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UCC256303: Burst mode disable/ seting minimum switching frequency

daniel pruna
Prodigy 65 points
Part Number: UCC256303

I am facing some problems while testing my LLC prototype and I would like to know:

- Is there a way to disable burst mode? This feature is not needed in my design.

- Is there a way to set the minimum switching frequency? Right now when I am powering the IC it starts switching with 33KHz which is way too low for my design.

Thank you in advance.

Daniel

  • 0 in reply to Mike O'
    Prodigy 65 points

    Hello Mike,

    many thanks for your elaborated reply!

    • -Disable burst-mode -> I have already tried to remove the upper resistor, anway if burst can't be completely disable I'll have to live with it.
    • Setting an absolute minimum frequency- >  I have tried already to put a resistor from  Feedback pin to ground, this is the way how most of the classic LLC controllers I have worked with  have the minumum freq. adjusted. For the UCC256303 I have tried with 33K and it didn't work. I will try again with 2M and let you know.

    Daniel.

  • 0 in reply to daniel pruna
    TI__Guru* 86106 points

    Hello Daniel,

    33 k should be too small.  Minimum frequency of 35 kHz is 0uA coming out of the FB pin.  Maximum frequency lowest gain is with 85 uA coming out of the FB pin.

    I estimated that the resistance should be around 2M ohms based on slope and estimated FB voltage.  Could you retry this?

    Regards,

    Mike

  • 0 in reply to Mike O'
    Prodigy 65 points

    Hello Mike,

    I have tried your suggestion, it didn't make any difference though.8176.Waveform.docx

    My Setup is as follows: 

    VCC = 16V, BLK = 3V (BLK tied to VCC trough 550k/130k divider), once it starts swichting I increase the input voltage of the PSU from 0 to 50V in order to prevent FET damage due to hard switching in capacitive region. Unfortunately I couldn't test yet with full input voltage (380VDC) since the FETs will blow (it happens once at around 160VDC).

    What I do not understand is why it goes into burst mode even before reaching the output regulation voltage? if I put some load at the output burst mode goes away but when I increase DC Link voltage further it goes back into burst. Please have a look at the switching waveforms attached.

  • 0 in reply to daniel pruna
    TI__Guru* 86106 points
    Hello Daniel,

    A resistor from the FB pin to ground should have limited the maximum switching frequency. I discussed this with another applications engineer at TI and he mentioned the FB pin voltage is around 5V. A 5M ohm resistance should ensure that is at least 10 uA coming out of the FB pin limiting the maximum frequency. If you used 2M I would have thought that it would limit the frequency to something higher than 35 kHz. The waveform you have attached shows the minimum frequency switching at roughly 35 kHz.

    Unfortunately there is no way to completely disable burst mode with this device. The best you can do is remove the resistor from RVCC to the LL/SS pin. This will ensure that the burst mode will trigger at the lightest load possible. There is an application note, "UCC25630x Practical Design Guidelines" ,at the following link that will discuss how to fine tune burst mode in section 3.1 if you are interested. www.ti.com/.../slua836a.pdf

    The UCC256303 is supposed to have a ZCS fault that should prevent against hard switching and keep you out of the capacitance region. I am curious if your Isns and VCR dividers are setup correctly. You might want to double check this to see if you have these setup correctly. There is an excel design tool that you may find helpful in double checking your VCR and Isns setup.
    www.ti.com/.../sluc634

    Regards,

    Mike