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INT- LM5118 EMI issue

Miteshkumar Dalabhai Patel
Intellectual 2450 points
Other Parts Discussed in Thread: LM5118

Hi ,

One of my customer is working on Automotive design for which they are working on LM5118. LM5118 is working ok as per specification & no issue with electrical specification. They are now planning for EMI/EMC test & during pre scan LM5118 was failed. When they bypassed LM5118 whole unit passed with comfortable margin. 

LM5118 is used as buck boost for 12V to 12V/2A output requirement. We have reviewed the layout & it is not the best one or we can say there is a scope for improvement. We have shared our inputs for the placement & layout. 

When we probe the switching node with active probe(ground lead is not short but not long also) we observed 60MHz ringing. We also tried with EVM & that is also showing the same with lesser amplitude.

We suggested to keep provision for pre & post LC filter in present schematic with ceramic close to switching elements.

Can you guys help with your expert comments, so we can share with customer? We also observed that after adding the 5 Ohms of gate drive resistance ringing amplitue came down.

If anybody has observed the same, can you help with the suggestions.

Regards,

Mitesh

  • 0
    TI__Mastermind 21720 points

    Mitesh,

    Just to clarify the only radiated emissions test is failing correct? Or is conducted emissions test failing too?

    The ringing on the switch node can cause failure of EMI testing. A RC snubber can be used on both or the switch nodes to eliminate the the ringing and help pass EMI. And like you mentioned, a gate resistor can be used to reduce the amplitude of the ringing due to the MOSFET being turned on slower than without a gate resistor.

    Usually EMI issues can be fixed with layout changes, but it sounds like you have footprints for EMI filters which can be easier then doing a complete layout change. Here is an article that goes into detail about how to design the filters.

    Let me know if you have any questions.

    Thanks,

    Garrett

  • 0 in reply to Garrett Roecker
    TI__Intellectual 2450 points

    Hi Garrett,

    On this board they are failing in both radiated & conducted.

    We have already tested with pre-filter & observed good improvement at input side. Now we are more worry for radiated, so I think sunbber on both side, decoupling very close to switching element & placement of all the major power component will be very critical.

    Thanks & appreciate your comments.

    We will update you once we for scanning.

    Thanks,

    Mitesh