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UCC25600: Lm Transformer Heating issue

deepak naiknavare
Prodigy 220 points
Part Number: UCC25600

Tool/software:

Hello Team,

I am facing the issue for transformer heating, both Lm and Lr upto 70 deg at 50 W output (designed for 150W) .

During proto, with multiple iterations and calculations we limited the thermal rise to 44 deg(@150W). but now its started heating again.

We tried to restore all the similar changes (even the handmade transformer) we used on proto, but no success.

We doubt on the new procured transformer (but similar worked on proto), but do you suggest any other parameter responsible ?

i have attached waveform for reference. the current looks within the design current limit of 4 A for transformer.

Transformer details:

  • 0
    TI__Guru 73715 points

    Hi,

    what is the waveform you show?

    What is your converter circuit?

    The current shape should be towards a sine wave so not able to know what this current is and where you measured.

  • 0 in reply to Hong Huang
    Prodigy 220 points

    Hello,

    I have attached the circuit and where the waveforms are attached.

    The circuit is mainly designed with VDC=665V, Vout=55V, 1A typical and 3A maximum.

    We have used LLC topology, Lm=260uH, Lr=64uH and Cr=25nF.

    The circuit is not operating in resonance and observed by us as well, but we had to just modify it from exixting 30V to 55V. Earlier with 30V output as well the waveforms were similar.

    The current waveform is mostly the magnetising current due to high input voltage and low magnetising inductance to have ZVS at turn ON.

    I have attached document to show the waveforms position and thermal rise of Lm and Lr

    3884.Thermal.pdf

  • 0 in reply to deepak naiknavare
    TI__Guru 73715 points

    Hi,

    I am not sure your converter has ZVS. Can you capture the waveforms like the below to help get your converter operation status.

    In the below: Q2 is low side MOSFET

    Your waveform Vpri shows high frequency ringing and rising seems a step which needs to check if it is from miller plateau - if so it is not ZVS.

    Not sure how your input filter capacitors, type, values, etc, which are required to design carefully as we had experience these capacitors if not able to flow the current properly they can cause additional power losses.

    I think you need to study each heated up component to compare the measured temperature and the calculation from their datasheet to see if these components are already at their best performance, and if so, the improvement would be only from change them to better components.