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UCC2803: UCC2803 Problem with startup. Bug in Power Designer?

Anatolii Dobryn
Prodigy 10 points
Part Number: UCC2803
Other Parts Discussed in Thread: UCC2802

Hi all. 

I made DC/DC converter with UCC2803, based at TI Power Designer WEBENCH Design Report.
I never used before flyback converters, so for first try I totally copied TI Power Designer references. 

At real test samples, I got startup troubles and some overheat problems, I suppose this due with wrong parts selection. 
TI Power designer suggested UCC2803 and IRFS4227PBF as power switch. 
 
Basically, UCC2803 have turn on threshold 4.1V, but   IRFS4227PBF have minimum Vgs voltage to turn on more than 5V. 
So at scope I see only short start-up pulses 4.1V. These pulses are not enough to start IRFS4227PBF, so this setup can not start. 

If I make force start  (Apply 10V to VCC pin of UCC2803D for short time), the design started to work, but I'm getting significant heat up at snub resistors. 
I will try to replace UCC2803 to UCC2802 with turn on threshold 12.5V and will check if this help. 

Just want to pay attention, developers of power designer, to this bug. Attached reports.  Thanks.  

CoreCoilformer221 (2).pdf

 WBDesign221.pdf

  
Best Regards,
Anatolii

  • 0
    TI__Guru 57365 points

    Hi,

    Your converter input min 36V. Suppose Vin will go all the way to minimum 36V when VCC should be > 5V, based on the Webench. 

    So in your test you should apply a voltage to VCC close to the auxiliary bias to supply in steady state instead of using a 4.1V to make test.

    So it seems nothing wrong with the Webench but your test setup.  

    Besides, Webench is to provide a design at start for bench test. You would have tune up the design to resolve any issues you may have. Webench cannot provide a tested design which replies on the designer who is using the Webench.

  • 0 in reply to Hong Huang
    Prodigy 10 points

    Thanks for reply. 
    Please see, what I mean.
     
    In default state, start pulses for UCC2803 have amplitude 4.1V, this not enough to open mosfet. So design can not start to work. 
    This waveform taken at 48V input supply voltage 


  • 0 in reply to Anatolii Dobryn
    TI__Guru 57365 points

    Hi,

    It looks your finding is VCC cannot reach the intended AUX 10V since based on AUX winding turns its steady state voltage should about 10 V. But due to switching the VCC cannot be charged higher than 4.1V.

    So UCC2803 is not a correct device for power on from its uvlo on.

    I believe these should be part of work of bench debugging. You can change to a different device in this family but you also can increase power current so allow VCC further charge up until AUX takes over. But it looks Webench for this design does not cover these and then these become a bench debugging task.

    Another possible reason is Webench design is based on typical without consider min and max. The MOSFET VGS is with min 3V and max 5V so from common sense 4V is most likely its typical as no clear spec in its datasheet. You just got a part with VGS not typical.

    I think it is normal as the Webench only provides a design to help your design starting but a design relies on the designer to find issues to resolve until acceptable.

  • 0 in reply to Hong Huang
    Prodigy 10 points

    Hi,
    Thanks.
    Yes, project debug for the moment in progress. 
    This mostly a note for TI Designer developers, I want to make it better. 

  • 0 in reply to Anatolii Dobryn
    TI__Guru 57365 points

    Hi,

    Ok. Will let the webench team know.