Current Sense Circuit Using Averaged Mirror Voltages
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Solution Overview
Problem
Conventional current sense circuits face inaccuracies in measuring high currents due to significant IR losses from implicit metallization resistances, leading to voltage differences at the drain and source terminals of switch transistors, especially when composed of arrays of unit transistors.
Innovation Solution
A current sense circuit design that couples the source and drain terminals of multiple constituent field-effect transistors together, using a mirror current branch with a mirror transistor, and applies averaged voltages at the source and drain terminals of the mirror transistor to approximate the average voltages of the constituent transistors, thereby improving measurement accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a sense resistor is used in conventional current sense circuits, then current measurement is achieved, but power dissipation increases and measurement accuracy decreases at high currents due to IR losses from implicit metallization resistances
Solution Approach 1:
The patent extracts the explicit sense resistor from the circuit and replaces it with implicit resistance elements that are inherent to the transistor structures themselves. The sense function is achieved through the source and drain resistances of the transistors in the current mirror configuration, eliminating the need for a separate power-dissipating sense resistor while maintaining measurement capability
Solution Approach 2:
The patent introduces a current mirror configuration as an intermediary mechanism that transfers the current information from the switch transistor to a measurement transistor. This current mirror acts as a mediator that allows accurate current sensing without requiring direct voltage measurement across a sense resistor, thereby reducing power dissipation while maintaining measurement accuracy
2Measurement precision
If the resistance of the sense resistor is increased to improve measurement signal, then voltage VSENSE increases for accurate measurement, but power dissipation increases and heat dissipation challenges arise
Solution Approach 1:
The patent replaces the traditional resistive voltage division mechanism with a transistor-based current mirror system. Instead of using Ohm's law voltage drops across resistors to sense current, the system uses transistor current mirroring properties, substituting a resistive sensing mechanism with a transmissive one that is less sensitive to power dissipation and heat generation
3Device complexity
If implicit metallization resistances are neglected in high current applications, then circuit design is simplified, but measurement accuracy breaks down due to significant IR losses causing voltage differences at transistor terminals
Solution Approach 1:
The patent applies equipotentiality by ensuring that the source terminals of the switch transistor and measurement transistor are held at the same potential through direct connection, and the drain terminals are held at the same potential through the current mirror configuration. This eliminates voltage differences caused by implicit metallization resistances, allowing accurate current measurement even at high currents without complicating the circuit design
Solution Approach 2:
The current mirror configuration provides implicit feedback by continuously adjusting the measurement transistor's operation to match the switch transistor's current. The gate terminals are connected such that the transistors operate in mirror mode, automatically compensating for variations and maintaining accuracy without requiring additional feedback circuitry that would increase complexity
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 design provides more accurate current measurements through the switch by minimizing the impact of implicit resistances, ensuring accurate operation even at high currents.
Implementation Method 1
a mirror transistor to generate a mirror current that is approximately proportional to the current through the switch transistor
Implementation Method 2
The gate terminals are coupled together; the source terminals are coupled together. The drain terminals are kept at the same voltage using the operational amplifier configured with feedback provided through transistor
Data Source
AI summary
A current sense circuit that measures current passing through a multi-transistor switch, each transistor in configured in parallel. The current sense circuit mirrors the current in the switch through a mirror current branch that includes a mirror transistor. The current sense circuit obtains an accurate measure of current through the switch by applying voltages at the source and/or drain terminals of the mirror transistor that more closely approximate the average source and drain terminals of the constituent transistors of the switch. Thus, relatively accurate switch current measurements may be obtained.