Evaluation Engineering - 25

specifications. A is rated at 50 amps, at
an operating voltage range from 5~50 V,
minimum 2 V, with a power rating of 1,000
W. B is a 70-A, 70-V, 10~70 V (5 V min)
device rated at 1,000 W. At first glance,
B looks better than A. The figures below
illustrate the operating area of each electronic load (Figures 2 & 3).
Operating areas are traced by the intersection of the upper limits of power, voltage, and current according to the specifications. Green lines show upper voltage
limits. Blue lines show upper voltage and
current limited by the power rating. Red
lines show upper current limits. Purple
lines show upper current limits in the
area outside of the voltage specifications.
Yellow lines show upper current limits in
ranges of the minimum operating voltage
(Figure 4).
It is important to know what operating
area will be needed for your application.
Choosing an electronic load with a capacity that is not much larger than what your
capacity requires may save you from having to pay unnecessary costs. Let us compare the operating areas of Load A and B.
In the range of 30-40V, neither load has
the advantage, because both loads have
the same power rating. However, load A
may be a better choice than B if we need
to use lower voltages, because load A has
a larger current range for lower voltages,
shown by the yellow line. Even though
more capacity might be appealing it often
comes with more cost. If our application

will not require more capacity than load
A offers, it is unlikely that load B's larger
capacity will offer any advantages. In addition, the higher capacity of load B could
result in less resolution and accuracy in
measurements and settings.

user manuals are helpful to understand
the basics of a function, but they cannot
explain all the limitations you may experience in practice. If possible, request to
demo the load before purchasing.

Power Consumption Methods

Other specifications of electronic loads
we need to consider before purchasing
are the slew rate, CR response, and CV
response. With regards to the slew rate,
what often concerns us is the difference
between the ideal slew rate and the actual slew rate. When electronic load suppliers list specifications in their catalog
or specifications manual the slew rate
value is often written as an ideal value.
When we use the load for our application, we may find that it is not the same
as the specification. The main reason for
this is that the slew rate is not a constant
value but changes depending on the current amplitude. For a larger current we
can expect a faster slew rate. If an application requires a smaller current amplitude the slew rate will be slower than
ideal value given in the specifications.

As mentioned earlier, electronic loads
employ two types of power consumption
methods, each with their advantages and
disadvantages. The linear regulator type
tends to have a faster response, because
it employs a linear amplifier, which has a
faster response than the switching power
devices used in regenerative types. The
absence of switching devices also leads
to linear regulators having lower noise.
Another advantage of a linear electronic
load is the ability to use common singlephase 120/240 Vac as input power, even
for large-capacity loads (20kW or higher).
Regenerative types have the advantage
of being more efficient and operating at
lower temperatures. The regenerative
type uses switching elements to redirect
power back to the grid, offering better
efficiency but also slower response, with
introduced noise. For the regenerative
type to put energy back on to the grid,
they require 3 phase and higher-voltage
AC connections, which may be a limited
resource in a lab.
The regenerative type also operates at
a lower temperature than the linear type
because energy isn't dissipated as heat,
saving on costs from air conditioning in
the lab. When choosing between the different types of power consumption, one
should also consider the facilities available in the lab, as well as energy costs
and application requirements.

Functionality

Figure 4: The operating area of both loads are
superimposed for a comparison of the respective
advantages and disadvantages of each load.

As mentioned earlier, the menu of electronic load functions available these days
is long, and it's not possible to explain
them in detail here. It is good practice
to read manufacturer datasheets to find
the functions most relevant to your application. If your application requires an
electronic load with special functions, it is
recommended you check with the manufacturer to be sure the needs of your application will be satisfied. Datasheets and

Response

Figure 5: The ideal rise time, often used in the
specification manual, can differ greatly from the
actual rise time. It is important to test the rise
time at the voltage and current settings in the
application.

Figure 5 shows an example, using an
electronic load with a 100 µs rise time
specification when the CC setting is
100% of the current rating. Ideally, the
rise time is 100 µs when the setting value
is 100%, and the rise time is 1us when
the setting value is 1%. But in reality, the
slew rate does not change linearly with
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Evaluation Engineering

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