# MOSFET bootstrap capacitor

I'm using a N channel gate driver with a bootstrap diode included, so I just need to add the capacitor. The FETs will be powered from a 3S lipo pack (12.6 to 9.0v) so I was wondering if I need 30v rated caps (2 x 12.6= 25.2) or because the capacitor sits between drain and source, and not ground, so a 16V capacitor could be used instead. What about the capacitor type? does it make a difference if I used tantalum vs ceramics (all the parts are 1206 SMD if it matters)? What about the capacitance? 1uF? 10uF? Any specific calculations?

• What low side PWM rate? Aug 4, 2018 at 13:39
• PWM will be 16-20KHz and 20-95% duty cycle Aug 4, 2018 at 13:52
• If the supply range is 9 volts to 12.6 volts why are you considering using a bootstrap circuit? Aug 4, 2018 at 14:03
• Dual Nch half-bridges need Vb Aug 4, 2018 at 14:56

I would estimate Cb =Qb/Vb such that Qb 20 to 50x Qiss of gate on the basis of charge dump but you can also consider

https://www.onsemi.com/pub/Collateral/AN-6076.pdf.pdf

You only have to worry about the input voltage. Because the capacitor is not connected to ground it will never see more than Vin between its terminals. So 16V should be OK, as long as the actual capacitance doesn't derate too much at the max Vin DC bias (12.6V). If that's the case, use a higher voltage capacitor just to be safe.

I would definitely pick a ceramic capacitor over tantalum: cheaper, more reliable and lower ESR for a switching circuit like this.

As far as capacitance, it needs to hold enough charge to turn the power FET on. So look at the FET's gate charge (Qg). A good rule of thumb is to use a capacitor at least 100 times the input capacitance of the FET. A typical capacitor value is 0.1uF. You don't want a capacitance so high so that there's not enough time for charging when the bottom FET turns on.