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TPS40305EVM-488: EMI Reduction

Part Number: TPS40305EVM-488
Other Parts Discussed in Thread: TPS546C20A, TPS543B20, TPS56121, TPS56221, TPS40305

Hi team,

A customer is evaluating TPS40305EVM-488. They already turned on Spread-Spectrum but are measuring too much EMI. Do you have suggestions on how to reduce EMI (input/output filters) or an alternative device suggestion for less EMI?

Thank you,



    Is the customer concerned with conducted EMI on the input/output, or are they concerned about radiated EMI?

    What frequencies are they seeing "too much EMI" ?

    Spread Spectrum on a switch-mode power apply is typically focused on reduction conducted RF energy at the switching frequency and it's first several harmonics - frequencies upto a couple of Megahertz.  By varying the switching frequency and the on-times accordingly, they can spread the noise from the switching frequency and its first several harmonics over a wider band by moving peak noise around rather than continually sustaining it at the same frequency.  It does not help with very high frequency noise that is emitted from the resonant tanks of the switching nodes, which is typically in the 50-200MHz range.

    To reduce radiated emissions from the switching node's parasitic tank circuits, there are some techniques that can be used to either reduce the energy in the resonant tank, or dissipate that energy as heat rather than emitted EM radiation.

    Adding resistors between the LDRV, HDRV, BOOT and SW pins and their corresponding nets on the high-side and low-side FETs.

    The EVM only has a location for the BOOT resistor (R5) which is defaulted to 0-ohms.  You can increase it as high as 4.7-ohms if needed.

    Adding an R-C snubber from the switching node (Source of high-side FET and Drain of low-side FET) to GND (Source of the low-side FET) or VIN (Drain of the high-side FET)

    The EVM has locations R7 and R12 for this, but they are not populated on the stock EVM.

    Improving the impedance of the bypassing from the drain of the high-side FET to the source of the low-side FET by rotating the FETs, or placing one on either side of the PCB.

    Adding additional input or output capacitors directly under C1, C2, C8, and C9.

    Adding a 10nF 0402 bypass capacitor from the drain of Q1 to the source of Q2, even if that means moving them slightly apart.

    You could also consider selecting one of TI's dual FET converters that keep the FETs themselves closer together with lower parasitics by stacking them inside a single package, or one of our integrated FET converters, such as the TPS56121 / TPS56221 which have similar control characteristics to the TPS40305, or the TPS543B20 / TPS546C20A which integrate an internally compensated Advanced Current Mode Control architecture with high-current integrated power FETs.