Selecting an Electronic Load for Power Supply DC to DC Converter Testing

Selecting an Electronic Load for Power Supply DC to DC Converter Testing
Contact: Steve Grodt
Marketing Manager
Phone: (949) 600-6400
Fax: (949) 600-6401
25612 Commercentre Drive
Lake Forest, CA 92630-8813
www.chromausa.com
Selecting an Electronic Load for Power Supply & DC to DC
Converter Testing
By Martin O’Donnell
Director of Business Development - North America, Europe
Having been around since the early 1960’s, electronic loads evolved out of a
necessity to emulate real world applications in power supply testing as well as batteries and
now fuel cells. They began using Bi-Polar Transistors and have now settled in utilizing FET
Technology to dissipate the power in the form of heat.
When selecting a load that best fits an application, the first and most important task
is to define the UUT (unit under test) and how it needs to be tested. Specific questions may
include:

What are the lowest voltage / highest current operating points? (Cable drop should
be considered)

What specific measurements are needed? (RMS measurements; digitize V&I, etc.)

One UUT or many? If many, true master/slave capability is critical.

Are highly accurate loads at both low and high current settings needed? If so, three
current ranges are necessary.

Are minimizing cable drops and series inductance critical? Front panel connections
are then needed for accurate loading.

Is a system that can be reconfigured quickly for changing applications needed? Front
panel modularity is critical to maintainability and configuration.
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Answering these questions will ensure that when the product is placed in the field, its
quality and reliability meets the required standards. Testing considerations include: Output
Regulation, Noise Measurements, High Line and Low Line AC, Transient Measurement and
Protection Circuit Operation.
Once it is decided which tests are required, a load may be selected. The four main
modes of operation for an electronic load are Constant Current, Constant Resistance,
Constant Voltage and Constant Power (fig 1). Most manufacturers also include specialty
modes for unique testing requirements. One important note to remember is that no matter
what mode the electronic load is in, it is always controlling the current. Constant Current
mode allows the test engineer to set a current level that the load will draw regardless of any
change in voltage. Constant Resistance mode allows the test engineer to set a resistance
value and the load will adjust the current draw inversely to compensate for any change in
voltage in an attempt to keep the “resistance” value at a constant (R=V/I). Constant
Voltage mode allows the test engineer to set a voltage that he wants to remain constant
and the load will sink the necessary current in an attempt to keep the voltage at the set
level. Constant Power mode allows the test engineer to set a wattage level that the test
requires and will adjust the current draw proportionately to compensate for any change in
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voltage (P=IV). This will all hold true unless something occurs, triggering one of the
protection modes.
Specialty features such as Dynamic Loading, Load Profiling, Constant Impedance,
Short Circuit and Low Voltage Operation may be required for unique load emulation.
Dynamic loading includes the ability to pulse load the power supply in an attempt emulate
the actual load. In Dynamic Loading, the parameters of your load profile may be
manipulated to get as close as possible to the actual load profile required by the device the
power supply is powering (Fig 2). Adjustable parameters may include: Slew Rate,
Frequency, Duty Cycle, Baseline Current, Amplitude and Triple Step Values.
The limitation of most dynamic load profile schemes is the inability to generate
anything other than a square wave. If the UUT requires a load profile that is more than an
ON/OFF square wave type, then the load chosen will need to include a 0-10V analog input,
which allows the test engineer to input an analog representation of his waveform at the
external modulation input. The load will then track this input and adjust the current draw to
the corresponding value. For example: The full scale of the load selected is 300A. The
external modulation’s input is rated for 0-10V. The 0-10V waveform put across this input
will have a resolution of 30A per volt (fig 3). For both Dynamic Loading and Load Profiling,
the load should have a bandwidth between 10KHz and 20KHz.
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Constant Impedance has been developed by Chroma and made available recently in
the 63600 series of modular loads. This mode provides the ability to simulate the
impedance of the actual load during current changes and startup. Some power supplies
react differently on initial turn on when impedance is present. This cannot be effectively
emulated by any of the four current modes of operation. The Constant Impedance (CZ)
mode allows the test engineers to set the capacitance, inductance and resistance values of
the actual load in order to test real world transients (fig 4).
The Short Circuit function will test the UUT’s current limit function by driving it to
beyond its maximum current rating. Measurement can be made in both the current limit
operation and the current trip level.
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Most electronic loads cannot provide full scale current draw at very low voltages.
Typically FET loads can provide maximum current draw at 10% of the rated voltage. For
example: The loads full-scale voltage is 100V and the full scale current is 100A. Typically,
this load can provide 100A of current draw at 1V (1% of FS). As the voltage potential
across the load is reduced below 1V, the current draw will be reduced exponentially all the
way down to 0V. Keep in mind that this is the voltage potential at the load, not the UUT.
Ultra low voltage testing is more common in fuel cell and battery testing; however, may still
be required for certain power supply or DC to DC converter applications.
The two cooling methods available are air and water. The air-cooled loads tend to be
more robust and easier built; however, the air-cooled loads are more expensive than the
water-cooled. One key issue regarding the cost of water-cooled loads that is frequently
overlooked is the cost of the chiller or chilling system. Typically the cost differential is
negligible until you reach about 30kW, unless of course your facility already has chilled
water.
Air-cooled loads are portable and can be shared between departments if needed,
while water-cooled loads are tied to a cooling system and cannot be moved without altering
the plumbing. At many companies, installing water-cooling also involves the intervention of
the facility manager.
Condensation is a design problem that has not been completely eliminated from
water-cooled systems. Many water-cooled load failures are attributed to condensation
forming on the components and shorting out the power section. This type of failure can be
catastrophic leaving your load irreparable. Water control valves and water servo’s attempt
to prevent the chilled water from flowing when the load is not hot; however, these designs
aren’t fool proof.
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Protection Features are essential to protect not only the electronic load, but also the
power supply (UUT). Protection features may include: Over current (OC), Overvoltage
(OV), Over power (OP), Under voltage (UV), Oscillation Control, Over temperature (OT) and
Reverse Current Protection (RCP).
Many test engineers utilize complete systems such as Chroma’s C8000 for their
power supply testing requirements. These systems provide a comprehensive way to
emulate the real world applications and collect essential data. These systems include a wide
range of hardware such as Oscilloscopes, AC and DC Sources, Electronic Loads, Power
Meters, Noise analyzers and DMM’s. The hardware married with flexible software (PowerPro
III) can provide for years of testing, which will grow with the end users products. These
systems can be utilized in both engineering (design evaluation) and production
environments (go/no go testing), which allows the R&D engineers and manufacturing floors
to coordinate their efforts.
***End***
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