...
What issues arise as a half-bridge begins to switch at a higher duty cycle?
A higher duty cycle means a larger portion of the time the circuit is in the on-state in one cycle. With respect to the bootstrap circuit, this means a larger portion of the time that the bootstrap capacitor is discharging rather than charging. This means that the duty cycle cannot keep increasing to any percentage that we would like, as we have to ensure that the bootstrap capacitor has enough charge to support the MOSFET gate during its discharging cycle.
What are some similarities and differences between bootstrap circuits and charge pumps? When would I consider using one over the other?
Both bootstrap circuits and champs pumps utilize diodes and capacitors to store voltage in a capacitor. They both have a charging and discharging state which enable the capacitors to store and release energy. Charge pumps require two diodes and two capacitors (for a doubler configuration), while bootstrap circuits only require one diode and one capacitor, and leverage the gate driving circuitry of the IC. Due to the forward bias on diodes, typically the more capacitors a charge pump has (for example triple or quadruple configuration), the less efficient the charge pump would be. Charge pumps are ideal when you want increased voltage rails, and don’t want to take up too much space on the PCB as you would by using an inductor for a boost converter. For driving high-side MOSFETs for half-bridges, bootstrap circuits are better than charge pumps as they are easier to implement, and since 0% and 100% duty cycles would not be used, that charge pump advantage serves no purpose.