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UCC28950-Q1: External Op-Amp based Compensator Usage

Kiran Kumar Valiveti
Prodigy 50 points
Part Number: UCC28950-Q1

Tool/software:

Hi Team...For PSFB Application, I am Planning to Design a Voltage Feedback Circuit followed by Compensator externally and feed that to the either COMP Pin or EA+ Pin.

When Feeding it to the EA+ Pin, EA- Pin and COMP Pin are Shorted to act as a Buffer and One advantage of this way of doing is Soft Start Will be Still Functional.....Does my Understanding Correct? Is it Recommended?

Where as While Feeding the External Compensator Output Directly to COMP Pin, the Soft Start Feature of IC Will be Not Useful and has to be designed externally.....Does this Correct? Is it Recommended?

  • 0
    TI__Prodigy 400 points

    Hi Kiran,

    Let me consider it as two configurations 

    • External compensation output is connected to EA+ pin, EA- Pin and COMP Pin are Shorted to act as a Buffer

    EA+ pin is supposed to be connected to the Vref and during a soft-start controller checks the EA+ pin voltage and SS pin voltage and consider the smallest out of this two as reference. When you connect EA+ pin to external compensation output with soft-start enabled, controller will consider the compensation output as the Vref and will compare with SS pin voltage. This will cause unexpected results during soft-start and I don't think it is a good idea to do this, if you need soft-start to be still functional with this configuration.

    • External compensation output is connected to comp pin

    Comp pin is sensitive and it is not a good idea to connect it directly to external compensation output. If you have such configuration, based on EA+ and EA- pin voltages controller error amplifier will try to control the comp pin voltage and at the same time external compensation output is also trying to control the comp pin voltage based on the feedback. This may result in unexpected and undesired results.

    Can you please explain this configuration better so that I can analyze it and give you a recommendation? I am more interested in understanding how are you planning to handle EA+ and EA- pins in this case.

    If possible, can you also let me know what is the reason for using an external compensation circuit for your application instead of using the controller's internal compensation logic?

    Regards,
    Deepak K

  • 0 in reply to Deepak K
    Prodigy 50 points

    Hi Deepak,

    Thank You for Detail Explanation.

    I remember Pulling Down both EA+ and EA- Pins While testing the Controller IC by feeding External Voltage at COMP Pin.

    The Reason for Using External Compensation is to verify is it practically implementable so that the old design can be modified with this controller and also the Controller IC is planned to be on the Primary Side thus Secondary Side V along with Compensator can be on Secondary ground and can be fed to this controller IC using Isolator.

    Thanks,

    Kiran

  • 0 in reply to Kiran Kumar Valiveti
    TI__Prodigy 400 points

    Hi Kiran,

    Thanks for sharing the details. 

    Even if EA+ and EA- is grounded when compensation output is fed to COMP pin, the issue I mentioned in my earlier response can still exist and not recommended to do that. My recommendation is to try using the first configuration where EA+ pin is fed with external compensation and use error amplifier as a voltage follower. You may have to add external circuit for direct control over startup in this case.

    Regards,
    Deepak K