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UCC28742: FEEDBACK RESISTOR VALUES

ABHISHEK ARRAGUNTA
Prodigy 70 points
Part Number: UCC28742
Other Parts Discussed in Thread: PMP30545

i would like to know how the resistor values R10 and R11 have arrived with respect to the feedback ciruit .

Why is it similar to the gate resistor value R12 

Please provide the explanation for the same 

  • 0
    TI__Mastermind 26870 points

    Hello Abhishek,

    Thank you for your interest in the UCC28742 flyback controller.

    This circuit is the PMP30545 reference design for a multi-output, non-isolated feedback application.  The three resistors you cite (R10, R11, R12) are all "place-holder" components.  They are not usually necessary, but included into the design in order to provide flexibility for future value adjustment, if it proves advantages to circuit optimization.  Their values are currently set to zero either because no optimization has been found to be necessary or because no optimization has been attempted yet. 

    A few ohms at R12 would provide damping to the gate-drive loop in the event that there is excess ringing from on/off switching due to parasitic inductance and capacitance.

    Q2 and Q3 form a current mirror that emulates an opto-coupler (normally used in an isolated feedback design).  R10 and R11 are gain-degeneration resistors in the emitter legs, used to reduce current gain in the mirror.  Design considerations for this arrangement are beyond the scope of this forum, however a web-search should result in references to this topic.

    Regards,
    Ulrich