The Engineer’s Guide To EMI In DC-DC Converters (Part 8): Common-Mode Noise Mitigation In Isolated Designs
by Timothy Hegarty, Texas Instruments, Phoenix, Ariz., How2Power Today, Feb 15 2019
Focus:
DC-DC converters operating at a high input voltage—such as the phase-shifted full-bridge and
LLC series resonant converter in applications such as electric vehicle onboard charging, data
center power systems and RF power amplifier supplies—can generate large CM currents. The effect
is more pronounced when applying gallium-nitride (GaN) switching devices, as they switch at
higher dv/dt than their silicon counterparts. A wide variety of techniques exist for mitigating
CM noise in isolated dc-dc designs, including symmetrical circuit arrangements, connecting a
capacitor between primary and secondary grounds, shielding, adding balance capacitors,
optimizing transformer winding design and using an adjustable CM cancellation auxiliary
winding. This article reviews these techniques, focusing mostly on flyback circuits.
What you’ll learn:
- How to mitigate common-mode noise in isolated dc-dc converters
- How to reduce CM EMI generation through choice of power supply topology, use of capacitors, and transformer design
- How to reduce CM EMI generation through transformer design including use of shielding and winding techniques
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