Design Con 2015

Testing a power supply (Part 1)

Robert Hanrahan -April 02, 2013

Read Part 2 of this series which covers noise, and Part 3 on stability.

This three-part series describes how to properly test a DC/DC power supply, and ensure that it works reliably over various operating conditions. This series is intended to provide the designer with a sufficient understanding about some, but not necessarily all, of the testing needed to verify a reliable power supply design. Be sure to check out the two videos embedded in this article for even more detail.

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Introduction  

In my 20 years of being a field application engineer, I have seen a fair share of power supply designs. In many cases these designs work without any issues.  Occasionally I found problems that could have been avoided with a little extra engineering effort before releasing a product to production. All too often system designers employ a power circuit without thoroughly ensuring its proper operation throughout operating extremes. Situations exist where prototypes work fine, so further power supply testing gets overlooked, or is the last item to be checked for proper operation. Sometimes issues don’t arise until the product is in production, resulting in field failures.

These power supply problems vary greatly, from system noise that didn’t show problems with a prototype, but later caused system performance issues, to power supply stability problems, which resulted in intermittent system shutdown. The focus of this article is on the different tests associated with DC-to-DC power supplies.

Why test?

The power supply is the foundation for any electronic product, so verifying its performance and design margins is necessary to ensure a high quality and reliable product. Not verifying a power supply leaves a designer vulnerable to a potentially unpleasant situation, if problems arise after products are in the field. Power supplies may operate fine under typical conditions, but may be at the edge of normal operation. When a power supply is heated or cooled, or when components age, its characteristics change to a point where a marginal design might fail.

No matter how basic a power supply may be, it should be tested by a qualified individual to ensure it meets system requirements. Although software might need to be written or FPGAs fully debugged, it is critical that the power supply be verified it is working properly and operating with sufficient design margins.

Power supply testing is not complex. One only needs a good understanding of which tests are needed, and how to properly perform them. A designer should establish a test specification and a test plan for the power supply. The test specification should include all acceptable operating limits and the various operating conditions (temperature, line conditions, and so forth), under which the system must operate. A test plan describes the process on how to ensure the design meets the test specification.  

System conditions (line, loads, etc.) and the environment vary greatly from application to application. Therefore, specific test specifications and plans vary from one system to another. This article does not discuss philosophy regarding design margins for quality products, but assumes the design test specifications are well understood. We focus on sound methods to test and verify that a design meets or exceeds predetermined specifications.

Simulation

Component modeling and simulations have made significant advancements, giving designers great design tools for expediting power supply designs. In some cases it is difficult to accurately model a system power load, especially complex systems, so simulations must rely on some level of assumptions.

Large systems, which include a wide range of impedances on the power rails, can cause unexpected power supply performance characteristics – which only accurate testing can uncover. Power supply simulation tools such as TI’s WEBENCH™ help expedite a sound design providing the engineer with an excellent starting point for hardware creation. However, only system bench testing can accurately provide actual circuit characteristics over operating extremes.

Test equipment

The test equipment needed for proper power supply testing varies with the type of power system being tested, as well as the financial budget for the equipment.  Here is an example list of equipment, which will be referenced later.

  • DC power supply capable of developing voltage and current for the specific design. A programmable version is preferred. ·        
  • Electronic or dynamic load capable of handling your system requirements: voltage and current. A programmable version with load-step capability is preferred.
  • Two volt meters, accurate to the desired specifications.
  • Two current meters, or low Ohm power resistors with additional voltmeters. A current meter within an electronic load may be sufficient for one meter.
  • Oscilloscope 500 MHz BW or greater with probe for noise measurements.
  • Frequency response analyzer or network analyzer designed for power supply stability measurements.

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