Switching Power Supplies And EMI: Debunking The Myths About Frequency And Slew Rate
by Gregory Mirsky, Continental Automotive Systems, Deer Park, Ill., How2Power Today, Apr 17 2018
Focus:
The majority of switching power supplies use fast switching of power components to
diminish dynamic losses in these components. It is conventional to think that fast
switching components may create issues with electromagnetic compatibility (EMC). It is
true that short pulses with very steep edges have widespread spectra. The process of
such switching may be represented as a rectangular pulse train, which may be described
with a pulse repetition frequency, switching slope slew rate and duty cycle. All of
these parameters affect the pulse-train spectrum but, as this article explains, seldom
does this spectrum produce components that exceed the FCC limits for conducted and
emitted EM radiation. It mathematically analyzes how pulse-train frequency, slopes and
duty cycle affect the signal spectrum and what part of the spectrum can be radiated.
Ultimately, it addresses the question of whether the reduction in EMI achieved by
slowing down switching edge rates is worth the added power dissipation.
What you’ll learn:
- How to obtain the signal spectrum of a trapezoidal pulse (representing a switching waveform) and analyze its dependence on the pulse parameters
- How to determine whether the improvement in EMI from slowing down switching edge rates is worth the added power dissipation
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