Cascode Current Sense Circuitry for GaN Transistors
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Solution Overview
Problem
Current sensing methods in high-performance power converter devices, such as those using sense resistors or RDS(on) sensing, suffer from power loss, source voltage variations, and accuracy limitations, particularly in gallium nitride (GaN) HEMT devices, where existing techniques like sense FETs face challenges with matching performance and noise introduction.
Innovation Solution
A current sensing circuit utilizing a cascode configuration with a scaled-down sense MOSFET monolithically fabricated on the same substrate as the main MOSFET, eliminating the need for a sense resistor and achieving a K:1 current ratio through equalized gate-source and drain-source voltages, ensuring accurate and noise-free current sensing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a sense resistor is used for current sensing, then the voltage drop is linearly proportional to the current, but power loss increases and efficiency decreases
Solution Approach 1:
The patent uses a sense MOSFET that is a scaled-down copy of the main power MOSFET, fabricated on the same substrate with matched characteristics. This copy approach allows current sensing without requiring a separate sense resistor, thereby eliminating the power loss associated with resistive sensing while maintaining measurement accuracy through the matched electrical characteristics between the sense and power MOSFETs.
2Loss of energy
If RDS(on) sensing is used to infer current, then no additional sense resistor is needed, but source voltage variations reduce measurement accuracy
Solution Approach 1:
The patent changes the operating parameters by ensuring both the sense MOSFET and power MOSFET operate with equal gate-source voltages and equal drain-source voltages. This parameter equalization approach, combined with the scaled channel width ratio, allows accurate current sensing through the sense MOSFET's drain current without being affected by source voltage variations, as both devices experience identical voltage conditions.
3Measurement precision
If a sense FET with smaller channel is used, then current sensing accuracy is improved, but matching performance degrades with larger scaling ratios
Solution Approach 1:
The patent merges the sense MOSFET and power MOSFET fabrication processes by fabricating both devices on the same semiconductor substrate using the same process steps. This integration ensures that both devices experience identical process variations and have matched electrical characteristics. The matched gate oxide thickness, threshold voltages, and mobility parameters maintain reliable matching performance even with scaled channel dimensions.
4Duration of action of moving object
If sample-and-hold capacitor is used in sense FET approach, then current can be held during off times, but transient spikes introduce noise
Solution Approach 1:
The patent extracts and eliminates the sample-and-hold capacitor from the sensing circuit by using the sense MOSFET's inherent channel characteristics to naturally hold and transfer the current signal. The continuous conduction through the matched MOSFET pair provides inherent signal holding capability during off times without requiring external capacitive elements, thereby eliminating the noise introduced by transient spikes across capacitors.
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 solution provides efficient and accurate current sensing without power loss, maintaining linearity and reducing noise, even at high current levels, by using a cascode configuration with equalized voltages across both MOSFETs, enhancing the accuracy and reliability of current measurement in GaN HEMT devices.
Implementation Method 1
The first MOSFET device has a channel size that is K times larger, where K is an integer greater than 1, as compared to the second MOSFET device. Circuitry is included that equalizes a voltage across both the first MOSFET device and the second MOSFET device, thereby producing a current ratio of K:1 between the first and second MOSFET devices in operation.
Implementation Method 2
Circuitry coupled to the first and second nodes equalizes a voltage across both the first MOSFET device and the second MOSFET device.
Data Source
AI summary
An integrated circuit (IC) for sensing a current flowing through a transistor device includes a substrate and a current scaling circuit that includes first and second MOSFET devices. The first MOSFET device has a drain coupled to the switched FET at a first node and a source coupled to the substrate. The second MOSFET device has a source coupled to the substrate and a drain coupled to a second node. The first MOSFET device has a channel size that is K times larger than the second MOSFET device. Circuitry is included that equalizes a voltage across both the first MOSFET device and the second MOSFET device.