Test Selection and Tester Save Time and Money

Today’s power supplies—whether destined for lab, system or built-in applications—provide better performance, more features and occupy less space than yesterday’s models. At the same time, manufacturers must keep their prices down and quality up to remain competitive.

Providing quality products requires a thorough design verification to ensure that the supplies perform as expected over their entire anticipated operating range, even if component values deviate from the norm. The more exhaustive these verification tests are, the lower is the likelihood that component or process variations will cause unsatisfactory production yields, high diagnostic costs or unacceptable field-failure rates.

While entailing increased initial expenditures, performing extensive design- verification tests ultimately decreases product cost. Conversely, performing too many tests during or subsequent to production can be wasteful.

Range of Test Requirements

Design-verification tests are typically categorized as mandatory compliance or product functionality. Production tests often are a subset of the design tests, tailored to provide maximum performance information in minimum time.

Compliance-Related Design-Verification Tests

Compliance requirements are usually safety or market related. They are imposed by the UL, the FCC, the EU or agencies from other countries. Verification tests are performed to prove that the power supplies meet specific requirements and to generate the formal documentation needed to obtain product-qualification approvals.

Compliance with regulatory requirements must always be demonstrated after design and first final-product completion, and only occasionally after that. Special instrumentation is often required to perform these tests, including variable AC power sources and EMC test setups.

Functionality-Related Design-Verification Tests

Functionality-related design-verification tests are performed to determine whether the power supply meets established performance requirements. Table 1 lists common tests conducted at this stage.

These tests alone may not be adequate to assure that the supply will function under all expected environmental, line, load or component tolerance limit conditions. Unfortunately, there is no set formula about which additional tests should be run. They depend on the particular product and the design margins used. You must search out potentially weak areas and identify the most critical circuits or the most interdependent functionalities, then devise more appropriate tests.

Safety or regulatory compliance features may indirectly affect the overall functionality of the supply and impose extra test demands. One of the many cases in point is the soft-start circuitry mandated by some customers and regulatory agencies.

“Many designs use soft-start circuits to limit the initial turn-on current,” said Bob Leonard, Director, Marketing and Sales at ELTEST. “This circuit provides the appropriate bias and gain to get things rolling while avoiding big turn-on spikes and capacitor-charging current demands.

“While the circuit may operate properly at nominal line voltage, the question is:

Will the supply latchup (not deliver full output) if it starts at low-line voltage, minimal loading and colder temperatures? Under these conditions, gain and bias are both at the low end of their range at turn-on. Alternatively, at low-line voltage at cold temperatures but at full output loading, the supply has to provide maximum load current while still using minimal bias conditions.”

If design margins are adequate to handle such end-of-range operating limit situations (preferably with worst-case tolerance components), proper proof of the soft-start circuit functionality has been obtained. Consequently, this test would not be carried over into production.

Similarly, a range of other critical operating situations must be identified, tests conducted and functionality proven. The more confidence gained at this stage means fewer production tests to be performed.

Production Tests

Most production runs today range from a few hundred to many thousands of power supplies. As the quantity increases, minimizing test-time-per-unit becomes paramount.

Test-time minimization may be accomplished by limiting the number of tests performed or by reducing the time per test. The former calls for ingenuity on the part of the test engineer, the latter may be accomplished by using power supply-oriented ATE.

“At this stage, keep in mind that the power supply design had been verified and it is merely necessary to confirm manufacturing-process correctness and proper over-all power supply functionality,” said Michael Becker, Product Manager at Hewlett-Packard. “You must optimize the test regimen, running as few tests as possible while still testing the unit to meet the company’s quality standards.

“To accomplish this, you directly and indirectly test the most critical areas of the power supply as efficiently as possible, often making inferences about the quality of the unit from the results of just a few tests,” Mr. Becker continued. “In general, Tests 1, 2 and 9 listed in Table 1 are considered mandatory, but Tests 3 and 4 are also significant. The remaining tests are useful to varying extents, depending on the level of information obtained during the design-verification tests.”

Power Supply ATE Configurations

Power supply test systems consist of a variable power-input source, one or several electronic loads, voltage, current and noise-measurement facilities, a controller, a UUT interface and software. A PC is often used today to control the system and to provide the user interface. Physical configurations range from PC plug-in board systems such as those developed by ELTEST, to self-contained test systems capable of interfacing with a PC as exemplified by the Prodigit 3600A, to rack-mounted configurations typified by the Hewlett-Packard HP Z6150A.

The ELTEST system consists of two PC plug-in boards, an external electronic load and an extensive software suite. The PC-467 Power Supply Measurement Board includes an analog preprocessing section, A/D converters and isolated analog and digital outputs. The PC-465 Electronic Load Board Controller contains buffered D/A converters and on-board registers mapped to the host PC. It controls the loads to provide resistive, constant/dynamic current, and short- and open-circuit loading.

The Prodigit 3600A tests AC/DC and DC/DC power supplies up to 630 W with up to four outputs. Test procedures can be generated via front-panel controls or from a PC using the application software. Test results may be read from the front panel or sent to a printer via the Centronic interface.

The HP Z6150A offers a customized test solution composed of industry-standard HP as well as third-party hardware. The UUT is connected to the system via a standard HP test adapter. “This system is well suited for users who test varying types of power supplies and require flexibility and ease of use,” said Mr. Becker.

Power Supply ATE Software

The software provided with each power supply ATE includes a set of typical power supply test programs and a user interface for easy insertion of the UUT-specific test parameters. Most power supply ATE software is now MS Windows-based and includes all drivers needed to control the power supply test hardware as well as IEEE 488 instruments. In the case of VXIbus-based systems, the software may run on embedded or external PCs and VXIbus instrument drivers are included.

Although each supplier’s software is different, most are Windows-based which provides a certain degree of commonality. For example, the HP Power Test Software that accompanies the HP Z6150A not only includes a standard library of 21 power supply tests but is also compatible with Microsoft Visual Basic and C++. This helps you incorporate additional custom tests into your programs.

The HP Power Test Software for Windows also includes user-configurable test sequencing; built-in data-reporting tools; a built-in data base and data-archiving tools; system administration tools, including user log-on, security lock-outs and back-up features; a built-in program for system self-test; and on-line help.

Similarly, the ELTEST POWERWIN software is Windows-based and provides more than 30 power supply virtual test panels. It interfaces with hardware and software from other companies.

“For example, POWERWIN software is linked to Pacific Power Source’s code used for control and measurements performed by its AC sources,” said Mr. Leonard. “These units provide the AC-related test capabilities demanded by the EU regulations and we can link our system with theirs using GPIB control.”

Trends

As power-consuming devices get faster (transitioning from mA to multiamps within a few ns), power supplies must be tested to determine whether they can keep up with such sudden load changes. Electronic load response time and the interconnections between the UUT and instrumentation will become critical in such test situations.

More power supplies will contain microcontrollers to perform smart functions. Again, this will impose additional test-instrumentation requirements. Fortunately, most power supply ATE has an architecture flexible enough to facilitate needed upgrades.

Finally, as more power supply manufacturers are planning to sell their products in Europe, obtaining CE marking approval becomes essential. Relevant EU and International Electrotechnical Commission specs include requirements for several AC input-distortion tests which can only be performed with controllable AC sources.

While most of these tests are not production tests, in some cases, an ATE tie-in may be beneficial. Consequently, some alliances may emerge between power supply ATE and controllable AC source companies. Other companies will use in-house resources to provide both capabilities.

Table 1

Copyright 1996 Nelson Publishing Inc.

November 1996