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SWIFT Power Supply: Is it better to shield or not to shield around the inductor??

Phil Rickert
Prodigy 20 points

Hi,

Does anyone have experience enough with Conducted and radiated EMI to answer this question for me? Recently a product I designed with the swift family converter failed emissions. The board has 4-layers with a ground plane under the inductor and another ground plane under the entire power supply on the second layer with lots of stitching vias. Is it better to leave an absence of copper through the board directly below the inductor and the active driving trace from the fet, or is it better to keep it shielded? Much of my conducted noise seemed to come from what appears to be inductive coupling into the plane from the inductor over it?

Any help would be appreciated.

Phil

  • 0
    Prodigy 30 points

    Never route under inductor even on 4-layer PCB with internal GND. This is a bad design practice. If it is impossible to accomplish this, try at least not to put vias in this area (or leave vias only  to GND layer).

    Good luck!

  • 0 in reply to Kostyantyn Koziy
    TI__Guru**** 191445 points

    Wit regard to inductor routing, the key gactor is to make the switching node connection (PH) the proper geometry.  It should be a s short as possible and somewhat wide to prevent it acting as an antenna.  I never route any signals directly under the inductor or switching node trace.  However with regard to backside ground planes, we typically pour those over the entire PCB area includig under the inductor.  If your particualr design is having issues due to coupleing on to the ground plane, you can certainly try removing the ground area from under the inductor.  Also, be sure to use a shielded type inductor.  Tracing EMI issues is always complex, especially with switching power supply circuits.

  • 0 in reply to JohnTucker
    Prodigy 20 points

    Hey thanks much for the replies! I did end up pouring the ground plane under the entire power supply area (The nice thing about multi-layer boards!), However on the immediate surface under the inductor and around the PH trace I left a relief of 50 mil. to the surrounding ground plane. I Also fattened the PH trace as much as possible. The layout looks pretty cookie-cutter out of the data-sheet. Also of course am not routing any signal lines near or under the PH trace or inductor. I laid out the feedback circuit to the data-sheet recommendation. I also added stitching vias where I could around the device and near the commons on all the filtering to the ground plane, again as recommended in the datasheet.

    Just a side note about the plane. I found a reference tracing back to National (forgive me) that says a ground plane under an inductor is a good idea to help absorb some of the EMI field. The important thing however is the plane must be a continuous as possible to avoid setting up currents and additional harmonics

    Thanks Again![:D]

    Phil