Vicor Micro Family of DC-DC Converter - Manual (Page 21)

Design Guide & Applications Manual
For Maxi, Mini, Micro Family DC-DC Converters and Configurable Power Supplies
Maxi, Mini, Micro Design Guide Rev 4.9 vicorpower.com
Page 20 of 88
Apps. Eng. 800 927.9474 800 735.6200
5. Current Sharing In Power Arrays
Whenever power supplies or converters are operated in
a parallel configuration — whether for higher output
power, fault tolerance, or both — current sharing is an
important consideration. Most current-sharing schemes
employed with power converters involve either artificially
increasing the output impedance of the converter module
or actually sensing each output current, forcing all of the
currents to be equal by feedback control. In a synchronous
current-sharing scheme, however, there is no need for
having a current-sensing or current-measuring device on
each module, nor is there a need to artificially increase the
output impedance, which compromises load regulation.
WHY IS CURRENT SHARING IMPORTANT
Most paralleled power components — transistors, rectifiers,
power conversion modules, offline power supplies — will
not inherently share the load. In the case of power convert-
ers, one or more of the converters will try to assume a
disproportionate or excessive fraction of the load unless
forced current-share 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 deliver
some current, but the load will be shared unequally. Built-in
current limiting may cause all or most converters to deliver
current, but the loading will remain unbalanced, and
potentially cause damage to the converters.
Consider the situation when one module in a two-module
array is providing all of the load. If it fails, the load on the
second module must go from no load to full load, during
which time the output voltage is likely to droop temporarily.
This could result in system problems, including shutdown
or reset. If both modules were sharing the load and one
failed, however, the surviving module would experience a
much less severe transient (one half to full load), and the
output voltage would be likely to experience no more
than a slight momentary droop. The dynamic response
characteristic of all forward converters, resonant or pulse-
width modulated, is degraded when the load is stepped
from zero (no load) where the output inductor current is
discontinuous.
In the same two-module array example, the module
carrying all of the load is also generating all of the heat,
resulting in a much lower mean time between failure
(MTBF) for that module. An often-quoted rule of thumb
says that for each 10°C increase in operating
temperature, average component life is cut in half. In a
current-sharing system, all of the converters or supplies
run at a lower temperature than some modules would in
a system without current sharing. As a result, all of the
modules age equally.
Current sharing, then, is important because it improves
system performance; it minimizes transient / dynamic
response and thermal problems and improves reliability. It
is an essential ingredient in most systems that use multiple
power supplies or converters for higher output power or
for fault tolerance.
CURRENT-SHARING IN POWER EXPANSION ARRAYS
When parallel supplies or converters are used to increase
power, current sharing is achieved by a number of
approaches. One scheme simply adds resistance in series
with the load. A more practical variant of that is the
“droop-share” method, which actively causes the output
voltage to drop in response to increasing load. The two
most commonly used approaches to paralleling converters
for power expansion are the driver / booster or master /
slave arrays and analog current-share control. They appear
to be similar, but the implementation of each is quite
different. Driver / booster arrays usually contain one
intelligent module or driver, and one or more power-train-
only modules or boosters. Analog current-share control
involves paralleling two or more identical modules, each
containing intelligence.
Droop Share. The droop-share method, shown in Figure
5–1, increases the output impedance to force the currents
to be equal. It is accomplished by an error signal, which is
interjected into the control loop of the converter causing
the output voltage to operate as a function of load
current. As load current increases, output voltage
decreases. All of the modules will have approximately the
same amount of current because they are all being
summed into one node. If one supply is delivering more
current than another supply, its output voltage will be
forced down a little so that it will be delivering equal
current for an equal voltage out of that summing node.
Figure 5–1 illustrates a simple implementation of this
scheme where the voltage dropped across the ORing
diode, being proportional to current, is used to adjust the
output voltage of the associated converter.
Droop share has advantages and disadvantages. One of
the advantages is that it can work with any topology. It is
also fairly simple and inexpensive to implement. A major
drawback, though, is that it requires that the current be
sensed. A current-sensing device is needed in each of the
converters or power supplies. In addition, a small penalty
is paid in load regulation, although in many applications
this is not an issue.