Analysis of Bootstrap Circuit Risks and Design Considerations for Half-Bridge Topologies
Analysis of Bootstrap Circuit Risks and Design Considerations for Half-Bridge Topologies
The bootstrap circuit is the most common drive power supply scheme for high-side MOSFETs in half-bridge, full-bridge and synchronous buck converters. It generates floating supply voltage for the upper switch driver through a bootstrap diode and bootstrap capacitor. Improper design will lead to insufficient gate drive voltage, abnormal upper tube operation and even bridge arm shoot-through faults.
Comparison of Bootstrap Component Selection
| Component | Key Parameter | Risk if Improperly Selected |
| Bootstrap Diode | Reverse recovery time, voltage rating | Large reverse leakage, slow charging, diode breakdown |
| Bootstrap Capacitor | Capacitance, ESR, voltage rating | Insufficient charge storage, gate voltage sag |
| Gate Resistor | Resistance power rating | Excessive oscillation or slow switching speed |
| High-side Driver IC | UVLO threshold | False shutdown during normal operation |
Main Failure Mechanisms of Bootstrap Circuits
Insufficient refresh time: the upper tube stays on for too long, the bootstrap capacitor discharges excessively
High leakage bootstrap diode: the capacitor loses charge quickly and cannot maintain gate voltage
Excessive bootstrap capacitor ESR: voltage drop occurs during instantaneous gate charging
Low-side dead time is too short, the bootstrap charging window cannot be established normally
Consequences of Abnormal Bootstrap Operation
When the bootstrap voltage drops below the UVLO threshold of the driver, the high-side MOSFET will be turned off unexpectedly. Intermittent drive failure will cause output waveform distortion, current surge and shoot-through between upper and lower switches. Long-term insufficient gate voltage also increases the on-resistance of MOSFET and causes severe thermal runaway.
Design and Parameter Calculation Principles
The bootstrap capacitor must store enough charge for multiple gate charging cycles. The diode should be fast recovery type with high reverse voltage and low leakage current. The maximum on-time of high-side switch must be limited, or an auxiliary floating power supply can be adopted for applications with 100% duty cycle requirements.
Layout and Test Recommendations
Place the bootstrap capacitor and diode as close as possible to the high-side driver power pin. Minimize the loop area of bootstrap charging path. During testing, use an oscilloscope to monitor the bootstrap voltage under maximum duty cycle and full load. Verify the voltage retention capability under extreme operating conditions to avoid intermittent hidden faults.