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UCC28710: How to handle ucc28710 in a buck converter design

Sandro123
Prodigy 10 points
Part Number: UCC28710
Other Parts Discussed in Thread: PMP22210, UCC28881

Dear Team

I'm a student from switzerland and i would like to design a buck converter (without isolated output) with the ucc28710 chip. My design should work in range of 85VAC to 275VAC input and the output should be 3.3VDC by a current around 100mA. Is the ucc28710 a ideal device for a low cost, high efficiency AC/DC converter?

The datasheet from ucc28710 treated only isolated topologies, so i found your reference design for a buck converter. Do you have any datasheets who treat non isolated topologies?

Next i simulated the reference design below with the ucc28710 transient model in the simulation programm TINA9, but it doesen't work. Even the start up proced doesn't work. What could be my failure?

After that i bought some ucc28710 to built the reference desgin. With a DC input (up to 110V) works nothing. Then i replaced the DC input with a AC input and the diode recifier with a bridge recifier. After that i saw the start up proced by a input voltage of 20VAC(make sense). Why it doesen't work with a DC input? 

How can i influence the output voltage? 

Best Regards

Sandro

ucc28710_refdes.TSC

  • 0
    TI__Guru 66135 points

    Hello,

    There is not an application note on how to do this that I can see.  However, it should not be that hard to setup.

    The UCC28710 used resistor divider R1 and R2 to regulate the output voltage.  The control voltage at the VS pin 4.05 V so 3.3 V is not possible.

    However, you could set the output to 5 to 12 V and use an LDO or series pass regulator to achieve 3.3 V.

    Resistor R2 and R3 set the output voltage Vout/(R1+R2) = 4.05V/(R1)

    I reviewed the schematic for PMP22210 using this device.  It has a 150 k ohm resistor across D2.  In your design you have 500K.  You might want to decrease to 150 k ohms.

    http://www.ti.com/lit/df/sluraq7/sluraq7.pdf 

    The UCC28710 has an input under voltage lockout when used in a fly back is looking for  225 uA coming out of the VS pin when the internal FET is on.  This current is specified in the data sheet as IVLS(run).  When the switch it on R1 is internally clamped on the UCC28710 to ground.  IVS > 225 uA = VL1/(R2+R3)

    The UCC28881 has control voltage of 1V on the FB pin so 3.3 V is possible.  This was also designed for buck converters and there is an excel design tool that can aid with the design.  If you wish to investigate using this device.  The following two links will get you to the UCC28881 data sheet and excel design tool.

    Have you researched the TI reference design library to see if their is an existing design that meets your needs already?  You can find these designs at the following link.

    If you chose to use the UCC28881 or another device for your application and have trouble using this new device.  Could you repost the e2e question with the new device number in the e2e query.

    Regards,