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LM5176: LM5176

Part Number: LM5176

I am in the EMC test lab.
The problem is that when the LM5176 VIN DC input is connected to various test equipment, in some setup’s it is fine and in others it is not. It does not matter which voltage input I use. The designation of 48Vdc is actually 24Vdc.

In the problem configurations, the symptom is that the powerup starts and then shuts down and then back up and down again. It never completes the start up.

Now that I have done additional testing I suspect that the problem is probably due to the clients product reacting to the Coupling-Decoupling-Network (CDN) circuit that is in the Surge test equipment. I do not have the measurements to confirm, but I suspect it has to do with inrush current during bootup the inductor value in the CDN used for a Surge Generator or Line-Impedance-Stabilization-Network (LISN) used for measuring conducted emissions. I am now investigating what the client will have to do to their product to fix this problem.

a typical LISN will have 50uH inductor. A problem LISN, used for low frequency conducted emission measurements, with an additional 250uH choke in front of the 50uH, the unit does not power up before it is shut down.

My concern is that if this clients product has too much inductance in front from an external filter, the product will not power up. I do not have a variable inductor to verify at which level causes this problem.

Have you seen this sort of problem before?
Will adjusting the SS capacitor value from 0.1uF to 1uF resolve this problem?
Suggestions?

  • Hello David,

    Thank you for sharing the schematic (is this ok for your customer?). It is somewhat blurry and therefore I am not sure, but it seems that there is a bulk capacitor on the output, but no bulk capacitor on the input.

    Now when the device starts up, it needs to charge this capacitor in the given softstart time which will cause high inrush current.

    If there is not enough current coming through the LISN, the input voltage will most probable drop and the device will fall into UVLO.

    So increasing the softstart capacitor will reduce the inrush current and therefore the converter more likely can start with a lower input current capabiltiy.

    If it solves the issue, is dependent on the source capability.

    In general increasing the softstart capacitor is the first solution I would check.

    Another possible solution would be to increase the input capacitance, that the inrush current is somewhat spread in addition.

    In addition, checking the load current at startup and if possible reducing it, would be good as well.

    Best regards,
    Brigitte