Parallel Sensing FET Current Detection in Switch Mode Power Converters
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Solution Overview
Problem
Conventional current sensing in power conversion circuitry incurs significant power loss and occupies a large area due to the use of series-connected sensing components, which increases cost and size.
Innovation Solution
The use of integrated circuits with sensing apparatus that includes FETs connected in series and parallel with power FETs, employing sensing circuitry to provide an output based on voltage across the FETs, reducing power loss and area by eliminating the need for series-connected sensing components.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If series-connected sensing components (sense resistor or sense FET) are used for current sensing, then current measurement is achieved, but power loss increases due to heat generated by conduction
Solution Approach 1:
The patent introduces an intermediary sensing FET (Qs) that is connected in parallel with the power FET (Q1) rather than in series. This sensing FET acts as a mediator that replicates the voltage conditions of the power FET without carrying the full load current, thereby enabling current sensing without the significant power losses associated with series-connected sense resistors or sense FETs.
Solution Approach 2:
Instead of the conventional approach of connecting the sensing component in series with the power FET, the patent inverts the connection topology by placing the sensing FET in parallel. This inversion allows the sensing FET to sense the voltage across its own channel (which mirrors the power FET's voltage) without having to conduct the full power current, thus eliminating the I²R power loss problem.
2Measurement precision
If sensing component is sized to accommodate maximum input current, then current sensing capability is sufficient, but sensing device occupies significant die area and board area
Solution Approach 1:
The sensing FET (Qs) serves as an intermediary device that does not need to be sized for maximum current conduction. Instead, it is sized appropriately for sensing purposes only, since it operates in parallel and senses voltage rather than carrying the full load current. This dramatically reduces the required die area compared to a series-connected sense FET that would need to handle the full power current.
Solution Approach 2:
The patent applies local quality by making the sensing FET (Qs) have different characteristics than the power FET (Q1). The sensing FET is optimized for voltage sensing with appropriate W/L ratios and threshold voltages, while the power FET is optimized for current conduction. This allows each device to be sized and characterized for its specific function, reducing the overall area requirement.
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 significantly reduces power loss and component size, allowing for more efficient and cost-effective current sensing in power conversion applications.
Implementation Method 1
sensing circuitry providing an output representing the current flowing in the power FET based at least in part on a voltage across the first FET
Data Source
AI summary
Methods and apparatus are presented for sensing current flowing in a power transistor of a switch mode converter, in which a voltage is sensed across a first field effect transistor connected in a series circuit branch in parallel with the power transistor, and the sensed voltage is used to generate output signal to indicate the current flowing in the power transistor.