Bootstrap capacitor - Explanation
At first glance, you can treat capacitor as some kind of a voltage source. 
The capacitor act very similarly to a voltage source. The key equation is this I = C*dV/dt

Capacitor current is proportional the rate of voltage change across it (proportional to how quickly the voltage across the capacitor is changing). 

The faster the voltage change (frequency of an AC signal is high) the large the current flow through the capacitor.



All this means that to sustain current through a capacitor the applied voltage must change. The more rapidly voltage changes the larger the current. On the other hand, if voltage is kept constant no current will flow no matter how large the voltage.
Likewise, if the current through a capacitor is found to be zero, this means that the voltage across it must be constant, not necessarily zero.

And this circuit will try to show how Bootstrap capacitor work in switching application.  



We have a switch in "B" position. So we apply a 1.5 volts to the circuit.
At the beginning of the charging phase the capacitor is empty, and so Vc1 = 0V

The voltage on capacitor cannot change suddenly form 0V to 1.5V. 

We need time to voltage on capacitor to grow (t ≈ 5*R*C).

So, immediately after we connect the supply voltage a current begins to flow.
The capacitor will now charge on the right hand side through R1 starting at 0V toward 1.5V. After this the capacitor will stop charging and no current flow in the circuit Vc = 1.5V. 

And now we flip the switch into A position, the lower side of the capacitor is immediately brought to 1.5V, but since the voltage across a capacitor can't change instantaneously, the voltage across it will remain 1.5V which will take (boost) the upper voltage to 3V 

This diagram explain everything. 



So, immediately after we flip the switch the voltage across the LED is equal to 3V.
Exactly at the same time discharge current will start to flow. So the voltage across to cap will start to drop. So the LED will blink very shortly. 

A path for capacitor&#39;s charging, and another for discharging it