The Engineer’s Guide To EMI In DC-DC Converters (Part 11): Input Filter Impact On Dynamic Performance
by Timothy Hegarty, Texas Instruments, Phoenix, Ariz., How2Power Today, Jan 15 2020
Focus:
A dynamic coupling between a dc-dc converter and its EMI filter effectively creates a feedback
loop, where the source-side “minor-loop” gain is the ratio of the filter’s output impedance to
the converter’s closed-loop input impedance. Part 10 showed that impedance shaping (via passive
damping of the input filter) is required for robust stability. Another concern is that input-
filter interactions may severely affect the transfer functions related to dynamic performance
of the converter. The impact of those input-filter interactions on the converter’s dynamic
performance are analyzed here. This discussion begins by presenting a two-port model that can
be used to analyze converter dynamics of any dc-dc converter and this model is used to derive
equations that express system sensitivities to source-side and load-side interactions. Transfer
functions are given that express the influence of the source-side (filter output) impedance on
converter loop gain and output impedance, as well as the impact of load-side impedance on
converter loop gain and input impedance. These expressions are used to explain the relative
immunity of a voltage-mode-controlled buck converter with input voltage feedforward to source-
and load-side interactions. For other cases, equations are given that define what conditions
must be met to minimize such interactions.
What you’ll learn:
- How to analyze and mitigate the impact of input filter interactions on the dynamic performance of a dc-dc converter
- How to understand why a voltage-mode-controlled buck converter with input voltage feedforward is relatively immune to source- and load-side interactions
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