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TIDA-010054: Clarification on equation 15 and 16

Danilo A.
Mastermind 28640 points
Part Number: TIDA-010054

Hi Team,

Our customer has some clarifications about the equation 15 and 16 of the TIDA-010054 reference design. According to our customer,

I'm trying to design dual active bridge converter from TIDA-010054

My problem is related to relations 15 and 16 in 2.3.4.4 Capacitor Selection. I can't plot the graph in Figure 2-20 by using these relation

i think there are problem in equation 16.

Also, I calculated the RMS relation for equation 12 and i think first sentence and second part of right part of equation need a fraction over 3.

(i1^2)/3+(i2^2)/3

Regards,

Danilo

  • +1
    TI__Expert 4400 points

    Hi Danilo,

    The data plotted in Figure 2-20 was found by experimental testing. The equations 15 and 16 will typically lead to smaller capacitor values, since they do not include circulating currents. Regarding output capacitor selecting a simulation of the system is recommended.

    Regarding equation 12 you are right the whole equation should be divided by 3 (i1^2 +i2^2 +i1*i2*(1-2(phi/pi))/3.

    Best regards,

    Andreas

  • 0 in reply to Andreas Lechner
    TI__Mastermind 28640 points

    Hi Andreas,

    I just received this follow up inquiry from our customer.

    thus, Figure 2-17 should be edited by regarding equation 12 and dividing by 3 (i1^2 +i2^2 +i1*i2*(1-2(phi/pi))/3.
    yeah I agree with you that The equations 15 and 16 will typically lead to smaller capacitor values but the range of capacitor will be in nano farad not in micro farad.
    by the way, there is so much difference between calculation and selected value.
    I use another equation with consider circulating currents C=deltaQ/deltaVout(5%*500V). the calculated value for output capacitor was not in range of micro farad.
    I sure this difference is not acceptable.
    could you tell me what was the range of output capacitor from your calculations and your experimental testing?

    Regards,

    Danilo

  • 0 in reply to Danilo A.
    TI__Expert 4400 points

    Hi Danilo,

    There will be a major update to this design guide in the coming month. I will include the update of Figure 2-17.
    The output capacitor for our design was selected with 250 micro Farad after testing. For different leakage inductances Figure 2-20 can be used as a starting point. Never the less the final value should always be verified with testing. 

    Best regards,

    Andreas 

  • 0 in reply to Andreas Lechner
    TI__Mastermind 28640 points

    Hi Andres,

    Please see the feedback of our customer below.

    Thanks a lot for your assistance.
    I will verified the final value of out capacitor with testing,too, but normally the calculation give us an value for starting point. I can't understand why the relation for calculation the output cap (delatQ/deltaV) doesn't work for this kind of converter. In other way there isn't an value for starting point and the designer should be relay on his/her experience and testing. My other concern is that there is a an incredibly a huge difference between the calculations and the testing value.

    Regards,

    Danilo

  • 0 in reply to Danilo A.
    TI__Expert 4400 points

    Hi Danilo,

    I am sorry that I do not have a definite answer to this question, since I am not the original author of this design. I did some more research and simulations on this topic. In general my findings are the following. The general deltaQ/deltaV approach is working with this topology as well, but these small capacitor values will not work with the TIDA-010054 Hardware. The reason therefore is that the output capacitor has an impact on how the control loop has to work. 
    In simulation I had to drastically reduce Kp and Ki in the PI controller to get the system working with lower output capacitance values. 
    I still recommend the the values mentioned in the design guide as a starting point since this is a tested working design.

    Best regards,

    Andreas