Current Sensing Circuit With Protection Current Offset
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Solution Overview
Problem
Conventional current sensing circuits face challenges in achieving a large current measuring range while maintaining a small burden voltage, which limits the signal-to-noise ratio due to the need for small shunt resistors and fuses with low impedance.
Innovation Solution
A current sensing circuit design that includes a protection resistor and capacitor, along with a protection circuit to draw a protection current that offsets the subject current, allowing for a larger shunt resistor to be used, thereby improving the signal-to-noise ratio and maintaining a small burden voltage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a large shunt resistor is used to improve signal-to-noise ratio, then measurement precision is improved, but burden voltage increases affecting the subject device
Solution Approach 1:
The patent divides the current sensing function into two separate paths: a main current path with a small shunt resistor for low burden voltage, and a sensing path with a large sensing resistor for high signal-to-noise ratio. The sensing path is isolated from the main current path, allowing independent optimization of each path's characteristics.
Solution Approach 2:
The patent introduces an intermediary coupling mechanism (such as a transformer or capacitive coupling) to transfer the sensing signal from the isolated sensing path to the output without requiring the large sensing resistor to be directly in the main current path. This intermediary allows the large resistor to serve its sensing function while not directly affecting the burden voltage in the main path.
2Object-affected harmful factors
If a small shunt resistor is used to reduce burden voltage, then the subject device is less affected, but signal-to-noise ratio deteriorates
Solution Approach 1:
The patent segments the circuit into a main current path with small shunt resistor and a separate sensing path with large sensing resistor. This segmentation allows the small resistor to minimize burden voltage while the large resistor in the isolated sensing path provides high signal-to-noise ratio for precise measurement.
Solution Approach 2:
The intermediary coupling mechanism enables the small shunt resistor in the main path to transfer its signal to the sensing path containing the large resistor, allowing the signal to be amplified with high fidelity without the large resistor being directly exposed to the main current.
3Reliability
If a fuse with non-negligible resistance is used for overcurrent protection, then protection function is provided, but burden voltage significantly increases for large currents
Solution Approach 1:
The patent places the fuse in the isolated sensing path rather than in the main current path. This segmentation allows the fuse to provide overcurrent protection for the sensing circuit while its resistance does not contribute to the burden voltage in the main current path, as the sensing path is electrically isolated from the main path.
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 design enables a high signal-to-noise ratio across a wide current measurement range by using a larger shunt resistor, while maintaining a stable burden voltage, thus enhancing the circuit's performance and reliability.
Implementation Method 1
The shunt resistor 55 is coupled in the current path for converting the subject current into an output voltage difference across the shunt resistor
Implementation Method 2
a capacitor coupled between the first output node of the first amplifier and the inverting input node of the first amplifier
Data Source
Figure 1~2
Figure 3~4
Figure 5
AI summary
A current sensing circuit (50) comprises a first input terminal (51) and a second input terminal (53) for introducing a subject current (IS) from a device coupled between the first and second input terminals; a first amplifier (57) having a first input node coupled to the first input terminal via a protection resistor (71), a second input node coupled to the second input terminal, and a first output node (63) coupled to the first input node via a capacitor (73), wherein the first amplifier has a current sensing path for sensing the subject current and converting the subject current into an output voltage that changes with the subject current; and a protection circuit (117) coupled between the first and second input nodes of the first amplifier, wherein the protection circuit is configured to draw a protection current from the first input terminal through the protection resistor to at least partially offset the subject current.