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UCC2897A: Bandwidth

Julian Chen
Expert 2550 points
Part Number: UCC2897A
Other Parts Discussed in Thread: LM5022

Hi E2E,

We use UCC2897A to design 2 isolated circuit and  measure the VOUT ripple  which is in our SPEC (+/- 5%), But the bandwidth is too slow. (< FSW/20)

Could you kindly check it and let us know that bandwidth is OK or still need to over than FSW/20?

Thanks!


1. 36~57V →12V/1.5A; FSW=250k

Bandwidth:

Transient: 0~50%


Transient: 50~100%

 

2. 36~57V → 54V/1A ; FSW=250K

Bandwidth:

Transient: 0~50%

Transient: 50~100%


  • 0
    TI__Mastermind 34465 points

    Hello Julian,

    It should be possible to get a higher bandwidth form this circuit but you will need the schematic and an accurate bill of materials in order to make any changes to your unit.
    The  Application Report SLUA535 section 5 and 6 detail the method of compensating the feedback loop for the UCC2897A.
    You can also look at any of the reference designs for this part and check the typical compensation component values used here.
    Then just compare them with the values used in your design. This will give you a clue as to the components that may be required to check.

    Regards

    John

  • 0 in reply to John Griffin1
    Expert 2550 points

    Hi John,

    the Circuit is below,

    Could you kindly let us know how to fine tune the Bandwidth?

    LM5022 for 12V I think that add C97 first, then fine tune C97, R131 and C100.

    UCC2897A for 54V I think fine tune C401, C405 and R352.

    Is it right?  or Please let us know more suggestion to complete it, Thanks!

    CircuitReview_TI.zip

  • 0 in reply to Julian Chen
    Expert 2550 points

    Hi John,

    About 12V, I add C97=4700pF and modify the R132 to 1k, the Bode plot as below,

    I also fine tune the R139 , C132 and C97 but no any improve.

    Could you kindly let us know how to increase the bandwidth? 

  • 0 in reply to Julian Chen
    Expert 2550 points

    Hi John,

    About 12V, although bandwidth just only 10kHz, but transient is in SPEC. Customer think that is OK.

    Could you kindly let us know how to fine tune  36~58V→54V (UCC2897A)?

    Thanks!

  • 0 in reply to Julian Chen
    TI__Mastermind 34465 points

    Hi Julian,

    On your 54V schematic it looks like there is a low frequency pole set at about 100Hz with C401 (15nF) and R352 (100k).

    If you reduce the value of C401 to 10nF or less you should get a corresponding increase in bandwidth.

    That's is the easiest way of improving the step response.

    Regards

    John

  • 0 in reply to John Griffin1
    Expert 2550 points

    Hi  John,

    We follow your method to modify C401 and R352...

    But Bandwidth don’t improve at all.

    We just want to increase bandwidth to make sure the transient better.

    Did you have another suggestion?

  • 0 in reply to Julian Chen
    TI__Mastermind 34465 points

    Hi Julian,

    Sorry about the delay in getting back to you.

    Your switching frequency is 250kHz and a reasonable bandwidth for your application is 10kHz or so with no overshoot in the transient response.
    This would be an acceptable value for most applications.
    You can continue to try and increase the bandwidth by reducing the RC value on the feedback amplifier but honestly I do not have any more ideas that may help you.

    I am going to close this post since there seems to be no new information available.

    Regards

    John