Check the Refresh Window of a Bootstrap Gate Supply

A bootstrap capacitor can support a high-side gate driver only while its stored charge and recharge opportunities meet the circuit’s needs. A design that works at a moderate duty cycle may fail during startup, an extended high-side on-time or a control mode that stops providing the expected refresh interval.

Identify when charging is possible

Trace the charging path through the supply, diode or internal charging element, capacitor and switching node. Record the conditions required for that path to conduct. Do not equate every commanded low-side interval with a successful recharge without checking the actual node voltage and timing.

onsemi AN-6076 discusses bootstrap circuit design and the constraints imposed by refreshing the capacitor. The useful design window depends on the specific driver and power stage. A single rule such as ‘keep duty cycle below a fixed percentage’ is not a complete calculation.

Budget charge during the on interval

Include MOSFET gate charge, driver consumption and relevant leakage over the maximum supported on-time. Relate the total charge loss to the permitted bootstrap-voltage drop. The elementary relationship is voltage drop equals charge removed divided by capacitance.

For an illustrative 100 nanocoulomb charge loss from an effective 100 nanofarad capacitor, the voltage drop is 1 V. This simplified example excludes additional losses and does not select a capacitor. Compare the resulting voltage against the driver’s requirements and the gate voltage needed by the MOSFET.

Use effective capacitance and real recharge time

Check capacitance tolerance, bias and temperature behavior at the operating conditions. Review diode drop, charging resistance and the available interval. Increasing capacitance can reduce droop while demanding more time or current to recharge, so a larger value is not an unlimited remedy.

Include startup from an uncharged capacitor, pulse skipping, burst operation, shutdown and recovery. Determine whether the controller has a deliberate bootstrap refresh mechanism and which operating modes depend on it. Avoid assuming a charge pump exists because another member of the product family includes one.

Verify with appropriate differential measurements

Observe bootstrap voltage relative to the switching node and gate voltage relative to source using equipment suitable for those nodes. A ground-referenced probe connection can be inappropriate and can alter or damage the circuit. Record the longest on-time and shortest useful recharge interval exercised.

Retain the charge budget, controller settings and measured minimum bootstrap voltage with the design review. Recheck the operating window after changing MOSFET gate charge, switching frequency, duty limits or low-power modes. Passing one steady switching condition does not establish that every supported control state maintains the gate supply.