Floating Gate Supplies in GaN Half-Bridge Circuits: Bootstrap, Isolated and Charge Pump Design
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Floating Gate Supplies in GaN Half-Bridge Circuits: Bootstrap, Isolated and Charge Pump Design
Table of Contents
- Introduction
- Why the High-Side Gate Needs a Floating Supply
- The Floating Reference Problem in a Half-Bridge
- Bootstrap Supply Method
- Bootstrap Diode and Capacitor Selection
- Bootstrap Capacitor Ripple Equation
- Limitations of Bootstrap Supplies
- Isolated Gate Drive Supplies
- Charge Pump Floating Supplies
- Comparison Table: Bootstrap vs Isolated vs Charge Pump
- UVLO for the Floating Domain
- Level Shifting Into the Floating Domain
- Common Mode Transient Immunity (CMTI)
- PCB Layout Guidelines for Floating Supplies
- Startup Behavior and Minimum ON-Time Limits
- GaN-Specific Design Considerations
- Design Checklist
- Applications
- Future Trends
- Frequently Asked Questions
- Conclusion
Introduction
Every half-bridge power stage has two switches stacked on top of each other. The low-side transistor has its source tied to a fixed ground, so driving its gate is straightforward — the gate driver simply references the same ground as the rest of the control circuit. The high-side transistor is a different story. Its source terminal is connected to the switching node, a point that swings from close to 0 V to the full bus voltage every switching cycle, often in a matter of nanoseconds in a GaN design. To turn that high-side device fully ON, the gate driver has to supply a voltage that is higher than the switch node by the full gate-drive voltage, and that reference has to move together with the switch node. This is what engineers mean by a "floating" gate supply — a small, isolated power source that rides on top of a rapidly moving voltage rail and still delivers a clean, stable gate voltage.
In silicon MOSFET half-bridges this problem is well understood and often solved with a simple bootstrap diode and capacitor. GaN transistors do not remove this requirement — if anything they make it more demanding, because GaN devices switch faster, have narrower safe gate voltage windows, and are frequently used in converters running at hundreds of kilohertz to several megahertz. This article explains how floating gate supplies work, walks through the three common implementation methods, and gives practical design guidance for GaN half-bridge and totem-pole circuits.
Why the High-Side Gate Needs a Floating Supply
A GaN transistor turns ON when its gate-to-source voltage, VGS, is raised above its recommended turn-on level. For the high-side switch in a half-bridge, the source terminal is the switching node, not ground. If the gate driver tried to use a fixed, ground-referenced supply to drive the high-side gate, it could only pull the gate up to that fixed rail voltage. Once the switching node rises above that rail during operation, VGS would collapse to zero or even go negative, and the device would turn itself OFF unintentionally. The only way to keep the high-side device properly enhanced throughout the switching cycle is to generate a small supply voltage that is referenced to the switching node itself and moves with it.
- The floating supply must track the switch node voltage without lag.
- It must deliver enough charge to fully drive the GaN gate capacitance every switching cycle.
- It must survive fast dv/dt transitions without losing regulation.
- It must be electrically isolated from ground by the full bus voltage rating.
- It must re-establish itself quickly enough to support high switching frequency.
The Floating Reference Problem in a Half-Bridge
Consider a simple half-bridge with a high-side and low-side GaN transistor. The low-side source sits at ground (0 V) permanently, so its gate driver supply can be a normal fixed rail. The high-side source, however, sits at the switch node voltage VSW, which alternates between roughly 0 V (when the low-side device conducts) and the bus voltage VBUS (when the high-side device conducts).
Low-Side Gate Driver Reference: Fixed Ground (0 V) High-Side Gate Driver Reference: Switch Node (0 V to VBUS) High-Side Gate Voltage Needed: VSW + VGS(on)
Because the high-side reference point moves by the full bus voltage on every switching transition, the supply that powers the high-side driver must move with it. A ground-referenced supply cannot do this directly — some form of isolated or bootstrapped energy transfer is required.
