Vicor VI-J00 Family DC-DC Converters and Configurable Power Supplies - Manual (Page 17)

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
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that it will be delivering equal current for an equal voltage
at the summing node. A simple implementation of the
droop-share scheme uses the voltage dropped across an
ORing diode, which is proportional to current, to adjust
the output voltage of the associated converter. (Figure 8–2)
Droop share has advantages and disadvantages. One of
the advantages is that it can work with any topology. It’s
also fairly simple and inexpensive to implement. Though, a
major drawback is that it requires that the current be
sensed. A current-sensing device is needed in each of the
converters or power supplies. Additionally, a small penalty
is paid in load regulation, though in many applications this
isn’t an issue.
In general, mixing and matching converters isn’t
recommended—especially those with incompatible
current-sharing schemes. The droop-share method,
however, is more forgiving in this regard than any of the
other techniques. With a little external circuitry, current
sharing can be achieved using arrays constructed from
different converter models or even from different suppliers.
Most systems can employ the Driver / Booster (or master /
slave) array for increased power. (Figure 8–3) The Driver is
used to set and control output voltage, while Booster
modules, as slaves to the master, are used to extend
output power to meet system requirements.
Driver / Booster arrays of quasi-resonant converters with
identical power trains inherently current share because the
per-pulse energy of each converter is the same. If the
inputs and outputs are tied together and the units operate
at the same frequency, all modules will deliver equal
current (within component tolerances).
The single intelligent module in the array determines the
transient response, which does not change as modules
are added. Slaved modules require only one connection
between units when their outputs are connected. No
trimming, adjustments, or external components are
required to achieve load sharing. The load sharing is
dynamic and usually guaranteed within 5%. It’s important
to remember that when using Boosters, the input and
output voltage and output power specifications of the
Boosters must be the same as the Driver.
Driver / Booster arrays have two advantages. They have
only a single control loop, so there are no loop-within-a-
loop stability issues. And, they have excellent transient
response. However, this arrangement isn’t fault tolerant.
If the Driver module fails, the array won’t maintain its
output voltage.
Analog current-sharing control involves paralleling two or
more identical modules, each containing intelligence. The
circuit actively adjusts the output voltage of each supply
so the multiple supplies deliver equal currents. This method,
though, has a number of disadvantages. Each converter in
the array has its own voltage regulation loop, and each
requires a current-sensing device and current-control loop.
Analog current-sharing control does support a level of
redundancy. But it’s susceptible to single-point failures
within the current-sharing bus that at best can defeat
current sharing, and at worst can destroy every module in
the array. The major reason for this is the single-wire
galvanic connection between modules.
Current sharing is an essential element in fault-tolerant
arrays. Yet regardless of the approach, there is an inherent
–OUT
–S
TRIM
+S
+OUT +IN
GATE
IN
GATE
OUT
–IN
–OUT
–S
TRIM
+S
+OUT +IN
GATE
IN
GATE
OUT
–IN Return
Zero Current
Switching
Converter
#1
Driver
Zero Current
Switching
Converter
#n
Driver
+VIN
+VOUT
–VIN
Figure 8–2 — Droop-share current sharing artificially increases converter output impedance to force the currents to be equal. Diodes on the
output of each converter provide current sensing and fault protection.
8. Using Boosters and Parallel Arrays