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I read some questions on this form regarding bootstrap capacitor e.g.: Bootstrap capacitor - Explanation and this Boost capacitor sizing, gate driver LM5106 used with N-MOSFET IPP075N15N3 G

the second one is a bit helpful but they are basing the cap size based on Qg but not showing any calculations. The mosfet I am using is IRF1405 and datasheet shows Qg: enter image description here

The driver I am using is HIP4081. I read the application notes for the driver and on page 6 has a formula for Cbootstrap: enter image description here

and at the end the formula boils down to this (we can ignore the marked term): enter image description here

assuming bootstrap cap voltage difference is 1 **QRR** = 12.5nC. Then QBS = 260 + 12.5 = 272.5nC or 0.27uC.

should I go with 0.33 uC or 0.22 uC?

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  • \$\begingroup\$ On the first look, I think the cap is way to small. \$\endgroup\$
    – Marko Buršič
    Commented Mar 28, 2020 at 22:43
  • \$\begingroup\$ would it be then safe to say that the cap should be >= 10 x Qg? which would give me 2.6uC. \$\endgroup\$
    – Grepsoft
    Commented Mar 28, 2020 at 22:47
  • \$\begingroup\$ Wait a minute, I can see now. You are searching a cap with Coulomb rating? There is no such. Coulomb is a unit of charge, Farads are capacitance units. \$\endgroup\$
    – Marko Buršič
    Commented Mar 28, 2020 at 22:59

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