GaN Current Mirror Sensing for Fast Overcurrent Protection
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Solution Overview
Problem
Current sensing technologies for GaN power switching devices are inadequate for providing fast and accurate overcurrent protection and short circuit protection, as existing methods such as desaturation detection, current sense resistors, and di/dt detection are either too slow or result in significant power loss and parasitic inductance, and are not suitable for GaN transistors due to their unique characteristics.
Innovation Solution
A current sensing circuit using a GaN sense transistor in parallel with a GaN power transistor, with a sampling circuit that maintains equal gate-to-source voltages for both transistors, employing operational amplifiers to convert and amplify current sense inputs, thereby ensuring accurate and temperature-independent current sensing for overcurrent protection and short circuit protection.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a current sense resistor is used for current sensing, then current sensing can be achieved, but significant power loss and parasitic inductance occur
Solution Approach 1:
The patent uses a sense transistor that replicates the electrical characteristics of the power transistor to sense current. The sense transistor is configured as a current mirror with the power transistor, allowing current sensing without directly measuring the main current flow, thereby avoiding power loss and parasitic inductance associated with current sense resistors
Solution Approach 2:
The patent replaces the passive current sense resistor with an active sense transistor that uses voltage sensing and current mirror technology. This substitution enables lossless current sensing by measuring voltage across the sense transistor rather than forcing current through a resistive element
2Reliability
If de-saturation detection is used for protection, then short circuit protection can be provided, but the response speed is too slow for GaN devices
Solution Approach 1:
The patent implements preliminary protection by sensing current through the sense transistor before catastrophic failure occurs. The sense transistor continuously monitors current conditions and provides early warning signals to the controller, enabling preventive action before the power transistor enters de-saturation or fails
Solution Approach 2:
The patent establishes a feedback loop where the sense transistor output is continuously monitored by the gate driver controller. This feedback mechanism provides real-time current information, enabling the controller to detect overcurrent or short circuit conditions and respond by adjusting the gate drive signal or shutting down the power transistor
3Measurement precision
If di/dt detection by voltage sensing across common source inductance is used, then current change rate can be detected, but source inductance cannot be minimized for GaN switches
Solution Approach 1:
The patent extracts the current sensing function from the main power path by using a separate sense transistor. This allows current monitoring without requiring voltage sensing across the common source inductance, enabling minimization of source inductance in the power path while still achieving accurate current change rate detection through the sense transistor output
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 provides high accuracy current sensing with minimal power loss and temperature independence, enabling effective current feedback and protection functions for GaN power switching devices, particularly in high-speed applications like switch mode power supplies.
Implementation Method 1
the GaN sense transistor being configured as a current mirror with the drain of the GaN power transistor being connected to the drain of the GaN sense transistor, the gate of the GaN power transistor being connected to the gate of the GaN sense transistor
Data Source
AI summary
High accuracy current sense circuitry for power switching devices comprising GaN power transistors provides for current feedback functions, e.g. current loop control, over-current protection (OCP) and short-circuit protection (SCP). The current sense circuitry comprises a current mirror sense GaN transistor (Sense_GaN) and a power GaN transistor (Power_GaN) and a sampling circuit. The sampling circuit comprises first and second stage operational amplifiers to provide fast response and improved current sense accuracy, e.g. better than 1%, over a range of junction temperatures Tj. The Sense_GaN, Power_GaN and first stage operational amplifier have a common ground referenced to a Kelvin Source of the Power_GaN, so that the Sense_GaN and Power_GaN operate with the same gate-to-source voltage Vgs, to provide an accurate current ratio. Applications include current sensing for switching mode power supplies that need high speed and lossless current sense for current protection and feedback.