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Flyback with current sense transformer ac coupled to UCC2807-2

Daniel Jinar
Prodigy 100 points
Other Parts Discussed in Thread: UCC2807-2

I have a legacy design that is driving the sense pin (CS) of the UCC2807-2 slightly negative. I understand that the "voltage rating" is -0.3V to Vdd+0.3V. I want to know if there is an associated current limit value?  Also, I am concerned that that the CS feedback going negative impacts the operation of the current mode control.   I do plan to correct the design, but I want to understand the current configuration.

  • 0
    TI__Mastermind 31580 points
    Hi Daniel,

    Looks like a question for the IC designers, I cant see any mention of a current been specified for the CS pin in the datasheet.

    Let me see what I can find out.

    Regards

    Peter
  • 0 in reply to Peter Meaney
    Prodigy 100 points

    Peter,

    I couldn't find any current limit in the datasheet.   I work with lightning protection quite a bit and I often rely on the absolute maximum ratings for transients.   Many times I will use a particular device based upon the current limit specification.    Generally an input/output pin will be specified at the maximum supply voltage plus the 0.3 V of a protection diode.   If your lucky, they will also specify a maximum input current, generally in the 5-30mA range.

    I am working through my distributor to get an answer directly from TI.

    thanks for your interest

  • 0 in reply to Daniel Jinar
    TI__Mastermind 31580 points
    Hi Daniel,

    Sorry for the delay. In general If the voltage on the CS pin drops below -0.3V the current sourced from the pin is close to zero.
    As soon as the voltage approach -0.5V and below, the current sourced increase.
    The most common problem is minority carrier injection that can generate offset in the internal analog circuit with possible mis-behavior.

    To get more precise informtaion than this will require a lot more time as this is an old device.

    Regards

    Peter
  • 0 in reply to Peter Meaney
    Prodigy 100 points

    Thanks for the update.  The information is both good and bad news.   While I haven't seen an example that we are driving the CS pin negative by more than 200mV, the unknown "mis-behavior" description has me concerned.   I believe if the CS pin is shorted or open (ie: no current feedback) the flyback will continue to operate in voltage control rather than current mode control.    This could lead to a stability issue but I would be able to characterize the expected performance for my system.   My worst fear is that the over current / duty cycle limit is lost and it leads to overstressing components due to excessive on time.

    I look forward to any further explanation of the expected UCC2807-2 failure modes.

  • 0 in reply to Daniel Jinar
    TI__Mastermind 31580 points

    Hi Daniel,

    In the event that the CS pin is shorted to ground then current mode  control is lost and the duty cycle will probably go to Dmax and there is no over current protection. If the CS pin is open then the voltage on the CS pin will be difficult to determine and the psu will be unstable due to variation in duty cycle and again there will be no over current protection.

    If the CS pin is connected to the PCB and there is some transient that causes the voltage to drop through 0V and low enough to turn on the parasitic diode in the substrate then it is very difficult to predict the exact issue that it will cause other than to say that it is likely to disrupt the normal operation of the controller.

    Regards

    Peter

  • 0 in reply to Peter Meaney
    Prodigy 100 points
    Just some feedback, I experimented and first shorted the CS pin to ground and found the flyback to continue to regulate. I assume this worked because the voltage feedback was still available and most likely operating in voltage mode control. I also opened up the CS feedback and saw the same results.

    I would agree that the over current sense shut down modes would be lost without any CS feedback, however from my testing I found regulation to be maintained (counter to how I understand a current mode controller to work??? ). Is it plausible to expect regulation to function for the UCC2807-2 without CS feedback?