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UCC28750: Removing Opto Coupler from the Feedback Loop

Keith Keller
Expert 6860 points
Part Number: UCC28750
Other Parts Discussed in Thread: ATL431, PMP31288, , TL431

Hello,

We're working on a UCC287505 flyback design.

One version of the solution is AC input and we need to isolate the secondary.

however a different version is using low voltage DC input and it would be nice to be able to remove the opto coupler need and several passive components.  (can ATL431 be removed as well?)

Is there a way to hook up this IC so that we can directly control the FB?

Thank you, Keith

  • 0
    TI__Expert 6860 points

    Hello, Please see this circuit replacing opto-coupler with 3 transistors.

  • +1 in reply to Keith Keller
    TI__Mastermind 24365 points

    Hi Keith,

    This circuit was developed by PDS team and can be used for non isolated design.

    If I remember correct there is a reference design PMP31288 developed by PDS team, not sure if it is released, but definitely it can be used. 

    Also shown below is another knaive version with a zener and npn

    Q1 goes to the FB pin which can be used to control.

    Thank you

    Regards,

    Harish

  • 0 in reply to Harish Ramakrishnan5
    TI__Expert 6860 points

    Hello,

    Could you please provide more details on the circuit:

    • Where is the Zener getting power?
    • Is that capacitor paired with the Zener?
    • Could you could elaborate on how the circuit is intended to work or give me a larger screenshot of the circuit?

    Thank you, Keith

  • 0 in reply to Keith Keller
    TI__Mastermind 24365 points

    Hi Keith,

    I do not have very fine details. I had tried this on a high Voltage buck circuit based on UCC28750.

    Thank you

    Regards,

    Harish

  • 0 in reply to Harish Ramakrishnan5
    TI__Expert 6860 points

    Hello Harish,

    Would it be possible to simulate the difference in Vout load regulation based on using your circuit versus an opto coupler?

    Thank you, Keith

  • 0 in reply to Keith Keller
    TI__Mastermind 24365 points

    Hi Keith,

    It will be difficult to simulate the cases to check voltage regulation difference as a lot of these depend on the choice of feedback components (their modelling). So it is better to implement this on the board but let me try to explain how this is implemented so customer can try to verify at their end.

    Shown below is the implementation for UCC28750 using TL431. In all cases GNDs can be changed for non isolated configuration.

    Now in the above we can replace TL431 with zener diode as shown below:

    The series drops across Vz, R1 and the forward drop of the LED of opto determine the output voltage. A diode is optional depending on the availability of a suitable Zener voltage. Resistor R2 provides a 0.5 mA bias current so that Vz is operated close to its knee voltage. Resistor R1 adjust the output voltage and limits the feedback current during output load transient.

    Vout = Vz+Vopto(forward drop)+VR1

    Output regulation is fully dependent upon the choice of these components but I thinkTL431 based scheme offers better regulation.

    Lastly,

    R2 basically is selected to set the bias current in Vz. R3 limits the base current to the transistor, the dotted components are optional which decide the roll off of high frequency gain.

    Thank you

    Regards,

    Harish