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UCC28070: Considerations to Place UCC28070 Underneath Boost Inductor

Mehdi
Prodigy 160 points
Part Number: UCC28070

Hello,

Although TI recommends UCC28070 not be placed underneath magnetic elements, due to some restrictions I have to place the UCC28070 underneath the boost inductor. 

Could you please let me know what kind of considerations I should take to minimize the adverse effect of boost inductor on the controller? (it's a 6 layer PCB) 

Thanks.

  • 0
    TI__Genius 9140 points

    Hi Mehdi,

    thanks for your interest in UCC28070.

    Yes it is recommended that the UCC28070 be away from the megnetic component. The reason is that it is possible that UCC28070 couple magnetic noise especially on Vinac, Vsense/rsynth/Imo pin and cause abnormal uncontrolled phenomeon.

    actually we do see customer place the UCC28070 under the PFC inductor because of the space limitation, and it can still works well with special care on layout.

    I have two suggestions on you case:

    1.suppose UCC28070 is on layer 6, then it is better to have ground copper on layer 5, and connect the ground copper to a stable point, such as PFC bus cap negative connection.

    2.please make sure the PFC inductor core‘s air gap should not be perpendicular to the controller and ground copper, it should be parallel to the controller and ground.

    hope this helps you.

  • 0
    TI__Mastermind 40120 points

    Hello Medhi,

    Thank you for your interest in the UCC28070 PFC controller. 

    The main reason we recommend to avoid placing controllers below magnetic components is to avoid the influence of stray magnetic fields on the control signals.  Unless well-shielded, most magnetic elements have some level of external leakage field (some lower level, some higher level). You are fortunate to have 6 layers at your disposal to help shield the controller from these stray fields.    

    I recommend placing a wide copper GND-plane on a layer just beneath the magnetic element (inductor in this case I presume). If it cannot be on the outer layer due to high voltage restrictions, then in the next layer next to the outer layer. I recommend to keep the controller interconnections confined to the controller outer layer and 1 or 2 inner layers next to that outer layer.  Although blind-vias would avoid punching holes in the GND plane/shield, they are very expensive.  Assuming you will use through-vias (pads on all 6 layers), then control nets will appear on the top layer under the inductor.  Your freedom to arbitrarily locate these vias will be limited by other system circuit constraints.  I recommend to check each via's position with respect to the inductor to see if there could be any significant capacitive coupling from a nearby winding or switching node and relocate it away if possible.   

    Strive to minimize loop areas of control signal paths and power paths (which is a good design practice in any case) and keep them well separated or shielded.
    Placing a controller (and associated components) under a magnetic element can be successful, provided that appropriate measures are taken to decouple/shield magnetic fields from the signals.  

    Regards,
    Ulrich