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UCC28950 open-loop operation?

Romero Andersen
Prodigy 10 points
Other Parts Discussed in Thread: UCC28950, UCC2895

We are developing a high-efficient dc-dc converter based on UCC28950 (Green Phase-Shifted Full-Bridge Controller With Synchronous Rectification). We are facing difficulties when trying to operate UCC28950 in open-loop configuration. We would like to set a fixed duty cycle without feedback for testing. Can anyone share information about how to operate this integrated circuit in open-loop configuration? Is it possible? Can we impose an external signal to pin 4 (comp) or is it going to damage the component?

  • 0
    TI__Guru 74115 points

    One way to make the open loop test is first to disconnect all the circuitry connected to EA- and COMP, then connect EA- to COMP together, then add an adjustable voltage to EA- (or COMP as the both now connected). The duty cycle will be inversely to the EA- voltage and adjustable. (Both EA+ and EA- require connecting a resistor before connecting a voltage and please refer to the next reply I made).

     

    Another way is still to connect EA-and COMP together as in the first method, then disconnect all external circuit to EA+. Then applying an adjustable voltage to EA+. In this way, the error op-amp is connected in voltage follower, the duty cycle will be proportional to the EA+ voltage, and adjustable.

     

    Please follow either of the above methods to make open loop test. Directly imposing an external signal to COMP risks damaging the part. 

  • 0 in reply to Hong Huang
    TI__Guru 74115 points

    On the first method, the error op-amp is configured as a differential input amplifier. So both of its EA+ and EA- are required to have input resistors connected in between the voltage in that regarding and the each pin, EA+ and EA-.

  • 0 in reply to Hong Huang
    Prodigy 40 points

    do you have a diagram for the UCC28950 used in open loop mode?

  • 0 in reply to rene osorio
    TI__Mastermind 25130 points

    Rene,

    I don't believe anyone has attempted to run it in that configuration. However, our customers are quite ingenious and someone may have done so.

    John

  • 0 in reply to John Bottrill
    Prodigy 40 points

    Ok, thank you very much, do you know another integrated circuit for phase shift control?, which can be used in open loop mode.

  • 0 in reply to rene osorio
    Prodigy 40 points

    UCC2895 can do the job:

  • 0 in reply to rene osorio
    TI__Guru* 86266 points

    I believe that this was discussed on this form previously.

    If you want to run the UCC28950 open loop you should be able to do it by putting voltage amplifier in a voltage follower configuration.  Just tying EA - to OUT.  EA out should track EA +.  Suggestions for EA + is to put a voltage divider on and control is with a separate bias supply.  Starting with 0V and bringing it up slowly.

    I would also suggest that you get a hold of one of the 600W EVMs to evaluate.  You can order these from the ti web site.

    Regards,

  • 0 in reply to Mike O'
    Prodigy 40 points

    UCC28950 is not working in Orcad (open loop operation). However UCCx895 is working. I leave you the diagram.

    Best regards
    René