EVL6563S-400W STMicroelectronics, EVL6563S-400W Datasheet - Page 14

• Manufacturer Part Number: EVL6563S-400W
• Description: EVAL BOARD FOR L6563(400W)
• Manufacturer: STMicroelectronics
• Type: Motor / Motion Controllers & Driversr

Specifications of EVL6563S-400W

• Main Purpose: Power Management, Power Factor Correction
• Embedded: No
• Utilized Ic / Part: L6563
• Primary Attributes: 400W Power Factor Correction and Preregulator Combination
• Secondary Attributes: Transition Mode & Active Tracking Boost Function.
• Maximum Operating Temperature: + 60 C
• Product: Power Management Development Tools
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• For Use With/related Products: L6563S
• Other names: 497-10488

Test results and significant waveforms
2.3
14/38
Voltage feed-forward
The power stage gain of PFC pre-regulators varies with the square of the RMS input
voltage. So 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 an fc of 20 Hz at 264 Vac means having an fc of
4 Hz at 88 Vac, resulting in sluggish control dynamics. Additionally, the slow control loop
causes large transient current flows 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 inputs at high lines, whereas a fixed power limit
requires the current limit to vary inversely with the line voltage.
The voltage feed-forward function can compensate for the gain variation with the line voltage
and allow overcoming 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/V
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 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 significantly improves
dynamic behavior at low lines and simplifies loop design.
The L6563S realizes a voltage feed-forward with a technique that makes use of just two
external parts and that limits the feed-forward time constant trade-off issue to only one
direction. A capacitor C
pin (5), complete an internal peak-holding circuit that provides a DC voltage equal to the
peak of the rectified sine wave applied on the MULT pin (pin 3). R
discharging C
However, a drawback of the V
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, thus causing a 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 is required. For reference,
of the input current shape and the measurement of the THD and 3RD harmonic amplitude
for different C
2
corrector) and providing the resulting signal to the multiplier that generates the current
FF
FF
values taken from a similar board using the former L6563.
when the line voltage decreases.
FF
(C18) and a resistor R
FF
Doc ID 15796 Rev 2
technique is an increase in the harmonics; in fact, deriving
Figure 21
FF
(R26 + R27), both connected to the V
and
Figure 22
FF
provides a means of
show a comparison
AN2994
FF