Current Sense Circuit for Bi-Directional Load Currents
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Solution Overview
Problem
Current current-sensing methods in power converters face challenges in accurately measuring bi-directional currents without introducing significant power losses or requiring high-speed, high-voltage amplifiers, and are susceptible to process variations and temperature changes.
Innovation Solution
A current-sense circuit that generates two sense currents, one for positive and one for negative values of the switch current, with a ratio independent of process variations and temperature, using matched sense FETs and amplifiers that operate independently of high voltages, allowing for accurate measurement of bi-directional currents without high-speed, high-voltage amplifiers.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If lossless current-sensing methods using parasitic resistances are used, then power losses are minimized, but measurement accuracy deteriorates due to tolerances and temperature dependency
Solution Approach 1:
The patent introduces a dedicated sense resistor as an intermediary element specifically for current measurement. This separate component allows accurate current sensing without relying on the parasitic resistances of power switches, which have tolerances and temperature dependencies. The sense resistor is designed with precise specifications to provide accurate measurements while minimizing impact on overall system efficiency.
Solution Approach 2:
The patent creates a scaled-down copy of the switch current through a sense FET that mirrors the behavior of the power switch. By using a smaller, matched FET device, the circuit generates a proportional current signal that accurately represents the main switch current without requiring direct measurement of the high-current path, thus maintaining both accuracy and efficiency.
2Measurement precision
If accurate sense-resistors are added to improve measurement accuracy, then current sensing precision is improved, but power losses increase
Solution Approach 1:
The patent applies local quality by using different resistance values in different parts of the circuit. The sense resistor has a precise, low value optimized for measurement accuracy, while the main power path uses components with higher resistance tolerances. This localized precision approach improves measurement accuracy without significantly impacting overall power efficiency, as the sense resistor carries only the measurement current.
3Measurement precision
If conventional current-sensing methods are used for bi-directional currents, then high-speed high-voltage amplifiers are required, but device complexity and cost increase
Solution Approach 1:
The patent inverts the conventional approach by using current-mode sensing instead of voltage-mode sensing. Instead of measuring voltage across a sense resistor and requiring high-speed high-voltage amplifiers to handle bidirectional signals, the circuit directly senses current through matched FET devices that naturally handle bidirectional current flow. This current-mode approach eliminates the need for complex high-speed amplifiers while maintaining measurement accuracy.
Solution Approach 2:
The sense FETs automatically adapt to bidirectional current flow without requiring external control or complex circuitry. The matched FET devices inherently handle both positive and negative current directions, generating proportional sense currents that directly represent the bidirectional switch current. This self-service capability eliminates the need for high-speed high-voltage amplifiers and reduces overall system 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
The solution enables accurate and efficient measurement of bi-directional load currents in power switching devices, independent of process variations and temperature, without the need for high-speed, high-voltage amplifiers, improving the reliability and efficiency of current-sensing in power converters.
Implementation Method 1
A first sense circuit may be coupled to the power switching device to generate a first sense current for positive and/or negative values of the switch current. A second sense circuit may be coupled to the power switching device to generate a second sense current for positive and/or negative values of the switch current.
Implementation Method 2
A first amplifier may have a pair of differential inputs and an output, with one of the pair of differential inputs receiving the first sense voltage. A second amplifier may have a pair of differential inputs and an output, with one of the pair of differential inputs receiving the second sense voltage.
Data Source
AI summary
A current-sense circuit for measuring a load current in a switching power regulator may operate independently of process variation and temperature, and measure bi-directional load currents without requiring high-speed, high-voltage amplifiers for operation. A positive sense voltage may be generated for positive and/or negative values of a switch current conducted by a power switching device in the switching power regulator, by applying a linear transformation to a switch voltage developed across the power switching device according to the switch current. A first sense current may be generated by applying the positive sense voltage across a matching switching device having a same channel length as the power switching device. A second sense current may be generated independently of the switch voltage, and a total sense current that is proportional to the switch current may be calculated by subtracting the second current from the first current. The ratio of the total sense current and the switch current will be independent of process variation and temperature.