Determining the behavior of your DC power supply under changing line/load conditions and checking its output ripple were all that you needed in the past to make a pass/fail decision. But these basic tests no longer are sufficient if your electronic equipment is to be sold worldwide.
Many countries now require a multitude of additional tests to prove that the power supplies you ship or are built into your end products meet new local or international standards. Some of these tests are performed only on preproduction prototypes, while others must be carried out on every power supply shipped.
New test equipment or new test setups are needed to demonstrate compliance with these additional standards. Some of the equipment, such as the latest AC sources required to conduct new regulatory requirement verifications, provide unique facilities not previously available in power supply ATE.
But not everything has changed. To test an AC-powered supply, we still require a controllable AC source, DC loads, and measurement instruments. Depending on the type of tests to be conducted, the AC source may merely provide a controllable output level or have all the capabilities needed to perform the latest international compliance tests.
Measurements may be performed with dedicated individual instruments or via switched shared test equipment. To assist you in making informed choices, we will review requirements and equipment capabilities, with special emphasis on the power consumption and regulatory compliance-related aspects of power supply testing.
Common AC Source Requirements
Since most power supplies are fed from common utility lines, their proper operation must be verified under fluctuating and contaminated line conditions. The AC source used to simulate the expected perturbations must be stable and not introduce any unwanted disturbances. The minimum features and functionality usually demanded from such a source are:
A dual programmable voltage range (commonly 0 to 150 V and 0 to 300 V) to test supplies for domestic and international applications.
The capability to simulate timed line sags, surges and dropouts.
Total harmonic distortion of less than 1% at the output.
Voltage regulation better than 1%.
Variable frequency (typically 45 Hz to 500 Hz, but in some cases 40 Hz to 5,000 Hz).
Programmability via an IEEE 488 or an RS-232 interface.
Of course, the AC source must have an adequate power rating to accommodate the range of supplies to be tested. However, when determining the needed VA and current ratings, several subtle factors should be considered.
“Specifically, the product of voltage and available rms current at a required output setting may not equate to the maximum VA rating of the source,” cautioned Herman van Eijkelenburg, product manager at California Instruments. “Maximum VA power is only available at the highest voltage setting, which is not necessarily the point where the source will be used.
“Also, the power factor of the load must be considered because a low power factor means the source needs to supply much higher VA than the watt rating of the load would suggest,” Mr. Eijkelenburg continued. “And, many AC loads use active input circuits which may cause current consumption with a high crest factor. It is not uncommon to need AC sources with a crest-factor capability of 4 to 1 or more to operate such loads.”
To simulate the real-world-environment, many AC power sources contain an arbitrary waveform generator (AWG) to produce controllable anomalies such as added harmonics, varying phase shifts, or discontinuous excursions. Some three-phase sources, such as the Elgar SW Series, even contain three AWGs. This allows us to create three independent waveforms on all three phases simultaneously to simulate real-world aberrations that could not be generated with a single AWG, according to Martin Sanders of Elgar.
Programming facilities and internal memory usually are provided to invoke and store often-used individual perturbations or complete waveforms. “For instance, sequence or event programming facilities may be used to create up to 1,000 time- or cycle-based transients from 1 ms to 7 days, including waveform, phase angle, and voltage/frequency ramps. Sequences may be stored in memory to be called up and executed at will,” added Mr. Sanders.
AC Sources for Regulatory Compliance Testing
Most of today’s DC power supplies use one of several switch-mode technologies, any one of which represents a nonlinear load. The current drawn by such a load is nonsinusoidal, exhibits high crest factors, and contains many harmonics.
The European Community (EC) established strict limits on allowable harmonics in IEC 1000-3-2 and EN61000-3-2. Also, a maximum allowable flicker requirement has been defined in IEC 1000-3-3 and EN61000-3-3. Flicker pertains to temporary voltage disturbances caused by a load—in this case, the power supply. Another required test is based on IEC 1000-4-11 which deals with voltage dips and power interruptions.
“With the advent of harmonic emissions testing in conformance with IEC 1000-3-2, engineers have become aware of the special demands these tests place on the AC power source,” commented Jerry Van Gessel, marketing manager at Pacific Power Source. “For these tests, the AC power source must have a very wide bandwidth and a low output impedance, even at the highest harmonic frequencies.”
When conducting the IEC 1000-3-3 flicker tests, the output impedance of the AC source must be set to specified values, and special isolation must be provided. To meet these and other IEC demands, compliance-test-dedicated AC sources had to be developed.
For instance, Elgar’s SW-106 models have selectable output impedance, low impedance for harmonic current emissions testing, or a standard reference impedance for flicker testing. An interharmonics waveform generator also allows you to add harmonic components to the AC output, as specified in EN61000-4-13.
