Bootstrap Diode Replacement with Depletion LDMOS Transistors
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Solution Overview
Problem
Conventional high-voltage half-bridge switching circuits face challenges with high forward voltage drop in bootstrap diodes, leading to inefficiencies and increased complexity, particularly in SiC and GaN power device technologies where stringent voltage supply constraints necessitate reduced diode drops.
Innovation Solution
The integration of high-voltage depletion LDMOS transistors in an isolating well pocket provides a low-forward voltage drop diode-like conductive path between the DC voltage supply and the bootstrap terminal, utilizing a cascode and decoupler transistor configuration with body polarization to avoid intrinsic bipolar transistor triggering, and a low-voltage switch to control the diode's operation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If conventional bootstrap diodes are used in high-voltage half-bridge switching circuits, then the circuit structure is simple, but the forward voltage drop is high leading to efficiency loss
Solution Approach 1:
The patent changes the fundamental operating parameters of the bootstrap diode by using depletion-mode LDMOS transistors with negative threshold voltage. This allows the transistor to conduct with very low voltage drop (close to zero) compared to conventional diodes, directly addressing the energy loss issue while maintaining circuit functionality
Solution Approach 2:
The patent replaces the passive diode component with an active depletion-mode LDMOS transistor circuit. This substitution enables controlled low-voltage drop operation and integrates the bootstrap function within the power device structure, reducing overall circuit complexity despite the more complex transistor operation
2Speed
If SiC and GaN power devices are used with stringent voltage supply constraints, then switching performance is improved, but the high forward voltage drop of bootstrap diodes becomes more problematic
Solution Approach 1:
The patent employs depletion-mode LDMOS transistors with negative threshold voltage parameters, enabling them to operate in a mode where they conduct with minimal voltage drop. This parameter change is particularly beneficial for SiC and GaN devices where fast switching is achieved but the bootstrap diode voltage drop becomes a significant efficiency limitation
Solution Approach 2:
The depletion-mode transistor acts as an intermediary between the DC voltage supply and the bootstrap capacitor, providing a low-impedance charging path that minimizes voltage drop. This intermediary component enables the high-performance SiC and GaN switches to operate efficiently by eliminating the bootstrap diode as a source of energy loss
3Loss of energy
If the forward voltage drop of bootstrap diode is reduced, then efficiency is enhanced, but the circuit complexity increases
Solution Approach 1:
The patent merges the bootstrap diode function with the power device structure by integrating depletion-mode LDMOS transistors that serve dual purposes: as switching elements and as bootstrap charging devices. This consolidation reduces the number of discrete components and simplifies the overall circuit architecture despite the sophisticated transistor operation
Solution Approach 2:
The depletion-mode LDMOS transistor is designed to perform multiple functions: power switching, bootstrap capacitor charging, and voltage level shifting. This multi-functionality eliminates the need for separate bootstrap diode and control circuitry, reducing overall circuit complexity while achieving ultra-low voltage drop operation
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
This configuration significantly reduces the forward voltage drop between the DC voltage supply and bootstrap pins, enhancing efficiency and reducing area occupation, thus addressing limitations in high-voltage gate driver IC development.
Implementation Method 1
high-voltage depletion LDMOS transistors, controlled in such a manner as to act as a high-voltage and ultra-low (forward) voltage drop diode
Implementation Method 2
integrated in an isolating well pocket, which sustains the high voltage stress between the high voltage supply and the integrated circuit ground potential
Implementation Method 3
a body polarization circuit, coupled with the bodies of the transistors, may be used, e.g., to avoid switch-on of LDMOS intrinsic bipolar transistors
Data Source
AI summary
A circuit provides a high-voltage low-drop diode-like conductive path between a DC voltage supply terminal and a bootstrap terminal in charging a supply capacitor for driving a power switch with the capacitor set between the bootstrap terminal and an output terminal alternatively switchable between a low voltage and a high voltage DC voltage. In an embodiment, the circuit includes first and second transistors such as LDMOS depletion transistors with the first transistor set in a cascode arrangement between the bootstrap terminal and the DC voltage supply terminal and the second transistor coupled with a sense comparator for comparing the voltage at the bootstrap terminal with the voltage at said DC voltage supply terminal. The first and second transistors have common control terminals coupled with the DC voltage supply terminal and common coupling terminals to the bootstrap terminal.