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TMS320F280025: TMS320F280025: LLC converter soft start-up problem

Biswajit Pattanaik
Prodigy 50 points
Part Number: TMS320F280025
Other Parts Discussed in Thread: UCC28180, UCC24624, UCC27714, TIDM-02013

Hii,

I am working on a 750 W battery charger project.
PFC topology

  • Boost topology with UCC28180 control IC from Texas instruments.
  • Switching frequency is 105khz

DC-DC topology

  • LLC half bridge converter(Split cap configuration) with secondary synchronous Mosfet.
  • Digital control technology is implemented using C2000 series MCU
  • Secondary side synchronous Mosfet drive is implemented through UCC24624.
  • Gate driver is UCC27714
  • Operating frequency range is from 93khz to 200khz
  • Resonant frequency is 153khz
  • Lm= 110uH,Lr = 27uH ,Cr= 40nF
  • Dead time is around 220nSec

Problem statement

  • During the standalone load test of individual converter, both converters are working perfectly fine with desired performance.
  • For standalone LLC testing , i am applying the pulses (250khz) to both high & low side mosfets first & then apply 390 V DC at the input.
  • When I am integrating both PFC & LLC , & apply 230Vac at the input there is a gate driver & mosfet failure of LLC converter at the starting itself with out any load.
  • LLC mosfet pulses are applied with a particular frequency(250khz) before 230V is applied in integration testing. As the power supply is independent of the input 230V supply.


Is it happening due to absence in softstart mechanism for LLC ?? If it is the reason, then why it is not failing in standalone testing

  • 0
    TI__Mastermind 41895 points

    Biswajit,

    This it a difficult problem to solve. Fundamentally without the hardware and software this type of debug is difficult. I personally operate all of my code disconnected from a power stage first to validate the correctness of the PWM output signals. Without this step you very likely could blow up FETs.

    As for why is there some dependency on the DCDC and PFC, this really comes down to code partitioning and structure. We offer some examples like TIDM-02013 to show how to run a variable and fixed frequency topologies together on one MCU.

    You must be very careful that all PWM values get updated before the end of the PWM period. If you do not update all values, then you can accidently output different frequencies on the different PWMs(an obvious reason for potentially catastrophic issues)

    Regards,
    Cody