Pacific Power Source developed the TMX Series Power Sources for compliance testing. Units providing up to 4.5 kVA are comprised of a high-performance linear AC power source, dual-impedance networks, dual analyzers, flicker meters, and software to form a one-box IEC Test System.
The HP 6840 Test Systems, which contain the features needed for single-phase compliance-level testing to EN 61000-3-2, EN 61000-3-3, and EN 60555 Part 2 standards, also include a range of diagnostic tools. “This allows regulatory test engineers to serve as consultants to their clients,” said Celeste Jenkins, product manager at HP. “It enables the client’s R&D engineers to quickly evaluate their equipment under test and determine the steps necessary to pass the compliance tests.”
California Instruments also offers special test systems for regulatory applications. Typical applications are AC-related IEC standard tests such as those defined by IEC 1000-3-2 (harmonics), IEC 1000-3-3 (flicker), and IEC 1000-4-11 (voltage dips and interruptions). The units also include software to perform tests to standards that still are in draft form, such as IEC 1000-4-14 (voltage fluctuations) and IEC 1000-4-28 (frequency variations).
Loads are connected to a power supply to verify that it can provide rated outputs. Switchable or variable loads also are needed to determine whether the supply meets input and output regulation specifications.
Banks of passive resistors may be used to provide a range of fixed loads for simple and noncritical power supply test applications. But being fixed sacrifices flexibility and since resistance values can change with temperature, accuracy may suffer. Consequently, instead of employing fixed resistive loads, many modern power supply test systems use electronic loads. These loads are voltage-controlled or current-controlled current sinks which, through feedback control, also can operate in a constant resistance, constant voltage, or short-circuit mode.
Being controllable, their values can be changed quickly which improves test throughput. They also can readily simulate time-variant current drains, such as those experienced when connecting capacitive or pulsed loads to the power supply.
The response of a power supply to fast transients also can be evaluated readily. Electronic loads such as the Analogic Model 2101A feature a bandwidth as high as 45 kHz, making it possible to simulate transients with slew rates of 5 A/µs at a repetition rate of 2,000 Hz.
Power supply performance evaluations require AC, DC, and frequency measurement facilities. Tests fall into three major categories: steady-state input-related and output-related measurements and assessment of regulation capability. Regulation refers to the degree to which the output of the supply is unaffected by changing line and load conditions.
Steady-state AC input evaluations include rms and peak current, crest factor, true and apparent power, and power-factor measurements. This requires AC voltage, current and power meters with rms, and peak reading capabilities.
In addition, for design verification or regulatory approval tests, a harmonic analysis of the power supply input current is required. These tests may be performed with dedicated harmonic analyzers or oscilloscopes with built-in FFT facilities.
Steady-state DC output tests require a determination of whether rated load current can be supplied, verification of the functionality of protection circuitry, and an assessment of the magnitude and frequency of periodic and random deviations (PARD) superimposed on the DC output. While the former tests are easily performed with DC meters, PARD evaluations require either a high-speed digitizer or an oscilloscope.
Input regulation is assessed by varying the AC voltage applied to the power supply while the load is kept fixed. An AC voltmeter monitors the input, and a DC voltmeter simultaneously measures the output. Similarly, output regulation is determined by measuring the output voltage while varying the load. To determine regulation capability under transient conditions again requires an oscilloscope or a digitizer.
Need for a Unifying Interface
While only a single AC source and one load per output is required, multiple instruments are needed and a vast number of measurements must be made to perform all required tests. The tests may be carried out under manual or semiautomatic control or under full program control. The former usually is the case when benchtop setups are used, the latter with ATE implementations.
To minimize extraneous interconnections or switching of resources, many AC sources now include all AC measurement facilities required to perform regulatory compliance tests. Similarly, some electronic loads feature built-in measurement capabilities.
But programming the equipment to carry out all tests in an efficient manner is not a simple task unless an overall test executive program is provided. And that is where the universal or application-specific power supply ATE can help.
“The flexible and universal power supply test system was created by ATE vendors to provide an adaptable and practical solution for testing a large variety of products,” said Robert Cox, president of Autotest. “Advantages include eliminating the need for a technician to rewire or modify the whole test setup for each different supply to be tested and the availability of full turnkey fixturing and application programming to meet specific test applications.”
Most power supply ATE companies have developed power supply test software suites, such as Autotest’s APG for Windows™ or Eltest’s POWERWIN™, to automate the programming task. And suppliers of AC sources provide the special software required to simplify the regulatory compliance testing process. To make it easy for users to meet all new power supply test requirements, AC source and ATE engineers are cooperating closely to provide well-integrated user interfaces.
