L6564 STMicroelectronics, L6564 Datasheet - Page 20

• Manufacturer Part Number: L6564
• Description: 10 pin transition-mode PFC controller
• 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 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 will be 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 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 will be affected by a
considerable amount of 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 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 L6564 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 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
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 C
avoiding long settling time before reaching the new voltage level. The discharge of C
stopped as its voltage equals the voltage on pin MULT or if the voltage on pin VFF falls
below 0.88 V, to prevent the “Brownout protection” function from being improperly activated
(see “Power management/housekeeping functions” section).
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 16202 Rev 4
2
) block diagram and transfer characteristic
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and a resistor
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L6564
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