Bootstrapping circuit in IR2101
Suppose u r going to drive a mosfet bridge (half or full) with all n-channel mosfets. Obviously there's no problem driving the low side mosfet, because its source is connected to GND and u can simply drive the gate pin with a 12v signal. But the challenge is how to drive the high-side mosfet, since its source is not connected to GND and is floating, and u have to apply a 12v on gate-source pins. For example with a bridge voltage of 100v, u need a 112 volt on its gate to drive it.  The solution is &quot;bootstrap&quot;. In this technique, as can be seen on schematic, a diode with a cap is used. When the low-side mosfet is ON, the cap is charged through diode and the ON mosfet. If Vcc of IR2101 is 12v, the cap is charged up to 11.3v (0.7v drop because of diode). Then the low-side mosfet is turned on and it's time to drive the high-side mosfet. In this case, we have a floating voltage supply of 11.3v stored in the cap which will serve the driver circuitry inside IR2101. So, the voltage on the minus pin of cap is 100v and the voltage on its plus pin is 111.3v (and the diode is in reversed bias). This voltage is used to apply on the gate-source of high-side mosfet.
This &quot;cycle&quot; continues over and over. The bootstrap cap is charged during low half-cycles and serves (and of-course discharges) in high half-cycles.