Current Sensor IC Biasing for Common-Mode Voltage Rejection
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Solution Overview
Problem
Current sensor integrated circuits face challenges in accurately measuring motor phase current due to large common mode voltages experienced by sense resistors, which can swing beyond the supply voltage range, complicating in-line current sensing and requiring effective voltage rejection techniques.
Innovation Solution
The proposed solution involves a current sensor integrated circuit with a substrate and tub structure, where the analog front end and tub are biased to a predetermined offset voltage greater than the common mode voltage, using a charging circuit with a boot capacitor and regulator to power the amplifier and isolate the circuitry from high voltages, preventing forward biasing and enhancing common mode voltage rejection.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the sense resistor is used to measure current in-line with motor winding, then current measurement is achieved, but large common mode voltages swing beyond supply voltage range causing measurement errors
Solution Approach 1:
An isolation tub structure is introduced as an intermediary between the sense resistor and the analog front end circuitry. The tub is biased to a voltage higher than the common mode voltage to create a protective potential barrier, preventing direct exposure of sensitive circuits to high common mode voltages while still allowing accurate current measurement through the sense resistor.
Solution Approach 2:
The bias voltage parameter of the tub is changed to be higher than the maximum common mode voltage. By adjusting the tub bias voltage to exceed the common mode voltage swing range, the circuit creates a safe operating window that rejects common mode interference while maintaining measurement functionality.
2Reliability
If multiple integrated circuits are used to electrically isolate sense circuitry from large common mode voltages, then isolation is achieved, but device complexity increases
Solution Approach 1:
The isolation tub structure integrates multiple isolation functions into a single semiconductor substrate. Instead of using multiple separate integrated circuits, the patent combines the sense resistor, tub structure, and analog front end into one monolithic device, achieving electrical isolation through the tub biasing scheme while reducing overall system complexity.
Solution Approach 2:
The analog front end circuitry is nested within the isolation tub, which itself is formed on the semiconductor substrate. This nested structure allows the sensitive circuits to be physically contained within the protected region, with the tub acting as an embedded isolation barrier that prevents high common mode voltages from reaching the inner circuits.
3Reliability
If the tub bias voltage is set to prevent forward biasing of junction, then circuit protection is achieved, but additional voltage headroom is required
Solution Approach 1:
The tub is pre-biased to a voltage higher than the maximum expected common mode voltage before any harmful effects can occur. This preliminary voltage establishment creates a protective buffer that prevents forward biasing of the substrate junction under all operating conditions, including voltage overshoots and undershoots.
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 approach improves the accuracy of current sensing by effectively rejecting common mode voltages, protecting the circuitry from high voltages and maintaining isolation, even during undershoots and overshoots in motor applications.
Implementation Method 1
a charging circuit configured to bias the analog front end and the tub to a bias voltage that is a predetermined offset voltage greater than a common mode voltage associated with the resistor
Implementation Method 2
The current sensor integrated circuit may further include a boot capacitor having a first terminal coupled to the charging circuit and to the regulator to provide the regulator supply input voltage
Data Source
AI summary
A current sensor integrated circuit to sense a current through a resistor includes a substrate, a tub disposed in the substrate, an analog front end disposed in the tub and comprising an amplifier having inputs coupled across the resistor and a charging circuit configured to bias the analog front end and the tub to a bias voltage that is a predetermined offset voltage greater than a common mode voltage associated with the resistor. In embodiments, the analog front end is biased to a first bias voltage and the tub is biased to a second, different bias voltage.