During my normal activity as an Analog Field Applications Engineer, I am called to help a customer solve system problems. These system problems sometimes relate back to the power supply. Last year, over a period of 3 months, I had 3 different situations at different customers where the root cause of a system problem was a relatively straight forward power supply design problem. Systems were in production, but problems were arising based on intermittent field failures. In each case, I discovered that the power supply needed a minor design change which could have easily been identified and implemented before the system was released to production.
What I found most interesting was that because of our advancement in software design tools, customers are placing much more trust in power supply designs without performing what used to be fairly standard bench testing. In cases, I observed system designers used the results from simulations tools as a strong indication of the robustness of a design, thus not placing as much time (if any at all) on actual bench testing of the power supply. This observation, with the fact that many designers have never gained experience with bench testing, left me with the clear realization that an article was needed on the subject. I sometimes wonder how much of this relates back to the current de-emphasis on hands-on design and test in universities, where software tools are replacing these tasks… but that is a topic for a future blog.
That being said, why is testing power supply so important?
As explained above, if a power supply is not fully tested it may leave a system in a marginal state of operation. A marginal design can result in system failures once the system is deployed and operating in different environments. System components vary slightly over time which can result in failures of a system that was not fully tested and/or analyzed.
Here is a simple procedure for accurately measuring efficiency:
- Before connecting the DC power supply to your power circuit, set the proper input voltage and verify correct polarity.
- Connect a voltmeter to the input and output of the power supply close to the input and output connectors.
- Connect a current meter to the input and output, see earlier comments.
- Connect the electronic load to the output and set it to the lowest value of interest.
- Turn on the input voltage and set it to provide exactly the nominal input voltage across the power supply input. Important: Input voltage accuracy may need to be within a few millivolts to ensure overall accuracy and needs to be adjusted after each time the load is changed.
- Record the input and output current along with the output voltage. The input voltage is fixed by step 5.
- Increase the load at regular intervals up to a load at or above 100 percent. Testing up to 110 percent of maximum load or greater is valuable to help understand operating margins.
- Plot curves of the output power over the input power (*100) to show the peak efficiency and efficiency at different loads (Figure 3)
Find more detail on how to measure efficiency of a power supply in my full article on EDN.
You can walk through the process with me in this Engineer It video on how to measure efficiency of a power supply:
Part 2 on measuring noise, and part 3 on measuring stability coming soon.
Why do you think fewer designers are testing their power supply?