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LM5017: Isolated DC/DC converter noise and temperature issues

MaxS
Prodigy 40 points
Part Number: LM5017

Hello, I designed an isolated DC/DC power supply with LM5017 and have noise and temperature issues. Transformer: LPD8035V-333 Coilcraft

Vin_min=36V

Vin_max=56V

ILoad_sec_max=250mA

Vsec=5V

Attached you can see the signals on feedback node, SW-Node and transformer current on the primary side.

  1. There is transients on feedback voltage and current I would like to eliminate. Could you give me a hint what could cause this transients and how can I avoid this?
  1. The transformer surface temperature is 52°C @Vin=56V, ILoad_sec=200mA. Because of small hermetic housing of the application, I would like to achieve max. 40°C. Besides the transformer parasitics, which design parameters affect the transformer temperature also?

Thank You in advance!

Max

  • 0
    TI__Guru**** 191445 points
    Moving this to the isolated forum...
  • 0 in reply to JohnTucker
    TI__Guru**** 191445 points
    Max,

    Do you still need help with this? Let me know and I'll get someone to help.
  • 0
    TI__Mastermind 31910 points
    Hello Max,

    The voltage on the FB looks typical. Please make sure you are measuring the voltage directly on the FB pin and AGnd with a kelvin connection making sure you have a short gnd lead on the scope. Suggest using a spring clip on the barrel of the scope probe for accurate measurement to ensure you are not measuring noise ?

    Regarding the transformer, peak current and switching frequency are the two parameters that will effect core losses, that you are in control of. and DCR winding resistance will effect RMS losses in the wire. I suspect you are dealing with core losses. usually core losses and RMS losses are traded off against each other.

    if you can select a higher Lrpimary with a DCR that is still relatively low, then this will certainly help reduce surface temp. another thing you can try is reduce the Switching frequency while increasing the Lprimary to keep the peak currents the same, this will certainly help...

    Core losses are effected by flux density in the inductor and flux density is effected by the peak currents, Switch frequency, the gap length, the core material and the cross section of the core as well as volume of the core itself. I have given you a couple of things to try that may be readily available to you in your transformer selection?

    Hope this helps?

    David.