Problem - The addition of an input filter to a
switching-mode power supply can cause the combination to go unstable and oscillate. The Bode plot of the voltage-loop gain may not indicate a problem even at the onset of instability for current-programmed converters.
Relevance - Adding an input filter to a
switching-mode power supply or installing it in a system with an EMI filter can degrade its performance or cause it to go unstable. Long input cables or added output capacitance may cause the same problems.
Solvability - To quickly determine if there is a problem, a graphical technique for duty-ratio programmed converters known as the Middlebrook Criterion is used. The criterion has been expanded to current-programmed converters by Jang and Erickson.
Solution - For optimum design, the
switching-mode power supply and the input filter or EMI filter should be designed together using criteria developed for this purpose. The criteria for both voltage-programmed and current-programmed converters is in a paper by Jang and Erickson.
Personal - A personal anecdote.
On the Web - Additional information on the Web.
References - Key papers including a timeline.