Dual Differential Amplifier Current Sensing for H-Bridge Common Mode Voltage
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Solution Overview
Problem
Current sensing in H-bridge motor controllers faces inaccuracies due to wide variations in common mode voltage, which can lead to errors in differential amplifier measurements, and existing solutions do not directly measure current in motor windings accurately.
Innovation Solution
A current sensing circuit with dual differential amplifiers that adjust their output based on common mode voltage levels, using a switching system to select the appropriate amplifier for low or high common mode conditions, and a switch timing/nulling circuit to maintain accuracy and compensate for drift.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a differential amplifier is used to measure the differential voltage across the shunt resistance, then the current measurement capability is provided, but the measurement accuracy deteriorates due to wide variations in common mode voltage
Solution Approach 1:
The patent divides the measurement task into two separate differential amplifiers: one optimized for low common mode voltage conditions and another optimized for high common mode voltage conditions. Each amplifier handles a specific segment of the common mode voltage range, preventing the harmful effects of wide voltage variations from degrading measurement accuracy.
Solution Approach 2:
The patent implements a dynamic switching mechanism that automatically selects between two differential amplifiers based on the instantaneous common mode voltage level. The switching system monitors the common mode voltage and dynamically connects the appropriate amplifier (low CMV or high CMV optimized) to maintain optimal measurement accuracy across varying operating conditions.
2Measurement precision
If a differential amplifier with high common mode rejection ratio is used, then measurement accuracy under varying common mode voltage is improved, but trade-offs between DC accuracy and AC performance are required
Solution Approach 1:
Instead of attempting to design a single amplifier that simultaneously optimizes for both DC accuracy and AC common mode rejection, the patent segments the functionality into two specialized amplifiers. Each amplifier is optimized for its specific operating condition (low or high common mode voltage), eliminating the need for compromising trade-offs in a single device.
Solution Approach 2:
The system achieves universal current measurement capability across wide common mode voltage ranges by implementing a multi-function switching architecture. The switching system enables a single measurement channel to universally handle both low and high common mode voltage conditions by selecting the appropriate specialized amplifier, providing broad applicability without requiring each amplifier to be universally optimized.
3Device complexity
If current is measured in ground or supply connection instead of directly in motor winding, then device complexity is reduced, but measurement accuracy deteriorates
Solution Approach 1:
The patent takes preliminary action by directly placing the shunt resistance in series with the motor winding before the current flows through the measurement path. This direct insertion enables accurate measurement of the actual winding current from the outset, rather than attempting to infer it from ground or supply connections later in the circuit.
Data Source
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AI summary
A current sensing circuit may include a shunt resistance through which current to be sensed travels. A first and a second differential amplifier may each provide an amplified output of the voltage across the shunt resistance. A switching system may deliver a current sensing signal output based on the amplified output of the first differential amplifier when the common mode voltage across the shunt resistance is low and based on the amplified output of the second differential amplifier when the common mode voltage across the shunt resistance is high. The first differential amplifier may provide its lowest output DC offset voltage when the common mode voltage is low, while the second differential amplifier may provide its lowest output DC offset voltage when the common mode voltage is high. The first and second differential amplifiers may both have a low common mode voltage rejection ratio, such as a ratio of less than 40 db at the switching frequency of switches that control the current that is sensed.