Multi-Level Converter Bootstrap Charging at Duty Cycle Extremes

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Solution Overview

Problem

Multi-level power converters face challenges in ensuring reliable charge replenishment of switch nodes at low load conditions, particularly at duty cycles of approximately 0.5, leading to unreliable operation of n-channel MOSFET switches.

Innovation Solution

A multi-level power converter design that includes a flying capacitor and bootstrap capacitors configured to share charge with other capacitors, ensuring reliable charge replenishment of switch nodes at various duty cycles, including duty cycles of 0.5, by using recharge switches controlled by delayed versions of switch control signals.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Strength

If floating supply voltages are used to recharge intermediate nodes in multi-level power converters, then higher output voltages can be generated with lower voltage-rated switches and capacitors, but unreliable switch actuation occurs at low load conditions and certain duty cycle configurations

Engineering Contradiction:

Improveoutput voltage capabilityVSAvoidswitch actuation reliability

Core Design Contradiction:

StrengthVSReliability

Solution Approach 1:

A bootstrap capacitor is introduced as an intermediary energy storage element connected between the intermediate node and ground. This capacitor acts as a mediator that stores voltage during high-load conditions and releases it during low-load conditions, ensuring the intermediate node maintains sufficient voltage for reliable switch actuation across all load conditions without requiring floating supply voltages

Inventive Principle:

Principle #24Intermediary (Mediator)

Solution Approach 2:

The patent changes the voltage storage mechanism from floating supply voltages to a ground-referenced bootstrap capacitor. By changing how the intermediate node voltage is maintained (from external floating supply to local capacitor storage), the system achieves reliable switch actuation while maintaining the ability to generate higher output voltages with lower voltage-rated switches

Inventive Principle:

Principle #35Parameter changes

2Device complexity

If conventional two-level converters are used, then circuit complexity is reduced, but the capability to generate higher output voltages with lower voltage-rated components is lost

Engineering Contradiction:

Improveconverter structure simplicityVSAvoidoutput voltage generation capability

Core Design Contradiction:

Device complexityVSStrength

Solution Approach 1:

The power converter is segmented into multiple voltage levels (three-level configuration) with intermediate nodes that can be independently managed. By dividing the voltage conversion function across multiple switches and capacitors, the system can generate higher output voltages while using lower voltage-rated individual components, and the bootstrap capacitor further segments the voltage management function

Inventive Principle:

Principle #1Segmentation

3Strength

If multi-level power converter configuration is implemented, then higher output voltages are achieved with lower voltage-rated switches, but additional circuit elements and complexity are introduced

Engineering Contradiction:

Improveoutput voltage capabilityVSAvoidcircuit element quantity

Core Design Contradiction:

StrengthVSDevice complexity

Solution Approach 1:

The bootstrap capacitor serves multiple functions: it acts as an energy storage element for intermediate node voltage, provides a discharge path during switch transitions, and enables the system to operate reliably across all duty cycles. This multi-functionality reduces the need for additional dedicated components, offsetting the complexity increase from the multi-level configuration

Inventive Principle:

Principle #6Universality (Multi-functionality)

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 ensures reliable operation of switches across different duty cycles, particularly at duty cycle extremes, enhancing the performance and efficiency of multi-level power converters.

Implementation Method 1

the second capacitor is configured to share charge with at least one of the first capacitor and the third capacitor to enable non-gate terminals of at least one of the first switch, the second switch, and the third switch to replenish with charge

Methodology Applied

Scientific EffectCharge sharing: Capacitance

Data Source

PatentUS12603573B2Switch bootstrapping for multi-level inductive power converter

Publication Date: 2026.04.14 CIRRUS LOGIC INC

AI summary

A multi-level power converter configured to receive a power supply and generate an inductor current may include a first switch coupled to a first capacitor, a second switch coupled to a second capacitor, a third switch coupled to a third capacitor, a fourth switch, an inductor coupled to a switch node between the second switch and the third switch, and a flying capacitor having a first terminal coupled to a first node between the first and second switch and a second terminal coupled to a second node between the third and fourth switch, wherein the second capacitor is configured to share charge with at least one of the first capacitor and the third capacitor to enable non-gate terminals of at least one of the first switch, the second switch, and the third switch to replenish with charge at or approximate to a duty cycle extreme of the multi-level power converter.