Post on 18-Aug-2020
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W H I T E P A P E R
Electronic Load FundamentalsIntroduction to the Polarity Convention of a Power SupplyBefore you learn more about electronic load, let’s begin with the power supply
polarity convention. This basic principle helps you to understand the operation of
an electronic load. Figure 1 shows a diagram of a power supply (a two-terminal
device) with the standard polarity for voltage and current. A standard power
supply typically is a source of power. To source power, current must flow out of
the positive voltage terminal. Most power supplies source energy by providing a
positive output voltage and positive output current. Polarity typically refers to the
polarity of the voltage and not the direction of the current flow. If current flows
into the positive voltage terminal, the power supply is sinking current and is acting
like an electronic load – it is absorbing and dissipating power instead of sourcing
power.
Power Supply and E-Load
If current flows into the positive voltage termi-nal, the power supply is sinking current and is acting like an electronic load – it is absorbing and dissipating power instead of sourcing power
Figure 1. Polarity convention of a power supply
Power Supply +V
+I+
-
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Figure 2. Cartesian Coordinate system
A bipolar power supply operates in all four quadrants. In quadrants 1 and 3, a bipolar
supply is sourcing power — current flows out of the more positive voltage terminal. In
quadrants 2 and 4, a bipolar supply is consuming power — current flows into the more
positive voltage terminal as shown in Figure 2.
Why Do You Need a DC Electronic Load?You have learned from the previous paragraph that when a power supply is sinking
(consuming power), it acts as an electronic load. An electronics load’s design mimics a
device that consumes power. The loading to a power supply allows the power supply
to “see” the load in the DUT (application and environment). They are programmable to
give you different types of loading; they include static and dynamic loading pattern. Real
loads are more complex and unpredictable, but an electronic load offers a stable and
organized load pattern.
A DC electronic load is a requirement for design, manufacturing, and evaluation of DC
power supplies (batteries, converter, and an inverter). Other applications include fuel-cell
and photovoltaic cell test.
The DC electronic load contains a bank of power transistors or metal-oxide-
semiconductor field-effect transistor (MOSFETs) to sink or absorb power. A current
amplifier regulates the e-load input current intake through turning on and off these
transistors.
+I-I
+V
-V
Quadrant II (2)• Voltage: Positive• Current: Negative (sink I)• Power: Consumed
Quadrant I (1)• Voltage: Positive• Current: Positive• Power: Sourced
“Load”
“Load”
“Power Supply”
“Power Supply”
V
+I
-
V
+I
-
V
+I
-
V
+I
-
Quadrant III (3)• Voltage: Negative • Current: Negative• Power: Sourced
Quadrant IV (4)• Voltage: Negative • Current: Positive (sink I)• Power: Consumed
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E-load ApplicationsAs technology advances, electrification of legacy mechanical drive energy is significantly
increasing the use of electrical power. The substantial increase in electrical power
demand is raising awareness of conservation of energy. Designers are racing to
produce high energy-efficient products. Rigorous testing on power source and power
consumption devices is widening the area of DC electronic load applications.
Listed below are typical applications and how they help industries measure their e-load.
Uninterruptible power supply (UPS) – a full test requires an AC source,
DC source, DC load, and AC load. The DC load uses the load bank to
test the backup battery and charger within the UPS. An AC load tests the
entire UPS system. A load bank test indicates the UPS’s ability to provide
the required power, voltage stability, and efficiency of control systems
under varying load conditions.
Power converter and inverter testing – a fast way to test DC-DC, AC-
DC, and DC-AC converter. E-load at the output simulates the device in the
process of powering up. You can test the minimum and maximum input
turn-on voltage level with different load levels. Measurements on ripple and
noise, load and line regulation, and over voltage and current protection
test are carried out with the e-load.
Batteries and fuel cells – gives you a constant loading to reduce the
time for test compared to resistor load banks. To test the capacity, use
CP mode to provide a consistent power drain as the battery voltage drops
over time. The capability of e-load to program different load profiles with
a fast transition enables profile testing for charge and discharge cycle for
batteries.
Solar panel – an excellent solution for high-power PV testing. It has
the capability to sink high current at a lower cost. Use the CV mode for
capturing the I-V curve and incremental voltages to measure the current.
Portable device – program the e-load to simulate various power states of
a device such as sleep, power conservation, and full power modes for a
power consumption test.
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A typical e-load has:
• Measurement functions – Voltage, current, power, peak value, average value, minimum, and maximum
• Panel display to show input readout and measurement readings
• Independent channel operation
• Built-in pulse generator for continuous, pulsed, and toggled transient operation
• Trigger input for synchronized measurement in a parallel configuration
• Remote programming through SCPI command language
DC Electronic Load Operation ModeWhen you decide the type of load test for the DUT, a load requires mode selection.
The most common operating modes of an e-load are constant current (CC), constant
voltage (CV), constant power (CP), and constant resistance (CR). When programming
the e-load to a mode, it remains in that mode until the mode changes. Alternatively, until
a fault condition, such as an overpower, or over-temperature occurs.
The current, resistance, voltage, and power mode parameters descriptions in
subsequent paragraphs are programmable if the mode is selected. When a mode
is selected via the front panel or programmed over the bus, most of the associated
parameters take effect at the input (exceptions noted in the mode descriptions).
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The heavy solid vertical line illustrates the locus of possible operating points as a
function of the input current. A constant current (CC) status flag indicates the output
current is at the specified setting.
Note that in CC operation, a voltage limit is not programmable. However, if the DUT
imposes a voltage that is above the allowable voltage for the specified current range,
the overvoltage protection trips, and the input turns off.
