# How does a bootstrap circuit work and how can I calculate the resistor and capacitor values?

#### xSyrax123

Joined Jun 24, 2021
11
I am designing an H-bridge with an N-channel MOSFET to control a 12V - 1.35A DC motor. The H-bridge will be controlled with the digital outputs of an Arduino. I did some research and found that I need to use a bootstrap circuit, which from what I understood is to improve the linearity of the MOS switch (on resistance of transistor) by making sure that the signals from the source and the gate of the NFET follow each other. For this, a capacitor is added between the gate and the source of the transistor, so that the gate and source signals follow each other.

Circuit diagram
I found a bootstrap circuit and included it in the H-bridge design, but didn't end up understanding how the boostrap works.

• Where is the capacitor supposed to find its way to ground?
• How do I calculate the value of the boostrap resistor and capacitor?
• Is the circuit correct? It works in the simulator, but I don't know if it works in real life.
Any other comments on the H-bridge design are also welcome.

Thanks in advance for the help.

#### Ya’akov

Joined Jan 27, 2019
8,976
Welcome to AAC.

Is this schoolwork?

#### xSyrax123

Joined Jun 24, 2021
11
Welcome to AAC.

Is this schoolwork?
No, it is a h-bridge that I was designing for learning purposes to control the motors of a sumo robot competition. In the end I determined that it is not feasible to do it with mosfets for different reasons and I am going to do it with transistors. Although in the future I will try to make the bridge with only n-channel mosfets because it is an interesting challenge but I don't feel ready to face it yet.

#### rfengineer28

Joined Apr 28, 2021
79
Hi Syrax, There are ways to calculate the needed bootstrap capacitor via formulas to make bootstrapping work.Now, bootstrapping can be difficult to achieve at first, I know I personally struggled. One solution I would recommend is to use an isolated DC-DC converter in place of the bootstrap capacitor to operate you high side FETs. If you wish to know how this operates let me know. Greetings.

#### crutschow

Joined Mar 14, 2008
34,055
Where is the capacitor supposed to find its way to ground?
When the load is connected to ground by the bottom bridge transistor.
Is the circuit correct? It works in the simulator, but I don't know if it works in real life.
Don't see how it can work, as the bootstrap capacitor voltage to the gate is connected to the signal source so the gate voltage can't go above the signal voltage, as needed for the bootstrap to work.

Below is the LTspice simulation of a bootstrap driver circuit that works to rapidly turn the MOSFET on and off.
It uses a transistor (Q2) to provide the large current needed to rapidly charge the MOSFET gate charge, along with a separate diode to allow Q1 to rapidly turn off the MOSFET.
The red trace is the MOSFET gate to source voltage, as provided by the bootstrap.
How do I calculate the value of the bootstrap resistor and capacitor?
The capacitor needs to be large enough to supply the large MOSFET gate charge, with only a small drop in its voltage.
(See green trace below, where the capacitor voltage drops only about 0.2 volt to charge the gate for the given capacitor size and the selected MOSFET gate charge of 114nC.)
The resistor (in my circuit) needs to be small enough to provide sufficient base current to the transistor to supply the large transistor collector current when switching.
The trade-off is that smaller resistors cause a higher current through Q1 when it
is on.

Note that a bootstrap circuit cannot control a long duration on-off, since the charge on the bootstrap capacitor will eventually leak off, so it has to be periodically recharged by switching the circuit one cycle.

Last edited:

#### ScottWang

Joined Aug 23, 2012
7,397
The direction of D1(1N4742A) is reversed.

#### rfengineer28

Joined Apr 28, 2021
79
also, why are you grounding your source? There's automatically a 'floating' ground on this configuration.