Most engineers use automatic test systems for production or receiving inspection testing of power supplies. But many companies are finding that ATE is equally beneficial for design verification testing (DVT).
DVT for a power supply is the process of assuring that it reliably meets its design specifications. It should be performed on new power supplies prior to release to manufacturing and after any changes which may affect function or reliability.
And the DVT process should not be limited to manufacturers of power supplies. Power supply users, even those who buy them off the shelf, can use design verification to assure that the product reliably meets their needs.
Testing Performed in DVT
In the production environment, the main need is to perform any required adjustments or calibration and verify that the product was manufactured with correct and functional components. In receiving inspection, the main focus is to assure that the lots of product meet their particular critical specifications and that the product was not damaged in shipment. For each of these cases, typical test groups are:
Output voltage set points.
Regulation.
Noise.
Timing.
Efficiency.
These tests are also important for DVT, but they need to be performed under all possible environments and conditions. More subtle characteristics must also be challenged to assure that the power supply is going to meet specifications in a variety of operating environments and with the allowable component tolerances.
In addition to performing these tests with the stress of physical environments, such as temperature or vibration, other characteristics are studied:
Monotonic rise/fall.
Common-mode noise.
Transient input response.
Transient load response.
Power factor.
Input harmonic currents.
How Can ATE Help?
ATE provides the capability to perform the tests faster, according to Nathan Wiscomb, a Senior Design Engineer at Dell Computer Corp. “We decide what is important for each power supply during the DVT process and make that the production test. The programs are written to our network and downloaded to the testers in each of our facilities,” he explained.
At Power-One, ATE allows all of the characteristics of its new power supply designs to be thoroughly evaluated in a variety of stress environments. The power supply is run in an oven or on a vibration table and massive amounts of data are collected over a long period. Also, engineers can see the data as it is accumulated on a LAN.
This information is used to generate spreadsheets and graphs which predict the product performance in a customer’s environment. With ATE, the tests results, data and methods also are predictable and repeatable.
Types of Instrumentation
For the most part, if the ATE has a versatile design to allow a variety of testing capabilities, there are few differences. The basics are a solid-state AC source, dynamic/static DC loads, a time measurement and sequencing instrument, a noise measurement instrument, a digital storage oscilloscope (DSO) for waveform capture and evaluation, a power line analyzer for evaluating harmonic distortion, and a simple and versatile interface for quick and simple adaptation to the power supply.
The ATE must also have software that completely controls the instrumentation in a simple and easy-to-understand high-level language. It should provide an interfaced manual software control system to easily manipulate the hardware in evaluating and diagnosing power supply performance.
The capability to pause the program and bring up manual control to look at what is happening is essential. Since the testing, by definition, is part of the unknown, debugging is part of the process. Being able to set up data collection files easily and write data to a variety of formats and applications makes good ATE software an invaluable DVT tool.
Difficult Tests
Obviously, changing the line and load conditions and making voltage measurements are easy for ATE to perform. With software-controlled instruments like line power analyzers, measuring the line current harmonics and evaluating the power factor are not difficult.
But what about monotonic rise at power up? Or what if an engineer wants to see the dynamic response of the output at a variety of line and load conditions?
These can be accomplished with the DSO capture capability. If the DSO saves the captured waveform with annotations, a series of bit-map files could be stored for later evaluation. This, along with other logged measurement data, provides a complete picture of how the power supply responds.
If the ATE has multichannel sequencing capture capability, then the relationships of multiple (three or more) outputs can be compared to logic signals such as power good or power fail. Verifying these relationships at both power up or power down with a single-cycle or multiple-cycle line dropout is possible with a solid-state source event program and sequencing capability.
Production Testing
If the product receives a good DVT and the process reviews the product on a periodic basis, many tests performed in DVT are not necessary at the production level. Such tests as line surge, line current harmonics, AC dropout, monotonic rise and frequency regulation (on the switch-mode power supplies) are not necessary if the design was thoroughly qualified.
These parameters are not likely to change as a result of manufacturing or component errors. If they do, simpler tests such as dynamic loading, static regulation and noise should indicate a problem.
The key is a good DVT process with reliable and repeatable methods. This process must recheck the power supply performance with every major engineering change order and on samples at random intervals. If ATE is used to perform the initial DVT, retesting and verifying data to that of the original tests can be done quickly and easily. This helps keep the product and manufacturing quality high and the customers happy.
About the Author
James Pennington joined Autotest seven years ago and has been the Applications Engineering Manager for the past four years. For more than 16 years, he has been affiliated with the power supply ATE industry in test engineering, applications engineering and sales, and as a private consultant. Mr. Pennington was educated as an electrical engineer at San Jose College. Autotest Co., 5347 Dietrich Rd., San Antonio, TX 78219-2997, (210) 661-8661.
Copyright 1995 Nelson Publishing Inc.
November 1995