Upcoming Trade Shows CHINA HI-TECH FAIR 2008 Location: Shenzhen Convention and Exhibition Center Date: 12-17 October 2008 Booth Number: 2C52 Contact : Mr. Gordon Chen
Power Supply Output Filter Design - Overshoot(4 of 4)
Personal Anecdote
During the 1970's, a major semiconductor vendor released a widely distributed application note on the design a switching-mode power supply. As I recall, it was for a 5V, 12A, forward converter. Their recommended output filter resulted in a 15V overshoot on the 5V output when the 12A load was removed. No overvoltage protection was shown. This degree of overshoot would destroy most logic circuits for which the supply was intended, or worse, severely degrade their reliability without destroying them.
In this time period, few design activities were familiar with switching-mode power supply design and the application-note design was widely copied into system proposals. Instead of indicating the design activity was competent in this type design, it indicated the opposite and lost the vendor points during proposal evaluation.
Virtually every experienced switching-mode power supply designer has heard of some horror story associated with overshoot. The worst I heard was during the 1960's when a designer of commercial main-frame computers was working on a new prototype, which was powered by 60 Hertz saturable reactor switcher feeding a huge LC filter. A short occurred in the load establishing a large current in the inductor, and then the short cleared. The resulting overshoot destroyed all the logic circuits. It cost the program over a million (1960's) dollars. Needless to say, independent overvoltage protection became mandatory.
Because of many such horror stories, for many years you never saw a power supply without overvoltage protection. Now you do, indicating the lesson will have to be relearned the hard way.
Finally, it is important to note that it is impractical to meet all requirements in some designs. More than once I have had to relax overshoot somewhat and rely on the overvoltage protections circuit for abnormal events.
Do not use this information for design without independent verification of the information.
