Personal Anecdote
R. David Middlebrook changed all this when he published a 1975 paper and expanded on it in a 1976 paper describing a measurement technique using a tuned voltmeter that eventually gave some credibility to gain-phase measurements of switching-mode power supplies. When I and others started to use Middlebrook's techniques, it took about 40 hours, starting with a simple RC circuit and graduating to switching-mode power supplies, to learn how to make them with confidence -- and they were always tedious to make. The next breakthrough came in the late 1970's and early 1980's when people started to successfully put together systems based on instruments from Hewlett Packard, Schlumberger, Nicolet, and Bafco. Some of these systems took man-years to put together and debug -- and then gave mixed results. In 1980 Dean Venable experimented with various systems, published a paper and founded Venable Industries that provided turnkey systems, including accessories and documentation, for making these measurements. Dean Venable is no longer associated with Venable Industries. However, Venable Industries is still a leader in this field and Dean Venable has kept publishing papers on the subject (see references). Some of Venable Industries computer controlled systems have gotten quite sophisticated. In the late 80's, some of the major measurement companies like Hewlett Packard, figured out how to do it and gave some seminars, but it still took some figuring out to use their equipment. Lately, some other companies, such as Ridley Engineering, have entered the field with low cost approaches. As a finishing touch to the problem, R. David Middlebrook has published his General Feedback Theory, which discusses the optimum method of injecting signals to measure loop gain. These developments have turned the once very difficult task of measuring the gain-phase of switching-mode power supplies into something manageable. But you still have to be careful. If the signal distorts on you, then things are no longer linear, and under these conditions even sophisticated equipment can give erroneous results. This is the most common mistake with the new equipment. I always monitor the input and output signals with an oscilloscope so I can either control distortion, or at least know not to trust the measurements when, as at some resonances, distortion is inevitable. Finally, a comment on injecting white noise across the reference. I noticed the problem when one of my 25 KHz switching regulators developed a 12.5 KHz subharmonic ripple when in the system. When in the lab or on an extender board, there was no subharmonic. Thinking it might be related to noise, I injected white noise into the reference and other circuit nodes. When the white-noise generator hit a certain level, the subharmonic would appear. Increasing the level started what I now know as bifurcation to other harmonics, probably on the way to chaos, which at the time, I had never heard of. On the WebArdem Associates, R. David Middlebrook's website, describes a seminar that covers his techniques of loop analysis and measurement. The site includes Dr. Middlebrook's annotation of his papers covering his techniques of analysis and measurement with the pioneering papers on measurement of switching-mode power supplies. Venable Industries provides many uses of a network analyzer and a list of his papers, many with full text. Ridley Engineering's pages provide additional information including his page on Design Tips which gives some solid information on closing the loop. It also has a list of his papers with some excellent annotations. References |
