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WEBENCH® Tools/TPS62136: Output capacitance value

Elder Costa17
Expert 2825 points
Part Number: TPS62136
Other Parts Discussed in Thread: TINA-TI

Tool/software: WEBENCH® Design Tools

Hello, I marked this as Webench related but I am not sure about it (bear with me :)  ).

I am using this part in several power supplies with different output voltages and currents. While creating the electrical schematics, I noticed the derated Cout varied widely in two cases, per Webench:

1- Vin 11-13V, Vout 5V@3A, Cout derated: 9.1uF.

2- Vin 11-13V, Vout 5.5V@1A Cout derated: 29uF.

I exported the two designs to TINA-TI and compared the simulation results, looking at the output voltage ripple and they matched. I even replaced the capacitor macro with equivalent derated C and R values and again results were very close so Webench does use the derated values on simulations.

BTW I expected the higher current supply to require bigger capacitances but Webench picked smaller capacitors for it, thus the smaller derated value.

I am asking this because the data sheet recommends 47uF but in these two designs the capacitor it picked makes the effective capacitance way bellow than that. So is it safe to use lower capacitance values? FWIW I am thinking of using a pair of 10uV/35V/1210 capacitors that derate ~20% at the voltages above. Simulation shows output voltage ripple is still low.

I am attaching links to the two Webench designs, for reference.

https://webench.ti.com/appinfo/webench/scripts/SDP.cgi?ID=04198E9F45AEC3F5

https://webench.ti.com/appinfo/webench/scripts/SDP.cgi?ID=C04C434075247722

  • 0
    TI__Genius 14455 points

    Hi Elder,

    WEBENCH uses derating curves based on information from component datasheets. In some cases, a pessimistic piecewise linear approximation can be used as well.

    Here are the charts that I have from the component vendor. In this design for Vin 11-13V, Vout 5V@3A, Cout derated is 9.1uF. The derating is higher and so the effective capacitance is lower because the Cout package is smaller and so is the VDC rating which is 10V. Comparing it to Cout in the design Vin 11-13V, Vout 5.5V@1A Cout derated: 29uF - the Cout package is much larger and also is it a 16V rated cap and hence does not derate as much and hence you have a higher value of effective capacitance.

    .  

    Regards,

    Gerold

  • 0 in reply to Gerold Dhanabalan - Online Design Tools
    Expert 2825 points

    Hi, Gerold.

    Thank you for your quick response.

    I do understand the derating thing. Lately I have looked a lot at the C x V graphs to pick the right capacitors for the designs I am working with.

    What is not clear to me is

    a) the criterion Webench  uses to pick a part; it picked a tiny part for the more demanding design (5V 3A) that derates so much that the capacitance is 20% of the nominal whereas the other, less demanding design it picks a much bigger part (that derates less)

    b) if that 47uF is the nominal capacitance and the effective one is not as important, within limits; the simulation produce seemingly good results in terms of ripple (18mVpp or less than 0.4%), stability and transient reponse, even with those 9uF.

    As I mentioned in my first post I am prone to using two 10uF capacitors with effective capacitance of 16uF, almost twice the capacitance in that design. Is it OK?