In Search of the Zero-Ohm Load
by Tom Lecklider, Senior Technical Editor
High-wattage resistors often are used as test
loads for power transformers. Resistors don’t introduce phase
shifts, and it’s easy to check that a transformer’s voltage
regulation remains within spec at different rms currents. The
effect of various primary tap and secondary loading combinations
also can be measured. On the other hand, actual loads don’t
always have a constant resistance (CR) characteristic.
Many switching power supplies present a
constant power (CP) load to the AC input. DC power supply
outputs are intended to provide a constant voltage (CV) or
constant current (CC) depending on the mode selected. To
simulate anything other than simple CR, you need an electronic
load.
Both AC and DC electronic loads are
commercially available with DC loads being more prevalent.
Either type can emulate a short circuit as well as the CC, CV,
CR, and CP modes of operation. According to Adrian Butoi,
western regional manager at NH Research, AC loads are used for
test applications that require linear or nonlinear AC loading
with power and crest-factor control. In addition to the five
basic modes of operation, the company’s Model 4600 AC Load also
provides unity power-factor loading and a complex nonlinear
waveform mode.
Built-in measurements include frequency,
voltage, peak voltage, current, peak current, crest factor,
apparent power, true power, peak power, reactive power, power
factor, and resistance. DC load measurements compose a subset of
this list that is not related to reactance.
Cliff Nazelli, the managing director of
marketing and sales at PPM Instruments, described a couple of
typical applications. In one example, vehicle electrical-system
power-distribution module testing must simulate various load
conditions. While the module’s durability and temperature rise
are monitored, the load current is switched on and off. In
another test, fuel-cell impedance is measured by modulating the
DC load current. The impedance is determined from the current
and voltage amplitudes and their phase relationship.
Isolation also is critical in many cases,
especially those that involve off-ground differential voltage
sources. Multitap battery load testing is an application that
requires this capability.
DC Load Basics
Typically, MOSFETs are used as the
dissipating devices in a DC load. These semiconductors have ON
resistance much less than 100 mΩ,
and several are operated in parallel to achieve the needed power
rating. Of course, paralleling devices also reduces the
resistance the load presents.
ON resistance is higher for high-voltage
MOSFETS than for low-voltage devices, so DC loads rated for 500
V generally will have higher resistance than 50-V loads even if
the same number of MOSFETS is used. Figure 1 shows this
effect for a 5-kW PPM 600-V load (dark green curve) compared to
a 5-kW 60-V load (red curve). The initial slope of the PPM
Modular Electronic Load (Mel) 5000-200-600 is 30 mΩ
compared to the 1-mΩ
resistance of the Mel 5000-600-60 even though there is only a
3:1 ratio relating their maximum currents. The resistances of
the lower wattage Chroma loads range from 5 mΩ
to 25 mΩ.
Figure 1. Safe Operating Areas for Various DC Loads
Source: Chroma Systems Solutions and PPM Instruments
All electronic loads have a safe operating
area (SOA) limited by voltage, current, and power. These areas
are indicated in Figure 1 for the PPM Mel 5000-600-60. As the
input current increases, the voltage across the load also
increases because of the load’s finite resistance. At the
maximum current, higher voltages can be supported but only to
the maximum voltage and power ratings.
On a graph with linear X- and Y-axis scaling,
the maximum power curve is a hyperbola. The load voltage can
continue to increase to the maximum voltage limit as long as the
current is low enough that the maximum power limit isn’t
exceeded. As the figure shows, the SOAs of loads having the same
power limit are bounded by the same hyperbolic curve although it
may be intersected at different places by the voltage and
current limits.
It’s also important to note the lack of
standardization regarding load specification in datasheets. The
NH Research Model 4750 DC Load and PPM Mel Series show the SOA
on a graph with voltage plotted vertically and current
horizontally on a log-log grid. Chroma’s 63600 Series datasheet
plots this information with the same axes but on a linear grid.
