IEEE Power Electronics Magazine - June 2018 - 15

him did not mean power efficiency
but rather some measure of process
efficiency, the ease with which the
factory produces a useful product. In
this sense, it is perfectly logical that
efficiency would be important if a
low-cost product is a goal. A related
trend, which was noted by one of the
parties at a booth in the APEC exposition hall, is to offer even physically
large resistors (a centimeter or more
in all dimensions) in surface-mounted packages.
According to Morris, one of the
bright spots in the resistor industry is
current sense resistors. These commonly are built to tight resistance
specifications. In recent times, these
have moved down a lot in the range of
available resistances, to as low as
100 μΩ. As switched-mode power
supplies are increasingly produced at
higher output power and at lower output voltage, the current that needs to
be sensed is rising. Also, as the output current increases, the loss in the
sense resistor becomes an increasingly important source of inefficiency. There is a strong incentive to
make the sense resistor have enough
resistance to get the sensed voltage
above the noise floor but no higher-
thus the need for lower resistance
components, which are difficult to
make in a cost-effective manner.
When asked about details of the production process, Morris answered
very politely, but in a way indicating
that follow-up questions were unlikely to provide further enlightenment.
Another growth industry within
the resistor marketplace is resistors
with improved pulse or surge ratings. These are used even in systems
far enough removed from central
station power that lightning surges
are not a concern. This is because
there are other sources of radiated
energy within an interconnected
system of power electronics components that can place a large temporary load on a resistor under some
infrequent conditions. The ability to
survive these occasional events and
remain fully serviceable is a highly
desirable attribute.

Morris also remarked that the
industry does not appear to be making significant advances in stability.
This is apparently due to a lack of
market demand for such resistors
rather than any inherent inability to
produce them. Additionally, precision resistor functions are increasingly being integrated onto silicon
rather than being performed with
discrete resistors.
In summary, it seems that resistors are an evolving technology. They
are getting smaller and less expensive. They are produced to specifications not formerly available. These
trends are being supported by manufacturing knowhow that is not widely
publicized and is certainly protected
as trade secret practices at some
level. Do not anticipate a change in
this marketplace comparable to the
wide-bandgap semiconductor revo-

June 2018

lution, but do expect the resistor
industr y to continue to evolve to
support the changing needs of power
electronics systems built with the
new technology.

About the Author
Tom Keim (tkeim@alum.mit.edu) is a
late-career engineer and a longtime
Member of the IEEE. His specialty is
high-performance electromechanical
systems and the power systems that
drive and control them. He has
worked for a worldwide conglomerate, for a small (50 employees) innovative research and development company, for a major research university,
and for an engineering consulting
company. He has 50 publications and
11 patents and is currently active as an
author, inventor, and consultant.

z IEEE PowEr ElEctronIcs MagazInE

http://www.itg-electronics.com

Table of Contents for the Digital Edition of IEEE Power Electronics Magazine - June 2018