22-02-2013, 02:39 PM
Distributed Power Systems
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Introduction to Distributed Power
Traditionally, power generation for an electronic
system was assigned a particular location in the
system's structure where a central power supply
would reside, powering all the system's elements
through a network of cables or buses as shown in
Figure IA. The advantages of this approach include
concentrating all the power processing technology
-including thermal I management -into a single box
which could RL then be designed, subcontracted, or
purchased as a stand-alone item.
Advantages of Distributed Power
While not all of the following list of potential
advantages are common to all distributed power
configurations, it is still a list worthy of consideration
during any power system definition phase.
1. Standardized designs: A centralized power
supply almost by definition must be designed
specifically for each new set of requirements. A
goal of distributed power is the availability of
standardized off-the-shelf modules or designs which
could be combined in a variety of ways to meet a
specific application. This has obvious benefits in
development time and engineering costs as well as
the confidence gained from using pre-qualified
power components.
2. Ease of customizing: If unusual requirements
are encountered, it is much easier to modify,
redesign, or replace a smaller power module allocated
to the unique portion of the system than to
redesign a larger central power supply. Customizing
a supply delivering common load voltages is often
as easy as paralleling the required number of
standard power modules needed for a given requirement.
A corollary of this benefit is the ease of
accommodating system growth, or recovering from
an overly optimistic initial estimate of the system's
power needs.
3. Maintainability: With distributed power it is
possible to localize and isolate faults much more
readily and, with properly designed parallel systems,
on-line replacement (hot-swapping) will allow
repairs to be made with a minimum of down time.
Applications for Distributed Power
While we may not often think of power utilities
in the same context with electronic systems. the
most obvious example of distributed power is our
nationwide 60 Hertz power grid. Clearly the problems
of distributing 110Vac power over hundreds of
miles would be insurmountable were it not for the
distributed network of step down transformers to
process the power from much higher transmission
voltage levels to household values.
Smaller examples where power is used at some
distance from its point of generation can be seen in
ships and airplanes where power is distributed at
higher voltage levels and converted at locations
closer to the point of use. These applications. like
the utilities. are typically designed for ac transmissions
where the local power processor involves a 60
or 400Hz transformer. a relatively bulky item. It is
interesting to note that a similar technique was
initially proposed for the Space Station but with the
transmission frequency changed to 20kHz to reduce
the size and weight of the line transformers. While
an interesting concept. this approach suffered
(perhaps fatally) from problems associated with
EMI generation and power factor control.
System Hold-up Time
Typically, most power systems have a requirement
to maintain some intelligence for a specified
period of time after removal of the input supply.
Without a backup power source, this means energy
storage somewhere in the power path. If only a
small and defined portion of the load has the holdup
requirement, it might well be provided with
capacitor storage at the point of use; but more often
the location of hold-up energy will be on or before
the distribution bus. So this may be another important
criteria for defining the bus voltage level.
Since energy stored in a capacitor is proportioned
to the square of the voltage, it is clear that the
higher the voltage, the less capacitance is required
for the same energy storage.
Load Regulators
The options for converting the 48V bus voltage
to useful low voltage levels are too numerous to
give an in-depth description. Clearly, the choices
will be made on the basis of each specific requirement
where performance and efficiency can be
weighed against cost and simplicity with power
level being an additional variable. Some possible
choices which might be considered are the following:
1. A simple buck, or step-down switching
regulator is a choice for non-isolated, single-value
output levels. A typical circuit is shown in Figure
15, implemented with a UC2575HV .([he "HV"
gives this device 63V input capability). Recognize
that the conversion of 48V to 5V means a duty
cycle of approximately 10% which will cause
efficiency to suffer because of higher peak currents.
By way of illustration, when powering a 5 Watt
load, the circuit of Figure 15 has an efficiency of
76% with an input of 12V, 75% at 48V, and 68%
at 6OV.