Bootstrap Capacitor Overvoltage Management for GaN Half-Bridge

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Solution Overview

Problem

Conventional bootstrap drive circuits for e-mode GaN transistors in half-bridge configurations face overcharging of the bootstrap capacitor during dead-time, leading to potential damage and reduced lifetime of high side transistors, and existing solutions either increase complexity, introduce high losses, or are not suitable for high voltage applications.

Innovation Solution

A modified drive circuit incorporating a shunt diode connected to the bootstrap capacitor with a shunt resistor, decoupled from the half-bridge midpoint, provides a low voltage drop path for charging during dead-time, avoiding overvoltage conditions and optimizing current handling without adding complexity or high losses.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If a conventional bootstrap drive circuit is used for e-mode GaN transistors, then the circuit is simple and operates normally during switching, but the bootstrap capacitor becomes overcharged during dead-time causing gate overvoltage that damages the high side transistor

Engineering Contradiction:

Improvetransistor reliabilityVSAvoidgate overvoltage

Core Design Contradiction:

ReliabilityVSObject-affected harmful factors

Solution Approach 1:

A shunt diode is introduced as an intermediary component connected in parallel with the bootstrap capacitor. This shunt diode provides an alternative discharge path for the bootstrap capacitor during dead-time, preventing overvoltage conditions. The diode acts as a mediator that safely dissipates excess voltage without affecting the normal bootstrap charging function during switching operation.

Inventive Principle:

Principle #24Intermediary (Mediator)

Solution Approach 2:

The invention changes the electrical parameters of the bootstrap capacitor by adding a parallel shunt diode path. This modifies the voltage-time characteristics of the capacitor, allowing it to maintain proper voltage during switching while safely discharging during dead-time. The parameter change enables the capacitor to adapt its behavior based on the operational phase without requiring complex control circuitry.

Inventive Principle:

Principle #35Parameter changes

2Reliability

If existing solutions like Zener diodes or active clamping are used to prevent bootstrap overcharging, then gate overvoltage is prevented, but the circuit complexity increases or switching losses are introduced

Engineering Contradiction:

Improvegate voltage protectionVSAvoidcircuit complexity

Core Design Contradiction:

ReliabilityVSDevice complexity

Solution Approach 1:

The invention uses a simple shunt diode, which is a low-cost, passive component with no moving parts or complex control requirements. Unlike active clamping circuits that require additional transistors and control logic, or Zener diodes that dissipate significant power, the shunt diode provides reliable protection through its inherent directional conduction property. This disposable-like simplicity makes the solution economically attractive and easy to implement.

Inventive Principle:

Principle #27Cheap short-living objects (Disposable)

3Object-affected harmful factors

If Schottky diodes are used in the power stage to prevent overvoltage, then voltage protection is achieved, but switching losses increase and the solution is not suitable for high voltage applications

Engineering Contradiction:

Improveovervoltage protectionVSAvoidswitching losses

Core Design Contradiction:

Object-affected harmful factorsVSLoss of energy

Solution Approach 1:

The invention segments the protection function from the power switching function. Instead of using Schottky diodes in the power stage that would affect switching performance, the protection function is separated into a dedicated shunt diode path connected to the bootstrap capacitor. This segmentation allows the power stage to operate efficiently with minimal losses while the separate shunt diode handles the protection function independently.

Inventive Principle:

Principle #1Segmentation

Applied Scientific Principles

This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.

Function Achieved in This Case

The solution effectively prevents bootstrap capacitor overcharging, enhances power stage performance, and is suitable for a wider range of applications without the limitations of prior art, such as requiring Schottky diodes or increasing switching losses.

Implementation Method 1

a shunt diode connected to the bootstrap capacitor at a node VB between the bootstrap capacitor and ground

Methodology Applied

Scientific EffectDiode conduction: Diode

Implementation Method 2

the shunt diode being decoupled from the midpoint node of the half bridge (VSW) by a shunt resistor. The shunt resistor controls and limits the current through the shunt diode

Methodology Applied

Scientific EffectElectrical resistance: Electrical Resistance

Implementation Method 3

A bootstrap capacitor 22 (CB) is coupled in parallel with the gate driver IC

Methodology Applied

Scientific EffectCapacitance: Capacitance

Data Source

PatentUS10454472B2Bootstrap capacitor over-voltage management circuit for GaN transistor based power converters

Publication Date: 2019.10.22 EFFICIENT POWER CONVERSION CORP

AI summary

A drive circuit for a half bridge transistor circuit formed of enhancement mode GaN transistors. A shunt diode is connected to the bootstrap capacitor at a node between the bootstrap capacitor and ground, the shunt diode being decoupled from the midpoint node of the half bridge by a shunt resistor. The shunt diode advantageously provides a low voltage drop path to charge the bootstrap capacitor during the dead-time charging period when both the high side and low side transistors of the half bridge are off.