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LM5121: Step Load Behavior

Chuchen Wang
Expert 3470 points
Part Number: LM5121
Other Parts Discussed in Thread: LM5122EVM-1PH

Hi,

My customer has the following question;

I was working with the LM5121EVM-1PH with 24V/4.5A output and noticed something about the response to load transients. If we give it a stepped load of say 40W-90W, the input PSU (set to 11V / 10A(max)) will latch at something like 5V/10A with the current maxed out and voltage pulled down. I think this happens at lower wattages too (don’t have it handy in front of me to confirm), so it seems like we inducing some kind of instability in the boost. The EVM performs fine across all loads if we use the “Soft” load-ON feature of the e-load to apply the load (vs just plugging the load in to connect like in our test).

  1. Is this the expected behavior for a step load on a boost regulator? What is actually going on here that causes the short circuit on the PSU? Is the inductor saturating?
  2. If so, is there anything we can change in the circuitry that makes it stable with a stepped load?
  3. Is there a circuit you can recommend for ramping a 100W load up at the right speed that is not huge (in order to dissipate 100W)? Perhaps an inline inductor?
  4. Do you think a possible solution might be to force passthrough mode while load is connected and then use some kind of soft ramp up in the feedback voltage loop to bring the boost up to our desired load voltage?

Thanks,

Chuchen

  • 0
    TI__Guru* 78200 points
    Do you mean the LM5122EVM-1PH? We do not have the LM5121EVM-1PH.

    From you description your PSU is the problem. At step load response the input current can rise up quickly to provide extra current to meet the output load requirement. This increased input current may exceed your PSE 10A limit, tripping its OCP and lose the supply voltage. The EVM user guide recommend 25A capability.
    1. It is normal. It does not look like inductor saturation, which is protected by the peak current limit.
    2. You should use a PSU has higher current capability. Please follow the EVM user guide recommendations.
    3. See above.
    4. You can slow down the loop response time by lowering the loop bandwidth.