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Original question:

UCC28780: More questions about Mathcad worksheets

UCC28780: More questions about Mathcad worksheets

Nisarg Mehta
Prodigy 160 points
Part Number: UCC28780

Hello,

 

I was trying to calculate switch node capacitance and I have few questions.

 

I found in the MathCAD sheet the list of capacitances which I need to consider as shown below:

 

 

It is followed by one formula as shown:

 

 

1. Can I get some kind of derivation of this formula?

 

2. Is this effective time-related capacitance?  

 

3. If it is Time related capacitance, is it also calculated by the same procedure as explained by Mr. Goerte in reply to one post named “UCC28780: More questions about MathCAD worksheets”?

 

Mr. Goerke explains :

 

 

4. Coss for depletion mode MOSFET should also be calculated with the same procedure as shown above? 

 

 

In the same post, Mr. Goerke suggests one way to find capacitance of primary MOSFETs individually as shown below:

 


 

I tried to calculate primary MOSFET Coss with both the formulae (As given in MathCAD sheet and as suggested by Mr. Goerke) but I got different results.

 

I selected MOSFET - Infineon IPP60R180C7

Vbulk_min - 68.04V

Nps – 8

Vo_nom – 12V

Vrfl – 96V

 

With the formula in Mathcad sheet, I got, 1.008*10^3 pF

 

 

With the formula suggested by Mr. Goerke, I got, 1.379*10^3 pF.

 

5. What could be the reason for different results?

 

  • 0
    TI__Genius 9710 points

    Dear Nisarg,

    Thanks for interested in TI's solution.

    1.The derivation is quite complex, I will contact the designer and feedback to you.

    2.Yes. It is the time-related capacitance.

    3.Yes, it is the same procedure, but slightly different concept which cause slightly difference of your calculate result.

    4.Because the depletion mode MOSFET have quite small capacitance, so it is not worthy to calculate it by time-related.

    5.The equation use slightly different concept, which cause have different input parameter.

    We will suggest you follow the equation:

    Best Regards

    Kevin

  • 0 in reply to Kuan Wei Chen (Kevin Chen)
    Prodigy 160 points
    Thank you Mr. Chen for your help. In addition to Cosstr_FET, if you can provide the derivation for Lmmax that would be great. So I will have in depth understanding why I am selecting particular value.
    After calculating Cswntr, and iterating Kres gives me Kres 28.5%. Isn't it too big?