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UCC28951-Q1: bias supply and grounding for onboard charger application with isolated gate driver

Natthapol Vanasrivilai
Prodigy 240 points
Part Number: UCC28951-Q1
Other Parts Discussed in Thread: UCC28700

Hello,

i'm designing an onboard charger, and came to the gounding confusion for the isolated gate driver for driving the UCC28951-Q1 signals.

In my application, I've implement the UCC28700 for bias flyback supply with isolated ground (Sec Bias Flyback). Upon drawing up the schematics, it seems like I am not allowed to use this supply to drive the MOSFET's gate. Otherwise high voltage ground will be joined with low-voltage ground through the MOSFET source pin.

I have watched the TI training series "How to Design Multi-kW DC/DC Converters for Electric Vehicles (EVs) (part 4)" and on slide #35, it shows that the gate drivers were driven by 2 supplies (VSEC_BIAS + VPRI_BIAS).

Do i actually have to implement the same UCC28700 bias supply again with common ground (Pri Bias Flyback) and have doubled the component count? Am i understanding the concept correct?

  • 0
    TI__Mastermind 25570 points
    Hi Natthapol

    You must use separate bias rails for primary and secondary circuits - otherwise as you say, the high voltage ground will be shorted to the low voltage ground. One possibility is to use a single UCC28700 and put an additional secondary winding on the transformer. One secondary winding generates a bias voltage referenced to the output (low voltage) ground. The other secondary winding generates a bias voltage referenced to the input (high voltage) ground. this way you get two outputs for the cost of a single primary controller (UCC28700).

    Regards
    Colin
  • 0 in reply to Colin Gillmor
    Prodigy 240 points

    Hi Colin,

    thanks for clearing this up.

    One more question regarding an extra secondary winding on the SMPS transformer.

    I have confirmed the design with Webench and it tells me that I need 4 turns on the secondary.


    If i add another secondary to the transformer, do i need to go back and do all the calculations on the primary side? or I can wind another secondary with the same turns count and change nothing on primary side.

    Do you have any suggested layup for this configuration, or can i just add 2nd secondary winding above the 1st secondary winding ann call it good?


    Regards,

    Natthapol Vanasrivilai