The quick answer to the above question is: because you will
likely damage at least one of the loads with excessive voltage! For the longer
answer, read on….
[By the way…this is a milestone post for Watt’s Up? since it
is post # 100, so thank you to our readers…and Happy Thanksgiving to those in
the US or celebrating elsewhere!]
I’ll start with a brief explanation of what an electronic
load is, and what it is used for. I am specifically talking about DC electronic
loads here. A DC electronic load is a two-terminal electrical instrument that
draws power from a DC source. Loads are used to test DC sources. Any device
that has a source of DC output power, such as a DC power supply, a DC-to-DC
converter, a battery, a fuel cell, or a solar panel, can have power drawn from
it with an electronic load. Click here to see Agilent’s DC electronic loads.
For example, to test a fixed-output DC power supply that is
rated for 20 V, 5 A, 100 W, you would connect the power supply output to an
electronic load with ratings that are equal to or greater than the power supply
ratings and that can draw a constant current from the power supply. Since the
power supply is regulating the voltage (20 V), the load must regulate the
current it draws from the power supply (up to 5 A). If your DC power supply is
a constant current source, the load must be capable of drawing power while
regulating voltage. You can set most electronic loads to draw power by
regulating either constant voltage (CV) or constant current (CC). You can also
set many electronic loads to regulate constant resistance (CR) across their
input terminals, and some can regulate constant power (CP).
If the power supply to be tested has a higher output voltage
than a single electronic load can handle, you may be tempted to put multiple
load inputs in series to accommodate the higher voltage. After all, you can do
this will power supply outputs to get higher voltage (click here)….why not with
loads?
Putting electronic loads in series can cause one of the load
inputs to be exposed to a voltage beyond its capabilities that could result in
damage to the load. You are putting loads in series because a single load does
not have a high enough voltage rating to handle the voltage of your DC power
source. But since one of the load inputs could become a low impedance (nearly a
short circuit) during test, all of the voltage from your DC source could appear
across the other load input in series. There are several scenarios that can
result in this destructive situation. To understand these scenarios, you first
have to understand how an electronic load works.
Loads work by controlling the conduction of FETs across
their input terminals. The control is realized by using a feedback loop to
adjust a measured level (such as the input current) so that it equals a
reference level (such as the set current). My colleague, Ed Brorein, posted
about this topic last year (click here).
When you put multiple electronic loads in series to
accommodate higher voltage, one problem scenario occurs when you set both loads
to operate in CC mode. You set the same current on both loads. The exact same
current flows through both loads (see figure below), but due to small errors in
the accuracy of the settings, the real set values will never be exactly equal.
Therefore, one of the loads will be trying to draw a higher current (Load 2 in
the figure) than the other load (Load 1 in the figure). Since Load 1 will limit
the current at the lower value (9.99 A in this example), Load 2 can never
attain its real set point (10.01 A in this example). So its internal feedback
loop continues to tell the FETs to conduct more and more current until the FETs
are fully on looking nearly like a short circuit. This results in nearly all of
the power supply voltage appearing across the Load 1 input which can damage it.
If you operate one load input in CC and one in CV, at first
this looks like it will result in a stable operating point. However you have to
think about how you get to that stable operating point. If you set the loads
first before you connect the voltage, before the voltage is applied, the CC
load is not satisfied (no current is flowing) so it goes to a short and the CV
load is also not satisfied (no voltage is present) so it goes to an open. When
the test voltage is applied, all of the voltage initially appears across the
open CV load and can damage it. There are other procedures to follow that could
temporarily result in a stable operating point (such as slowly increasing the
test voltage if you have that ability), but if any fault condition occurs in
any of the loads, they try to protect themselves by either turning the FETs on
hard (a short) or opening the FETs. In either case, the large destructive
voltage will appear across one of the loads in the series connection resulting
in damage.
One of my colleagues, Bob Zollo, wrote an article entitled
“Why Can’t You Put Electronic Loads In Series To Get More Voltage?” that
appeared in Electronic Design on November 4, 2013. For some additional
information about this topic, click here to read the article.
So you can see that putting loads in series can too easily
result in damage to at least one of the load inputs. I strongly recommend that
you do not do it!
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