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TIDA-00229: general questions about the circuit

Amr Elhadidy
Expert 1005 points
Part Number: TIDA-00229

hello,

I have checked the document tidu304.pdf and have some related questions please.

what is the maximum load in mA of the circuit under rectified current input?

why is there a coil of 3.3uH (L3) in page 10 inserted before the buck stage?

what is the benefit of R75 and R87 in page 10?

It is mentioned in the document tidu304.pdf that the VA rating of the CT will be less by using higher voltage. I can't understand why. The one thing that I can see is that the capacitor after the MOS shunt will discharge in this case at high voltage and so the current can be less and so some losses are saved. But how the VA rating will be less is still unclear. 

Last question is why to use the shunt mosfet while it seems that for higher values of C28, the voltage at the terminals of C28 seems not varying that much

Thank you very much.

Regards,

Amr Elhadidy

  • 0
    TI__Guru** 124058 points

    Hello Amr Elhadidy

    Thank you for considering TI design. Here are my comments.

    what is the maximum load in mA of the circuit under rectified current input?

    There is no concern on the max current in mA due to the use of shunt regulator. With increasing secondary current, the hunt regulator on time increases.

    The high current are expected to be available for the fault duration only.

    why is there a coil of 3.3uH (L3) in page 10 inserted before the buck stage?

    This is optional. The output is expected to have ripple and L3 is used as a filter at the DC/DC converter input. This is good to have when the breaker is turned on with fault.

    what is the benefit of R75 and R87 in page 10?

    I assume you are asking R75 and R78

    The shunt FET can be controlled by PWM from MCU or the comparator sensing the DC/DC input or both. Diode is used to OR when both options are used. Diode is DNP when any one of the option is used. These resistors are made as DNP based on the selected regulation option. If both options of regulation are used, then both resistors are made DNP. and diode is used.

    It is mentioned in the document tidu304.pdf that the VA rating of the CT will be less by using higher voltage. I can't understand why. The one thing that I can see is that the capacitor after the MOS shunt will discharge in this case at high voltage and so the current can be less and so some losses are saved. But how the VA rating will be less is still unclear. 

    One of the  major advantage of using higher voltage is reduction in heating due to shunting of the MOSFET. it the shunting happens only during overload,  losses are minimized during normal operation  The CT is rated based on the trip unit consumption + losses. Although not significant, VA optimization is possible and this needs testing. 

    Last question is why to use the shunt mosfet while it seems that for higher values of C28, the voltage at the terminals of C28 seems not varying that much

    In the current implementation, C28 voltage is regulated by the zener diode + comparator that controls the MOSFET. A CT in open condition, depending on the VA rating can generate  high voltage. Using shunt regulation is the simplest approach to regulate the DC/DC input but there could be other approaches.

    Please note that this design is for a circuit breaker trip unit.  What is the application you are looking to use the design. Please note that the current transformer design is out of the scope of this reference design.

    Regards,

    Sreenivasa 

  • 0 in reply to Kallikuppa Sreenivasa
    Expert 1005 points

    Dear Sreenivasa,

    Thank you very much for your reply.

    The intention is to find a circuit to be used as a general auxiliary supply for other 3.3V IC's for metery.

    Regards,

    Amr 

  • 0 in reply to Amr Elhadidy
    TI__Guru** 124058 points

    Hello Amr Elhadidy

    Noted and thanks for the mail.

    Regards,

    Sreenivasa 

  • 0 in reply to Kallikuppa Sreenivasa
    Expert 1005 points

    Hello Sreenivasa,

    from your point of view, is the circuit suitable for this purpose or are there some other factors that should be considered?

    Regards,

    Amr

  • 0 in reply to Amr Elhadidy
    TI__Guru** 124058 points

    Hello Amr Elhadidy

    The focus application of this design is circuit breakers or overload relays which are expected to work and protect during overload.

    Other applications needs understanding of the requirement, testing and analysis.

    Regards,

    Sreenivasa 

  • 0 in reply to Kallikuppa Sreenivasa
    Expert 1005 points

    Hello Sreenivasa,

    Thank you very much.

    If you please there are some other questions. If I rightly understood, through the microcontroller, one could have adjustable voltage up till 39V. But if one doesn't use the microcontroller, then the voltage will be regulated at the 39V level only?

    If one uses only the current sense input, what would be the maximum and minimum output current required at the secondary of the CT.

    Is it so designed that the supply of the hysteresis comparator can be energized either from the 12V output or from the 39V?

    Regards,

    Amr

  • 0 in reply to Amr Elhadidy
    TI__Guru** 124058 points

    Hello Amr, 

    Is it so designed that the supply of the hysteresis comparator can be energized either from the 12V output or from the 39V?

    Yes, this is to take care of some f the starup conditions specifically to circuit breaker application. the feature is breaker being turned on with fault, current available is the short circuit current.

    If one uses only the current sense input, what would be the maximum and minimum output current required at the secondary of the CT.

    Max current, as i mentioned is not a concern due to shunting. Higher current means more shunting and heating losses.

    Min current, depends on the load and the CT VA. A number cannot be specified. We have not designed the magnetic in this design. This is only emulated using DC source.

    If I rightly understood, through the microcontroller, one could have adjustable voltage up till 39V. But if one doesn't use the microcontroller, then the voltage will be regulated at the 39V level only?

    yes, you are right.

    Regards,

    Sreenivasa 

  • 0 in reply to Kallikuppa Sreenivasa
    Expert 1005 points

    Hi Sreenivasa ,

    thank you very much for the comments.

    I iwsh you a nice weekend.

    Regards,

    Amr

  • 0 in reply to Amr Elhadidy
    TI__Guru** 124058 points

    Hello Amr Elhadidy

    You too have a great week end.

    Regards,

    Sreenivasa