• State Resolved
  • Locked Locked
  • Replies 7 replies
  • Answers 1 answer
  • Subscribers 63 subscribers
  • Views 471 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

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: about LLC

ACGUY
Guru 11170 points
Part Number: UCC256404
Other Parts Discussed in Thread: UCC28065, UCC28056, UCC28064A, UCC256302, UCC256402

Hello E2E,

Q1:PFC necessary for LLC? If not necessary that I don't like to operation PFC when light load. (Any load current)
Q2:Can UCC256404 decreases Vout from 16.5V to 6V when light load? or by external operation.
Q3:I want to do suppress input power under 0.25W when ouput power under 0.1W. Please let me know TI recommended PFC-controller.

Regards,
ACGUY

  • 0
    TI__Mastermind 26840 points

    Hello ACGUY,

    Thank you for your interest in the UCC256404 LLC controller.

    A1:  PFC is not necessary for LLC, however most LLC designs work best with a narrow input voltage range.  A PFC stage helps make the input voltage range to the LLC very narrow.  The UCC256404 controller may operate with a wider-than-usual input range.

    A2: The output voltage of the LLC is controlled by a secondary-side regulator with feedback through an opto-coupler.  To change output voltage, the regulator must be controlled to adjust the feedback properly.  

    A3:  PFC controllers with very low standby bias power are UCC28064A, UCC28065, and UCC28056.

    Regards,
    Ulrich

  • 0 in reply to Ulrich Goerke
    Guru 11170 points

    Hello Ulrich,

    Thank you for your reply. Can I ask more questions?

    What is the concern if the PFC is stopped at a light load with 90V AC input?

    Regards,

    ACGUY

  • 0 in reply to ACGUY
    TI__Mastermind 26840 points

    Hello ACGUY,

    One concern would be "how light?" is the light load when the PFC is stopped.

    When the PFC is not boosting, the ripple voltage on the bulk cap must be low enough to not trigger the UV detect at the BLK pin.

    Another is: how fast will a load step occur on the LLC while the PFC is off?  The bulk cap must provide energy to the LLC and remain above the UV trigger until the PFC can be turned on and resume boosting.  A slow increase in load can be okay, or advanced warning to turn on the PFC before a heavy load step.

    Regards,
    Ulrich

  • 0 in reply to Ulrich Goerke
    Guru 11170 points

    Hello Ulrich,

    Output power;
    Typ : 300W (16.5V/18A)
    Light load : less than 9W (6V/1.5A) <- After switch output voltage by feedback resistor

    Input
    AC90V - AC264V WW range

    Idea from customer. Reason for switching output voltage and stop PFC is to reduce standby power consumption.

    I think that if become to reduce standby power by TI's power solution (PFC and LLC) that not necessary to stop PFC.

    Design requirements;
    Input power less than 250mW (at output power 100mW).

    Can TI power devices solve this challenge?

    Regards,
    ACGUY

  • 0 in reply to Ulrich Goerke
    Guru 11170 points

    Hello Ulrich,

    Our customer has concern with UCC256302. Could you check question from customer on following URL?

    https://e2e.ti.com/support/power-management/f/196/t/912152

    Best regards,

    ACGUY

  • 0 in reply to ACGUY
    TI__Mastermind 26840 points

    Hello ACGUY,

    I've referred your question to another engineer to assess the waveforms.

    Regards,
    Ulrich

  • 0 in reply to Ulrich Goerke
    TI__Mastermind 26225 points

    Hi ACGUY,

    Under normal switching conditions, the high side gate will turn on as soon as the controller detects the switch node has finished slewing up to Vin. When the controller is not able to detect the slew rate, the next gate turn-on will be delayed and you will obtain a skinny high side pulse. The reason for the skinny pulse is because the internal frequency compensation current ramp on VCR is active during the dead time. So even though the high side gate has not been turned on yet, the internal current source is already charging up the VCR pin. When the high side finally does turn on, the VCR voltage is already quite close to the VTH turn off threshold. There are a few items that can help improve the consistency of the slew rate detection:

    1. increase your MOSFET turn off speed

    2. use MOSFETs with lower Coss

    3. increase your burst threshold slightly. This will make the controller burst at a slightly higher power. The higher power burst energy will have a larger magnetizing current amplitude to drive the switch node

    4. Use smaller Lm to increase the magnetizing current

    5. swap to UCC256402 which is pin to pin compatible with UCC256302, only a few passive components around the IC need to change. UCC256302 has a minimimum detectable slew rate of 1V/ns while UCC256402 can detect down to 0.1V/ns. 

    Best Regards,

    Ben Lough