L6564T STMicroelectronics, L6564T Datasheet - Page 20

• Manufacturer Part Number: L6564T
• Description: 10 pin transition-mode PFC controller for outdoor applications
• Manufacturer: STMicroelectronics

Application information
Figure 28. Voltage feedforward: squarer/divider (1/V
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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
significantly improves dynamic behavior at low line and simplifies loop design.
In fact, 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 L6564T 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
RFF, both connected from 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
impedance of the internal diode; in case of line voltage drop, an internal “mains drop”
detector enables a low impedance switch which suddenly discharges C
settling time before reaching the new voltage level. The discharge of C
voltage equals the voltage on the MULT pin or if the voltage on the VFF pin falls below 0.88
V, to prevent the “Brownout protection” function from being improperly activated (see
Section 6.3
As a result of the VFF pin functionality, an acceptably low steady-state ripple and low current
distortion can be achieved with a limited undershoot or overshoot on the pre-regulator's
output.
).
Doc ID 022671 Rev 1
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) block diagram and transfer characteristic
FF
is rapidly charged through the low
FF
FF
is stopped as its
avoiding long
FF
and a resistor
L6564T