Bootstrap Supply Method
The bootstrap method is the simplest and most widely used way to create a floating gate supply. It uses a diode and a capacitor connected between a fixed low-voltage rail and the switch node.
VCC (fixed rail) │ Bootstrap Diode │ ▼ Bootstrap Capacitor (CBOOT) │ Switch Node (VSW) ── High-Side Source
When the low-side transistor is ON, the switch node is pulled close to ground. This allows the bootstrap diode to conduct and charge the bootstrap capacitor up to approximately VCC minus the diode forward drop. When the high-side transistor turns ON, the switch node jumps up toward the bus voltage, carrying the bootstrap capacitor and its stored charge along with it, and the diode blocks so the charge cannot flow backward into VCC. The floating driver simply draws its gate-drive current from this charged capacitor, which now sits at approximately VSW + VCC.
Bootstrap Diode and Capacitor Selection
| Component | Selection Criteria |
|---|---|
| Bootstrap Diode | Fast recovery, low forward drop, blocking voltage rated above the full bus voltage. |
| Bootstrap Capacitor | Low ESR, sized to limit voltage droop over the maximum high-side ON time. |
| Placement | As close as possible to the driver supply pins to minimize loop inductance. |
| Voltage Rating | Should exceed the floating supply voltage with adequate margin for ringing. |
Bootstrap Capacitor Ripple Equation
The bootstrap capacitor must supply the gate charge and any quiescent driver current for the entire time the high-side device stays ON, without the floating rail sagging below the driver's undervoltage lockout threshold. A simple first-order sizing approach is:
CBOOT ≥ (QG + IQ × tON(max)) / ΔVBOOT Where: QG = total gate charge delivered per switching cycle IQ = quiescent current drawn by the floating driver tON(max) = maximum high-side ON time (worst-case duty cycle) ΔVBOOT = allowed droop on the bootstrap rail
Because GaN transistors have very low gate charge compared to silicon MOSFETs of similar current rating, the QG term is small, which is an advantage for bootstrap sizing. However, GaN converters are frequently operated at high switching frequency and sometimes at high duty cycle, so the tON(max) term can still be significant and must be evaluated at the actual worst-case operating point, not just at 50 percent duty cycle.
Limitations of Bootstrap Supplies
- Requires the low-side switch to turn ON periodically to recharge the capacitor.
- Struggles at very high duty cycle, where the low-side ON time becomes too short to refresh the charge.
- Not suitable for 100 percent duty cycle or DC-holding high-side operation.
- Diode forward drop and recovery charge add small losses.
- Bootstrap ripple can affect gate drive accuracy at light load or very high frequency.
- Additional startup circuitry may be needed to pre-charge the capacitor before the first switching cycle.
Isolated Gate Drive Supplies
An isolated gate drive supply uses a small transformer-based or capacitively-isolated DC-DC converter to generate the floating rail directly, without relying on the switching pattern of the half-bridge itself. The isolated supply provides a continuously available floating voltage, regardless of duty cycle.
Ground-Referenced Input
│
Isolated DC-DC Converter
(transformer or capacitive isolation)
│
Floating Output ── High-Side Driver Supply
- Works at any duty cycle, including near 0 percent or near 100 percent.
- Provides stable voltage independent of switching activity.
- Better suited to resonant converters where duty cycle can be extreme.
- Adds cost, size, and a second magnetic or isolation barrier component.
- Isolation barrier must be rated for the required dv/dt and voltage isolation.
Charge Pump Floating Supplies
A charge pump supply uses switched capacitor stages to step up a lower voltage into the floating domain without a magnetic component. It is often used in monolithically integrated GaN power stages where a transformer is impractical.
- Compact, no magnetics required.
- Well suited to integrated or module-level GaN half-bridge designs.
- Output current capability is generally lower than a transformer-based isolated supply.
- Efficiency depends on switching frequency of the charge pump stage itself.
