Agilent Technologies AN 372-1 specifications An Overview of Power Supply Testing Needs

An Overview of Power Supply Testing Needs

Power supplies are used in a wide variety of prod- ucts and test systems. As a result, the tests per- formed to determine operating specifications can differ from manufacturer to manufacturer, or from end user to end user. For instance, the tests per- formed in an R&D environment are primarily for power supply design verification. These tests require high performance test equipment and a high degree of manual control for bench use. In contrast, power supply testing in production envi- ronments primarily focus on overall function based on the specifications determined during the prod- ucts design phase. Automation is often essential due to large volume testing, which requires high test throughput and test repeatability. Power sup- ply test instruments must then be computer pro- grammable. For both test environments, measure- ment synchronization is necessary to perform some tests properly and to obtain valid data. In addition, considerations such as test set reliability, protec- tion of the power supply under test, rack space, and total cost of ownership may be of equal impor- tance to the power supply test set designer. Proper selection of testing instrumentation will provide the best combination of measurement sophistica- tion and test set complexity.

Power Supply Testing Instrumentation

The power supply testing methods and configu- rations discussed in this application note are cer- tainly not the only means of obtaining the desired measurements. However, certain instruments are essential to all tests, regardless of the implementa- tion. Some commercially available turnkey power supply test systems incorporate custom board level instrumentation and hand wiring. However, power supply test systems based on standard products afford greater benefits. These systems are more reliable and provide repeatable, high performance measurements because of their low noise environ- ment. A system which utilizes standard instrumen- tation is modular, allows configuration flexibility based on performance needs, and is easier to upgrade. In addition, the service, replacement, or calibration of separate instruments in the system can be performed with minimal system down-time.

The tests covered in the following section are con- figured with standard instrumentation: electronic loads, digital oscilloscopes, digital multimeters, true rms voltmeters, wattmeters, and AC power sources.

Electronic loads can facilitate power supply testing in several ways. They are typically programmable, although most require external DAC programmers.

This capability enables finer control over loading values during testing, and can provide the test set operator with valuable status information. These loads are often designed with FETs, which provide increased reliability over less sophisticated solu- tions consisting of relays and resistors. Also, these products offer a selection of operating modes: constant current (CC), constant voltage (CV), and constant resistance (CR). The more sophisticated electronic loads provide all three modes in one product for optimum testing flexibility. They pro- vide a versatile solution for testing both DC voltage and current sources. A final advantage is provided by loads with readback over the bus. This can elim- inate the need for digital multimeters for voltage and current measurements in some tests. As men- tioned, there are varying degrees of electronic load sophistication. The Agilent Electronic Load family provides all of the most sophisticated features and high level performance in one box.

Several other instruments are required for power supply testing. The performance criteria (accuracy, resolution, stability, bandwidth, etc.) vary for each test. In general, the measurement capability of the instruments should ensure an error no greater than 10% of the measured specification. Table 2 on the next page provides a guideline for instrument per- formance levels for each test discussed in this application note.

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