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Original question:

PMP8740

PMP8740

anjana salgaonkar
Expert 1460 points
Other Parts Discussed in Thread: UCC28950, PMP8740

Hello Roberto,

Please clear my doubt. In PMP8740 design R34 is Rcs(as per UCC28950 typical application figure in Ucc28950 datasheet) and R69 is used to reset CT right ? if yes then Why R34 selected as 16.2ohm becaouse as per datasheet reset resistor =100* Rcs, hence 16.2*100= 1620 and reset resistor used in PMP8740 is 100K which is confusing. Please help.

Anjana

  • 0
    TI__Expert 5245 points
    Hello Anjana,
    You are right: R34 is Rcs and R69 used to reset CT.
    In the datasheet is explained how to calculate R69, which is the 100 * Rcs. This is the standard way.
    I prefer to do it differently.
    In practice, I calculate the current stored in the CT, according to the voltage I have on the output (which is the output current of CT + Vf of the diode D2). This stored current is the magnetizing current of CT (in Lm). During Toff, I calculate the resistor R69 so that the peak magnetizing current (Ipm) is discharged in less than 5 or 10 times the Toff time (the discharge time = Lm / R69). The higher the resistor R69, the faster is the demagnetizing time, which I prefer to calculate as fast as possible for reliability reasons (a saturated CT is catastrophic).
    It is also important that R69 * Ipm < max. rating voltage of D2, otherwise D2 will be damaged. This is why R69 cannot be too high.
    Please let me know if that answers your question.
    Regards,
    Roberto
  • 0 in reply to Roberto S.
    Expert 1460 points
    Sorry Roberto I am not getting your answer. how do you calculate value of magnetizing current of CT? also please correct me if primary side fsw is 100kHz then with Dmax 90% ton= 9usec, and toff =1usec, demagnetizing time=1usec/5=200nSec (because Ipm discharged in less than 5 times the Toff time) so R69= 20mH/200nSec=100K,

    What is secondary voltage of CT?

    Anjana
  • 0 in reply to anjana salgaonkar
    TI__Expert 5245 points
    Sorry Anjana,
    You are right: the value of R69 can be calculated exactly as you suggested, by considering R69 = Lm / demagnetizing time of 1usec.
    The story with output voltage of CT can be applied when you considering a zener diode instead of R69 (therefore a constant voltage applied during Toff).
    Regards,
    Roberto
  • 0 in reply to Roberto S.
    Expert 1460 points

    Dear Roberto,

    in PMP8740 design 

    1) how do you get value of  R34= 16.2ohm.?

    2)how do you calculate c1, c2, r3, C12, C13, r37? because in fs=200Khz, and considering load impedance at 10% load i am getting value of R3=32.7K, but in design it is 68.1K, simillarly i am getting values of c1=234pF, C2=2.3nF  

    3) Value of R11 should be 49.9K or it should be 21.9K? beacause if Tabset=100nS, then Tafset=50nS, then R6 and R17 gives correct values (8.25K, 348Ohm resp), but value of R11 calculated as 21.5K which is shown in PMP8740 design as 49.9K. Please clear my doubts.

    Anjana

  • 0 in reply to anjana salgaonkar
    TI__Expert 5245 points
    Dear Anjana,
    Let's reply each point:

    1) R34 should have been higher value, in the range of 21 Ohm (consider that my power supply is 2KW, while the excel sheet refers to 1920W). I selected a lower value because I don't want the peak current limit to trigger before the average current limit (CL loop, managed by U1B) otherwise two current limits will fight against each other. Therefore the peak current limit is intended to protect the power stage against dangerous peak currents, while U1B defines the actual current loop.

    2) I didn't use the values from excel sheet because this sheet refers to normal power supply, while mine is a battery charger. Therefore the impedance of a battery is way smaller than normal load. This way I preferred to measure the bode plot and adjust poles and zeroes accordingly.

