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UCC28881 - non isolated AC/DC input AC100V output DC5V@300mA

Hirotaka Matsumoto
Guru 27525 points
Other Parts Discussed in Thread: UCC28881, UCC28880

Hi all

Would you mind if we ask UCC28881?

Our customer would like to design non isolated AC/DC solution with the condtion from input AC100V to output 5V@300mA using UCC28881.
On the datasheet P23, Fig22 shows Low-Side Buck-Boost Converter.
We guess that it is possible to use for our customer's request.

<Question1>
Fig22 on the datasheet P23, Vout- is not connected to GND.
We guess that the Vout- ground level will be unsettled, right?
Is it no problem? 

<Question2>
Could you let us know how to calculate constants of D1, L1, RFB1, RFB2, CL, and Q1?

Kind regards,

Hirotaka Matsumoto

  • 0
    TI__Genius 9130 points
    Matsumoto-san,

    Q1 - in the buck-boost configuration as shown in Fig. 22, the Vout- is connected back to Vin+, so the DC output voltage is floating high wrt Vin-, and will move up with Vin voltage level.

    E.g. if Vin = 150 Vdc, then Vout- will sit at 150 V wrt Vin-, and Vout+ will sit at 155 V wrt Vin-. This may be ok for you, but it depends on your load and what the Vout is connected to.

    Q2 - the buck-boost configuration may not be the best solution is your case, since you need to buck down a lot to 5 V, and your load current is quite high. Since the UCC28881 Ipeak max is worst-case min of 315 mA, it would be hard to meet 300 mA load current.

    It might be better to use the non-isolated Flyback configuration shown in Fig. 24.

    You could use these existing TI reference designs as a starting point for your design:

    www.ti.com/.../TIDA-00708
    This is a 5-W design using UCC28881 for isolated and non-isolated 12-V outputs. The isolated 12 V can be removed, and non-isolated 12 V can be modified to 5 V by reducing the sec turns, and changing the FB resistor.

    www.ti.com/.../PMP10143
    This is a 1.7-W design using UCC28880 for 12 V & 3V3 isolated outputs. Again, the 12 V can be reomved, add a turns to the 3V3 to get 5 V, and remove the isolated feddback, and connect FB directly to output voltage divider.


    I hope this helps.

    Thansk,
    Bernard
  • 0 in reply to Bernard Keogh
    Guru 27525 points

    Bernard san

    Thank you for your very kind reply!

    E.g. if Vin = 150 Vdc, then Vout- will sit at 150 V wrt Vin-, and Vout+ will sit at 155 V wrt Vin-. This may be ok for you, but it depends on your load and what the Vout is connected to.
    -> In case of switching operation, will Vout flutter?

    And then, our customer don't want to use the trans, because the trans is more expencive compared with the inductor.
    Furthermore, it is not required isolation.

    If you have cheaper non-isolated AC/DC solution which is 100V input to DC5V/0.3A, could you advice us?

    Kind regards,

    Hirotaka Matsumoto

  • 0 in reply to Hirotaka Matsumoto
    Guru 27525 points

    Bernard san,

    If you have some comment, could you let us know?

    Kind regards,

    Hirotaka Matsumoto

  • 0 in reply to Hirotaka Matsumoto
    TI__Genius 9130 points
    Matsumoto-san,

    For the buck-boost question, sicne the negative terminal of the output cap is tied to Vin-, and the output cap has DC voltage across it, the Vout voltage will move up or down if vin move. However, there will be no PWM/switching freq movement in Vout.


    For non-isolated bias supply for 100 V ac to 5 V, I searched the available TI Designs suing the parametric tool here:
    www.ti.com/.../refdesignsearchresults.tsp

    You can use this too lto search for available designs and refine by input voltage, output voltage, output current, AC vs DC input, isolated vs non-isolated, etc.


    From this tool, there are 2 designs for 100 V to 5 V @ 0.25 A, they can be adjusted slightly to meet your spec of 0.3 A output current.
    www.ti.com/.../PMP5298

    This uses a coupled-inductor L1, but this is not a custom part, it is an off-the-shelf 2-winding inductor availabel from Coilcraft.

    This is a very similar design, just using a different PWM IC:
    www.ti.com/.../PMP5422

    I hope this is helpful for you.

    Thanks,
    Bernard