GaN Power Electronics Masterclass – Part 43

This lesson is part of the Complete GaN Power Electronics Masterclass.

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Bootstrap Gate Driver Design for GaN Transistors: Working Principle, Circuit, Design Steps and Practical Guide

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Table of Contents

• Introduction
• What is a Bootstrap Gate Driver?
• Why Bootstrap Drivers are Used
• Need for High-Side Gate Driving
• Bootstrap Driver Basic Circuit
• Main Components of a Bootstrap Driver
• Working Principle
• Low-Side ON Interval
• High-Side ON Interval
• Bootstrap Capacitor Charging Process
• Complete Switching Cycle
• Advantages of Bootstrap Gate Drivers
• Limitations of Bootstrap Gate Drivers
• Design Summary of Part 1

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Introduction

A bootstrap gate driver is one of the most widely used methods for driving the high-side switch in half-bridge, full-bridge, buck, boost, synchronous rectifier, inverter, and motor-drive circuits. It is popular because it provides a simple and cost-effective way to generate a floating gate-drive supply for the upper transistor without using a separate isolated power supply. In Gallium Nitride power converters, bootstrap gate driver design becomes especially important because GaN transistors switch much faster than silicon MOSFETs. The fast switching speed of GaN reduces switching loss, improves efficiency, and allows higher switching frequency, but it also makes the circuit more sensitive to gate ringing, bootstrap voltage droop, parasitic inductance, dv/dt noise, false turn-on, and PCB layout errors. A poorly designed bootstrap driver can cause incomplete turn-on, excessive R_DS(on), gate overvoltage, bootstrap capacitor undervoltage, shoot-through, EMI problems, and even device failure. Therefore, understanding the bootstrap driver working principle is essential before moving to component selection, bootstrap capacitor calculation, diode selection, and PCB layout design.

Key Takeaway A bootstrap gate driver uses a diode and capacitor to create a floating supply for the high-side transistor. It is simple, compact, and low-cost, but it requires careful design when used with fast-switching GaN devices.

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What is a Bootstrap Gate Driver?

A bootstrap gate driver is a gate-driving circuit that generates a floating voltage supply for the high-side switch in a half-bridge power stage. The word "bootstrap" means that the circuit charges a capacitor during one part of the switching cycle and then uses that stored charge to drive the high-side gate during another part of the cycle. In a typical half-bridge circuit, the low-side switch source is connected to ground, so it is easy to drive using a ground-referenced gate driver. However, the high-side switch source is not fixed. It moves up and down with the switching node. When the high-side transistor turns ON, its source rises close to the DC bus voltage. Therefore, the high-side gate voltage must also rise above the switching node by the required gate-to-source voltage.

For Low-Side Switch:

Source = Ground

Gate Drive = Ground Referenced


For High-Side Switch:

Source = Switching Node

Gate Drive = Floating

Gate Must Be Higher Than Source

The bootstrap driver solves this by creating a floating supply between the high-side driver supply pin and the switching node. This floating supply moves with the high-side source, allowing the gate-to-source voltage to remain properly controlled.

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Why Bootstrap Drivers are Used

Bootstrap drivers are widely used because they are simple, inexpensive, compact, and efficient. Instead of using a separate isolated supply for the high-side driver, the bootstrap circuit uses the low-side switching interval to recharge a capacitor. This capacitor then powers the high-side driver when the high-side transistor is ON.

• They eliminate the need for a separate isolated high-side power supply.
• They reduce circuit cost and component count.
• They are compact and suitable for high-density converters.
• They work well in half-bridge and synchronous buck topologies.
• They are widely supported by commercial gate driver ICs.
• They are effective when the switching node periodically returns low enough to recharge the bootstrap capacitor.

For GaN-based power converters, bootstrap drivers are commonly used in synchronous buck converters, LLC resonant converters, totem-pole PFC circuits, half-bridge DC-DC converters, and compact fast chargers. However, because GaN devices switch very fast, the bootstrap network must be designed with low impedance, low parasitic inductance, and sufficient voltage margin.

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Need for High-Side Gate Driving

In a half-bridge circuit, the high-side transistor is connected between the DC bus and the switching node. The low-side transistor is connected between the switching node and ground. The switching node moves between approximately 0 V and the DC bus voltage.

        +VDC

          │

      High-Side GaN

          │

      Switch Node

          │

      Low-Side GaN

          │

        Ground

To turn ON the high-side GaN transistor, the gate must be driven above its source. Since the source terminal is connected to the switching node, and the switching node rises close to the DC bus voltage when the high-side device turns ON, the high-side gate must rise above the DC bus by the required V_GS. For example, if the DC bus is 400 V and the GaN device requires 5 V gate-to-source voltage, then the high-side gate may need to reach approximately 405 V with respect to ground. This does not mean the gate oxide or gate structure sees 405 V. The device only sees the difference between gate and source, which is about 5 V. The bootstrap driver creates this floating gate signal safely.

Engineering Insight The high-side gate voltage may look very high with respect to ground, but what matters for the transistor is V_GS, the voltage between gate and source. A bootstrap driver keeps this voltage within the proper range while the entire high-side driver floats with the switch node.

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Bootstrap Driver Basic Circuit

A typical bootstrap gate driver contains a bootstrap diode, bootstrap capacitor, high-side driver, low-side driver, and supply capacitor. The bootstrap capacitor is connected between the bootstrap supply pin and the switching node. The bootstrap diode charges the capacitor from the gate-driver supply when the switching node is pulled low.

