The Engineer’s Guide To EMI In DC-DC Converters (Part 9): Spread-Spectrum Modulation
by Timothy Hegarty, Texas Instruments, Phoenix, Ariz., How2Power Today, Aug 15 2019
Focus:
Spread-spectrum frequency modulation (SSFM), or frequency dithering, is a way to
distribute the spectral energy produced by a switching power supply’s switching
frequency and its harmonics. It flattens the fundamental and harmonic noise peak
amplitudes to achieve electromagnetic compliance. SSFM is an additional and
complementary method of noise reduction with respect to the EMI mitigation techniques
described previously in this series. Here the author explains the math behind SSFM and
details of its practical implementation. This includes explanation of the three
different modulating functions (sinusoidal, triangular and exponential), the spectra
they produce, and tradeoffs in modulation parameters. The example implementation is a
four-phase synchronous buck regulator based on the LM5143 controller with triangular
spread-spectrum modulation. Different methods of achieving SSFM are noted, example
modulation parameters are calculated, and spectrum results are presented.
What you’ll learn:
- How to understand the principles and application of spread-spectrum frequency modulation to EMI reduction in power supplies
- How to implement spread spectrum frequency modulation in a dc-dc converter
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