Today’s DC power supplies used in electronic products and systems usually are of the low-voltage, high-current variety. Voltage regulation is standard, and built-in features such as overvoltage protection, current limiting, and undervoltage lockout now are common. Would it be possible for a power supply like this to produce unwanted behavior in an electronics system? Let’s examine the possibilities.
Power Supply-Induced Failures
Many electronic devices behave unpredictably when subjected to a voltage outside specified limits. For example, most logic circuits require a nominal 5-V supply (4.75 to 5.25 V). What if the voltage drops to 4.5 V? Another problem occurs when an electronic device shuts down independently due to overvoltage, undervoltage, or thermal overload. In such a case, system behavior could be indeterminate.
Aberrations also can occur with voltage regulation. The output voltage is compared to an internal reference, and the control loop tries to maintain them at a fixed ratio. As in any system with feedback, the loop might become unstable, and the resulting oscillations could produce voltage excursions harmful to sensitive electronics.
Static and Dynamic Testing
An automated test setup provides stimulus to and measures response from the power supply. Software controls source and load values via a control bus.
To uncover potential power supply problems, you may have to run dynamic tests as well as static tests. Certain stimuli may render the power supply temporarily or permanently unstable or cause it to go into a protection mode. On the other hand, you need to test protection features to make sure they work correctly.
Key dynamic measurement techniques are required to characterize power supplies accurately. According to Herman van Eijkelenburg, product marketing manager at California Instruments, “A true power analysis requires that both voltage and current be acquired simultaneously at high sampling rates and with minimal timing skew. To ensure proper time alignment of signals, we use a separate analog-to-digital converter (ADC) for each channel, rather than multiplexing which introduces significant delays at higher frequencies.”
For dynamic tests, sources and loads are required to change outputs quickly. Programmable power sources and electronic loads are used for that purpose.
Also, application-specific instrumentation, such as data acquisition, usually is required to capture transients. An event in the power supply triggers the data capture. Once captured, the data may be post-processed for future analyses.
Perhaps you don’t need power analysis, but other dynamic measurements may be required. AC-to-DC converter power supplies contain input diode rectifiers rated in peak as well as average current. The maximum peak current, called inrush current, occurs when a fully loaded power supply is first turned on.
The output capacitors aren’t charged yet, and this presents a momentary short circuit to the input. “Inrush current and load transient response are important parameters,” indicated Jim Pennington, applications engineering manager at Autotest. “For inrush current, we load the power supply to maximum, turn it on, capture the peak input current, and hold it for evaluation by the software. The peak current is measured by sending the signal to the peak detector and the timer simultaneously.”
In testing the load transient response, it is necessary to produce a fast change in load current and then measure the output voltage deviation. The easiest way to bring about a fast change in load current is with an electronic load which employs solid-state instead of mechanical switching.
“The electronic load has made testing of power supplies easier and faster,” said Rick Parizot, sales manager at Dynaload. “A single electronic load can replace many resistors, and the built-in fast switching is the best representation of real-world stimuli in measuring transient response, recovery time, and load regulation.”
During dynamic testing, ensure that the resulting disturbances you observe are normal and not peculiar to the test setup. To eliminate the measurement technique as a source of error, simulate field conditions by using representative cables and layouts.
For the control and analysis functions of automated power supply testing, look into various software packages. Start with power supply test software for Windows that runs on a PC and provides a graphical user interface that does not require programming.
“Test software controls AC input sources and output loading,” pointed out Bob Leonard, director of marketing and sales at Eltest. “Various test screens allow you to test for inrush current, transient load response, ripple, and many other parameters. You also can imbalance multiple output supplies and check the results.”
Indeed, many power supplies for electronics have multiple regulated outputs, such as +5 VDC and ±12 VDC. Check the data sheet for legitimate current loading conditions to maintain the specified output voltages. For accurate results, test at the actual load.
Technological Advancements
With improved software, today’s power supply testing is easier to automate and interface to ATE. In most cases, test time has been drastically reduced.