             VDD

              │

        Bootstrap Diode

              │

             VB ──────── High-Side Driver

              │                 │

        Bootstrap Capacitor     │

              │                 │

             VS ───────────── High-Side Source / Switch Node

              │

          Low-Side GaN

              │

            Ground

Here, VB is the floating high-side supply node and VS is the switching node. The voltage across the bootstrap capacitor is approximately:

VBOOT = VB - VS

This V_BOOT voltage supplies the high-side driver and provides the gate-to-source voltage needed to turn ON the high-side GaN transistor.

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Main Components of a Bootstrap Driver

Component	Function
Bootstrap Capacitor	Stores charge and provides floating supply voltage for the high-side driver.
Bootstrap Diode	Allows the capacitor to charge from VDD when the switch node is low.
High-Side Driver	Drives the gate of the high-side GaN transistor relative to the switch node.
Low-Side Driver	Drives the low-side GaN transistor relative to ground.
Driver Supply Capacitor	Provides local energy to the gate driver IC.
Gate Resistors	Control turn-on and turn-off speed.
Switch Node	Floating reference point for the high-side driver.

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Working Principle

The bootstrap driver works by charging the bootstrap capacitor when the switching node is low and then using the stored capacitor charge to drive the high-side device when the switching node rises. The process can be understood in two main intervals:

• Low-side ON interval: bootstrap capacitor charges.
• High-side ON interval: bootstrap capacitor supplies the high-side driver.

This method works only if the switching node periodically returns low enough to recharge the bootstrap capacitor. If the high-side switch remains ON for too long, the bootstrap capacitor voltage may drop due to gate charge consumption, driver quiescent current, diode leakage, and other losses.

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Low-Side ON Interval

When the low-side transistor is ON, the switching node is pulled close to ground. During this interval, the bootstrap diode becomes forward biased and charges the bootstrap capacitor from the driver supply VDD.

Low-Side ON

↓

Switch Node VS ≈ 0 V

↓

Bootstrap Diode Forward Biased

↓

Bootstrap Capacitor Charges

↓

VBOOT ≈ VDD - Diode Drop

This interval is essential because it refreshes the floating high-side supply. The bootstrap capacitor must charge sufficiently before the high-side transistor is commanded ON.

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High-Side ON Interval

When the high-side transistor turns ON, the switching node rises toward the DC bus voltage. The bootstrap diode becomes reverse biased because the bootstrap node also rises with the switch node. The bootstrap capacitor now floats on top of the switching node and supplies the high-side driver.

High-Side ON

↓

Switch Node VS Rises

↓

Bootstrap Diode Reverse Biased

↓

Bootstrap Capacitor Floats

↓

High-Side Driver Uses Stored Charge

↓

High-Side Gate Driven Relative to VS

During this time, the bootstrap capacitor voltage slowly decreases because it supplies gate charge, driver current, leakage current, and other small losses. The capacitor must be large enough so that this voltage droop remains within the acceptable gate-drive range.

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Bootstrap Capacitor Charging Process

The bootstrap capacitor charges through the bootstrap diode when the switching node is low. The final voltage across the capacitor is slightly less than the gate-driver supply due to diode forward voltage drop and any series resistance in the charging path.

Approximate Bootstrap Voltage:

VBOOT ≈ VDD - VF

Where:

VDD = Gate driver supply voltage
VF  = Bootstrap diode forward voltage

For GaN devices, bootstrap voltage accuracy is important because the gate voltage margin is narrow. If the bootstrap voltage is too low, the high-side GaN may not fully turn ON, increasing R_DS(on) and conduction loss. If the bootstrap voltage is too high, the gate may be overstressed.

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Complete Switching Cycle

A full bootstrap-based half-bridge switching cycle can be summarized as follows.

Step	Condition	Bootstrap Action
1	Low-side switch turns ON.	Switch node goes low and bootstrap capacitor charges.
2	Low-side switch turns OFF.	Dead time begins; bootstrap capacitor remains charged.
3	High-side switch turns ON.	Bootstrap capacitor floats and powers high-side driver.
4	High-side switch remains ON.	Bootstrap voltage slowly droops due to charge consumption.
5	High-side switch turns OFF.	Switch node transitions downward after dead time.
6	Low-side switch turns ON again.	Bootstrap capacitor recharges for next cycle.

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Advantages of Bootstrap Gate Drivers

• Simple circuit structure.
• Low cost compared with isolated supplies.
• Compact design suitable for high-density converters.
• Efficient high-side gate drive generation.
• Widely available driver IC options.
• Suitable for half-bridge and synchronous topologies.
• Reduced component count.
• Good performance when properly designed.

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Limitations of Bootstrap Gate Drivers

• Requires periodic switch-node refresh.
• Cannot support true 100% high-side duty cycle.
• Bootstrap voltage droops during long high-side ON time.
• Bootstrap diode must withstand high voltage and fast dv/dt.
• Layout is critical for GaN switching speeds.
• High dv/dt can inject noise into the driver.
• Not always suitable for very slow switching or static high-side ON operation.

Part 1 Design Summary A bootstrap gate driver is ideal when the switch node regularly returns low enough to recharge the bootstrap capacitor. For GaN devices, the driver must be designed with strict voltage control, short current paths, low parasitic inductance, and sufficient bootstrap capacitance.

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