Vicor VI-J00 Family DC-DC Converters and Configurable Power Supplies - Manual (Page 15)

Design Guide & Applications Manual
For VI-200 and VI-J00 Family DC-DC Converters and Configurable Power Supplies
VI-200 and VI-J00 Family Design Guide Rev 3.5 vicorpower.com
Page 14 of 98
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OVERVIEW
The VI-/ MI-200 Family of DC-DC converters are available
as Driver or Booster modules. The Driver can be used as a
stand alone module, or in multi-kilowatt arrays by adding
parallel Boosters. Booster modules do not contain
feedback or control circuitry, so it is necessary to connect
the Booster GATE IN pin to the preceding Driver or
Booster GATE OUT, to synchronize operation. Drivers and
Boosters have identical power trains, although Drivers
close the voltage loop internally while Boosters do not.
The concept behind Driver / Booster operation is that two
or more ZCS power trains driven at the same frequency
will inherently load-share if their inputs and outputs are
tied together. Slaved modules require only one connection
between units when their outputs are connected
together; no trimming, adjustments or external
components are required to achieve load sharing. The
load sharing is dynamic and typically within 5%.
For additional information, refer to Electrical Considerations
– High Power Arrays in the
Module Do’s and Don’ts .
(Section 3)
IMPORTANT: It is important to remember that when
using Boosters, the input voltage, output voltage and
output power of the Boosters must be the same as
the Driver.
Whenever power supplies or converters are operated in a
parallel configuration—for higher output power, fault
tolerance, or both—current sharing is an important
consideration. Most current-sharing schemes employed
with power converters involve analog approaches. One
analog method artificially increases the output impedance
of the converter modules, while another actually senses
the output current of each module and forces all of the
currents to be equal by feedback control.
Synchronous current sharing offers an alternative to
analog techniques. In a synchronous scheme, there is no
need for a current-sensing or current-measuring device on
each module. Nor is there a need to artificially increase
output impedance, which compromises load regulation.
There are advantages and disadvantages associated with
each approach to current sharing. In choosing the best
approach for a given application, designers should be
aware of the tradeoffs as well as tips for implementing a
successful design.
Most paralleled power components, such as transistors,
rectifiers, power conversion modules, and offline power
supplies, will not inherently share the load. With power
converters, one or more of the converters will try to
assume a disproportionate or excessive fraction of the
load unless forced current-sharing control is designed into
the system.
One converter, typically the one with the highest output
voltage, may deliver current up to its current limit setting,
which is beyond its rated maximum. Then, the voltage will
drop to the point where another converter in the array—
the one with the next highest voltage—will begin to
deliver current. All of the converters in an array may
Figure 8–1 — Parallel array
INPUT LOAD
+S
TRIM
–S
–OUT
+IN
GATE
IN
GATE
OUT
–IN
+S
TRIM
–S
–OUT
+IN
GATE
IN
GATE
OUT
–IN
+S
TRIM
–S
–OUT
+IN
GATE
IN
GATE
OUT
–IN
+
–
Zero-Current-
Switching
Driver
VI-2xx-xx
Zero-Current-
Switching
Booster
VI-Bxx-xx
Zero-Current-
Switching
Booster
VI-Bxx-xx
+OUT
+OUT
+OUT
8. Using Boosters and Parallel Arrays