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LMZ1420xH High Frequency Radiated Emissions

Prodigy 160 points

Replies: 14

Views: 1090

Hello,

I use an LMZ14201H in a certain circuit with the following parameters:

Vin: 16-32V

Vout: 11V

Iout: 600mA (max)

Frequency: 330KHz 

For some reason we have a peak of Radiated EMI at 70MHz approximately.

Are there any known issues or solutions?

We had in the past serious problems with conducted noises in low load conditions, due to the DCM/CCM transitions, we solved this problem, but the 70MHz radiated is hard to explain.

Thanks

  • In reply to Emanuel Marinescu:

    If you are in shielded enclosure then radiation from the PCB may not be the issue but ,in general, the return circuits should be on the ground plane not coplanar with the current carrying trace. Also, the components on the "clean" side of T2 ( C4,C5,FB1,FB2) should be away from the "dirty" ground; no ground copper should be adjacent or below these components. The purpose of T2 is to reduce common mode noise so you don't want to re couple it afterwards.

    As far as the filters go, I think there is a potential for a resonant circuit to be formed at these high frequencies. It might be useful to rearrange the caps C9,C10,C11 and C12 to potentially change the high-frequency behavior. This won't affect your low-frequency performance.

    EMC testing is quite frustrating at times, sometimes the very littlest things can make a big difference.
  • In reply to joe baldwin:

    I got your point about the grounding plane and I agree. And also I think maybe to reduce the number of capacitors, in similar designs they work and behave much better. By the way, speaking about ground plane sensitivity, I have several circuits assembled on single sided MCPCB and also work with no EMI noise at all.
    The problem I had here is a combination of two requirements:
    1- To work in standby mode (no load) with a very low current consumption
    2- The load changes from 10mA minimum to 600mA maximum.
    In standby and minimum load I had serious problems with the conducted emissions, as the LMZ is switching between DCM/CCM modes and creates a lot of noise at low frequencies, 10-150KHz.
    I solved part of the problem by shutting down the LMZ completely at standby, and filtering at minimum load.
    But this created the present problems.
  • In reply to Emanuel Marinescu:

    Emanuel,

    The easiest way to tackle radiated EMI noise is to place very small capacitances, very close to the input and output of the IC. Start with values that are roughly between 470pF and 4.7uF. One or two small capacitors should be sufficient and will provide a high frequency bypass route.

    The other option is to employ a LISN device for each DUT. Very frequently the cabling to and from the board carry large chopping currents that radiate out and interfere with the readouts. I had this happen to me once and solved it by looping the input and output wires from the DUT around a ferrite bead or torroid.

    Let me know if this helps,
    Thanks,
    Anston
  • In reply to Anston Lobo:

    Hi Anston,

    Thanks for the answer, I will try this of course and also the ground plane change as suggested by Joe.

    Let me add that the situation we are facing is quite complicated. I had a lot to say about the test procedure which was made by the customer, which I think it was very wrong. The 461F allows reproducing the real installation conditions, but what they did is checked 2 units, first each one independently and somehow they passed, not brilliant results but below the limits. When connecting both of them the plots were as shown, and the over the limit peak at about 150MHz was only in horizontal antenna polarization, vertical passed. This clearly shows the noise is going out through the cables.

    They connected the cables separately for each DUT, giving a total of 4-5m non shielded cable. Of course, only 1 pair of LISNs was used.

    The problem is they won't agree to change anything, adding ferrite or using shielded cables. 

    They also claim the reputable lab insists the procedure is correct and above all they tested a competitor's product which passed the test with the same setup.

    I wish the solution will be as simple as adding some capacitors or changing the ground plane, given the expected quantities I think both us and TI will be happy.

    I will test the suggested solutions and keep you posted.

    Thanks,

    Emanuel

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