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UCC28740EVM-525: The reason to need L1 220uH inductor on UCC28740EVM

Motoyasu Hattori
Expert 8675 points
Part Number: UCC28740EVM-525

Hi,
On UCC2840EVM-525 schematics, we can find  L1 220uH inductor between diode bridge and transformer.  Could you please let us know the reason why L1 220uH inductor is needed?

Please advise us.

Thanks and best regards,
M.HATTORI.

  • 0
    TI__Mastermind 26730 points

    Hello Hattori-san,

    The L1 inductor (220uH) is used to reduce differential-mode (DM) conducted-EMI from the EVM.  It is not required to be placed between the bridge and the transformer, but there is some advantage to placing it here.

    By splitting the bulk capacitor into two smaller caps, the DM choke between them creates a pi-filter which helps to keep the high-frequency switching currents local to the inside loop.  L1 blocks most of the HF current from getting past the diode bridge onto the AC line. 

    A DM choke can also be placed on the AC side of the bridge, but the X-cap values must be much smaller there, so the pi-filter would not be as effective.  Using the much-higher bulk capacitance helps to make the poles of the DM filter much lower.

    The value of the DM choke is chosen based on making a trade-off between the effectiveness of the filter and the cost and size of the filter. Higher inductance will attenuate the DM noise more (provided parasitic winding capacitance is low) but will be larger in size to avoid saturation at peak current and will cost more.

    Regards,
    Ulrich