The Engineer’s Guide To EMI In DC-DC Converters (Part 10): Input Filter Impact on Stability

Focus:

Complying with regulations designed to limit conducted electromagnetic interference (EMI) usually requires the insertion of a low-pass EMI filter between a switching power converter and its source. Part 2 of this EMI article series provided a detailed perspective of noise propagation and the requirement for both differential-mode and common-mode input filtering as an essential part of switching power-supply design. However, dynamic interactions may occur due to a poorly damped EMI filter subsystem when connected to a regulated dc-dc converter. Here in part 10, the interaction between EMI filter and dc-dc converter is addressed to explain its impact on overall system stability. Following an analysis of cascaded systems and the impact of impedance interaction dynamics on stability, this article presents simulation results using a synchronous buck controller with voltage-mode control to illustrate the important characteristics of the stability criteria. The discussion covers Middlebrook's criterion for stability as well as some less conservative criteria/design rules for stability.

What you’ll learn:

• How to analyze and simulate the effect of input EMI filter impedance on power supply stability
• How to determine power supply stability using Nyquist stability criteria
• How to eliminate EMI-filter-induced power supply instability by applying a damping network

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Author & Publication:

Timothy Hegarty, Texas Instruments, Phoenix, Ariz., How2Power Today, Nov 15 2019

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