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UCC28950: SHIM INDUCTOR CALCULATION

ilia hadzhiev
Prodigy 160 points
Part Number: UCC28950

Hello, 

I am in designing a 18KW ev charger, i am using UCC28950 full bridge power circuit.
My problem is with the calculation of the shim inductor Ls   the value i get for the inductor is negative.

I will attached i file with the input/output parameters of my design. 

SHIM INDUCTOR.doc

 

 

 

 

 

 

 

 

  • 0
    TI__Mastermind 25570 points

    Hello Ilia

    I ran your input/output parameters through the UCC28950 Excel calculator and it gives me a shim inductance of -4uH - is this a similar value to the negative result you got ?

    The transformer turns ratio was 0.56:1  - BE VERY CAREFUL WITH THE EXCEL CALCULATOR HERE. The calculator calculates the correct turns ratio in line 25 (T1 Transformer Turns Ratio=NP/NS) BUT it rounds it to the nearest integer !. This is just about ok where then input / output ratio is around 21:1 or so (400V/12V) but is completely wrong for your case 400Vin/450Vout where N = 0.56:1 !  You should cut and paste the value from cell 25C into cell 26C - I don't think you fell for this but I mention it just in case.

    Anyhow, if the value is negative then you won't need an additional shim inductance because there is enough energy available to drive the ZVS transition - the 4uH leakage inductance value sounds a reasonable assumption too.

    Regards

    Colin

  • 0 in reply to Colin Gillmor
    Prodigy 160 points
    Thank you

    I have one more question becouse i not going to use SR and i gonna use ADAPTIVE DELAY figure 52, page 55 . Can i leave floating this pins of the controller OUTE, OUTF, DELF, ADELF, DCM if no how i should connect them. How ADELF affect on CS if i dont use Synchronous Rectification.

    Regards
    Ilia
  • 0 in reply to ilia hadzhiev
    TI__Mastermind 25570 points

    Hi Ilia

    You should leave OUTE and OUTF open circuit. Tie DCM to VREF - this stops the SR drive signals so that there won't be a signal at the pins.

    If OUTE and OUTF are disabled as above then the ADELEF pin should be tied to GND to disable adaptive delays and to prevent the pin picking up noise.

    I would recommend that you do not leave the DELEF pin open circuit but instead tie it to GND via a resistor anywhere in the allowed range (13k to 90k) for example a 47k resistor.

    Regards

    Colin