Several manufacturers’ graphs showing
low-voltage characteristics generally have linear grids with
current plotted vertically and voltage horizontally. Of course,
depending on the DUT, current or voltage could be the
independent variable, so it really doesn’t matter how the graphs
are drawn as long as you understand what they mean.
Finally, model numbers may include power,
voltage, and current limits but not always in the same order.
PPM’s Mel Series lists power, current, and voltage: Model
5000-200-600 is a 5-kW load with 200-A and 600-V maximum limits.
Chroma’s Model 63630-80-60 is a 300-W unit with 80 V and 60-A
capabilities.
DC Load Selection
You must choose a load with an SOA sufficient
to handle the maximum current, voltage, and power expected from
the DUT. The actual combination of current and voltage can lie
anywhere within the SOA. Nevertheless, the trend in
semiconductors is toward low voltage and high current so
supporting this combination often is a major consideration.
Jim Dougherty, senior engineer at Chroma
Systems Solutions, explained, "Up to a maximum current limit, a
DC load presents a constant minimum resistance. For example, if
a load can support a 10-A current and has a 10-mΩ
resistance, the DUT output voltage must be at least 100 mV even
if the connections and wiring were perfect. Taking into account
the finite resistance of the connections and wiring further
increases the minimum DUT output voltage required for the load
to sink the full current. Of course, the load can be programmed
to represent a higher resistance but you cannot get less than
the minimum.
"Suppose you have designed and characterized
your DUT interconnect and cable resistance and found that
RSeries = 2 mΩ.
Further, assume you need to support 0.5 V @ 60 A. To do this,"
he continued, "the DC load must represent a resistance RLoad
<(0.5/60)-0.002 or RLoad <~6 mΩ.
The sloping lines in Figure 2 show the resistance
associated with three models of Chroma 63600 Series Loads and
correspond to the area circled in Figure 1. As expected, the
higher the output current rating, the lower the resistance. And,
loads can be operated in parallel to achieve higher current
capacity as well as lower resistance."
Figure 2. Low-Voltage Resistance Characteristics of DC Loads
Courtesy of Chroma Systems Solutions
Zero-Volt Operation
What if you need to sink the full rated
current but the voltage drop associated with the load resistance
and wiring is large compared to the DUT output voltage? Adding a
boost supply in series with the load effectively increases the
DUT output voltage and makes possible even zero-volt full
current loading.
However, Mr. Dougherty cautioned, "Boost supplies
should be considered only as a last resort. The effective power
of the load that otherwise would be available to the DUT is
reduced; you no longer can perform transient tests; noise from
the supply affects ripple and noise measurements; and complexity
and cost are increased."
Commenting on his company’s true zero-volt
PLZ-4WA Series of DC loads, Takuya Takeda, vice president of
Kikusui America, said, "The PLZ-4WA employs a bias supply
installed directly in the electronic load. It supports true
zero-volt operation by stepping up the input voltage and
applying a load voltage adequate to allow the internal current
source to operate correctly.
"A switched-mode power supply is small enough
to fit inside the load," he continued. "Typically, a
switched-mode supply can create noise issues, but that is not
the case with this bias supply. It has been designed using
zero-volt switching technology and other special techniques to
reduce noise."
An extensive feature set has developed around
the basic DC load function to address a wide range of
applications. Four-wire Kelvin connections ensure that the DUT
terminal voltage is used in power calculations, not the load
voltage that is reduced by wiring and connection IR drops. Also,
because many tests require switching the load on and off to
stimulate DUT transient response, this aspect of DC load design
has become very sophisticated.
Transient Response
A DC load’s internal wiring and terminations
must present a low impedance, not just a low resistance. Mr. Nazelli explained that PPM uses a heavy-duty laminated copper
bus structure internally in conjunction with a proprietary FET
circuit board layout to ensure the lowest possible impedance. In
particular, the laminated copper bus minimizes the impedance
increase caused by skin effect that otherwise would occur at
high frequencies.
