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UCC2897A for 60V-160V input application.

Yipin Xu
Prodigy 30 points
Other Parts Discussed in Thread: UCC2897A, UCC2894

Hi all

I got a case recently, its requirements are as below:

Input voltage:60VDC-160VDC.

Output Voltage:24VDC

Output Current:5A

Efficiency:89%(typical)

I want to use UCC2897A as the main controller and adopt synchronization rectification in secondary side. My question is:

1) As the output voltage is as high as 24V, is it more efficent with synchronizaion rectificaiton?

2) As UCC2897A is designded for 36V-75V input voltage application area, is it suitable to use at so high voltage range,60V-160V?

3) The Vin pin of the UCC2897A can only stands for 120V, recommended voltage is 110V, how can I deal with the start up circuit for the bias voltage for this IC?

I would like to hear  the comments from you all!

Thanks in advance!

  • 0
    TI__Guru 74115 points

    1) Yes, synchronous rectifier will give more efficiency benefit to some extent compared to diode rectifier, but may not be significant increase for 5A load at 24Vo. The savings can be estimated roughly as (0.7V x 5A - 0.1V x 5A) = 3W at full load, where we assume diode forward voltage drop 0.7V. If you use a Schottky, the savings may beome less, say about 2W. So at full load, you are expecting efficiency increase somewhere about 1.5%.  The cost increase is obvious. So you would have to make decision with the trade-offs between the cost and the performance.

    2) The concern is from the avalability of P-channel MOSFET. For 160V input, a rule of thumb is to use a FET 320V rating. There may not be many options for P-channel MOSFET with such a voltage rating. Cost is also high.

    3) If you can find a P-channel MOSFET suit for your design, this is not a problem. You do not have to use Vin pin for your design.

    I would recommend using UCC2894 which uses N-channel MOSFET. Then you have many options to obtain a proper MOSFET with much lower cost.

     

  • 0 in reply to Hong Huang
    Prodigy 30 points

    Hi Hong Huang

     

    Thanks for your comments.

     

    Once more, if UCC2894 is adopted and traditional diode recitification is used, what about the control loop stability when the load is light enough that diode enters into discontinuous mode? Especially the no load condition.

    Thanks in advance and your comments are expected.

  • 0 in reply to Yipin Xu
    TI__Guru 74115 points

    If diodes are used for the secondary rectifier,  the operation will be in DCM at light load and no load. The contol loop should be compensated in that regarding.

  • 0 in reply to Hong Huang
    Prodigy 30 points

    okay, do you have any suggestions on the loop compensation on this no load and light load condition?

    Or do you have some papers on this topic?

    Thanks!

  • 0 in reply to Yipin Xu
    TI__Guru 74115 points

    I think the book, Fundamentals of power electronics, authored by Bobert W. Erickson and Dragan Maksimovic, can be a good reference. You could also search the internet to find various experiences.

    Usually I make the loop design with the CCM and then bench test with trial-and-error to fine tune the loop to cover DCM at light load.