📈📈📈 𝐁𝐨𝐨𝐭𝐬𝐭𝐫𝐚𝐩𝐩𝐢𝐧𝐠 𝐜𝐚𝐩𝐚𝐜𝐢𝐭𝐨𝐫𝐬 𝐢𝐧 𝐃𝐂-𝐃𝐂 𝐜𝐨𝐧𝐯𝐞𝐫𝐭𝐞𝐫𝐬
In a DC-DC converter, a bootstrapping capacitor is often used with a high-side gate driver to provide a voltage to drive the gate of a high-side MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor). The high-side MOSFET is used in configurations like a buck converter or a synchronous buck converter, where the switching element is connected between the input voltage and the inductor.
🔋The bootstrapping capacitor serves as a temporary energy storage element to supply the necessary gate drive voltage to turn on the high-side MOSFET properly. Since the gate-source voltage (Vgs) required to turn on the MOSFET is usually higher than the input voltage, a separate voltage source is needed to drive the gate.
The bootstrap capacitor is charged when the low-side MOSFET is turned 🟢 ON, and the high-side MOSFET is turned 🔴 OFF. In this state, the bootstrap capacitor charges through a diode connected between a stable voltage source (usually a lower voltage supply or an auxiliary winding) and the bootstrap capacitor. The other terminal of the bootstrap capacitor is connected to the source of the high-side MOSFET (which is at ground potential when the low-side MOSFET is on).
When the low-side MOSFET turns 🔴 OFF and the high-side MOSFET turns 🟢 ON, the voltage across the bootstrap capacitor is applied to the gate of the high-side MOSFET. The voltage across the capacitor adds to the input voltage, providing the gate drive voltage (Vgs) to turn on the high-side MOSFET fully.
The bootstrap capacitor works as a „floating“ voltage supply for the high-side gate driver, allowing it to drive the high-side MOSFET effectively, even though the source of the high-side MOSFET is not at a fixed voltage.
It is essential to select an appropriate value for the bootstrap capacitor to ensure adequate charge storage for driving the high-side MOSFET. The capacitor value should be large enough to provide the required gate charge for the MOSFET while accounting for the leakage currents and voltage drops in the driving circuit.