Current is programmable in either of two overlapping ranges, a low range, and a high
range. The low range provides better resolution at low current settings. The load selects
the range that corresponds to the range value that you program. If the range value falls
in a region where ranges overlap, the load selects the low range. If the present current
setting is outside the low range, the load automatically adjusts the input to the highest
value available in the low range. If you subsequently program an input value that is
outside the low range, an OUT OF RANGE error appears on the front panel display.
Constant Current Operation (CC)In this mode, the load module sinks a current by the programmed value regardless of
the input voltage.
InputVoltageSinking power
Programmable+OV
LoadCurrent
Max PowerContour
I set
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Constant Voltage Operation (CV)In this mode, the load module will attempt to sink enough current to control the source
voltage to the programmed value.
The heavy solid vertical line illustrates the locus of possible operating points as a
function of the input voltage.
Note that in CV (constant voltage) operation, a current limit is set. As shown by the
horizontal portion of the line, the output voltage remains regulated at its programmed
setting if the load current remains within the current limit setting. A CV status flag
indicates that the output current is within the limit settings.
When the output current reaches the current limit, the unit no longer operates in
constant voltage mode and the output voltage is no longer held constant. Instead, the
e-load now regulates the output current at its current limit setting. A negative current
limit status flag is set to indicate that a current limit is reached. If the voltage continues
to increase until it exceeds the allowable voltage or maximum power contour for the
specified current range, the overvoltage protection trips, and the input turns off.
InputVoltageSinking power
Programmable+OV
LoadCurrent
Max PowerContour
V set
I limit
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Constant Resistance Operation (CR)In this mode, the load module sinks the current proportional to the input voltage per the
programmed resistance.
The heavy solid line illustrates the locus of possible operating points as a function of
the resistance setting. The resistance slope line operates down to R min (dotted line) to
allow the e-load to produce a low voltage while still loading high current.
Resistance is programmable in any of the three overlapping resistance ranges — low,
medium, and a high. The load selects the range that corresponds to the resistance
value that you program. If the resistance value falls in a region where ranges overlap, the
load selects the range with the highest resolution. If the present input setting is outside
the range that you select, the load automatically adjusts the input setting to the closest
available value within the newly selected range. If you subsequently program an input
value that is outside the newly selected range, an OUT OF RANGE error appears on the
front panel display.
InputVoltageSinking power
Programmable
LoadCurrent
Max PowerContour
R set
R max
R min
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Constant Power Operation (CP)In constant power operation, the load module regulates the power drawn from the DUT
according to the programmed constant power value.
The load module regulates the input power by measuring the input for the voltage and
current; and adjusts the input power based on data streamed from the measurement
ADCs. Note that the corresponding programming ranges override the voltage and
current measurement range selections when in CP mode. The e-load automatically
selects the lowest range possible to give better resolution and accuracy.
The load modules have a built-in over-power protection function that will not let you
exceed the output power rating of the load module (the maximum power contour) by
more than 2%.
InputVoltageSinking power
Programmable
LoadCurrent
Max PowerContour
CPset
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How to Select the Right E-loadThere is a vast variety of DC e-load in the market. Getting the right e-load to serve the
application needs can be quite a task. Below are the basic consideration factors when
you select the right e-load ;
• Capacity rating
- Maximum voltage, current, and power rating has the necessary information required for your application to ensure your e-load can handle the capacity of your power source
• Dynamic testing
- Some loads are dynamic, an arbitrary function of an e-load allow you to perform different functions like pulse, step, and ramp
• Operating modes — CR, CC, CV, and CP
- CC for a power consumption test
- CR is a direct replacement for the resistor
- CV to test current source
- CP to test storage capacity
• Computer control
Use an RS232, GPIB, or USB for a complex test program when you are unable to
set parameters through the front panel
• Form factor
Use a form factor to give you the flexibility to scale for future expansion
DC Electronic Load Form FactorThe DC electronic load is available in a few form factors related to power supplies.
Below are the three common form factors of e-load:
1. Benchtop
The benchtop is the most common instrument in a lab environment. This low-cost
instrument gives you basic features; most settings are through the front panel. The
e-load comes with basic connectivity such as GPIB or RS232 for remote programming.
Newer models may have USB functionality.
Benchtop single input e-load is a low-cost instrument with basic features
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2. System e-load
These are commonly built into a rack system, mainly used in automated test system
(ATE). This system integrates in a manufacturing environment with high throughput and
higher-power capability (more than 5 KW) to test multiple devices at the same time. The
system has advanced connectivity; such as LAN.
3. Modular e-load
Typically consists of a mainframe with computing processor — the modular e-load
works together with the power supplies unit in the same mainframe. It allows flexibility
to scale up and down by changing different modules according to the application
requirements. Usually, the modular e-load comes with a large display that allows you to
view fundamental analysis on a trend chart. The N6705C shown in the figure below has
USB and LAN connectivity.
Modular e-loads are popular in manufacturing environments — you can easily add this
instrument to the 1U high mainframe. Most test systems already have a power supply
to make it easy to add a load. The mainframe uses front to back cooling so that it only
needs 1U of rack space. The N6700C shown in the figure above has LAN and USB.
System e-load used in a rack configuration; primarily for ATE testing
Modular e-load help you test in a manufacturing environment
Page 11This information is subject to change without notice. © Keysight Technologies, 2019, Published in USA, January 18, 2019, 5992-3625EN
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ConclusionAn e-load is an instrument that sinks current from the power source, operating in
quadrant 2 and 4 of the Cartesian coordinate system. Its applications are appliable in
the power electronics and energy market. A good e-load can simulate a real complex
and dynamic load with stable and reliable input. There are four common operating
modes; CC, CV, CP, and CR. You can choose the different types of e-load depending
on your application needs.
For more information, visit DC Electronic Loads at K.com