Fully Differential Current Sensing for SMPS Noise Rejection

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Solution Overview

Problem

Conventional current sensing circuits in switch-mode power supplies (SMPS) face issues with transient output spikes, leading to false high current alerts and delayed alerts due to their single-ended feedback sense technique, which requires filters and offsets to manage common mode noise.

Innovation Solution

A fully differential current detection system that includes a high side circuit with a transconductance amplifier and a low side circuit, which generates a signal indicative of the input current without the need for filters in the alert signal path, providing improved common mode noise rejection and eliminating the requirement for specific polarity adjustments.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Device complexity

If a single-ended feedback sense technique is used in conventional current sensing circuits, then the circuit structure is simpler, but transient output spikes occur leading to false high current alerts and delayed alerts due to common mode noise

Engineering Contradiction:

Improvecircuit structureVSAvoidcurrent sensing accuracy

Core Design Contradiction:

Device complexityVSReliability

Solution Approach 1:

The patent divides the current sensing circuit into differential pairs (positive and negative sensing paths) that independently measure voltage drops across the inductor. By segmenting the sensing function into separate differential channels and combining their outputs, the circuit achieves noise rejection while maintaining structural organization.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The patent introduces an intermediary differential amplifier stage that processes the voltage signals from the inductor terminals before generating the current sense output. This intermediary stage converts voltage differences into current signals while providing immunity to common mode noise, acting as a mediator between the voltage sensing and current output stages.

Inventive Principle:

Principle #24Intermediary (Mediator)

2Object-affected harmful factors

If filters are added to the alert signal path to manage common mode noise, then noise rejection improves, but the time constant requirement increases causing delayed high current alerts

Engineering Contradiction:

Improvecommon mode noiseVSAvoidalert delay

Core Design Contradiction:

Object-affected harmful factorsVSLoss of time

Solution Approach 1:

The patent applies differential sensing at the input stage before signals are processed further through the circuit. By establishing the differential measurement approach at the beginning of the signal path, common mode noise is rejected early in the process, eliminating the need for subsequent filtering stages that would introduce time delays.

Inventive Principle:

Principle #10Preliminary action

Solution Approach 2:

The patent replaces the mechanical/filter-based noise rejection approach with an electronic differential amplification approach. Instead of using filters to remove noise after it enters the system, the differential architecture electronically rejects common mode noise at the source, substituting passive filtering with active differential signaling.

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

3Ease of operation

If deliberate offsets are required in conventional current sensing circuits, then polarity management becomes more complex, but the sensing range is limited

Engineering Contradiction:

Improvepolarity adjustmentVSAvoidsensing range

Core Design Contradiction:

Ease of operationVSAdaptability or versatility

Solution Approach 1:

The patent inverts the conventional approach by using differential voltage inputs that naturally handle both positive and negative current directions. Instead of requiring external offset adjustments to manage polarity, the circuit is designed to inherently sense voltage differences in either direction, with the differential amplifier automatically adapting to the polarity of the input signal.

Inventive Principle:

Principle #13The other way round (Inversion)

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 solution reduces the time constant requirement for filters, allowing for immediate detection of high current alerts without delay and eliminating the need for deliberate offsets, thereby enhancing the accuracy and speed of current sensing in SMPS systems.

Implementation Method 1

The transconductance amplifier is configured to receive a differential voltage indicative of a voltage drop across the capacitor and output a differential output current proportional to the differential voltage

Methodology Applied

Scientific EffectTransconductance:

Implementation Method 2

The inductor is configured to receive an input signal that includes an input current and generate a voltage across the inductor

Methodology Applied

Scientific EffectElectromagnetic induction: Electromagnetic Induction

Data Source

PatentUS10996256B2Fully differential current sensing

Publication Date: 2021.05.04 TEXAS INSTRUMENTS INC

AI summary

A current detection system includes an inductor and a detection circuit coupled across the inductor. The inductor is configured to receive an input signal that includes an input current and generate a voltage across the inductor. The current detection circuit includes a sensing network and a transconductance amplifier. The sensing network includes a capacitor and is configured to monitor a voltage across the inductor. The transconductance amplifier is configured to receive a differential voltage indicative of a voltage drop across the capacitor and output a differential output current proportional to the differential voltage.