# Bootstrap capacitor calculation - Which formula should I pick?

In order to calculate capacitance of bootstrap capacitor, I searched around and found two different formula from Ti and Infineon. The Ti version states:

while Infineon version say:

I think the basic idea behind these two formulas are same: dividing maximum charge may loss when high side MOS conducting, by minimum change of voltage. But I failed to figure it out on myself that why $$\Q_g\$$ was multiplied by $$\2\$$ in Infineon's version and again the whole numerator was doubled.

So far the Ti version makes more sense for me as $$\Q_g\$$ itself is the charge we loss in a switching cycle and also OnSemi seems to give similar formula (pg 5). But I believe there is some reason why Infineon engineers write this formula. I am currently studying how to design half bridge driver and I have selected IR2103 from Infineon. And that's why I want to ask which formula should be used and why?

• I just thought it was a fudge factor. Jul 21, 2019 at 4:55
• the germans have te ability to complicate even very simple things. Jul 21, 2019 at 5:52