FE175D480M033FP-CB Datasheet, PDF (19/20 Pages) Vicor Corporation – VI Brick® AC Front End

Product Details and Design Guidelines (cont.)
The timing diagram in Figure 30 shows the output voltage of the VI
Brick® AC Front End module and the PRM® PC pin voltage and output
voltage of the PRM regulator for the power up and power down
sequence. It is recommended to keep the time delay approximately 10
to 20 ms.
VI BRICK™
AC Front End 49V – 3%
VOUT
PRM UV
Turn on
PRM™
Regulator
PC
tDELAY
PRM™
Regulator
VOUT
tHOLD-UP
Figure 30 – PRM Enable Hold off Waveforms
Special care should be taken when enabling the constant-power load
near the auto-ranger threshold, especially with an inductive source
upstream of the VI Brick AC Front End. A load current spike may cause
a large input voltage transient, resulting in a range change which could
temporarily reduce the available power (see Adaptive Cell™ Topology
below).
Adaptive Cell™ Topology
The Adaptive Cell topology utilizes magnetically coupled “top” and
“bottom” primary cells that are adaptively configured in series or
parallel by a configuration controller comprised of an array of switches.
A microcontroller monitors operating conditions and defines the
configuration of the top and bottom cells through a range control
signal.
A comparator inside the microcontroller monitors the line voltage and
compares it to an internal voltage reference.
If the input voltage of the VI Brick AC Front End crosses above the
positive going cell reconfiguration threshold voltage, the output of the
comparator transitions, causing switches S1 and S2 to open and switch
S3 to close (see Functional Block Diagram on page 6). With the top cell
and bottom cell configured in series, the unit operates in “high” range
and input capacitances C_IN-T and C_IN-B are in series.
If the peak of input voltage of the unit falls below the negative-going
range threshold voltage for two line cycles, the cell configuration
controller opens switch S3 and closes switches S1 and S2. With the top
cell and bottom cells configured in parallel, the unit operates in “low”
range and input capacitances C_IN-T and C_IN-B are in parallel.
Power processing is held off while transitioning between ranges and
the output voltage of the unit may temporarily droop. External output
hold up capacitance should be sized to support power delivery to the
load during cell reconfiguration. The minimum specified external
output capacitance of 6,000 μF is sufficient to provide adequate ride-
through during cell reconfiguration for typical applications.
FE175D480C033FP-00
Source Inductance Considerations
The AC Front End Powertrain uses a unique Adaptive Cell Topology
that dynamically matches the powertrain architecture to the AC line
voltage. In addition the AC Front End uses a unique control algorithm
to reduce the AC line harmonics yet still achieve rapid response to
dynamic load conditions presented to it at the DC output terminals.
Given these unique power processing features, the AC Front End can
expose deficiencies in the AC line source impedance that may result in
unstable operation if ignored.
It is recommended that for a single AC Front End, the line source
inductance should be no greater than 1 mH for a universal AC input of
100 - 240 V. If the AC Front End will be operated at 240 V nominal only,
the source impedance may be increased to 2 mH. For either of the
preceding operating conditions it is best to be conservative and stay
below the maximum source inductance values. When multiple AC
Front End’s are used on a single AC line, the inductance should be no
greater than 1 mH/N, where N is the number of AC Front End’s on the
AC branch circuit, or 2 mH/N for 240 Vac operation. It is important to
consider all potential sources of series inductance including and not
limited to, AC power distribution transformers, structure wiring
inductance, AC line reactors, and additional line filters. Non-linear
behavior of power distribution devices ahead of the AC Front End may
further reduce the maximum inductance and require testing to ensure
optimal performance.
If the AC Front End is to be utilized in large arrays, the AC Front Ends
should be spread across multiple phases or sources thereby minimizing
the source inductance requirements, or be operated at a line voltage
close to 240 Vac. Vicor Applications should be contacted to assist in the
review of the application when multiple devices are to be used
in arrays.
VI Brick^® AC Front End
Page 19 of 20
Rev 2.0
07/2015
vicorpower.com
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