Evaluation Engineering - 26

AUTOMATED TEST

Figure 6: A circuit diagram to show the wiring of an electronic load and power supply as DUT. One
application of electronic loads is to test the impedance of a power supply.

respect to the current setting. Figure 5
shows a representation of slew rate and
current setting as a percent of the max
setting, where the blue line represents
the ideal case and the red line represents
a realistic case.
When the rise and fall time of our
electronic load is important to our application it is important to find out the
limitations of the slew rate within the
current range we will be using the electronic load and when the load is being
used, make sure the appropriate range
setting is being used to get the fastest
slew rate possible.
We also need to understand the importance of slew rate in an application.
It's important to have a fast slew rate
because it helps discover the impedance
that a DUT has. An ideal constant voltage source has zero impedance however,
real constant voltage sources do have an
output impedance, which affects the DUT

transient response. For example, Figure
6 shows the DUT and electronic load
connected.
If the electronic load is switching CC
level, the waveform will be like the figure
below, where the blue line represents the
CC level of the load and the yellow line
represents the DUT output voltage. As
the current rises the voltage level drops
by the inductance of the DUT. Next, the
DUT tries to restore the output voltage
by feedback control and regain the CV
setting. When an electronic load with a
fast rise/fall time is used with large current fluctuations, it allows the user to observe the transient voltage fluctuations
and the associated impedance of the DUT
(Figure 7).
Incidentally, when a large voltage drop
is caused by a DUT's impedance, it can
sometimes cause malfunction of DC electronic load because the voltage will drop
below the operating area of the load and

Figure 7: If an electronic load is used with a
fast rise/fall time, large current variations can be
used to observe the transient voltage fluctuations.
It is important to consider whether the voltage
drops in the DUT will cause the voltage to be
outside the operating area of the electronic load.

Figure 8: In CR mode the current will lag the
voltage due to the feedback loop used by the
electronic load. Contact the manufacturer for an
accurate CR response for the range needed by the
application.

EVALUATION ENGINEERING SEPTEMBER 2020

it will be unable to sink current. If we are
testing a DUT that we suspect to have
voltage drops we should make sure that
it will not be below the operating area of
the electronic load. We can also look for
loads made with built in bias power supplies that are designed to sink current at
low or even 0 V.
In regards to CR response, the load current swing lags the voltage swing because
of the feedback control in the electronic
load. The lag is different for each electronic load. If we need a fast CR response,
we should check the CR response listed
in the manufacturers specifications and
ask suppliers for information on the response within the range we plan to use
(Figure 8).
When considering the CV response, the
most important thing to be careful of is
oscillation. The CV mode is achieved by
using feedback control that includes the
wire's impedance and DUT's impedance.
Since the point at which oscillation occurs differs depending on the test environment, suppliers have a difficult time
accurately listing the response specification in their catalog. However, when we
don't need a fast response, it is possible
to use loop gain by connecting a large
capacitor to the electronic load terminal
to make the response slower and prevent
oscillation.

Expanding Operating Area
Another important thing to understand
when purchasing electronic loads is how
multiple units can be connected to expand the capacity. If you have experience
using power supplies, you may be used to
connecting them in series or parallel to
expand respectively the operating voltage
or current range. It is not as straightforward with electronic loads.
Electronic loads cannot be connected
in series to expand the voltage capacity
because of the methods electronic loads
use to control current to achieve the CC/
CR/CV/CP operating modes. If two loads
are connected in series it will cause erratic operation such as oscillation, corruption, and test failure. If an application
needs a higher voltage it is necessary to
purchase an electronic load with a larger
voltage operating area (Figure 9).

Evaluation Engineering

Table of Contents for the Digital Edition of Evaluation Engineering