• State
  • Locked Locked
  • Replies 6 replies
  • Subscribers 64 subscribers
  • Views 134 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.

UCC256404: UCC256404 HB -LLC always in burst mode

Rahamadullah A
Prodigy 160 points
Part Number: UCC256404
Other Parts Discussed in Thread: UCC25640EVM-020, ISOM8110

Tool/software:

Hi 

We have implemented a power supply of 100W  PFC +LLC using UCC2818EP and UCC256404.

When we tested our PCB  it is giving us less efficiency than the test results performed with UCC25640EVM-020 and custom build UCC2818EP PFC.

When we looked into where the efficiency is leaking we observed that in LLC side the bottom mosfet in Half bride is heating up a lot and when observed the Vds waveform of bottom mosfet it is operating in burst mode always even when load is increased. The LLC circuit is implemented as same as that of UCC25640EVM-020 circuit except the resonant inductor and transformer which is from different manufacturer but same value. We also made sure the new Lr and transformer is not lossy.

We also tried to solder the eval board transformer and checked but still the LLC is lossy. 

When probed across the VCR pin the voltage after removing the LL/SS top resistor  is greater than 0.2V. Is there any reason that the UCC256404 can enter into burst mode even at higher powers? Please advice how we can solve this issue.

Regards

Rahamadullah

  • 0
    TI__Genius 12795 points

    I am not sure if you used the same BOM like our EVM (not just the transformer).

    Please read the Q&A section 1.14 and 1.15 for burst mode setup.

    https://www.ti.com/lit/an/sluaal2/sluaal2.pdf

  • 0 in reply to Ning Tan
    Prodigy 160 points

    Hi Ning

    Yes we used the same BOM (all IC pin programming resistors and capacitors, mosfets and resonant caps).

    We have gone through this 1.14 and 1.15 and tried the same but the pulses are skipping.

    Is that anything related to instability?

  • 0 in reply to Rahamadullah A
    TI__Intellectual 1190 points

    Hi,

    In EVM, this is working fine. I dont know why this is happening in your board. 

    Also please check input voltage of LLC  converter when it goes into burst mode in high power. Is it dropping or not ?

    Regards,

    Sougata

  • 0 in reply to Sougata Nayak
    Prodigy 160 points

    No the input voltage is not dropping

  • 0 in reply to Rahamadullah A
    Prodigy 160 points

    Hi Nayak

    We tried to disable the burst mode using BW pin resistors programming, we got the gate pulses continuous but the output voltage is out of regulation. So when FB pin is probed the voltage is observed to be zero and it is kind of going to open loop mode. 

    We are suspecting that it could be optocoupler bias current issue. Since in UCC25640EVM-020 Eval kit VO618A opto is been used which has CTR range of 50% to 600% Whereas we are using ISOM8110 optoemulator whose CTR range is 100% to 155% range. Is there any way that I can bias the optocoupler collector side connected to FB pin of ucc256404 IC to operate in that CTR range. What's your recommendation on biasing that and how to choose the bias current for collector (like in case of resistor and zener diode biased from RVCC in PFCLLCSREVM-020 Eval kit) ?

  • 0 in reply to Rahamadullah A
    TI__Intellectual 1190 points

    Hi, If FB pin voltage is observed near 0 voltage, Can you please try adding a ~2.2kΩ resistor and 8.2V Zener in between RVCC and the FB pin of the controller. It should help limit the swing of the FB signal due to delays from opto-coupler saturation.

    Regards,

    Sougata