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UCC28951-Q1: Change of the current transformer from +DC bus to output of the bridge

Adriyan Manev
Prodigy 40 points
Part Number: UCC28951-Q1

Dear Support,

I am designing a high power converter with UCC28951-Q1. Because of that, and due to high di/dt and the leakage inductance of the current transformer, I expect to have high over voltages across primary switch. That is why, I would like to move the current transformer from the +DC bus to the primary of the main transformer. However, I do not know whether the controller uses the current sense signals for any other control of the converter besides dead time and cycle by cycle over current protection. 

I will be very happy if you could give me some further insight of the operation of CS pin and the risk of moving the transformer to the primary sides, especially in case of abrupt load changes.

Sincerely,

A.M.

  • 0
    TI__Expert 3460 points
    Dear Manev,
    If you select PCM mode, The current sense signal was used by the PCM mode control beside dead time and cycle by cycle over current protection. in addition, you can move the CT to the primary of the main transformer using full bridge diode for CT secondly side.

    thanks
  • 0 in reply to Bliss Zhou
    Prodigy 40 points

    Thank you very much for your answer.

    Just to be sure. Should I assume that the IC does not use the current sense signal for any type of peak current regulation, besides protections, and the risk of step saturation of both transformers is not present if I move the current transformer in series with the primary of the main transformer?

    Best Regards

  • 0 in reply to Adriyan Manev
    TI__Expert 3460 points
    Dear Manev,
    CT saturation or not which depend on the CT and reset circuit. I suggest you calculate the Bmax of CT using primary max current.

    thanks
  • 0 in reply to Adriyan Manev
    TI__Mastermind 25570 points

    Hi Manev

    If the current transformer is in the primary of the main transformer then it will not protect against shoot through events.  In reality this is a minor concern.  One advantage of having the CT in the main transformer primary is that the CT primary current is bo-directional so that core reset is not an issue..

    You are correct, the CS signal is used only to terminate the switching cycles, preventing core saturation. It is not used to provide a constant current output. If you want a CI/CV characteristic then you should have a look at Fig 29 (reproduced below) in https://www.ti.com/seclit/ml/slup348/slup348.pdf

    regards

    Colin

  • 0 in reply to Colin Gillmor
    Prodigy 40 points

    Hi Colin,

    Many thanks for your answer. So the UCC28951-Q1 uses indeed the current signal to control the turn on time of the switches.
    I have one last questions, and it has to do with the step saturation of main transformer. I have read, though I have never seen that, that if the turn on times of the switches, or more precisely if one "half wave" has a higher on time than the other, or there some unsymmetry in the rectification, the transformer can slowly go to step saturation. I have concerns that if the current transformer is in the primary side, the step saturation will not be detected from the controller, as the current transformer cannot transfer the DC value of the current. While if the CT is on the positive/negative bus, the difference in the currents can be sensed from the IC and the controller can turn off the switches appropriately. Is that correct or it is just myth?

    Best Regards,
    Adriyan

  • 0 in reply to Adriyan Manev
    TI__Mastermind 25570 points

    Hello Adriyan

    Any imbalance in the Ton times of the switches in the bridge will be cause an imbalance in the currents in the two half cycles of the bridge, ie. the current when the AD switch pair are conducting will be different to the current when the BC pair are conducting. (I'm using the switch naming convention in the UCC28951-Q1 data sheet here). This imbalance will be corrected automatically by the current control loop because the PWM comparator will trip at the same peak current level for the two half cycles even though the peak current may be reached sooner on one half cycle than the other. I'd expect to see a small amount of duty cycle jitter as the controller makes this adjustment.

    I take your point about DC imbalance when the CT is in the transformer primary - I'm going to ask some colleagues about this. I hope to have an answer in a day or two.

    Regards

    Colin

  • 0 in reply to Colin Gillmor
    TI__Mastermind 25570 points

    Hello Adriyan

    I haven't found any example where we have used the CT in the primary circuit of a PSFB in the TI literature. One of my colleagues did do this in the past and his experience was that there were oscillations due to noise on the CT waveform. If the CT is in the primary of the transformer then its primary winding will experience high dv/dt rates as the power bridge switches. The high dv/dt rates coupled noise onto the CT secondary (the CS signal) through the interwinding capacitance and this then caused a stability problem.

    Even if the noise coupling issue could be addressed with appropriate screening, we don't have any definitive answer whether the CT in the primary of the PSFB is inherently stable or not so our reccomendation is to place it in the DC bus like we do on our EVMs.

    Placing the CT in the DC bus will protect against shoot through events and the primary current will always be limited on a cycle by cycle basis. The only concern is that there is enough time available to reset the transformer core and this is certainly possible at normal switching frequencies and maximum duty cycles.

    Regards

    Colin

  • 0 in reply to Colin Gillmor
    Prodigy 40 points

    Many Thanks! To you and to your colleague! 

    That answers my question.

    Have a dice day.

    Best Regards,

    Adriyan