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UCC256403: Questions Regarding Open-Loop Testing of UCC256403 (Based on PMP40766)

Jeon Eun Bin
Prodigy 170 points

Part Number: UCC256403
Other Parts Discussed in Thread: PMP40766,

Hello TI Team,

I am developing an LLC battery charger using the UCC256403. I followed the PMP40766 reference design, but I encountered several issues, so I am currently testing the system in open-loop mode.

I referred to section 1.7, “How to Operate the UCC25640x Controller in the Open Loop,” in the Most Frequently Asked Questions About the UCC25640x LLC Resonant Controller document, but the waveform I expected did not appear.

 

RigolDS12.png

My test conditions are as follows: I applied 15 V externally to the VCC pin. The main input of 390 V is not applied. I supplied 3.1 V to the BLK pin. I also connected a 364 k full-down resistor to the FB pin.

I would like to confirm whether my test setup is correct for open-loop operation. Additionally, is it acceptable to remove the ISNS upper capacitor during this test? I plan to reconnect the ISNS upper capacitor when I test again with the full 390 V input.

Sheet1.pdf 

Thank you very much for your support.

  • 0
    TI__Expert 3965 points

    Hi Jeon,

    Allow me some to look into the attached sheet, will get back to you by tomorrow (IST).

    Regards,

    Yamini.

  • 0 in reply to SIRIPURAM YAMINI
    Prodigy 170 points

    Hello Yamini,

    I conducted some tests over the weekend and realized that the incorrect operation was due to improper oscilloscope probing.

    I was able to confirm that the HS node is switching at approximately 68 kHz.

    To achieve 100 kHz switching, I removed the VCR upper capacitor and installed a 10 nF capacitor on the VCR lower side.

    I also installed a 36 kΩ resistor on the FB pin.

    Based on the calculations below, I cannot understand why the switching frequency is coming out as 68 kHz.
    The FB pin voltage is measuring 5.23 V.
    The BLK pin voltage is measuring 3.1 V.

    Am I calculating something incorrectly?

    Thank you.

  • 0 in reply to SIRIPURAM YAMINI
    Prodigy 170 points

    Hello Yamini,

    I am sending you the HS node output waveform.
    I am aiming for a 100 kHz switching frequency, but it is only switching at around 65 kHz.
    For testing, I have reduced the input using a 1/10 ratio and am applying 39 V.

    Should I also remove the ISNS capacitor?

    Additionally, when I test for 100 kHz output with the feedback network connected, is it okay to operate without the VCR capacitor?

    Thank you.

  • 0 in reply to Jeon Eun Bin
    TI__Expert 3965 points

    Hi Jeon,

    To disable the charge control , VCR upper resistor has to be removed. As switching frequency is dependent on internal 2mA ramp current and  VCR lower resistance, I don't think you can achieve switching frequency by totally removing VCR capacitors.

    Can you try installing  39Kohm at FB pin and VCR lower cap 10nF.

    Regards,

    Yamini.

  • 0 in reply to SIRIPURAM YAMINI
    Prodigy 170 points

    Hello,

    The waveform above was measured after removing the upper VCR capacitor, using a 10 nF capacitor on the lower VCR node, and installing a 36 kΩ resistor on the FB pin.

    I also tested with a 39 kΩ resistor on the FB pin, and in that case, the switching frequency dropped to around 40 kHz.

    I confirmed that lowering the FB resistance allows the controller to reach 100 kHz when the target frequency is 33.78 kHz, but the waveform becomes distorted.

    I have several questions:

    1. Is there any reason why the actual FB resistor value behaves differently from the calculated value?

    2. I am having difficulty designing the transformer. I manufactured a transformer with 240 µH magnetizing inductance, but the leakage inductance measured about 5 µH. I then adjusted the resonant inductor from 40 µH to 35 µH. Is it correct to subtract the leakage inductance from Lr like this?

    3. Do you have a more complete guide for designing an LLC transformer? I referred to “1.18 How to Design Magnetics for LLC?” in the document Most Frequently Asked Questions About the UCC25640x LLC Resonant Controller, but some explanations seem to be missing, and I am struggling with the design.

    Thank you.

  • 0 in reply to Jeon Eun Bin
    TI__Expert 3965 points

    Hi Jeon,

    I will check internally and get back to you.

    Regards,

    Yamini.