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Input filter interaction with SMPS (2 of 13)

Relevance

Cause. Problems are usually caused by the addition of an input EMI filter, but can also be caused by long cables to the input power source, and in some cases, by added load capacitance.

Input filter interaction with SMPS (1 of 13)

Summary

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.

Chaos in Power Electronics & Power Supplies (5 of 5)

Personal Anecdote

I remember when chaos entered my life in the '60s (a chaotic decade). I reported to my manager that, as far as I could tell, the circuit behavior of my 20kHz "bang-bang" regulator was controlled by the phases of the moon. He did not take kindly to blaming the moon for my circuit's misbehavior. I changed to a PWM control and the problem went away. Later, when working with a discontinuous mode boost converter similar to that reported by Tse, I saw similar waveforms. In hindsight I feel vindicated in being perplexed. It was 20 years later, in 1984, when the problem was first discussed at a power electronics conference. We now know a lot more about it and more is becoming available.

Chaos in Power Electronics & Power Supplies (4 of 5)

Solution

The following are some considerations for solving chaos problems.

Stable
Investigators wanting to create chaos often start with nearly unstable circuits. This suggests avoiding chaos by making sure the circuit is robustly stable.

Chaos in Power Electronics & Power Supplies (3 of 5)

Solvability

Nonlinear
Chaotic circuits are always nonlinear, which means there is no general analytical methods for solving them. There is no a priori criteria for determining if a nonlinear circuit will be chaotic.

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