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LM5180EVM-DUAL: radiated emission

Davide Tosatto
Prodigy 100 points
Part Number: LM5180EVM-DUAL
Other Parts Discussed in Thread: LM5180, LM25180

Tool/software:

Hello everyone,

I am evaluating a LM5180EVM-DUAL and I saw something strange. With the device turned ON my oscilloscope is measuring a signal without having connected any probe.

The measured signal is this:

If you can appreciate its shape it has the same shape as the voltage measured between the SW pin, or across the primary side inductance.

If I get closer to the evaluation board with the voltage probe the noise increases.

If I remove the voltage probe "hat" and I use a short ground loop voltage probe the noise is reduced, so I think the "hat" is acting like an antenna.

Have you ever faced this behaviour? Could these emissions cause problems in nearby circuits? How could I lower these emissions?

Thank you,

Davide

  • +1
    TI__Guru* 78300 points

    Hi Davide,

    Thank you for testing with the LM5180 EVM.  You are right that the long ground lead of the probe makes an antenna.  Since the probe is basically a high impedance circuit (10 Meg Ohm and 10 pF?), the unrealistic noise may appear large on the scope. 

    Generally, the switch mode power supply including the flyback is always a source of EMI generator, but with proper treatment in circuit design and PCB layout optimizations, the EMI can be well attenuated both in the conducted emission and radiated emission. Regarding your last few questions, let me answer from a different angle: the LM25180 has been used by many customers in mass production. 

    Depending your system requirement, you may add additional EMI filter or shielding if necessary, but avoiding overdesign is another aspect to consider.  In other word, you do have a valid concern, but treat it further only if you see the need by test, other than by speculation. 

    Hope this clarifies.

    Regards,

    Youhao

  • 0 in reply to Youhao Xi
    Prodigy 100 points

    Thank you so much Youhao for the reassurance, you clarified my doubts.

    Best regards,

    Davide