Current Sensing Circuit Using Ohmic Mode Transistors
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Solution Overview
Problem
Current sensing technologies face challenges in achieving stable and accurate current measurement over temperature and supply voltage variations due to the unavailability of integrated precision resistors that can handle high current levels, leading to inefficiencies and inaccuracies in applications like power converters and switched mode power supplies.
Innovation Solution
A circuit arrangement using three transistors configured to operate in ohmic mode, where the current through one transistor is proportional to the current through the other two, with matching characteristics such as type, temperature, and production process, and an operational transconductance amplifier to convert voltage drops into currents, reducing the impact of resistance variations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a low-valued sense resistor is placed in series with the current path to sense current, then current sensing capability is achieved, but the voltage drop across the sense resistor is very small and requires amplification, increasing circuit complexity
Solution Approach 1:
The patent replaces the traditional voltage-based sensing mechanism (sense resistor + amplifier) with a current-based sensing mechanism using transistors operating in ohmic mode. The sense transistor converts the voltage drop directly into a proportional current signal, eliminating the need for voltage amplification and reducing circuit complexity while maintaining sensing accuracy.
Solution Approach 2:
The patent changes the operating parameter from voltage measurement to current measurement. By using transistors in ohmic mode where the drain current is proportional to the voltage across the transistor, the system transforms the small voltage drop into a measurable current signal that can be directly processed without complex amplification circuits.
2Measurement precision
If a current mirror is used to reduce current levels for sensing, then the current for sensing is reduced, but the mirrored current does not contribute to load current, reducing overall circuit efficiency
Solution Approach 1:
The sense transistor operates directly in the main current path and utilizes the existing voltage drop across the switching element to generate the sensing current. The transistor's own characteristics (ohmic mode operation) provide the sensing mechanism without requiring separate current mirrors or additional current paths, thereby maintaining full load current efficiency while enabling accurate sensing.
3Measurement precision
If portioning-off a small amount of current through the sense resistor is used, then the sensing current is obtained, but the resistance values differ by several orders of magnitude and depend on production process and temperature, reducing stability
Solution Approach 1:
The patent changes the operating region of the transistor to ohmic mode, where the device exhibits linear resistance characteristics. This parameter change enables the transistor to function as a stable, temperature-compensated sense element whose resistance is determined by its bias conditions rather than by process variations, significantly improving sensing stability.
Solution Approach 2:
The patent uses transistors that are matched in type, temperature, and production process characteristics. By ensuring homogeneity in the transistor parameters and operating conditions, the system achieves stable and accurate current sensing that is insensitive to process variations and temperature changes.
4Ease of manufacture
If the on-resistance of switching elements is used for sensing, then integrated sensing is possible, but the on-resistance depends strongly on temperature, production process and overdrive voltage, leading to large spread in sensed current
Solution Approach 1:
The patent changes the operating parameter by biasing the sense transistor in ohmic mode with a controlled gate voltage. This parameter change transforms the highly non-linear switching resistance into a stable, linear resistance value that is determined by the bias conditions rather than by temperature or process variations, enabling accurate integrated sensing.
Solution Approach 2:
The patent implements a feedback mechanism where the gate voltage of the sense transistor is controlled to maintain a constant overdrive voltage. This feedback control compensates for temperature and process variations, ensuring that the transistor's on-resistance remains stable and the sensed current remains accurate across different operating conditions.
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 stable and accurate current sensing by minimizing the effects of resistance variations and temperature sensitivities, ensuring consistent performance across different conditions.
Implementation Method 1
The first transistor, the second transistor and the third transistor may be configured to operate in an ohmic mode
Data Source
AI summary
A circuit arrangement including a first transistor, a second transistor and a third transistor. The first transistor and the second transistor are configured so that the current flowing through the first transistor is proportional to the current flowing through the second transistor and the third transistor. The first transistor, the second transistor and the third transistor are configured to operate in an ohmic mode. The second transistor and the third transistor are coupled in series to each other. The first transistor, the second transistor and the third transistor match each other in at least one characteristic.