    3) Also here, I think I didn't use the values from excel sheet but from another design, and I verified that the values I employed, were setting up all delay times, probably a bit conservative but everything was working fine. In your case, please follow the values you get from the excel sheet.

    Best regards,
    Roberto
  • 0 in reply to Roberto S.
    Expert 1460 points

    Dear Roberto,

    Thanks for reply.

    My application is also battery charger not power supply only thing is its output voltage rating is 40V-54V-58.5Vdc (min-typ-max resp). so should I go with your values for setting up delay time? Please help.

    Anjana

  • 0 in reply to anjana salgaonkar
    TI__Expert 5245 points

    Dear Anjana,

    If your application is battery charger, with Vout range 40V...58.5V and same power, I would keep the same value of voltage and current loop compensation.

    Regarding the values that set up delay times, I suggest also to keep my values, even if they are not optimized.

    Then, after testing extensively the charger interms of max and min Vout, as well as 0 to full charging current, you can have a look to delay times in the switch nodes and adapt the setup on the controller. This way you get an optimized efficiency curve.

    Thanks,

    Roberto

  • 0 in reply to Roberto S.
    Expert 1460 points

    Dear Roberto, 

    1) You mean I should check waveform for Tabset and Tcdset  as shown in your design sheet named as "TABSET Valley Switching"?

    2) Regarding to current sense resistor R34, if I modify power rating Pout in your design sheet from 1920W to 2000W then i get Ip1=7.8A, and considering VP(cycle by cycle current limit_min)=1.94V then I get Rs=20.2 Ohm . Can I follow this method?

    I have not changed the multiplication factor of 1.1 in that formula. but if i change that multiplication factor to 1.35 then i get Rs=16.5 Ohm and closest resistor value given by design sheet is 16.2 Ohm. Is this way  correct?

    Anjana

  • 0 in reply to anjana salgaonkar
    TI__Expert 5245 points
    Dear Anjana,
    The answer is simply "yes" on all three questions :-).
    Regards,
    Roberto
  • 0 in reply to Roberto S.
    Expert 1460 points

    Thanks Roberto, Thank you so much.

    Anjana

  • 0 in reply to anjana salgaonkar
    Expert 1460 points

    Hello Roberto , 

    please tell me in PMP8740 design at secondary side you used synchronous rectification, what exactly called to that topology used in secondary side?

    Anjana 

  • 0 in reply to anjana salgaonkar
    TI__Expert 5245 points

    Hello Anjana,

    I am afraid I didn't get exactly what is your question. Did you mean:

    1) Why I used synchronous rectification?

    2) Why I selected a secondary side center-tap sync. rectification with only two FET (actually 4, with 2 in parallel)

    3) Why I didn't use a single winding and 4 FETs?

    Please let me know.

    Regards,

    Roberto

  • 0 in reply to Roberto S.
    Expert 1460 points
    hi Robetro,

    No..no.. I want to know what is the name of topology used in secondary side? I got how it is working? I just want its name.

    Another doubt. Please correct me. for pmp8740 pin out for T4 transformer should be
    primary (start- pin 1; end- pin 6);
    secondary1 ( start-pin7 &8 ; End-pin9)
    Secondary2 ( start-pin10 ; End-pin11&12)

    Anjana
  • 0 in reply to anjana salgaonkar
    TI__Expert 5245 points

    Hi Anjana,

    The topology of the whole DC/DC converter is Phase Shift Zero Voltage Transition Full Bridge with synchronous rectification.

    The way the synchronous rectification is used on secondary side is "center tap".

    Regarding the transformer T4, as can be seen from attached datasheet, the primary starts on pin 1 and ends on pin 6, while first half-secondary starts on pin13 and ends on pin 15, while the second half-secondary starts on pin14 and ends on pin 16.

    This is congruent with schematic and also datasheet of T4.

    Best regards,

    Roberto

    145400-a_english.pdf

  • 0 in reply to Roberto S.
    Expert 1460 points

    Ok , thank you Roberto.