Bootstrap Supply (Bootstrap Circuit)
It is the diode–capacitor arrangement used to lift the gate of the high-side N-channel MOSFET above the supply voltage. While the low-side switch is conducting the capacitor charges, and it delivers this energy to the gate when the upper switch turns on. The typical capacitor value is between 100 nF and 1 µF.
Using an N-channel MOSFET in the upper leg of an H-bridge has one difficulty: for this type of switch to conduct, the gate voltage must typically be 10–12 V higher than its source terminal. Since the source terminal is at almost the supply voltage while the upper switch is conducting, in a 12 V system a voltage of about 22–24 V has to be applied to the gate.
Bootstrap solves this without an additional power supply. While the low-side switch is conducting, the bridge midpoint is close to chassis; at this moment the bootstrap capacitor charges from the logic supply through a diode. When the upper switch turns on, this capacitor behaves like a battery floating above the source terminal.
The method has a limit of its own: for the capacitor to recharge, the low-side switch must conduct periodically. For this reason, at 100 percent duty cycle, that is at continuous full throttle, classical bootstrap circuits cannot hold their voltage. To get around this, drivers either limit the duty ratio to 95–99 percent or use a separate floating supply.
The context in which this term is used: DC Motor Driver Selection Guide