Power Supply Test Products
Uses Standard Components
The Series 7000 Power Supplies ATE is a custom-configured turnkey test system consisting of standard components. Input power is provided to the UUT by source and control modules, and measurements are made through matrix scanners with waveform analyzers, scopes, and application-specific modules. Electronic loads are standard, such as the Series 2000 which has eight independent floating load channels that can be combined to accept up to 2,000 W. Each system is supplied with a standard interface fixture kit. $60,000 to $80,000. Analogic, (508) 977-3000.
VXI-Based System Tests
AC/DC Power Supplies
The HP Z6150A is a custom-configured Power Test System that contains standard HP, third-party, and custom hardware. The HP Power Test Application Software for Windows includes a program test library with standard HP and user- developed tests, test-development tools, a UUT test-results data base, a test sequencer, and report-generation and administration tools. Systems employ a B- or C-size VXI mainframe architecture. Virginia Panel interconnect panels are standard; other interfaces are supported. Call company for price. Hewlett-Packard, (800) 452-4844.
AC Power Sources/Analyzers
Combine Multiple Instruments
The ix Series AC Power Sources/Analyzers generate and analyze complex waveform and perform voltage, current, crest factor, power factor, phase angle, and distortion measurements. An arbitrary waveform/transient generator simulates line disturbances and a graphical LCD shows applied arbitrary waveforms and graphical/numeric harmonic analysis results. The series offers programmable output impedance and low output distortion to accommodate IEC compliance testing. The 5000ix furnishes 5,000-VA single-phase power from 16 Hz to 500 Hz; the three-phase 15003ix provides 5,000 VA/phase. From $12,475. California Instruments, (619) 279-8620.
Programmable Loads Mated With
PC-Boards Test Power Supplies
The EL Series of Voltage-Controlled Electronic Loads consist of three models rated from 50 V to 500 V with load currents from 0.1 A to 25 A. Up to 400 W dissipate with external heat sinking. Loads can be paralleled for higher current and power applications. Operating modes include constant-current, constant-resistance, constant-voltage, and short circuit. The modules combined with the company’s EL-465 Load Controller Board, an EL-467 Power Supply Measurement Board, and a PC form a power supply ATE. $999. Eltest, (508) 339-8210.
Power Source/Analyzer Includes
Programmable LISN, Instruments
The HP 6811A AC Power Source/Analyzer includes a built-in power analyzer, a programmable line-impedance stabilization network (LISN), an arbitrary waveform generator, and a power amplifier optimized for AC power applications. It provides 375 VA AC with peak current-sourcing capabilities up to 40 A, 12× its maximum rms current rating. The capabilities include rms-, peak-, and inrush-current measurements with 16-bit accuracy and easy generation of dropouts and arbitrary waveforms. $5,500. Hewlett-Packard, (800) 452-4844.
Programmable AC Sources
Cover Worldwide Power Ranges
The EasyWave™ EW371 and the EW801 AC Power Sources supply adjustable voltage and current at frequencies from 45 Hz to 500 Hz. The EW371 has an output power of 375 VA; the EW801 has an output power of 800 VA. Both models generate clean power with typical distortion of <0.3% and offer high-speed measurement of true rms voltage, true rms current, power, frequency, power factor, and crest factor. Starts at $2,595. Elgar, (800) 733-5427.
System Performs Compliance
And Precompliance Tests
The TMX Series Harmonic and Flicker Test Systems, offered in power ranges from 750 VA to 12 kVA, perform measurements per IEC 1000-3-2 and 1000-3-3. They combine a linear AC power source, dual synthesized and discrete IEC 725-compliant impedance networks, dual IEC 1000-4-7-compliant harmonic analyzers, and dual IEC 868-compliant flicker meters in one unit. The series facilitates simultaneous monitoring of the UUT and the power source. Dip and interrupt, harmonic and interharmonic, and fluctuation and frequency variation testing are supported. Pacific Power Source, (800) 854-2433.
Universal UPS ATE Addresses
Smart Technology Test Needs
The UPT 2000 Universal UPS Tester addresses testing requirements of Smart UPS products. Solid-state AC/DC sources, loads, and instrumentation resources create the stimulus and measurement environment to calibrate and test UPSs using modern technologies. A computer-interactive interface allows the tester to software configure and command UPS, or UPS PCBs. All aspects of UPS production and verification testing are automated. The test system uses the company’s APG for Windows™. Autotest, (210) 661-8661.
Copyright 1997 Nelson Publishing Inc.