The depth at which AC current density has
been reduced to 37% of its value at the conductor surface is
given by
where: δ = skin effect depth
ρ = conductor resistivity
ω = 2πf
µ = conductor absolute magnetic permeability
For a copper conductor, skin depth varies
from more than 8 mm at 60 Hz to only 66 µm at 1 MHz. Engineers
have used Litz wire for many years to minimize the resistance
increase caused by skin effect. Litz wire is stranded, but each
strand also is insulated. A laminated bus achieves a similar
result in a form that may be more easily manufactured and
terminated, especially for high current levels.
The PPM Mel units are specified with a 15-µs
to 20-ms rise time, selectable in 36 discrete steps, and a DC to
10-kHz frequency response. A 600-A load has a maximum slew rate
of 600/15 or 40 A/µs. When loads are connected in parallel, the
slew rates add. On the other hand, the highest practical slew
rate is limited by the inductance of the wiring needed to
connect the loads in parallel.
Kikusui’s Model PLZ1004W is a 1-kW load with
a maximum current rating of 200 A and a slew rate of 16 A/µs.
For the PLZ-4W Series, the slew rate is variable over a 100:1
speed ratio and guaranteed to be accurate to within 10% for
current within 2% to 100% of rated value. This series also
supports frequency range selection.
According to Kikusui’s Mr. Takeda, "Dynamic
response only is required for transient response tests of power
supplies. A wide bandwidth isn’t necessary for static tests such
as load variation tests and foldback characteristic tests.
Excess bandwidth affects load stability so the PLZ-4W/4WA load
includes selectable bandwidth, and it can be optimized to match
the kind of test and test condition."
NH Research’s Mr. Butoi explained that
because the load manufacturer cannot control DUT and cable
inductance, the most straightforward solution to mitigate
voltage spikes, ringing, and oscillation is to allow
load-current slew-rate programmability. Usually, slowing this
slew rate eliminates the problems.
PPM provides two types of filtering as
discussed by Mr. Nazelli, "The architecture of the Mel employs a
control board with programmable loop response to vary the
control-loop speed where you need to adjust rise/fall times for
pulse tests and stimulus/response testing. This is one type of
filtering. Separately, filtering is used on the FET circuit
board assemblies to control the feedback loop response of the
power-dissipating devices. The operator can adjust the loops to
shape the response either to further control rise/fall times or
eliminate oscillations induced from external reactive
components."
Power to the Load
Regardless of the other characteristics a DC
load may have, it must dissipate power—sometimes a lot of power.
The products in PPM’s Mel Series can handle 1 kW to 5 kW, and
master-slave configurations up to 80 kW are standard. Eight
models in NH Research’s 4700 Series range in capacity from 1 kW
to 36 kW. Chroma’s Series 63200 High Power DC Loads are
available in sizes from 2.6 kW to 15.6 kW. These three load
series are air-cooled.
Most manufacturers of heavy-duty loads
support paralleling for higher power handling. Loads feature
individual device protection against over-temperature,
over-voltage, and over-current conditions and further ensure
performance through active current balancing. For larger power
ratings, master-slave systems usually are rack mounted and up to
6 ft high.
One alternative to a big air-cooled unit is a
water-cooled unit. AMREL’s PLW Series handles up to 250 kW, and
several versions of the 36-kW model are available in a 4U-high x
27.5" deep rack-mount size. As a comparison, a 5-kW AMREL Series
PLA Air-Cooled Load is approximately the same size.
Another solution appropriate for high-power
applications is Kikusui’s Model PLZ6000R Regenerative DC
Electronic Load. The basic unit acts as a 6-kW load although
only about 15% of this is dissipated. The rest of the power is
regenerated as a synchronous AC current fed back into the AC
mains. Up to five units can be combined in a master-slave system
to provide a 30-kW capacity.
