• State
  • Locked Locked
  • Replies 3 replies
  • Subscribers 63 subscribers
  • Views 270 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

Original question:

UCC256404: Disabling Burst mode resulting in loss of regulation at Output

UCC256404: Disabling Burst mode resulting in loss of regulation at Output

Ahmad El Hajj Hassan
Prodigy 70 points
Part Number: UCC256404

Tool/software:

Hi,

I've read the forum below and I have the same issue:

UCC256404: Disabling Burst mode resulting in loss of regulation at Output - Power management forum - Power management - TI E2E support forums

I believe Euan's question in the aforementioned forum was misunderstood.

I'm making an LLC converter from 48V to 24V with maximum load 6.25A, and to simplify the qualification process, I disabled burst mode which might be what's causing my issue.

The issue is that output voltage regulation is lost, the maximum switching frequency is at around 170 kHz, and it doesn't go any higher.

So, when Vin is 48V, and the load is lower than the maximum load (≈4.4 A), Vout goes above 24V (up to 28V), so the gain is too high but the controller can't increase the frequency anymore to bring the gain down.

So the question is:

What can be causing the maximum frequency to be limited to a certain value and not any higher? 

How is that related to disabling burst mode?

  • 0
    Prodigy 70 points

    I tried decreasing BMTH (from 1.5V to 0.6V) and increasing CLL/SS (from 100nF to 1uF) and this didn't help.

  • 0 in reply to Ahmad El Hajj Hassan
    TI__Guru* 86266 points

    Hello,

    Your inquiry is under review and I will get back to you shortly.

    Regards,

  • 0 in reply to Mike O'
    TI__Guru* 86266 points

    Hello,

    I agree, if the frequency is not decreasing when the output voltage is going high and out of regulation, the issue is not burst mode.

    Your FB current (IFB) needs to vary from 0 to - 82 uA (IFBclamp) to control the oscillator frequency.  At 0 uA the LLC will be operating at the maximum frequency and and -82 uA the LLC will be operating at the minimum switching frequency. 

    There may something drawing current out of your FB pin when it should not be.  If you study the IFB current, Vout, ISNS and VCR during this condition it should help you determine what is causing the issue.  Please note that if your output voltage is high the IFB current should approaching 0 uA. 

    The other thing I would recommend is checking your VCR, ISNS and loop compensation with the excel design tool that can be found that the following link.  Just to double check your loop compensation.  You should also check your loop compensation with a network analyzer.

    The excel design tool can be found at the following link.

    https://www.ti.com/tool/download/SLUC675

    Thank you for interest in Texas Instruments (TI) products.  If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

    Regards,