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LM5023: Noise issue in the transformer..

kranthi reddy
Prodigy 30 points
Part Number: LM5023

Hi,

Currently we are using the LM5023 IC in our application.. When we are loading the unit , we  observed the burst pulses of 1 KHz with switching frequency of 100 KHz..  These 1 KHz burst pulses leads to audible noise which is undesirable, How can we increase this frequency greater  than audible range (>20 KHz ) from 1 KHz ?

Is there any other ways to reduce the noise?

  • 0
    TI__Expert 6080 points
    Hello Karnthi,

    The frequency can be modulated easily by these two methods:
    1. Increasing the output capacitance, that will reduce the freq.
    2. Increasing the load, normally putting a dummy load can also help to reduce te slope of Vout.
    Along with these points I would suggest focusing on the VCOMP and VOUT waveforms to analyse if they are following the suggested changes.

    I hope this helps.

    Regards,
    Sonal
  • 0 in reply to Sonal Singh
    Prodigy 30 points
    Hello Sonal ,
    Thanks for your quick reply..
    Our concern is to increase the frequency of the burst pulses from 1 Khz to >20 KHz , so that it can't be audible..
  • 0 in reply to kranthi reddy
    TI__Expert 6080 points
    I can understand that, please follow the recomendation and hopefully that will help you out.

    Sonal
  • 0 in reply to kranthi reddy
    TI__Mastermind 34465 points
    Hello Karanthi,
    The LM5023 has a burst mode feature that bypasses the PWM comparator and causes an asynchronous mode of operation in the converter.
    This should not normally cause an audio problem because the peak primary current is normally very low in this mode but if there are an issue I can suggest the following:
    (1) Re-evaluate your transformer design. Perhaps you can reduce the airgap and use a larger current sense resistor.
    This will reduce your peak currents and increase the effective frequency but you must ensure that the transformer can also deliver full power.
    (2) Audio noise is caused by the magnetostriction effect of the core and the interaction of the windings.
    Ensure that the core halves are glued and the transformer assembly is vacuum varnished.
    The core can also cause the pcb to vibrate sort of like an acoustic drum so you might place a piece of damping material like a soft plastic washer between the transformer and the pcb.

    The LM5023 goes into burst mode when COMP falls below about 0.7V (I think).
    So what you can also do is to dynamically switch in a higher value of current sense resistor when COMP < 0.7V
    This will reduce the peak current and increase the switching frequency.
    And hopefully take you out of the audio band.
    Regards
    John
  • 0 in reply to John Griffin1
    Prodigy 30 points

    Hello John,

    Thanks for your answer... I Will implement and observe the response ..