I am designing a H-bridge to drive a motor that is running at 12V / 3A.

To prevent shoot-through I decided to select a IC with integrated dead time - IR2111.

What I got so far is the following circuit (showing just one half of the full bridge to conserve space): circuit What I am curious about is the need for the bootstrap capacitor between \$V_B\$ and \$V_S\$ and the diode as shown in the datasheet: typical connection

My understanding is that usually the drain of the high-side MOSFET is connected to a much higher voltage as the supply voltage for the IR2111. To switch the MOSFET a higher voltage has to be generated, therefore the bootstrap capacitor.

But in my case the voltages (drain of the hide-side mosfet and supply voltage of the IR2111) are all 12V. So the voltage is high enough to switch the high-side MOSFET and I do not need the bootstrap capcitor. Am I right?

  • 1
    \$\begingroup\$ Why do you put D1 and D2? The MOSFETs already have internal body diodes. \$\endgroup\$ Oct 29 '14 at 10:13
  • 1
    \$\begingroup\$ @hkBattousai Body diodes are not optimized for the task. However, if you're going to take the effort to add external diodes then 1N400x diodes are certainly not the diode you should be using and is just putting that extra effort to waste. Use one suited for the task. \$\endgroup\$
    – DKNguyen
    Jun 24 '19 at 16:39

The datasheet shows this for a reason. If you are not completely sure what it's purpose is and know that you don't need it, then you have to include it.

In this case, the FET driver is intended for N channel FETs both top and bottom. When the top FET turns on, it's drain and source voltage will be close. After all the purpose is for the FET to act like a switch. N channel FETs require a higher gate voltage than the source to turn on. This means a higher voltage than the top rail. The diode along with the cap between Vb and Vs are part of a charge pump to produce this higher voltage.

If you happen to have the higher voltage already available, then you might be able to use it and not include the charge pump. To do so, you need to read the datasheet very carefully and see if this is possible with that device, and if so, exactly what requirements and restrictions there might be.

One drawback of the charge pump method is that the FET needs to be regularly switched for the charge pump to maintain a voltage ready to switch the FET. If you will have long periods at 100% PWM, for example, you may have to consider other methods. Again, the datasheet for the FET driver should go into detail on all this.


The bootstrap capacitor is used because the voltage on the high-side driver's gate needs to be about 10-15 volts higher than the voltage on its source even the source is near the positive supply rail (drain). Suppose the transistors require VGS to be VGSsat to conduct the desired amount of current without excessive voltage drop, and require that it stay below VGSmax to avoid breakdown, and suppose the motor supply voltage is Vmotor. To do without the bootstrap capacitor, you would need to have a supply available that was between Vmotor+VGSsat and VGSmax. Most applications will not have such a supply available.


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