Power Electronics
hand, they are installed directly on the printed circuit board and
manufacturers therefore wish to mount them in an automated
process together with the other components.
DC/DC converters essentially convert the incoming direct current
into a square wave signal of several hundred Hertz, which is then
transferred through a small toroidal transformer and
subsequently rectified and smoothed. There are numerous free
software products available that purport making dimensioning
easy. However, analogue technology is a bit more complicated
than it might appear at first sight, as it deals with switching
transistors, charging capacitors, transformers, inductors and
rectifiers that need to be carefully positioned to each other. The
length of each conducting track, its distance to other such tracks,
to the ground and to the individual components must be carefully
considered to minimise parasitic capacitance and inductance.
None of the readily available circuit diagrams provides this
information. Transformers are tricky devices, as their
performance is determined primarily by the qua lity of the ferrite
in the core and the hysteresis curve section at which it is
operated. If run near its saturation point, the ferrite becomes hot,
losing its magnetic capacity. The fact that most characteristics are
only available for sine waves, while the transformer is actually
operated with a square wave signal, must also be taken into
account. There are a number of alternative circuit topologies that
address these issues, but they cannot be discussed here in
detail.*
The general principles of analogue technology have been known
since the 1950s, but a lot of this knowledge has been lost as
digitisation and "Industry 4.0" became the buzzwords. Even
where engineers with a solid knowledge of analogue technology
are at work, they might be struggling to find an optimised design
that meets the latest standards. Leading manufacturers such as
RECOM estimate that around 75 to 80 percent of the
development time needs to be spent on design optimisation. For
companies that are not specialising in this technology and still
wish to design their own converters, this approach would be
ruinous.
Ready-made modules are more efficient
When considering the efficiency of DC/DC converters, technicians
first of all think about electrical efficiency. With 1W converters, it
is around 85 percent, provided that they are operated at full load.
Specialist manufacturers, however, spend a lot of time and effort
on achieving equally impressive values at the often more
important medium load range. This is normally done by
optimising the design, which is a task that few non-specialist
companies have mastered so far.
Let's have a look at a practical example: A cascade of twelve
discretely designed 2W converters is operated at an average load
of 75 percent. As the efficiency at this point is only 69 percent,
26W are to be provided at the input to obtain 18W at the output.
If this cascade is replaced by R2S converters from RECOM, the
efficiency jumps to 84 percent under the same operating
conditions. While, at first glance, this might not be a huge
difference, the power loss along the cascade is nevertheless
reduced by nearly 60 percent from 8 to 3.4W! Apart from the
obvious advantage of lower power consumption, this brings
additional advantages as regards the overall design lifetime as the
temperature inside the housing is significantly reduced.
At this point, we might have a look at efficiency in a broader
context, focussing initially on power density and module size. The
packing density of resin-moulded modules is generally much
higher that what can be achieved with units that are directly
mounted on the printed circuit board. Modular converters
therefore often only need half the space taken up by a discrete
assembly, which is a major advantage as space tends to be very
limited on circuit boards.
Does certification have anything to do with efficiency? In a strictly
technical sense, the answer is no. From a commercial point of
view, certification, however, is crucial as the use of certified
converters speeds up the certification process for the end
product. This applies especially in cases where the supplier is able
to submit relevant test reports such as the CB reports and the UL
test results in advance. To avoid nasty surprises, these documents
should be requested as early as possible. As a leading
manufacturer in the field, RECOM is going one step further
offering its customers the option to have their product tested at
its in-house EMC lab prior to submission for certification.
Is there a quantity threshold beyond which modular
converters are no longer a commercially viable
option?
Fig. 2: For the design of its converters, RECOM aims at optimising the
efficiency of the modules across a broad load range. This helps reduce
heat loss to a minimum.
There is no simple answer to this question as each case must be
evaluated individually. Taking into account the costs for
development, materials and production, discretely manufactured
converters are around half as expensive as fully assembled
modules.
Let's look at a concrete example: Over five years, 15,000 units of a
diagnostic device are to be sold at a price of CHF 3,000. The
printed circuit board is equipped with three different converters
that can be bought as fully certified units for a combined price of
CHF 8.00. The actual material and production costs for these
converters is CHF 4.00. In other words, the manufacturer could
potentially save around CHF 60,000. What has not yet been taken
into account are, however, the unavoidable costs for
development and testing, in particular the EMC tests. For each
ELE Times | 56 | November, 2016