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UCC29002: Why is voltage offset not needed in load share controller?

Andrew Massey
Prodigy 120 points
Part Number: UCC29002
Other Parts Discussed in Thread: UC3907, UCC39002, TL431

Hi,

How can the UCC29002 get away without the 50mV voltage offset that is needed in uc3907 to stop the master trying to adjust its own reference?

  • 0
    Prodigy 120 points

    Also, do you know why none of the App Notes show the UCC29002 being used with paralleled opto-feedback power supplies?

    Ie, they dont show the power supply's opto current being adjusted by the UCC29002, do you kjnow why this is?

  • 0
    TI__Mastermind 30330 points

    Hello Andrew, 

    Thank you for your interest in the UCC29002 load-share controller. 

    Such an offset DOES exist on the ADJ amplifier (buffer) of the UCC29002 (60mV in this case), but it is not explicitly mentioned in the text of datasheet as it is in the UC3907 datasheet. 

    In the Electrical Characteristics table of UCC29002 datasheet, page 5, it shows this:   

    Although the implementations of the ADJ amp are a bit different, I believe the offset concept is the same.  

    Regards,
    Ulrich

  • 0 in reply to Andrew Massey
    TI__Mastermind 30330 points

    Hello Andrew, 

    Let me think about your second question, and I'll get back to you on it soon.

    Regards,
    Ulrich

  • 0 in reply to Ulrich Goerke
    TI__Mastermind 30330 points

    Hello Andrew, 

    I believe that the UC3907 device is designed to specifically part of the power supply voltage control loop, including being the error amplifier for the power supply.  This device modifies the loop's reference voltage based on the load-share bus information.  The power supply doesn't have its own control loop and cannot work without the UC3907.

    On the other hand, the UCC29002 device is designed to be an "add-on" to an existing supply that already has its own voltage control loop.  The UCC29002 simply modifies the feedback voltage that is being sensed by the power supply's loop.  The power supply can operate independently without the load share controller.

    I hope this clarifies the difference between the 2 controllers.

    Regards,
    Ulrich

  • 0 in reply to Ulrich Goerke
    Prodigy 120 points

    Hi,

    Thanks, also  , if we want to do opto feedback then we can do it via the ADJ pin?...ie, use the ADJ pin to adjust the reference voltage  of the error amp that drives the opto? Also, it looks like UCC39002 cant be used if vout >15v?

  • 0 in reply to Andrew Massey
    Prodigy 120 points

    Hi,

    Thanks, also  , if we want to do opto feedback then we can do it via the ADJ pin?...ie, use the ADJ pin to adjust the reference voltage  of the error amp that drives the opto? Also, it looks like UCC39002 cant be used if vout >15v?

  • 0 in reply to Andrew Massey
    Prodigy 120 points

    Sorry to  ask again, if we are doing opto feedback then it looks like the UC3907 woudl be better for us?

    We are doing five 75w isolated flybacks in parallel (vin is 10-175vdc, vout = 32v).

    So we need HV FETs as 175vdc, but then need to pllel the flybacks due to rdson loss too high otherwise.

    Are we better using UC3907 for this?

  • 0 in reply to Andrew Massey
    TI__Mastermind 30330 points

    Hello Andrew, 

    I am not sure if the range of the UCC29002/39002 ADJ pin is sufficient for controlling an opto-coupled feedback.  In any case, as I mentioned above, the UCC29002 ADJ pin is intended to modify the voltage sense of an external feedback loop.  There are no provisions in that device to be embedded in the controlling part of that loop.  For that you'll need a regulator, such as TL431 or similar, to drive the optocoupler.  
    The UCC29002 simply modifies the voltage sensing to the regulator.  Each power supply module can operate independently without a UCC29002 circuit.

    The UC3907 would become a main part of the voltage loop and a TL431 would not be needed.  However, each power supply module would always require a UC3907 as part of it since it contains the voltage error amp and reference.  The power supply is not independent of the load-share device.
    Plus, the UC3907 is made with much older bipolar technology with higher bias currents and higher cost. 

    I can't say which approach is better for you.  I can only describe the differences between them. 

    Regards,
    Ulrich