2•f ) ripple appearing across C is triangular with a peak-to-
L
FF
peak amplitude that, with good approximation, is given by:
2VMULTpk
ΔVFF
=
1+ 4fLRFFCFF
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Doc ID 16047 Rev 2
STMICROELECTRONICS [ ST ]
