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WEBENCH® Tools: Output capacitor DC bias derating - Effective value used in Compensation Designer?

Jonathan Bender
Expert 7895 points

Tool/software: WEBENCH® Design Tools

Hello,

Some past threads mention Webench uses some DC bias derating on the output capacitors, either via a generic curve or more specific to the capacitor used (if that info was available and already in the system).

Is the derated, effective output capacitance value used by the Compensation Designer tool?

If so, is there any way to see what output capacitance value Webench is working with in the Compensation Designer tool?

I didn't see the derated value in the Operating Values tab, and the entry window in Compensation Designer (shown below) just shows the nominal value.

Thanks,

Jonathan

  • 0
    TI__Mastermind 24700 points

    Jonathan,

    Yes, most output capacitors use the generic derating curve that you are referring to. That same value is used by the compensation designer tool by default. However, there is no way to see the derated value in the tool today. 

    Regarding the Auto/Manual design window in the compensation designer tool - The default curves you see when you enter the compensation designer tool without any user updates on the forms are based on the generic cap derating curve as mentioned before. The Cout value shown in the"Power stage components" form is the nominal value, in this case 47uF. Once you click calculate button, the compensation designer now recalculates the power stage loop response based on the value in the form - so for example, if you make no changes, the default value is 47uF in this case. Now, if you click calculate, the power stage response will get updated with the 47uF cap. So, currently, you will need to update the Cout value to the derated value "manually" to get the accurate results. So, you could enter a 33uF (let's assume that is the derated value) and then look at the loop response curves to analyze the stability.

    As a side note, we are working on updating our ceramic derating logic for calculations of operating values and also for component selection like Cin, Cout, compensation network, etc. This derating will be more specific to the capacitor chosen and will be more accurate as opposed to the generic curve we use today. Once we finish this, you will be also able to see the derated value being used in the tool. In addition, we shall preload the Cout value to the derated value in the "power stage components" form in the compensation designer tool. And finally, we will also be displaying the derated value in the capacitor properties so you can make decisions accordingly.

    Regards,

    Amod Vaze

  • 0 in reply to Amod Vaze
    TI__Expert 7895 points
    Hi Amod,

    Many thanks for the quick and detailed reply.

    I'm glad to hear the Compensation Designer does apply some derating by default, and I like that I can override it with my own effective capacitance in the Power Stage Components section if I have an aggressive derating to include. Looking forward to the future revisions that make the effective value calculated by Webench more transparent.

    Thanks again,

    Jonathan