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UCC28730-Q1: consult QR and DCM condition, and simulation feasibility

Yin Wang
Prodigy 10 points
Part Number: UCC28730-Q1
Other Parts Discussed in Thread: UCC28730, TINA-TI

Hi, I want to consult -

For the QR-flyback using UCC28730-Q1, when it goes to DCM mode, would the UCC28730-Q1 have the capability to always detect the valley voltage on MOSFET and then turn on at the valley voltage point (or around)?  And if the flyback has wide input and different load condition, would UCC28730-Q1 always guarantee to catch the valley voltage and then turn on MOSFET?

Since we're evaluating several QR Flyback controllers.

But find that, for some controller, it can only detect & control the turn on moment when flyback is running in Critical Mode. Once it goes to DCM, the controller cannot control the turn-on moment, which means the MOSFET would have certain chance to turn on at really high Vds point and lead to large loss.

Also, is there a way to simulate UCC28730-Q1 ? I try to import the spice model into LTspice but it doesn't work then.

Pls indicate how to simulate... Thank you!

  • 0
    TI__Mastermind 40350 points

    Hello Yin,

    Thankyou for your interest in the UCC28730-Q1 flyback controller.

    This controller does not detect the DCM resonant valley directly, but rather it detects the zero-crossing of the negative-going auxiliary waveform at the VS input. When the zero-crossing is detected, the controller applies a delay time of about 500ns to turn-on at the expected valley.

    The fixed delay time is based on the typical magnetizing inductance and MOSFET Coss used for most 20-40W flyback applications.   Some special applications may require much higher Lm or large MOSFETs with extreme Coss.  These exceptions may result in resonant frequencies much lower than the usual ~500kHz.  In this case, the zero-crossing is still detected and the ~500ns delay is applied, but a low-frequency resonance may not fully reach the minimum valley point before turn on. 

    Since this controller uses variable frequency (among other schemes) to regulate output voltage, and still must achieve valley-switching, it will engage in "valley-skipping" in order to turn on at the first available valley that falls outside of the average switching period.  The controller will continue with valley-switching as long as the ringing amplitude is strong enough to be detected at the VS pin.  For operation in deep DCM when a zero-crossing can no longer be reliably detected, the tZTO zero-crossing time-out delay timer (2.2us) will force gate-drive turn-on anyway.
    These turn-ons could be at a high bulk voltage, but usually the switching frequency is already very low then and the loss is also low.

    The UCC28730 can be simulated using the TINA-TI model available here: www.ti.com/.../slum527

    The TINA-TI tool can be downloaded from here: www.ti.com/.../TINA-TI TI DOWNLOAD&tisearch=Search-EN-everything

    Unfortunately, we cannot support running LTSpice.

    Regards,
    Ulrich