Introduction to the Polarity Convention of a Power...

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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+

-


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


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.


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).


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


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.


Programmable+OV

LoadCurrent

Max PowerContour

V set

I limit


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.


Programmable

LoadCurrent

Max PowerContour

R set

R max

R min


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%.


Programmable

LoadCurrent

Max PowerContour

CPset


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


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

This information is subject to change without notice. © Keysight Technologies, 2019, Published in USA, January 18, 2019, 5992-3625EN


Learn more at: www.keysight.com

For more information on Keysight Technologies’ products, applications or services,

please contact your local Keysight office. The complete list is available at:

www.keysight.com/find/contactus

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

http://www.keysight.com/find/dmm

Introduction to the Polarity Convention of a Power...

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