Bootstrap Capacitor Calculator

While designing an H-Bridge MOSFET configuration, you can use this calculator for quickly estimating the value of your H-bridge bootstrapping capacitor values, depending on the specified parameters.

What is Bootstrapping Capacitor in H-Bridge MOSFET Configuration

When we are using high-side gate driver like IR2110, IR2184, IR2101, etc., for driving N-channel MOSFET at the high side, we cannot directly give 10V or 12V to the gate because the source pin of high side MOSFET is not fixed, it is jumping up and down with the load voltage.

So then to provide correct gate voltage (Vgs), we need a bootstrap circuit which is basically one fast diode + one capacitor — this capacitor stores charge and gives gate drive to high-side FET when needed.

Now how to calculate the exact value of bootstrap capacitor (Cb or Cboot) which will not fail even in worst-case condition?

Then we have to use this full equation which is a little long but it covers all current/charge losses from this cap per cycle.

The Formula:

C ≥ 2 × [ 2Qg + Iqbs(max)/f + Qls + Icbs(leak)/f ] / (Vcc − Vf − Vls − Vmin)

Now let us break and explain this one-by-one in crude style:

• C: This is the value of bootstrap capacitor in Farads. We want to find this.
• Qg: Gate charge of the high-side MOSFET. This is the main thing that consumes charge. Usually given in datasheet in nC.
• f: Operating frequency of the switching system. Higher frequency = more charge required per second.
• Iqbs(max): This is the max quiescent current of the driver high-side bootstrap supply. Driver IC eats this internally, even when not switching.
• Qls: This is the charge consumed during level-shifting process by the driver per cycle. Mostly small like 5 nC for 600V drivers.
• Icbs(leak): This is the leakage current of the bootstrap capacitor. Tiny but included for safety.
• Vcc: This is the supply voltage to the driver logic (like 12V or 15V).
• Vf: This is the forward drop of the bootstrap diode (like 0.7V or 1V).
• Vls: This is the drop across low-side FET or load when ON because source of high side is sitting at this voltage.
• Vmin: This is the minimum required Vbs voltage between VB and VS pin, needed for proper gate drive, usually around 8V or 10V minimum.

Why multiply by 2?

We multiply the whole numerator by 2 just for safety margin, to ensure even after aging, temperature effects and ripple, we still have enough charge stored.

Example:

Suppose:

• Qg = 40 nC
• f = 50 kHz
• Iqbs(max) = 200 µA
• Qls = 5 nC
• Icbs(leak) = 10 µA
• Vcc = 15V
• Vf = 1V
• Vls = 1.5V
• Vmin = 8V

Then:

Numerator:
= 2 × [ 2×40nC + 200uA/50kHz + 5nC + 10uA/50kHz ]
= 2 × [ 80nC + 4nC + 5nC + 0.2nC ] = 2 × 89.2nC = 178.4nC

Denominator:
= 15 − 1 − 1.5 − 8 = 4.5V

C ≥ 178.4nC / 4.5V = 39.64nF → So choose 100nF or more

We must use non-electrolytic, preferably X7R ceramic capacitor, rated above Vds of the MOSFET.