Software has improved, not only in ease of use, but in the ability to have programs work together. Where you previously needed different versions of a program for various applications, you now can use one integrated program.
Mr. Leonard of Eltest explained that users like the intuitive interface for specifying the test and limits and reading or graphing the measurements. “AC-to-DC and DC-to-DC converter power supplies now can be tested on the same platform. In the past, you had to change versions of software to test different power supplies,” he added.
And it isn’t just software that is forging ahead. Hardware developments also are underway. Hardware circuit improvements, especially improved processors and digital signal processing (DSP) circuits, have made power supply testing easier. Doing the number-crunching in hardware frees up the CPU for other tasks and generally proceeds faster than running computational algorithms in software.
“DSP has made it possible to integrate measurement and analysis functions which were performed externally with DMMs or stand-alone analyzers,” said Mr. van Eijkelenburg of California Instruments. “This reduces cost and facilitates the time correlation between stimulus and response.”
By contrast, some aspects of testing modern power supplies have become more challenging. Typically, the stimulus and electronic load need to be an order of magnitude faster than the response of the circuit to be measured.
The improved response times of today’s power supplies require a super fast stimulus to test it properly as Dynaloads’ Mr. Parizot pointed out: “The technology in today’s power supplies has forced us to design electronic loads that border on the laws of physics. To test the loop-response characteristics of the regulator, the speeds have become so fast that even simple interconnections may become too inductive for proper testing.”
So how important is testing the power supply? Since it is built into almost every electrical and electronic system today, proper operation of the power supply is indeed important. When the supply fails, everything fails. Whether you are a manufacturer, specifier, or user of power supplies, you need to test them to ensure performance.
Power Supply Test Products
AC Power Source/Analyzer
Tests Loads to 5 kVA
The 5001iX AC Power Source/Analyzer provides 5 kVA of single-phase power at frequencies from 16 Hz to 500 Hz, voltages from 150 to 300 V, current to 37 A, and peak current to 110 A. The front panel has a functional keyboard and an LCD. Standard interfaces include IEEE 488 and RS-232-C, and instrument software drivers are available. AC outputs include sine, square, clipped sine, and phase control. A transient generator permits testing to IEC 1000-3-2 and -3. A digital storage scope mode facilitates measurements. $12,475. California Instruments, (800) 422-7693.
All-In-One UPS Tester Has
Automatic Program Generator
The UPT-1000 Tester for UPSs features embedded PC architecture, Windows 95-based software, and simultaneous input/output measurement. The tester is equipped with front-panel keypad, real-time instrument control and display, calibration self-test, IEEE 488, and RS-232-C interfaces and additional connections for an SVGA monitor and a mouse. The standard source is a 2.1- kVA autotransformer from 0 to 270 VAC and up to 10 A. The standard electronic load is 300 W single-phase from 0 to 280 VAC at 5 A. Higher power and three phase options are available. From $20,000. Autotest, (210) 661-8661.
Electronic Load System
Has Four-Mode Operation
The MCL488 System consists of up to 10 configurable loads rated at 100 V, 400 V, or 600 V at 60 A and 350 W. Modules can be paralleled or operated as constant voltage, constant current, constant resistance, or constant power. The system can be controlled through the front panel and the IEEE 488 and RS-232 interfaces. The response time is 10 µs, the slew rate is adjustable from 10 µs to 4 ms, the pulse mode frequency is 0 to 20 kHz, and external modulation is 0 to 10 V. Chassis: from $2,000; loads: from $1,600. Dynaload, (973) 361-2922.
Rack-Mount System Tests
AC or DC Supplies to 1.5 kW
The EL-ADSYSR1 is a complete power supply test system in a 6’, 19″ rack. It includes an industrial PC with a 17″ monitor, PC plug-in cards, a keyboard, power supply test software for Windows, and programmable 1.5 kVA AC and 3-kW DC sources. Also featured are four electronic loads to 1.5 kW. The enclosure can accommodate 12 loads to 3 kW. $37,400. Eltest, (508) 339-8210.
Copyright 1998 Nelson Publishing Inc.