Traditionally, power supply ATE vendors have designed systems to target either the engineering or the production environment. Production ATE typically uses proprietary instrumentation to achieve maximum throughput when performing a limited number of tests. Usually, engineering ATE uses slower, off-the-shelf GPIB instrumentation to provide more extensive testing for design validation.
Due to this specialization, engineering and production groups have not been able to use the same ATE. Many power supply manufacturers use a production-oriented ATE in manufacturing and an engineering-oriented ATE or benchtop instruments for product development and characterization.
To compound the problem, electronic applications increasingly require multiple-output DC power supplies. Power supply purchasers are demanding customized, multioutput, switch-mode DC supplies even when produced in low volumes.
Power supply manufacturers have responded with modular product design approaches to meet these needs in a cost-effective manner. These manufacturers have found themselves producing a wider mix of products in smaller production runs. This has exacerbated the inefficiency of using and maintaining separate engineering and production test systems, test programs, and test harnesses—driving up the cost of test per unit.
Using VXI instrumentation helps develop a single test platform that supports both engineering and production applications. VXI provides a combination of accurate, off-the-shelf instruments and high throughput.
Tests developed in engineering can be transferred easily to the production floor, which then can run as subsets of the engineering tests. This approach lowers the cost of test and speeds the time to market for manufacturers producing a range of power supplies.
A power supply ATE that meets the needs of both engineering and production can be constructed using:
Programmable electronic loads.
Input power simulation supply.
Power supply to initiate overvoltage protection (OVP).
Oscilloscope and digital multimeter (DMM).
Digital I/O for miscellaneous power supply logic.
A high-speed VXI system is suitable for the oscilloscope, DMM, switching, and digital I/O portions of ATE. Systems requiring maximum throughput can be fitted with a MXI-2 interface to the system controller. Lower-cost systems can be equipped with an IEEE 1394 (Firewire) interface that is still much faster than GPIB.
Programmable Electronic Loads
Generally, VXI-based loads are restricted to less than 100 W/slot by the cooling available in commercial VXI chassis, but also could be used if just a few low-wattage outputs will be tested. For a flexible test system, GPIB programmable load mainframes complementary to the VXI system are available.
The quantity and rating of loads installed in the mainframe can be selected to support a range of tests. A single mainframe can be used to test supplies with up to 10 outputs of 350 W each. Or several loads can be connected in parallel to test supplies with a higher wattage on a particular rail. The selected load mainframe should include voltage and current measurement capability to simplify supply-output measurements; for example, to measure the output power to determine supply efficiency.
Input Power Simulation Supply
Power supply ATE must simulate input power to the supply-under-test. This can be DC, single-phase, or three-phase; a general-purpose ATE platform must produce all three.
Arbitrary waveform generators within such supplies enable them to simulate any desired line condition. Make sure that the input power supply is impedance-controlled if IEC 1003 testing is required.
Extremely flexible ATE supplies are available to perform this function. For example, supplies in the Elgar Smartwave series can meet this criteria and produce up to 5,250 VA total—312 VAC or VDC at up to 6 A on each of three independent outputs.
These outputs can test supplies with three-phase power or be paralleled to test supplies with AC inputs up to 18 A using a single supply mainframe. Figure 1 illustrates a system with a paralleled output configuration for testing supplies with AC and DC inputs only.
A power analyzer is required to measure the input power. Such power analyzers continuously sample current and voltage and include several built-in measurement functions. Power analyzers can measure input power for efficiency and be used to perform IEC 1003-2 harmonic and IEC 1003-3 flicker testing if required.
In addition to their measurement capabilities, power analyzers include analog outputs which can trigger the oscilloscope to make time-interval measurements, such as ramp-up.
Along with the input power simulator and analyzer, the ATE needs a robust programmable DC supply to drive the supply being tested and, if applicable, its regulation load into an overvoltage condition to ensure proper shutdown. Such supplies are widely available.
