How Bootstrap Capacitors Work in Switching Regulators
Switching regulators are susceptible to a particular form of inefficiency known as switching losses. One of the challenges in switching regulator design is to balance the various sources of radiated and conducted noise against system complexity and switching/conduction losses. Simple tricks like running at higher frequency, interleaving/multi-phasing, and using better MOSFETs will help increase power conversion efficiency. Another simple option is use of a capacitor in the switching stage, also known as a bootstrap capacitor.
This capacitor is often connected across two terminals on integrated switching regulators that include a gate drive stage integrated onto the semiconductor die. If you are building a DC/DC converter from discrete components, a bootstrap capacitor can also be used to provide reduced switching losses.
Bootstrap Capacitors in a Switching Regulator
Bootstrap capacitors are used in synchronous or asynchronous switching regulators to aid switching action in a MOSFET. Most often, they are used in synchronous switching regulators. The placement of the bootstrap capacitor in these circuits is such that it discharges into the gate driver’s power port for the high side MOSFET. An example with a synchronous buck converter is shown in the circuit diagram below.
The placement across the switching node and driver input ensures that the gate driver voltage remains above the input voltage during switching. This ensures that the high-side MOSFET remains fully ON during switching, which decreases switching losses. This works because the reference voltage for the bootstrap capacitor is the switching node, not the ground node.
Bootstrapping Process
Bootstrap capacitors operate in three stages:
Charging phase: When the low-side MOSFET is ON (high-side is OFF), the bootstrap capacitor charges. One side of the bootstrap capacitor is connected to a fixed voltage source (usually the input voltage or a derived voltage), and the other side is connected to the switching node, which is essentially ground when the low-side MOSFET is on. So, the bootstrap capacitor charges up to the fixed voltage.
Bootstrap phase: When the low-side MOSFET turns OFF and the high-side MOSFET turns ON, the switching node voltage rises. As long as the capacitor maintains a nearly constant voltage, the voltage seen at the other terminal of the bootstrap capacitor (connected to the high-side gate driver) also rises. The driver rails out at this elevated voltage as it drives the high-side MOSFET gate.
Discharging phase: The bootstrap capacitor discharges slowly to supply the necessary gate charge to the high-side MOSFET. The high-side MOSFET remains ON as long as the bootstrap capacitor can maintain a voltage high enough to keep it in saturation.
This process repeats with each switching cycle as the high-side and low-side MOSFETs switch ON and OFF.
How to Size a Bootstrap Capacitor
Sizing of a bootstrap capacitor is rather simple and it depends on the level of charge compensation required to maintain the high-side MOSFET in its ON state. First, we look at the amount of charge required to maintain the high-side MOSFET gate in the ON state.
This exact amount of charge required (QG) depends on the construction of the MOSFET and it is generally determined through measurement in PMICs or DC/DC converter ICs. Typically it will be on the order of 1-10 nF for integrated DC/DC converters, but it could be much greater for physically larger discrete power MOSFETs. The ID term is the discharge current from the capacitor during switching between HIGH and LOW sides, D is the duty cycle, and f is the switching frequency.
The minimum required value for the bootstrap capacitor is:
VBS is the minimum value the gate voltage is required to be above the input voltage during switching to maintain the MOSFET in saturation. Typically, datasheets for DC/DC converter ICs will provide a lower limit on the capacitor as a function of VBS. You may also find some measurement data in the datasheet that provides a better idea of expected performance.
The above calculation assumes that the bootstrap capacitor has minimal inductance and resistance along the discharge path and in the body of the capacitor. Therefore, bootstrap capacitors are usually placed as small-case SMD ceramic capacitor (e.g., 0402 with C0G or X7R dielectric). For integrated buck converters that have BOOT pins, the inductance and resistance will almost always be minimal along the discharge path as long as the bootstrap capacitor is placed close to the BOOT pin.
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