Diotec Semiconductor AG

Why the Bootstrap Supply Matters

Driving a high-side SiC MOSFET presents a unique challenge. Unlike the low-side device, the source terminal of the high-side device is not referenced to ground. Instead, connected to the switching node, it experience voltage swings of several hundred volts within nanoseconds.

To turn the MOSFET on, the gate voltage must remain several volts above this floating source potential. One of the most widely adopted and cost-effective methods for generating this floating supply is the bootstrap circuit.

The bootstrap capacitor is charged while the low-side MOSFET conducts. When the high-side device turns on, the charged capacitor effectively "rides" the switching node, providing the necessary gate-drive voltage. Because this process repeats every switching cycle, it places continuous demands on the bootstrap charging circuit.

The Role of the Bootstrap Diode

Although often considered a simple component, the bootstrap diode directly influences both switching performance and system reliability. It must recharge the bootstrap capacitor during every switching cycle, frequently under high repetitive current conditions.

If the diode cannot withstand these repetitive peak charging currents, designers typically introduce a series bootstrap resistor to limit stress. While this resistor protects the diode, it also slows capacitor charging, potentially reducing the available gate-drive voltage under demanding operating conditions.

The SI02C120SMA by Diotec was developed to address these requirements. Its ability to withstand higher repetitive peak forward currents allows designers to eliminate the additional bootstrap resistor in suitable applications. The result is faster recharging of the bootstrap capacitor and improved availability of the gate-drive supply.

Reducing Ringing and Improving UVLO Robustness

Another important parameter is the diode's junction capacitance. During the fast voltage transitions typical of SiC MOSFET switching, parasitic capacitances and inductances within the circuit can cause voltage ringing and increase electromagnetic interference (EMI).

Selecting a diode with lower junction capacitance helps reduce capacitive coupling during these switching events, resulting in lower ringing and improved EMI performance. Reducing ringing also minimizes disturbances on the bootstrap supply, providing a cleaner and more stable gate-drive voltage.

This is particularly important for ensuring reliable operation of the gate driver’s  under-voltage lockout function. In some systems, excessive ringing on the bootstrap supply can momentarily lower the sensed supply voltage, triggering a false UVLO event even though the converter is operating normally.

Small Component, Significant Impact

As SiC converters continue to operate at higher switching frequencies and faster edge rates, the performance of the components surrounding the power semiconductor become increasingly important. The bootstrap diode is one such component. Selecting a device with adequate repetitive current capability and low junction capacitance can improve bootstrap capacitor charging, reduce EMI and enhance the robustness of the gate-drive supply.

For engineers designing high-performance SiC power stages, careful consideration of the bootstrap charging path can help prevent intermittent gate-driver issues that are often difficult to diagnose. Although small in size, the bootstrap diode can have a significant impact on the overall performance, reliability and efficiency of the converter.