Switching Power Supplies And EMI: Debunking The Myths About Frequency And Slew Rate

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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Author & Publication:

Gregory Mirsky, Continental Automotive Systems, Deer Park, Ill., How2Power Today, Apr 17 2018

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