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UCC25600: problem with soft start

paul brooking
Prodigy 10 points
Part Number: UCC25600

Problem with Texas Instruments Ucc25600 resonant mode controller at around 150 to 350KHz operation.pdf

The soft start facility on the chip does not appear to work as would be expected and described on the data sheet. please see attached file for tests and details of problem.

I can make the IC do what we require, but if the parts we have are really behaving as TI expects then the datasheet is not explaining their operation properly.

I can't think why TI would purposely design the control function to work in this way it is rather inconvenient.

Please could someone comment at least to say if this is how the chips would be expected to behave.

We take precautions against fake parts and dont have any reason to think it is a fake part.

BR Paul 

  • 0
    TI__Genius 13245 points

    Hi Paul,

    Thanks for reaching out to us. I will look into this. In the meanwhile, could you share the schematic with me.

    Regards

    Manikanta P

  • 0 in reply to Manikanta Pallantla
    TI__Genius 13245 points

    Hi Paul,

    Please see my responses below for the issues:

    1. The startup frequency is given in the spec table:

    2. This controller won't be able to go into the burst mode while the controller is in the soft start. So, only way to go into burst mode would be quickly ending the soft start phase. But this can increase the startup currents.

    If your current prototype is in the initial evaluation phase, I would recommend using our latest generation LLC Controller (UCC256404 data sheet, product information and support | TI.com) through which you can get rid of all the issues that you are currently having.

    Regards

    Manikanta P

  • 0 in reply to Manikanta Pallantla
    Prodigy 10 points

    Hi Manikanta,

    Thanks for the quick response, this is helpful in showing that the part we are using is working as would be expected, and there is not a concern that it is a fake. However I take a poor view of the  way it is presented on the datasheet, it is not what I would expect from TI. I will look at the later parts, but this one is designed in and not pin compatible with the new one.

    The part is stated as active status by TI.

    I had not noticed the SS frequency spec you point out. I cant see any reason why it would be designed for the SS to start in the middle of the control range with burst mode disabled, are there any applications where this would be useful ?   Had I not actually tested the part I would be confused about what the SS frequency spec actually means.   In the applications shown where the resonant frequency is specified to be 130KHz a controller which may start below 130KHz is not suitable.

    Originally we were using your recommended circuit, with a resonant frequency of 150KHz, it is not possible to avoid excess current at start up no mater what SS cap is used. It was originally designed to use repeated current trip at start up to control starting current, but this has been found unreliable because on some units the second stage current trip gets triggered.

    If you look at my original attached file I have shown a very simple circuit which completely resolves both these problems.

    It is actually a good little chip, I like the small size, but people need to understand how it works.

    Best wishes Paul

  • 0 in reply to paul brooking
    TI__Genius 13245 points

    Hi Paul,

    Thanks for your feedback. This chip was designed long time ago. At that time, the customer that we were working with was specifically looking for such a startup frequency. So, the system engineer ended up implementing that though he was aware of the consequences.

    Also, Thanks for sharing your circuit. 

    Please reach out to us if you have any further questions.

    Regards

    Manikanta P

  • 0 in reply to Manikanta Pallantla
    Prodigy 10 points

    Hi Manikanta,

    thanks, I understand now it was designed for a specific application, this is not what an engineer would be expecting in a chip designed for general applications.  The unusual operation of the chip, is not explained on the datasheet. The soft start circuit shown on the datasheet will not work properly unless the chip is being used at a fairly low frequency.  I would like to see you clarify the datasheet, not just close the thread, so that other people do not have the inconvenience that I have had in working out what the chip is doing and how to make it work in my application. It is a good little chip with a small footprint and will fit some applications for which the newer chip you mentioned is not suitable.

  • 0 in reply to paul brooking
    TI__Genius 13245 points

    Hi Paul,

     Thanks for your comments. I will let team know about this. 

    Regards

    Manikanta P