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LM5022: PMP7764 TI reference design question

Tomasz Chumiecki
Genius 3055 points
Part Number: LM5022

HI TI experts,

could you be so kind and tell me what if the function of the R116 resistor in the PMP7764 TI reference design:

  • 0
    TI__Guru 68655 points
    My understanding is this resistor add some dc bias to the CS and hence helps balancing the peak power when input voltage increases.
  • 0 in reply to Youhao Xi
    Genius 3055 points

    Hi Youhao,

    Thank you for your replay but, is it just your understanding of the application or it is really so?
    Could you explain a bit more how can I calculate the needed resistor value?
    How can I calculate the resistor value? Is there any defined dependency I should consider between input voltage and switching current?

    There is other reference design from TI - NSC1024 (old National Semiconductor nomenclature) - very similar one but without this resistor???

    could you tell me why one reference design have this feature and the other one haven't?

    Thank you for your support!
    and have a nice weekend :)

    Best regards,
    Tomasz

  • 0 in reply to Tomasz Chumiecki
    TI__Guru 68655 points
    Hello Tomasz,
    In the past few days I have been trying to dig out the transformer info, however I could not find it. It is also not listed on GCI's website. I have sent an email inquiry to GCI but no reply yet. Could you also check with GCI? I will let you know if I get an answer. Could we close this post for now?

    Thanks,
    Youhao
  • 0 in reply to Youhao Xi
    TI__Guru 68655 points
    Hi Tomasz,

    This is the reply got from GCI: the transformer turns ratio is 1:0.125. The transformer p/n G095028LF.

    Hope this helps.

    Thanks,
    Youhao
  • 0 in reply to Youhao Xi
    Genius 3055 points
    Hi Youhao,

    thanks :) every single parameter of the transformer helps.
    The transformer p/n G095028LF is known and already included in the TI reference design BOM, but I cannot find it on CGI web page...
    The second very important information would be the primary side inductance. Could you ask CGI about it?

    Thanks,
    Tomasz
  • 0 in reply to Tomasz Chumiecki
    Genius 3055 points
    Hi Youhao, any news about the transformer? Were you able to get something more from CGI?
  • 0 in reply to Tomasz Chumiecki
    Genius 3055 points
    anyone else can give me a hint what is really the R116 resistor for and how to calculate the right value... ??? Tomasz
  • 0 in reply to Tomasz Chumiecki
    TI__Guru 68655 points

    Answers to your earlier post regarding the transformer:  The inductances of the winding of (1-3) is 3mH +/- 12%.  Leakage 50uH. 

    Regarding your question of R116, its purpose can be seen in the following analysis. Assume Lm is the inductance, Fsw the switching frequency, k the transformer turns ratio, and the load current Io.  Then, the duty cycle at a given Vin is:

    D = Vo /(Vo+Vin/k)

    The transformer peak current in the primary winding is:

    Ipk = Io / [k x (1-D)] + Vin x D / (2 x Fsw x Lm)

    Therefore, the peak current limit (IPK) is tripped, the load current is

    Iomax = [ IPK - Vin x D / (2 x Fsw x Lm) ] x k x (1-D)

    The load power when peak current limit is tripped:

    Pomax = Vo x Iomax

    Substitute Io and D into above equation, you can find that Pomax will increase with Vin.  To prevent Pomax from being excessive, you should reduce IPK when Vin increases.  The simple way to do this is to use R116.

    As I mentioned earlier, R116 introduces an DC bias voltage to adjust the peak current limit: Vin x (R108+R109) /(R108+R109+R116) to VCC, superimposed to the CS current signal  Ipk x R115.   Higher Vin will add more dc bias, effectively reducing the actual peak current IPK allowed through R115.  

    Roughly, the effective peak limit will become (converting to sense voltage):

    IPK x R115 = 0.5V - Vin x (R108+R109) /(R108+R109+R116) - 45uA x (2k + R108 + R109) x D.

    Where 45uA is the internal slope compensation current, and the 2k resistor is also internal.

    You can see, having R116 prevents the overload from being excessive when Vin increases.  Working out the math you can find out the proper R116 needed.

    Hope this clarifies.

  • 0 in reply to Youhao Xi
    Genius 3055 points
    Hi Youhao,

    thank you! it sounds absolutely plausible.
    The primary inductance looks like quite high for fixed frequency flyback... maybe for CCM one.
    My target was to drive FF flyback deep in DCM but I will try to play a bit with primary inductance too.

    Thank you one more time!
    Tomasz