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ISOLATED 12W FLYBACK WITHOUT OPTOCOUPLER USING LM3481

robin gangopadhya
Intellectual 855 points
Other Parts Discussed in Thread: PMP8977, UCC28700, TINA-TI, PMP4480

I need to give up trying to make LNK6777 work. It wont start up to troubleshoot. So I cannot find out what I am doing wrong.

So need to make this work reals fast.

-input 180 to 400 vdc

ouput 15v dc@ 500ma max with possibility of no load at all

no opto for feedback.

I can use 1 winding to control, second winding to external load with some min.  internall dc current dissipation.

The converter will drive an LDO whose output is loaded with 1500uF( for reasons outside my control)

anyone has a circuit/simulation with transformer designs I can utlize?

thnx

 

  • 0
    TI__Expert 5900 points
    Hi
    Please refer to PMP8977, its out is 15V/1A.
    Its input is 85V to 265V AC.
    http://www.ti.com/tool/pmp8977

    Br
  • 0 in reply to Kening Gao
    Intellectual 855 points
    Kening
    Sorry for the delay.
    I have paid attention to your response.
    I got samples of UCC28700
    Fabbed PCB.
    made transformer.
    I have a LTSPICE model of a flyback. I used that to define Lp, Ls, aux winding, ratio etc. My leakage is <10uH. Lp=1200uH. Laux=19uH, Lsec=11uH, Input range is 165V to 380V coming from 3phase rectified source. My output is to be 16.5 V @500mA
    No brainer.

    installed all parts.
    but of course it does not work!
    here are my observations
    -value of RS1 is 118k, Rs2=22.6k
    In second reading of the d/s. seems it is not correct.
    If you use Vin(DC)=165, NPA=7.6, Ivsl=.25mA, you get 86K,
    Rs2 comes out to be 38.5K

    So I got all wrong values from first cut calculations.
    Am I right?
    Next issue is how d/s defines current & voltage at VS pin
    What is the meaning of VSR=+ 4 V?
    At VS pin, one expects to see Aux dc value + Vsec and -Vdrain/NPA peak during OFF duration, both multiplied by (Rs2/(Rs1+Rs2)
    Which of this can get to +4V?
    In recommended operating condition, it is given I(VSR)= min -1 mA
    This is CURRENT GOING OUT OF VS PIN( - sign), right?
    Then in Tables, it is given as -300uA in negative clamp level of max 365 mV, what does this imply in a circuit ?
    With VSR=4V at VS pin, we cannot have Rs2=38K & Rs1=86K
    something does not jive here.

    Do you have a spread sheet I can use to get correct values for my operating conditions?
    Thnx
    robin
  • 0 in reply to Kening Gao
    Intellectual 855 points
    another question: I am running TINA-TI example of UCC2860 with my UCC28700 parameters. But of course, I get 0 in all outputs. How do I convert this to UCC28700 transient model using the SPICE model you have? This will answer a lot of my questions and support CDR level analysis before I get to put this design in a system.
    I am using Lp=1100uH, NP=80, NS=8, NB=11, Lkg=10uH, Rs1=86k, Rs2=19k, Rcs=1, R13 in example connected to 21V output.
    did not change opto driver, VG1=165V, Bias R11=1Meg, Cbias=4.7uF
    How do I upgrade TINA_TI to have this circuit in the examples?
    thnx
  • 0 in reply to robin gangopadhya
    TI__Expert 5900 points
    Hi Robin,
    Why don't you use UCC28700 simulation circuit which can be eaisly got from TI site. you may run and look at the waveforms first.
    www.ti.com/.../slum455

    Another suggestion is to read the 5W UCC28700 design review guide. I am sure it will help you a lot. (of course, you may goback to the datasheet for some explanation)
    www.ti.com/.../slua653

    UCC28700 is not a complicated part, but you still need some patience to understand it.
    I will be glad to answer your question after you read the design review guide.

    Br
  • 0 in reply to Kening Gao
    Prodigy 60 points
    Dear Robin,

    I tried to use Reference Design PMP4480 and I had some problem.
    Trasformer indicated in Ref.Des. and VS/CS setting are for a 5V@1A AND NOT for the declared 12V@1A.
    I tried to solve reducing minimum Vin to 150V and using same trafo with different Rs1/Rs2: am I right or I lost something in 28710 Datasheet or App.Notes ?

    Valerio