Technology / Technical guide
High-Side Gate Drive & Bootstrap Supply Principles
A gate driver transfers a switching request into the voltage and charge behaviour needed at a device’s control terminal. For a high-side device, that voltage is referenced to a moving electrical node rather than automatically to ground. Bootstrap supply is one way to provide local floating energy, with its own recharge boundary.
The gate voltage needs the device’s local reference.
A logic command, a floating bias supply and a power-conversion path have different electrical roles.
01 / Local voltage
Control depends on a difference at the device.
For a conceptual high-side MOSFET, gate-to-source voltage is Vgs = Vg − Vs.
For a conceptual high-side MOSFET, gate-to-source voltage is Vgs = Vg − Vs. The source potential can move with a switching node. A fixed ground-referenced gate voltage therefore does not necessarily maintain the same Vgs as the source moves.
This is a voltage-reference statement, not a recommended gate voltage. Source potential relative to ground and gate potential relative to source are two different quantities. The appropriate control-terminal relationship also depends on the actual device type and arrangement.
02 / Drive role
A switching request must also move charge.
A gate is not merely a symbolic Boolean input in a physical circuit.
A gate is not merely a symbolic Boolean input in a physical circuit. Changing its voltage involves charge associated with the device and surrounding capacitances. A driver provides that charge flow under its own supply and output conditions.
Command information and the energy used by the driver are separate. Likewise, gate-drive energy is not the main power delivered to an external load. A high-power conversion path can be controlled by a much smaller command path without their voltage references becoming identical.
03 / Floating supply
Bootstrap charge moves with the local reference.
In a conventional bootstrap concept, a capacitor stores bias energy referenced to the high-side node.
In a conventional bootstrap concept, a capacitor stores bias energy referenced to the high-side node. Under appropriate circuit states a charging path refreshes that storage. When the local node moves, the stored differential voltage can provide the high-side driver’s floating supply.
Infineon’s gate-driver explanation describes this charge-and-float principle. The capacitor’s absolute terminal potentials can change together while their difference remains useful locally. This does not imply the circuit is galvanically isolated from the other domain.
04 / Charge balance
Finite stored charge creates a time-dependent boundary.
For an ideal constant capacitor, a net removed charge ΔQ changes its voltage magnitude by ΔQ/C.
For an ideal constant capacitor, a net removed charge ΔQ changes its voltage magnitude by ΔQ/C. Drive events and other consumption can remove charge between refresh opportunities. A larger stored charge and an available recharge path are distinct aspects of the model.
As a purely proportional illustration, twice the removed charge gives twice the ideal voltage change at unchanged C. No capacitance, gate charge, maximum on-time or recharge interval is specified. A conventional bootstrap alone should not be treated as an indefinitely sustained supply.
05 / Signal isolation
Moving a reference and crossing an insulation barrier differ.
Level shifting can express a command in another reference domain.
Level shifting can express a command in another reference domain. Galvanic isolation concerns an insulating boundary between domains. The isolated output side still needs a source of local drive energy; isolating information does not create that energy.
The insulation-boundary guide separates barrier and assembly meanings. A bootstrap label, a floating output and an isolation claim are therefore not interchangeable descriptions, and none alone establishes complete converter insulation performance.
06 / Implementation scope
Do not infer a driver from a converter photograph.
The Miller-coupling guide explains another gate-related mechanism, while the dead-time guide concerns complementary timing.
The Miller-coupling guide explains another gate-related mechanism, while the dead-time guide concerns complementary timing. Neither establishes the installed driver’s supply arrangement or protective behaviour.
No MARSTON gate-driver chip, bootstrap circuit, isolation rating, bias voltage or implemented protection feature is assigned. This article gives no circuit connection, capacitor sizing, gate-resistor adjustment or startup sequence; it describes reference and charge-flow principles only.
Related technical guides: Miller Capacitance, Gate Charge & Switching-Node Coupling · Inverter Dead Time: Commutation Interval & Waveform Error · Parasitic Inductance, Current Slew & Switching Overshoot.
Extend the gate-drive supply review with gate charge and driver supply power.
General background: Infineon: local driver reference, bootstrap storage and command isolation. These references explain industry concepts; they are separate from the applicable MARSTON product and project documents.
Key concepts and conditions.
Read each quantity with its reference, units and stated conditions. A general relation does not establish a model-specific value.
| Concept / quantity | Meaning / relation | Conditions / limits |
|---|---|---|
| Device reference | Vgs = Vg − Vs for the stated MOSFET | Ground voltage alone is insufficient. |
| Command path | Information about requested state | Different from the energy supply. |
| Floating bias | Stored differential voltage | Absolute potentials can move together. |
| Charge change | ΔV magnitude = ΔQ/C | Ideal constant capacitor and net removed charge. |
| Isolation | Insulation barrier between domains | Not established by a bootstrap label. |
Engineering questions.
Is high-side gate voltage necessarily referenced to ground?
No. The relevant device control voltage uses its local source or emitter reference.
Does a Boolean request provide gate charge by itself?
No. The physical driver and supply provide charge flow.
Does floating mean galvanically isolated?
No. Reference movement and an insulation barrier are different concepts.
Can conventional bootstrap storage last indefinitely without recharge?
No. Its finite charge and consumption require their own boundary.
Does signal isolation provide the output-side bias energy?
Not by itself. Local energy supply remains a separate function.
Does this identify a MARSTON gate-driver implementation?
No actual driver, bias arrangement or operational setting is assigned.
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