Comparison Table: Bootstrap vs Isolated vs Charge Pump
| Method | Duty Cycle Range | Complexity | Typical Use Case |
|---|---|---|---|
| Bootstrap | Limited at very high or very low duty cycle | Low | Buck, boost, and standard half-bridge converters |
| Isolated DC-DC | Full range, including near 0% or 100% | Medium to High | Resonant converters, wide duty cycle applications |
| Charge Pump | Moderate range | Medium | Integrated GaN power stages, compact modules |
UVLO for the Floating Domain
Undervoltage lockout, or UVLO, on the floating driver supply prevents the high-side GaN transistor from being driven with insufficient gate voltage. If the bootstrap or isolated rail droops below the UVLO threshold, the driver output is forced OFF rather than allowed to operate with a weak, partially-enhanced gate drive that would increase RDS(on) and generate excess heat.
- Floating UVLO must reference the floating rail itself, not ground.
- UVLO threshold should include hysteresis to avoid chattering near the trip point.
- Bootstrap-based designs need UVLO tuned to the expected ripple envelope.
- Isolated supplies can use a tighter UVLO band since ripple is generally lower.
Level Shifting Into the Floating Domain
The PWM control signal for the high-side device is almost always generated on the ground-referenced side of the circuit. That signal has to cross into the floating domain safely, without letting the fast dv/dt of the switch node corrupt the logic level. This is handled by a level-shift stage inside the gate driver IC, typically using high-voltage level-shift transistors and pulse-based signal transfer designed to reject common-mode transients.
Ground-Referenced PWM Logic
│
High-Voltage Level Shifter
│
Floating-Domain Gate Drive Logic
│
High-Side GaN Gate
Common Mode Transient Immunity (CMTI)
Common mode transient immunity describes how well the level-shift and floating supply circuitry can tolerate the fast dv/dt of the switch node without producing false signals or corrupting the floating supply voltage. GaN half-bridges routinely produce switch-node slew rates of tens of volts per nanosecond, so CMTI is a critical driver specification, not a secondary detail.
- Choose a gate driver IC with CMTI ratings that comfortably exceed the expected switch-node dv/dt.
- Minimize parasitic capacitive coupling across the isolation barrier in isolated designs.
- Keep bootstrap diode reverse recovery fast enough to avoid injecting noise during transitions.
- Route the floating supply return path away from high dv/dt copper.
PCB Layout Guidelines for Floating Supplies
- Place the bootstrap capacitor as close as possible to the driver's floating supply pins.
- Keep the bootstrap diode loop short to minimize parasitic inductance and ringing.
- Route the floating ground return along a controlled, low-impedance path back to the switch node pin.
- Physically separate high dv/dt switch-node copper from the floating supply decoupling network.
- For isolated supplies, respect the creepage and clearance distances required by the isolation barrier.
- Use local decoupling capacitors directly across the floating driver supply pins.
- Avoid routing floating supply traces parallel to noisy high-current paths.
Startup Behavior and Minimum ON-Time Limits
At power-up, the bootstrap capacitor is uncharged, so the high-side driver initially has no supply voltage. Most converter designs solve this by forcing an initial low-side ON pulse, or by using a dedicated pre-charge path, so that the bootstrap capacitor reaches a safe operating voltage before the high-side device is ever commanded ON. Designers must also respect a minimum low-side ON time during normal operation so that the bootstrap capacitor gets refreshed often enough at every operating point, including light load and transient conditions.
GaN-Specific Design Considerations
- Lower gate charge reduces the current demand on the floating supply compared to silicon MOSFETs of similar power rating.
- Narrower gate voltage margins mean floating supply ripple must be tightly controlled.
- Faster switching edges increase CMTI requirements on both the level shifter and the bootstrap diode.
- High switching frequency operation favors compact charge pump or bootstrap solutions over bulky isolated magnetics.
- Cascode GaN devices may inherit gate drive requirements closer to a conventional silicon MOSFET, easing floating supply design.
Design Checklist
| Checklist Item | Status |
|---|---|
| Bootstrap or isolated supply selected based on duty cycle range | Confirm before layout |
| Bootstrap capacitor sized for worst-case ON time | Verify with ripple calculation |
| UVLO threshold set with correct hysteresis | Check driver datasheet |
| CMTI rating exceeds expected switch-node dv/dt | Confirm driver specification |
| Startup pre-charge sequence verified | Bench test at power-up |
| Floating supply layout loop minimized | Review PCB layout |
Applications
- Half-bridge and full-bridge DC-DC converters.