Several ranges of benchtop DC loads also are
available with ratings to a few hundred watts. Three models from
Chroma’s 63600 Series are shown in Figures 1 and 2. B&K
Precision’s 150-W Model 8540 handles up to 60 V and 30 A in the
CC, CR, and CV modes with current, voltage, and power
measurements presented on an integral display. Model 8510 has a
600-W capacity with 120-V and 120-A limits. It, too, provides an
integral display and includes a CP mode as well as battery test
capability.
Kikusui’s PLZ-4W Series features 165-W,
300-W, 660-W, and 1-kW models. In addition to the basic
products, the 165-WA and 660-WA models are available with a
built-in bias supply and support true zero-volt operation.
The 300-W Model LD300 DC Load manufactured by
Thurlby Thandar Instruments and available in the United States
from Saelig has 80-V and 80-A maximum ratings. It supports the
CC, CV, CR, and CP modes and provides a transient generator, a
variable slew rate, soft start, and a current monitor output.
Controlling the Power
In addition to a load’s fundamental
capabilities, the extra features it offers can be important
depending on the types of tests you need to run. Kikusui’s
PLZ-4W Series includes soft start, a variable slew rate, a
switching function, a preset memory function, 100 setup
memories, and a sequence function.
B&K Precision’s 8500 Series Loads also
support battery testing by measuring total battery discharge in
amp-hours. Jeremy Lo, an application engineer at the company,
said, "Software is available to control the load for this test.
It plots battery discharge curves in real time as well as gives
you the option to export raw data in text or Excel format for
further analysis. The software also can be used to monitor and
plot power, voltage, and current levels at the load inputs."
In NH Research’s Model 4700 DC Electronic
Load, an auto mode provides glitchless switching among the CR,
CC, CV, and CP limits. Further, you can programmatically control
the mode of operation and its duration via a 100-step
customizable macro with 10-µs timing resolution.
PPM’s Mel Loads have several means of
control. RS-422 and USB 2.0 ports are standard with both GPIB
and Ethernet optionally available. You can log into a load’s IP
address to perform remote control and diagnostics. According to
Mr. Nazelli, "This also enables PPM to send feature improvements
without hardware intervention and without requiring you to
return units to PPM. In addition, you can add modules in the
field to upgrade a load. The load automatically reconfigures
itself to its new capabilities."
Chroma’s Model 63472 High Slew Rate DC Load
incorporates Intel’s power test tool (PTT), which can simulate
microprocessor load changes of up to 150 A at a 1,000-A/µs slew
rate. Because the PTT is small enough to fit into a
microprocessor socket, it cannot dissipate the required power
without significantly changing its temperature and operating
characteristics. The 63472 provides measurement hardware and
over-current and over-voltage protection as well as the
automatic calibration required to ensure test-result accuracy.
Unless you use a device such as the PTT,
there’s no way to connect a high slew-rate load that will not
introduce significant errors. The PTT mimics the load presented
by a microprocessor, which may have 100 or more power and ground
pins. The slew rate at each pin is only a few amps/µs, and most
of the transient current is provided by local capacitors. With
the PTT, you are testing the capability of the power supply in
combination with local capacitors to cope with the overall 150-A
changes and 1,000-A/µs slew rate.
Summary
Many models of DC electronic loads are
available, some with very specialized capabilities. Determining
the load that will best fit your test requirements starts with a
list of the specifications you must have. Power dissipation,
maximum current and voltage, and the minimum resistance the load
presents are key to most applications. So, too, are the modes in
which you will operate the load and how it changes from one to
another.
| FOR MORE INFORMATION |
|
Click below |
| AMREL Power Products |
PLW Water-Cooled DC Loads |
Click here |
| B&K Precision |
Series 8500 DC Loads |
Click here |
| Chroma Systems Solutions |
Series 63600 DC Loads |
Click here |
| Kikusui |
PLZ6000R Regenerative DC Load |
Click here |
| NH Research |
Model 4700 DC Load |
Click here |
| PPM Instruments |
Mel Series DC Loads |
Click here |
| Saelig |
Thurlby Thandar LD300 |
Click here |