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TINA/Spice/UCC28C42: Duty cycle is always very low ?

John Khan32
Prodigy 45 points
Part Number: UCC28C42
Other Parts Discussed in Thread: TINA-TI,

Tool/software: TINA-TI or Spice Models

Hi,

I'm trying to simulate a switch mode power supply using the UCC28C42 PWM Controller.

But for some reason the PWM output from UCC28C42 always has a duty cycle of roughly around 20-30% from rough estimation.

Can anyone tell me how the controller sets the duty cycle? What voltages at the pins set the duty cycle?

The only mention in the data sheet that I could see is that voltage the FB pin lower than 2.4V will cause close to 100% duty cycle.

But even if I manually set Vfb to 2.4V, the duty cycle is still around 20-30%.

  • 0
    TI__Intellectual 1490 points
    Hi John,

    Can you please share your TSC file ? That will help to debug the issue.

    I tried simulating the Transient Startup Reference design (with Vin reduced to below 50V) on www.ti.com/.../toolssoftware and observed that duty cycle does go up (above 50%) as expected. I have not observed any particular issue with the model.

    Best Regards,
    Srikanth Pam | Online Design Tools
  • 0 in reply to Srikanth Pam
    Prodigy 45 points

    Hi Srikanth

    I'm trying to simulation a circuit that I got from WEBENCH. I have replicated the whole circuit on TINA TI.

    The input is set to 120Vrms. I'm expecting a 19V 2.1A output. But the simulation only gets me 14V and I think its due to the duty cycle.

    If I manually set a PWM input to the MOSFET with a 55% duty cycle I get a 20V output.

    But for this simulation the duty cycle is only like 20-30%.

    ucc28c42_simulation.TSC