Dear Roberto,
I am stucked at output capacitor as per the formula given in SLUA560C it gives value greater than selected one. Please explain how do you calculate output capacitor value?
Anjana
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Hi Anjana,
The output capacitor, calculated in the excel sheet, has the same calculation procedure as in the SLUA560C: it's made by setting the maximum deviation during a transient response (in my case 5% of Vout, 32V, which is 1.6V). The result is 1936uF. I selected in the excel sheet 4 x 470uF, which are barely sufficient. In the final prototype I selected 6 x 470uF to have more margin.
Regards,
Roberto
DEAR ROBERTO ,
ATTACHING EXCEL SHEET FOR PMP8740 DESIGN. PLEASE CHECK AND SUGGEST CORRECTIONS.
THANKS,
ANJANAUCC28950 _PMP8740.xlsx
Dear Anjana,
I see that in your excel sheet the time "Thu" has a 10^-6 factor, which should not be there, because the normal calculation takes into account already that the result is in usec and not seconds. If you correct the formula in the cell, the it should give ~ 13usec and the output capacitance several thousand uF. This big capacitor comes from the fact that you set a small voltage deviation during output load transients (VTRAN) compared to my design, and also because you are switching at 50KHz (on the bridge) and 100KHz (on the output inductor), which is "fs", cell C18.
Please let me know if you get reasonable values.
Best regards,
Roberto
UCC28950 _PMP8740_ti.xlsxHello Roberto ,
1)In attached excel sheet power rating of DcDc converter considered by you for PMP8740 design is correct? because I am not able to get IPS=60.3 A as you mentioned in reply for how to calculate R98.
2) I am still not able to get value of Ls inductor =300uH as used in PMP8740 design. As per your suggestion I considered 15% load for calculating Ls, but still not get around 300uH value. Please help.
3) Also please explain output capacitor selection.
Thanks,
Anjana
Hello Anjana,
Here are the answers about your questions:
1) The excel sheet you attached is correct. You get a different IPS number because your output nominal voltage is lower than my case.
2) Please consider that the 300uH value is placed in the BOM coming from a placeholder (in order to route the board layout). The right value is described in the Power Point Presentation file and also in the white paper, which is 10uH. In those documents is explained how it is built. Therefore I suggest you use also 10uH in your design and in the excel sheet.
3) I already replied to this question few answers ago, see:
The output capacitor, calculated in the excel sheet, has the same calculation procedure as in the SLUA560C: it's made by setting the maximum deviation during a transient response (in my case 5% of Vout, 32V, which is 1.6V). The result is 1936uF. I selected in the excel sheet 4 x 470uF, which are barely sufficient. In the final prototype I selected 6 x 470uF to have more margin
Please let me know if that answers all your questions.
Regards,
Roberto
Dear Roberto,
Attached is the excel sheet in which I am trying to match values selected in PMP8740. This Excel sheet is not as per my design specifications. I am trying to consider an input specifications which you have considered while designing PMP8740. Once this sheet get matched with PMP8740 design. I will use it by putting my inputs. So please check and tell me, inputs i am considering in the attached sheet are as same as those you have considered for PMP8740 design. Beacuse i am still not able to get values of Ls, Lout, Cout, in excel sheet as those used in PMP8740 design.
Hello Anjana,
Please find attached the excel sheet about UCC28950, used to design the DC/DC stage of PMP8740.
It's a bit different from yours, but basically it's the same file with different parameters.
Best regards,
Dear Roberto,
Hope you are doing well.
I have procure P0584ANL transformer to replace AF4779 but footprint is not matching at all. is there any alternate part no which matches footprint and specifications of AF4779?
Anjana
Dear Anjana,
I am fine, thanks....hope the same about you.
I checked out and, since AF4779 part number from Vitec is a custom, I didn't find a pin to pin replacement.
I am afraid you have to adapt your actual transformer to the layout, sorry.
Regards,
Roberto
Hello Roberto!
Hope you doing well!
What is function of Precharge signal? How it is used in PMP8740 design?
Anjana
Hello Anjana,
The digital signal "Pre-Charge" is enabling all FETs Q12, Q17, Q25...Q31: these form the "precharge" function, which means they are OFF at the beginning and let, by means of thermistors RT1 & RT2, charge the output capacitors C53...C56, C19 and C29 from the attached battery. Without this pre-charge function you get a spark on output terminals during battery connection. Furthermore the FETs Q12...Q31 provide reverse polarity protection, in case on output terminal you have negative voltage.