Vicor Micro Family of DC-DC Converter - Manual (Page 23)

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 22 of 88
Apps. Eng. 800 927.9474 800 735.6200
5. Current Sharing In Power Arrays
CURRENT SHARING IN FAULT TOLERANT ARRAYS
Current sharing is an essential element in fault-tolerant
arrays, and regardless of the approach, there is an
inherent additional cost incurred by the addition of at
least one redundant converter or supply.
Most applications today that require fault tolerance or
redundancy also require Hot-Swap capability to ensure
continuous system operation. Hot swappable cards must
be designed so that the operator cannot come in contact
with dangerous potentials, currents or thermal hazards. It
is also essential that when a module fails, the failure is
detected and identified by an alarm or notice to provide
service. A Hot-Swap system must ensure that during swap
out there is minimal disturbance of the power bus.
Specifically, the affected voltage bus must not drop
enough to cause errors in the system, either on the input
bus or the output bus.
N+1 Redundancy. A power supply failure can cripple an
entire system, so a redundant converter or supply can be
added to ensure that, in the event of a failure, the system
will continue to operate. Adding an extra module (N+1) to
a group of paralleled modules will significantly increase
reliability with only a modest increase in cost.
How redundant converters are implemented is determined
in part by the available space and cost requirements. Two
500 W Maxi modules, for example, could be used to
provide a 1 kW output with an additional 500 W module
for 2+1 redundancy a total of 1.5 kW in a volume of
about 16.5 in
3 (270 cm 3 ). Four 200 W half-size modules
might be used instead with a fifth 200 W module for
4+1 redundancy, a total of 1 kW and 14 in
3 (229 cm 3 ).
Although the second solution uses less space, it increases
the accumulated failure rate because it employs more
converters, more ORing diodes, more monitoring circuitry,
and more assembly.
ORing diodes may be inserted in series with the +Output
of each module in a N+1 array to provide output fault
tolerance (Figure 5–1). They are important in a redundant
power system to maintain fault isolation. Without them,
a short-circuit failure in the output of one converter could
bring down the entire array. As well, fusing the input of
each converter prevents a converter input short from
compromising the entire array.
ORing diodes, however, add losses to the power system,
reducing overall efficiency (and, potentially, decreasing
reliability). To ameliorate this negative effect on efficiency,
ORing diodes should run hot, thereby reducing forward
voltage drop and improving system efficiency. Reverse
leakage current will be an issue only if the output of a
converter shorts and the diode is reverse biased. This is an
important consideration with regard to operating temperature.
Current sharing, required to ensure system reliability, can
be implemented by a multiplicity of methods. Figure 5–1,
shown earlier as an example of the droop-share method,
is also an example of N+1 redundancy using ORing diodes.
Synchronous Current Sharing. Synchronous current
sharing is available with Maxi, Mini, Micro converters —
converters that use the zero-current-switching and zero-
voltage-switching topology. Each module has the capabili-
ty to assume control of the array, that is, they constitute a
democratic array. The module that assumes command
transmits a pulse on the parallel bus to which all other
modules on the bus synchronize.
The converters use this pulse as a current-sharing signal
for power expansion and fault-tolerant applications. The
pulsed signal on the parallel bus simplifies current-sharing
control by synchronizing the high-frequency switching of
each converter. The parallel pin is a bi-directional port on
each module used to transmit and receive information
between modules. If the lead module relinquishes control,
another module in the array will transparently take
command with little or no perturbation of the output bus.
A pulsed signal gives designers the option to use capaci-
tors (Figure 5–2) or transformers between parallel pins,
providing DC-blocked coupling. Such coupling
prevents certain failure modes internal to a single module
from affecting the other modules in the array, thus
providing an increased level of fault tolerance.
Use of a current-share bus transformer (Figure 5–3)
enables arrays of Maxi, Mini, Micro converters to current
share when they are widely separated or operated from
independent sources. Since the current-share signal is a
pulsed signal, it can be transformer coupled. Transformer
coupling this pulsed signal provides a high level of
common-mode noise immunity while maintaining SELV
isolation from the primary source. This is especially
useful when board-to-board load sharing is required
in redundant applications.
Synchronous current sharing eliminates the need for
current-sensing or current-measuring devices on each
module, and load regulation is not compromised.
Additional advantages of the synchronous current sharing
architecture includes excellent transient response, “no
loop within a loop” control problems, and, a high degree
of immunity from system noise. The availability of synchro-
nous current sharing in democratically controlled arrays
offers power architects new opportunities to achieve
simple, non-dissipative current-share control. It provides
options that simplify current sharing and eliminates the
tradeoffs — such as the need to sense the current from
each individual module and adjust each control voltage —
as is the case with other current-sharing methods.