Simple Tests Aren’t Simple Anymore

In the simple world of a few years ago, it was easy to test a power supply.

You just provided the rated input and verified that the output voltage across a simple resistive load was correct. Oh, you checked the output ripple, too.

But that was back when simple products were powered by simple power supplies. Now products are much more complex, imposing more difficult loading on their power supplies. This means new, innovative types of supplies and thorough, dynamic tests.

Test Categories

While any given supply must be considered independently to determine its operating characteristics, there are some generic test categories:

  • Current-Limiting Test—An analysis of the supply’s capability to reduce its output voltage if the load is too great to avoid damaging the supply.
  • Efficiency Measurement—A calculation of the ratio of output power to input power with nominal input voltage and various loads. This clearly is linked to thermal performance since the excess power dissipates as heat.
  • Line Regulation—A measurement of the percentage change of the output voltage when the input voltage changes, typically from nominal to a low or a high value.
  • Load Regulation—An evaluation of the percentage change in output voltage when the load changes from a specified level, such as 50%, to another specified level, possibly 100%.
  • Overvoltage Protection—A test to ensure that if the input voltage becomes higher than the specified upper limit, then the output voltage will not increase enough to damage a load.
  • Periodic and Random Distribution (PARD)—The total noise on the DC output, typically measured over a 20-MHz bandwidth with nominal input and full load.
  • Power-Up Check—A verification of the supply’s capability to start under a full rated load.
  • Ripple Amplitude Measurement—A check of the peak-to-peak amplitude of AC disturbances on the DC output, typically at nominal input voltage and full load.
  • Short-Circuit Test—A measure of the product’s capability to withstand the prolonged application of a short circuit.
  • Transient Test, Low-to-High (L-H) and High-to-Low (H-L)—A measurement of how long it takes to reach the proper output voltage when the load switches from low (typically 50%) to high (100%) or from high to low. If the supply has multiple outputs, typically all but the one of interest are kept at 50% load for the test.
  • Voltage Accuracy Measurement—A check of the deviation of the supply’s output from its nominal value.

Priorities in Selecting a Test System

In the definition of a power supply test system, certain factors are critical in making your choice. After considering accuracy, most of the concerns relate to the cost of testing. How many products can you test in an hour? What is the length of a shutdown when going from one model to the next?

Accuracy

With any test, the validity of test results must be unquestionable. Therefore, the measurement capabilities of the tester are critical.

Speed

Each of us has traveled across a continent or ocean at 600 mph, only to wait for 30 minutes on the tarmac because the assigned terminal gate is not available. The power supply manufacturer doesn’t want this same experience at the end of a production line as products manufactured at high rates bog down at a slow test system. Testing speeds are important to the test-system manufacturer. Today, most power supply test systems are optimized for fast testing with high-speed computers, parallel measurement capability, and streamlined software.

Versatility

“Fast test speeds are essential to remain competitive,” Peter Swartz, president of NH Research, noted. “These vary depending on the type of supply, the number of tests, the number of adjustments made per test, and the scheduled power-on cycles. Notwithstanding these qualifiers, however, many DC-DC converter manufacturers expect each test of a suite of 15 to 20 to take less than 5 s.”

As the computer and telecom industries continue their relentless push for faster processing speeds and greater bandwidth, power supplies must be designed and tested for proper performance in these applications. Already, operating voltages of less than 3.3 V at greater than 60 A with load transient slew rates as high as 300 A/µs are driving a new generation of test and measurement equipment. This necessitates new approaches to ATE design and fixturing.

“We have introduced a load family that can slew at 300 A/µs down to 0.6 VDC,” Fred Sabatine, ATE business development manager at Chroma, said. “With less than 10-nH fixture inductance, the loads can step from zero to full scale in 500 ns without ringing, overshoot, or undershoot.”

Fast Adaptability to New Test Conditions

The manufacturer of several types of supplies needs the capability to change quickly from one model to the next. Hardware and software environments must be designed with fast changeover. For example, it may mean several sources and an extra load plus versatile software.

System Architecture

The power supply test system consists of a power source, load, controller, and software. The hardware elements may be separate or an integrated package. The functions, however, are generic:

Power Source

Some power supplies will operate from a 12-V or a 42-V automobile battery and others from commercial power in St. Petersburg, FL, St. Petersburg, Russia, or any other point on earth. The test-system source must simulate the basic power of each location.

There is more to a source than just supplying power. “Typically, customers also require that an AC source simulate voltage sags, surges, and dropouts,” noted Herman van Eijkelenburg, vice president for product development at California Instruments. “In addition, testing for voltage dips and interruptions per IEC 61000-4-11 and current harmonics per IEC 61000-3-2 is combined with production testing using a common test system.”

Load

Programmable loads for dynamic testing of supplies with DC outputs generally offer a choice of maximum-power levels and the capability to switch loading rapidly from one state to another. In response to the well-recognized trends, they operate at various DC levels, even as low as 0.6 VDC.

The AC load has unique capabilities when testing an AC-to-AC or DC-to-AC product such as a UPS or an inverter. A versatile load offers constant-power, constant-current, constant-voltage, or constant-resistance modes. The crest factor and power factor can be programmed on some models to simulate worst-case loads. Some loads also have short-circuit modes.

Controller

The PC is the controller of choice in the typical power supply test system. With RS-232, IEEE 488, or a higher speed VXI or PXI architecture, it issues commands, receives and analyzes test results, prints reports, and archives the transactions for retrieval and in-depth study.

Software

“Customized test reporting and statistical process control (SPC) are the keys to getting maximum benefits from power supply ATE,” Mr. Sabatine of Chroma ATE reported. “We provide the tools so you can create customized test reports in Excel or other familiar formats. The system has built-in SPC functions so it isn’t necessary to export data to another software package. This gives quality engineers the tool for monitoring production trends in real time rather than by delayed batch processing.”

Integrated System

The rack-and-stack configuration has advantages in some applications, notably those where a wide range of power supplies is being developed or where there is a broad mix of products on a production line.

The other configuration consists of an AC stimulus, a programmable DC source, and a power analyzer in one package. Generally, the speed of this arrangement is greater than the larger system, and it has a smaller footprint.

Trends

ATE vendors will continue to look carefully at the power supply technology, preparing new versions or modifications of hardware and software as they are needed. For example, they will test multiple-output products in a variety of scenarios.

The major forces in development relate to higher currents, faster load-switching speeds, and lower DC output voltages. “Semiconductor supply voltages are on the downward path from their 5.0-V beginning to 3.3 V, 2.5 V, 1.8 V, 1.5 V, 1.2 V, 1.0 V, 0.8 V, and even 0.6 V,” according to Bob Leonard, product marketing manager at Datel. “This was projected as a 10-year sequence but it is ahead of schedule. Power supply manufacturers must stay ahead of the migration, and of course, that establishes a target for manufacturers of test equipment.”

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Published by EE-Evaluation Engineering
All contents © 2001 Nelson Publishing Inc.
No reprint, distribution, or reuse in any medium is permitted
without the express written consent of the publisher.

June 2001

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