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LMR16006: How do this ringing occur?

Tristan Gao
Prodigy 60 points
Part Number: LMR16006

Tool/software:

I am testing a buck converter evaluation board LMR16006EVM. This LMR16006 has an ECO mode which terminates switching when the load current is very low.

This is the schematic of LMR16006EVM.

The board is connected to an electrical load with constant current mode. The current condition ranges from 0.001A to 0.100A.

I am testing the SW signal, and I get this waveform as follows.

There seems ringing occur in this scenario.

1. I am confused whether this waveform is caused by ringing or resonant.

2. If this is caused by resonance, what is the mechanism by which it is formed and which devices are involved? 

3. If this is ringing, why it is different with the overshoot of PWM signal.

4. How to eliminate this anomaly?

Thank you every much!

  • +1
    TI__Guru* 76295 points

    Hi Tristan,

    Thank you for posting.  The expert of this device is currently out of office and I have notified him to take care of follow-on support after he returns.  Here let me try to answer you.

    The waveform is typical in DCM operation, in which the inductor current falls to 0A before the next cycle starts. The inductor and the Coss of the high side FET as well as the parasitic capacitor of the rectifier diode will resonate.  

    Hope this clarifies.

    Regards,

    Youhao

  • 0 in reply to Youhao Xi
    TI__Prodigy 60 points

    Hi Youhao,

    Thank you for your answer! I now know it is caused by resonance. So does that mean this resonance is caused by the previous switch?

    If I want to reduce this resonance, should I add a resistor in series?

    How is this problem solved in practical way?

    Thank you very much!

    Best regards,

    Tristan

  • 0 in reply to Tristan Gao
    TI__Guru* 76295 points

    Hi Tristan,

    Why do you want to eliminate it?  Like said, it is the natural resonance, which is a normal behavior during dead time. All buck with diode as the freewheeling diode would behave like this at light load in DCM. 

    To reduce the resonance frequency and maybe magnitude, they can increase the inductances to slow down the resonance, but I don't think it is necessary.

    If the customer really dislike it, they should choose a circuit with synchronous rectifier operating in FPWM, then there will be no dead time and such resonance is eliminated completely. 

    Thanks,

    Youhao