Bootstrapping is a crucial aspect that you will find in all H-bridge or full bridge networks with N-channel mosfets.
It is a process in which the gate/source terminals of the high side mosfets are switched with a voltage that's at least 10V higher than its drain voltage. Meaning, if the drain voltage is 100V, then the effective gate/source voltage must be 110V in order to enable the full transfer of the 100V from the drain to the source of the high side mosfet.
Without bootstrapping facility an H-bridge topology with identical mosfets simply won't work.
We will try to understand the details through a step by step explanation.
A bootstrapping network becomes necessary only when all the 4 devices in the H-bridge are identical with their polarity. Commonly these are n-channel mosfets (4 p-channel is never used due to obvious reasons).
The following image shows a standard n-channel H-bridge configuration
The main function of this mosfet topology is to switch the "load" or the transformer primary in this diagram, in a flip-flop manner. Meaning, to create an alternating push-pull current across the connected transformer winding.
To implement this the diagonally arranged mosfets are switched ON/OFF simultaneously. And this is cycled alternately for the diagonal pairs. For instance the pairs Q1/Q4 and Q2/Q3 are together switched ON/OFF, alternately. When Q1/Q4 is ON, Q2/Q3 is OFF, and vice versa.
The above action forces current to alternately change its polarity across the connected transformer winding. This in turn causes the induced high voltage across the secondary of the transformer also to change its polarity, producing the intended AC or alternating output on the secondary side of the transformer.
What are High-Side Low-Side Mosfets
The upper Q1/Q2 are called the high side mosfets, and the lower Q3/Q4 are called the low side mosfets.
The low side mosfet have their reference leads (source terminals) appropriately connected with the ground line. However the high side mosfet have no access to the reference ground line directly, instead are connected to the transformer primary.
We know the "source" terminal of a mosfet or the emitter for a BJT must be connected to the common ground line (or the common reference line) in order to enable it to conduct and switch a load normally.
In an H-bridge since the high side mosfets are unable to access the common ground directly, switching them ON effectively with a normal gate DC (Vgs) become impossible.
This is where the problem arises, and a bootstrapping network becomes crucial.
Why this is a Problem?
We all know that a BJT requires a minimum 0.6V between its base/emitter to conduct fully. Similarly, a mosfet requires around 6 to 9V across its gate/source to conduct fully.
Here, "fully" means optimal transfer of the mosfet drain voltage or the BJT collector voltage to their respective source/emitter terminals, in response to gate/base voltage input.
In an H-bridge the low side mosfets have no problems with their switching parameters and these can be switched normally and optimally without any special circuitry.
This is because the source pin is always at zero or ground potential allowing the gate to be elevated at the specified 12V or 10V above the source. This meets the required switching conditions of the mosfet and allows it to pull the drain load to ground level fully.
Now, observe the high side mosfets. If we apply 12V across its gate/source, the mosfets initially responds well and begins conducting the drain voltage towards the source terminals. However, while this happens, due to the presence of the load (transformer primary winding) the source pin begins experiencing a rising potential.
When this potential rises over 6V, the mosfet begins stalling, because it has no more "space" to conduct, and by the time the source potential reaches 8V or 10V the mosfet just stops conducting.
Let's understand this with the help of the following simple example.
Here the load can be seen connected at the source of the mosfet, imitating a Hi-side mosfet condition in an H-bridge.
In this example if you measure the voltage across the motor you will find it to be just 7V, although 12V is applied at the drain side.
This is because the 12 - 7 = 5V is the bare minimum gate/source or Vgs that's being utilized by the mosfet to keep the conduction ON. Since the motor here is a 12V motor it still rotates with the 7V supply.
If suppose we used a 50V motor with 50V supply on the drain and 12V on the gate/source, we may see just 7V on the source, producing absolutely no movement on the 50V motor.
However, if we apply around 62V across the gate/source of the mosfet. This would instantly turn ON the mosfet, and its source voltage would rapidly start rising until it reaches to the maximum 50V drain level. But even at 50V source voltage, the gate being 62V would be still 62 - 50 = 12V higher than the source, enabling a full conduction of the mosfet and the motor.
This implies, the gate source terminals in the above example would require something around 50 + 12 = 62V to enable a full speed switchng on the 50V motor. Because this allows the gate voltage level of the mosfet to be properly elevated at the specified 12V level above the source.
Why doesn't the Mosfet Burn with such high Vgs
It's because as soon as the gate voltage (Vgs) is applied, the drain side high voltage is instantly switched ON and it rushes at the source terminal cancelling the excess gate/source voltage. Finally, only the effective 12V or 10V is rendered at the gate/source.
Meaning, if 100V is the drain voltage, and 110V is applied on the gate/source, the 100V from drain rushes at the source, nullifying the applied gate/source potential 100V, allowing only the plus 10V to operate the procedures. Therefore the mosfet is able to operate safely without burning.
What is Bootstrapping
From the above paragraphs we understood why exactly we need around 10V higher than the drain voltage as the Vgs for the high side mosfets in an H-bridge.
The circuit network that accomplishes the above procedure is called a bootstrapping network in an H-bridge circuit.
In standard H-bridge driver IC, the bootstrapping is achieved by adding a diode and a high voltage capacitor with the gate/source of the high side mosfets.
When the low-side mosfet is switched on (high-side FET is off), the HS pin and the switch node are grounded. The Vdd supply, through the bypass capacitor, charges the bootstrap capacitor through the bootstrap diode and resistor.
When the low-side FET is switched off and the high-side is on, the HS pin of the gate driver and the switch node get connected to the high voltage bus HV; the bootstrap capacitor discharges some of the stored voltage (collected during the charging sequence) to the high-side FET through the HO and HS pins of the gate driver as shown in.
For more info on this you can refer to this article
From the above discussion we understand that bootstrapping is crucial for all H-bridge topologies in order to allow effective switch ON of the high side mosfets.
In this process an appropriately selected capacitor across the gate/emitter of the High side mosfet is charged to 12V higher than the applied drain voltage level. Only when this happens the high side mosfets are able to switch ON and complete the intended push pull switching of the connected load.