Oscilloscope and DMM
A high-speed VXI-based oscilloscope can perform most of the measurements required in the system. A four-channel, 8-bit digitizing oscilloscope with an analog bandwidth in excess of 100 MHz can make measurements such as noise and ripple, transients, and timing on up to four power supply outputs at a time. A two-channel oscilloscope can lower the system acquisition cost if longer test times for systems with more than two outputs can be tolerated.
Oscilloscope probes must be selected based on the types of measurements to be made. Differential probes are required to measure the entire output range of the supply being tested. Their function is twofold:
To isolate the scope ground from the ground of the power supply being tested. This eliminates ground loops, ground currents, and the dangerous lab practice of floating-the-scope ground.
To maximize common-mode noise rejection for accurate supply noise measurements.
Systems requiring more precise noise/ripple testing may use a low-voltage probe. Since the amplitude of the noise is smaller than the DC amplitude of the power supply, the use of a low-voltage, AC-coupled probe can increase the resolution of noise measurements. An extremely accurate, yet high-speed, ATE could use a low-voltage probe on one scope channel for noise/ripple testing and higher-voltage probes on the remaining channels for all other tests involving the oscilloscope.
A high-speed, 6.5-digit DMM such as Racal Instruments’ Model 4152A can supplement the oscilloscope with very accurate static measurements. For example, the DMM can precisely measure the power supply output voltage.
Power supply ATE generally requires two types of switching:
High bandwidth coaxial switching—This connects the necessary outputs of the supply-under-test to the oscilloscope and DMM. A bandwidth of 100 MHz is recommended to ensure accurate transient analysis. A differential switch complements the differential probes, providing good common-mode rejection for noise and ripple testing.
High-power switching—This connects the OVP supply to the outputs to be tested.
In addition to switching, most systems will require basic digital I/O capability to test any logic on the supply. Modular switching platforms are suitable for meeting these requirements. For example, Racal’s Adapt-a-Switch line offers the capability to mix and match 13-A power-switch plug-ins, high-bandwidth coaxial-switch plug-ins, and any necessary digital I/O plug-ins in a single, two-slot VXI module. A register-based interface provides maximum switching system throughput.
The VXI system should be housed in a high-quality VXI chassis. It should implement all seven voltage rails and specify them as required in the VXI Specification Revision 1.4.
The subsystem also must offer a high level of cooling, using more than one fan to ensure maximum system reliability. The power supply and fan assemblies should be removable and replaceable in less than five minutes to minimize any repair and maintenance downtime.
A rugged interface receiver simplifies mating to the supplies-under-test. The ATE can be wired to a single interface connector assembly. Multiple interface test adapters (ITAs) then can mate to the supplies-under-test. Sharing standard ITA and harness kit designs between engineering and production eliminates significant test costs.
Power supply ATE typically includes application software with the capability to perform 30 to 50 standard tests. The software should accommodate all standard tests without writing lines of code.
Data entry can be simplified by using spreadsheet-style input for power supply specifications as well as selecting and sequencing desired tests. Custom tests should be developed with a minimum of programming experience.
Finally, the application environment should offer the capability to output results in various formats. For example, engineering may want to print extensive scope screen shots, and production may simply want a pass/fail indication on the monitor. Network connectivity may be necessary to distribute test programs to the ATE under revision control or collect and analyze results for statistical process control.
A VXI-based ATE can be developed to perform both engineering and production test of power supplies. Reduced test development time results in significant savings for high-mix supply manufacturers.
About the Author
Kevin Leduc is a business development manager at Racal Instruments. He has held a wide range of sales and marketing positions since joining the company in 1991. Mr. Leduc received a B.S.E.E. from the University of Arizona in 1986, and prior to joining Racal, he was an avionics maintenance and repair engineer at Boeing. Racal Instruments, 4 Goodyear St., Irvine, CA 92618, (949) 460-6710.
Copyright 1999 Nelson Publishing Inc.