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LM5045: Unstable light load operation - 2kW full bridge converter

H Win
Prodigy 10 points
Part Number: LM5045

Hi

We are using the LM5045 in one of our projects, with an output power of about 2kW. The converter operates across a wide output range, including CC or CV operations at 0-100%.

The issue we are encountering is instability and oscillation at light loads (low impedance load for CC operation). We have verified the stability of the PFC section, and the bulk voltage remains very stable. Loop analysis for the DC-DC side shows good gain and phase margins (>10dB for Gain and >60 deg for Phase) until the output becomes unstable.

Is there anything specific we should consider for this light load condition (less than 10% of full load)?

Any assistance would be appreciated. Thank you.

  • 0
    TI__Guru 54055 points

    Hi,

    What is the unstable behavior? 

  • 0 in reply to Hong Huang
    Prodigy 10 points

    The output oscillates a lot with a peak-to-peak value of 200% of the current drawn. eg., Set current = 5A, the peak to peak current ripple is about ± 5A. 
    The oscillation frequency is around 10kHz while the switching frequency is about 250kHz.  

    Is there any FAE support we can get in Singapore? We want to solve it asap. Thank you.

  • 0 in reply to H Win
    TI__Guru 54055 points

    Hi,

    I am not understanding your description - you said you measured the bode plots with > 60-deg phase margin while at this condition your had +/- 5A oscillation - how were you able to measure the bode plots in such an operation?

  • 0 in reply to Hong Huang
    Prodigy 10 points

    "Loop analysis for the DC-DC side shows good gain and phase margins (>10dB for Gain and >60 deg for Phase)" This condition is just before the output becomes unstable. 

    When the output becomes unstable, I couldn't get the bode plot anymore. 

  • 0 in reply to H Win
    TI__Guru 54055 points

    Hi,

    I am still not able to understand - what do you mean by just before the output becomes unstable - how much is before ? 1A, 0,5A, or what, can you reduce the number before to check the phase margin?

    Did you measure the current waveform "before" unstable? Was your transformer in saturation when unstable happened? 

  • 0 in reply to Hong Huang
    Prodigy 10 points

    For example, for CC operation, the load is set at 1 ohm. When the current is set at 10A (this is a stable operating point), we can get the bode plot with good margins. 

    When the load current is reduced, say, to 9A (this point becomes unstable). The output current oscillates and we can't measure the bode plot anymore. 
    This instability happens at light load only. 

  • 0 in reply to H Win
    TI__Guru 54055 points

    Hi,

    It sounds your operation mode is in transition - is it right? What about you reduce CC entering point from current threshold to be further lower? If your CC mode change is transition back and forth to another mode (CV?) without enough hysteresis then you could have unstable operation from mode transition back to forth.

    Can you check and increase the hysteresis for entering and coming out the CC?

  • 0 in reply to Hong Huang
    Prodigy 10 points

    It is very far from the transition point. Yes, it oscillates a little at the transition point. But this is happening at a point far from transition. 
    Is there anything in particular we should take note of for light load operations?

  • 0 in reply to H Win
    TI__Guru 54055 points

    Hi,

    You need to check both feedback loops although you think far away from transition point. 

    Please provide your schematics and the transition waveforms in order to provide further support.

  • 0 in reply to Hong Huang
    TI__Guru 54055 points

    Hi,

    As no response for a long time, I will close this thread. You can re-open this thread or create a new one if further support is needed.