With each new generation of processor, the
trend is toward lower voltages, higher
currents, and faster dynamic loads. As a
result, power-system designers are challenged
to provide ever-faster transient response. They also have
to do it using less board area while providing cost-effective and
efficient power-systems solutions that offer the requisite performance.
The question is if power devices can keep up.
Power designers traditionally responded to the need for
fast, dynamic loads by putting very simple energy storage—
one or more capacitors—right at the point of load. This often
addresses the issue of delivering energy very quickly to a device,
but there are tradeoffs. Capacitor technology hasn’t moved forward
as quickly as many other technologies, so a large amount
of real estate is required to accommodate this capacitance.
The reliability of the capacitor, especially when there may be
dozens of them in high-performance applications, is another
concern. Simply adding more parts affects mean time before
failure (MTBF) calculations, decreases available board space
(most often in critical areas of the board), and adds cost.
Capacitor failure could compromise the system.
DESIGN REQUIREMENTS
Often while addressing severe dynamic loading conditions, you
may need to maintain the voltage within a certain regulation
band. Also, you may need to be able to deliver energy instantaneously,
or absorb load dumps, where the current at full load
drops down to no load at all.
Typically, power-supply control loops are limited in bandwidth
to a frequency significantly less than the switching
frequency. Nevertheless, power-conversion manufacturers
have been trying to achieve faster control loops and higher
frequency to achieve better transient response.
One of the first steps in improving the transient response of
“brick” converters was removing most of the internal capacitance
within the converter and locating it at the load. The parasitic
inductance between the converter and the point of load
capacitance could then appear as part of the output inductor.
This allowed a great increase in the closed loop bandwidth
of the converter while permitting it to be some distance away
from the point of load. For some applications, this approach
provided the desired faster dynamic performance, but it was
gained at the expense of significant additional design and
implementation complexity.
A lot of work is going on with digital power conversion right
now, and there eventually could be some significant improvement
in faster dynamic load response. So far, there have been
small increments of improvement.
While some designers are trying to improve control loops
and develop better controllers, the most prevalent way to
improve converter transient response is with multiphase-type
buck converters. Using multiple power trains controlled by a
control chip, the apparent frequency of the combination can
be multiplied, resulting in a number of favorable advantages
over the single power-train approach. Most important, the
frequency is increased without also increasing switching losses,
helping to achieve a faster load transient response.
Bulk energy storage is still needed with this approach, and
more phases add complexity, both in terms of control and more
components. The control device not only manages the phasing
of the individual voltages, it also is needed to ensure current
sharing. The additional components play into the reliability
equation as well. The multiphase solution has survived, at least
in part, because the intense competition has driven costs down.
At this point, multiphase is an acceptable approach.
Rethinking the architecture/topology of power conversion
has produced significantly faster transient response. The Factorized
Power Architecture employs a separate voltage transformation
stage that enables a module with no external control
loop. It’s a fixed-ratio converter. This, and an effective switching
frequency of 3.5 MHz, creates a very powerful platform for
delivering energy very quickly to a dynamic load.
WHAT’S NEXT?
The Factorized Power Architecture was a rather significant
leap. Over time, there will be improvements in multiphase
buck converters or other solutions. It’s inevitable. There’ll be
better products. More clever design will go on.
Finally, there must be a practical limit on processing speed.
The single-core microprocessor demanded higher and higher
currents: over 100 A, with talk about well over 150 A. Manufacturers
recognized that microprocessors demanding such high
currents could be at the mercy of fewer power-supply makers
who could achieve that performance and name their price.
So, they rethought the concept and made a conscious effort
to divide and conquer, yielding the birth of dual-core and multicore
processors. When they split the cores up within the processor,
they can split up the power supply as well. Now you may
only need 50 A. Of course, they had to make the cores work
together, and they did! As a result, fast dynamic load response
isn’t quite the problem that it could have been with the singlecore
processor following Moore’s Law.
MICROCHIP • www.microchip.com
MOXIA ENERGY LTD. • www.usbcell.com
TARGUS • www.targus.com