- Totem-pole power factor correction stages.
- Synchronous buck converters with high-side GaN switches.
- LLC resonant converters.
- Motor drive inverters using GaN half-bridges.
- Onboard EV chargers.
- Solar microinverters and power optimizers.
- Telecom rectifier and DC-DC modules.
Future Trends
- Monolithically integrated floating supplies inside GaN power ICs.
- Higher CMTI gate driver ICs to match ever-faster GaN edge rates.
- Digitally monitored bootstrap voltage with adaptive refresh timing.
- Coreless transformer isolated supplies for smaller form factors.
- Wide duty cycle bootstrap-free architectures for resonant GaN converters.
- Increased adoption of GaN-optimized driver ICs with built-in floating regulation.
Frequently Asked Questions (FAQs)
Why can't the high-side GaN gate use the same supply as the low-side gate?
Because the high-side source is connected to the switch node, which moves between roughly 0 V and the bus voltage. A fixed, ground-referenced supply cannot maintain the correct gate-to-source voltage once the switch node rises, so a supply that floats with the switch node is required.
What is the simplest way to create a floating gate supply?
The bootstrap method, using a diode and capacitor charged whenever the low-side switch is ON, is the simplest and most common approach for standard duty cycle ranges.
When does a bootstrap supply fail to work properly?
At very high duty cycle, where the low-side device stays OFF for extended periods, the bootstrap capacitor does not get recharged often enough and its voltage can droop below the driver's UVLO threshold.
What is the advantage of an isolated gate drive supply over a bootstrap supply?
An isolated supply provides continuous floating power regardless of duty cycle, making it suitable for resonant converters or applications that need extreme or near-100-percent duty cycle operation.
Does GaN change how the bootstrap capacitor should be sized?
The lower gate charge of GaN transistors reduces the charge demand per cycle, which helps bootstrap sizing, but higher switching frequency and tighter gate voltage margins still require careful ripple analysis.
What is CMTI and why does it matter for GaN designs?
CMTI, or common mode transient immunity, describes how well the level-shift and floating supply circuitry tolerate fast switch-node dv/dt without producing false signals. GaN devices switch faster than silicon MOSFETs, so CMTI requirements are stricter.
Can a charge pump replace a bootstrap circuit in a GaN half-bridge?
In many integrated or compact GaN designs, yes, particularly where duty cycle range is moderate and no external magnetic component is desired. Its output current capability should be checked against the gate drive requirements.
Why is UVLO applied on the floating supply rail specifically?
Floating UVLO prevents the high-side driver from operating with insufficient gate voltage, which would otherwise increase RDS(on) and generate excess conduction loss or thermal stress.
How is the bootstrap capacitor pre-charged at startup?
Most designs force an initial low-side ON pulse, or use a dedicated pre-charge path, so the bootstrap capacitor reaches a safe voltage before the high-side device is ever commanded ON.
What PCB layout mistake most commonly causes floating supply problems?
Placing the bootstrap capacitor or diode too far from the driver's floating supply pins, which increases loop inductance and allows ringing that can trip UVLO or exceed the gate voltage rating.
Conclusion
Floating gate supplies are a small but essential piece of every half-bridge GaN converter. The high-side gate driver has to stay powered, regulated, and referenced correctly as the switch node swings through the full bus voltage on every cycle, and getting this wrong leads to false turn-on, excessive RDS(on), or outright device failure. Bootstrap supplies remain the simplest and most cost-effective solution for the majority of standard duty cycle converters, while isolated and charge pump supplies fill the gap for wide duty cycle, resonant, or highly integrated GaN designs. Whichever method is chosen, careful attention to capacitor sizing, UVLO thresholds, CMTI, and PCB layout is what separates a reliable GaN half-bridge from one that fails under real-world transient and duty cycle conditions.
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