MOSFET Current Sensing Circuit With Replica Transistors
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Solution Overview
Problem
Current sensing in MOS transistors for switching systems faces challenges such as noise immunity, high-amplitude sensing signal handling, reduced sensing errors, and a complex circuit structure, particularly in high-efficiency applications like regulators and audio amplifiers.
Innovation Solution
A current sensing circuit utilizing a pair of field effect transistors, where one transistor's drain and gate are coupled with the power transistor's drain and gate, and the second transistor's current path is in series, with a sensing signal generated at a node between them, providing improved noise immunity and reduced errors without the need for additional resistors or complex circuitry.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If intrinsic resistance of MOS transistor is used for current sensing, then additional resistors are avoided and circuit complexity is reduced, but noise immunity and sensing accuracy deteriorate
Solution Approach 1:
The patent uses a replica transistor that copies the electrical characteristics of the power MOS transistor. The replica transistor is identically sized and configured to mirror the drain-source voltage, providing an accurate sensing signal without requiring additional resistors. This copying approach maintains sensing accuracy while avoiding the complexity of external sensing components.
Solution Approach 2:
The patent introduces a sensing transistor as an intermediary element that translates the drain-source voltage into a usable sensing signal. This sensing transistor acts as a mediator between the power transistor and the sensing circuit, improving noise immunity and measurement precision while maintaining circuit integration.
2Device complexity
If simple current sensing is implemented without additional components, then device complexity is reduced, but noise immunity deteriorates
Solution Approach 1:
The replica transistor creates a clean copy of the drain-source voltage that is inherently more immune to noise. By copying the voltage signal through an identically configured transistor rather than using passive resistors, the sensing signal achieves better noise immunity while maintaining circuit simplicity and integration.
3Quantity of substance
If intrinsic resistance sensing is used, then additional resistors are eliminated, but high-amplitude sensing signal handling becomes problematic
Solution Approach 1:
The replica transistor copies the drain-source voltage in a manner that produces a well-defined, manageable sensing signal. This copied signal has predictable amplitude characteristics that are easier to handle and process compared to direct intrinsic resistance sensing, while still eliminating the need for external resistors.
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 enables simple, compact, and accurate current sensing with enhanced noise immunity and reduced errors, suitable for high-efficiency applications like regulators and audio amplifiers, by effectively mirroring the power transistor's drain-source voltage, thus maintaining accuracy and reducing circuit complexity.
Implementation Method 1
a first sensing field effect transistor arranged with its drain and gate coupled with the drain and the gate of the field effect transistor, respectively
Implementation Method 2
a second sensing field effect transistor having a gate configured for receiving a replica of the drive signal. The second sensing field effect transistor is arranged with its current path in series with the current path of the first sensing field effect transistor
Data Source
AI summary
A circuit includes a field effect transistor having a gate driven via a drive signal. The field effect transistor has a drain-source voltage drop indicative of the intensity of a current flowing in the current path through the field effect transistor. The circuit also includes a pair of sensing transistors that include a first sensing field effect transistor arranged with its drain and gate coupled with the drain and the gate of the field effect transistor, respectively, and a second sensing field effect transistor having a gate configured for receiving a replica of the drive signal. The second sensing field effect transistor is arranged with its current path in series with the current path of the first sensing field effect transistor. A sensing signal at a sensing node is indicative of the current intensity flowing in the current path of the field effect transistor.