Active EMI Filter Using Negative Impedance Converter
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Solution Overview
Problem
Existing active EMI filters require bulky inductors for sensing and injection, which occupy space, weigh heavily, and have bandwidth limitations due to parasitic coupling and self-resonance, while passive filters are large and inefficient.
Innovation Solution
An active EMI filter with a single-point connection through a capacitor that acts as both sensor and injector, utilizing negative capacitance to sense current changes and inject compensation back into the power line, along with variable components for gain compensation across temperature and frequency bands.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If passive EMI filters are used, then EMI reduction is achieved, but the filter becomes bulky and heavy
Solution Approach 1:
The patent replaces mechanical inductors with an operational amplifier-based active circuit that synthesizes inductive impedance. The op amp circuit with feedback network creates a negative impedance that, when combined with a small capacitor, produces the desired inductive filtering effect without requiring physical inductors, thereby eliminating the bulk and weight of passive filters while maintaining EMI reduction capability
Solution Approach 2:
The patent changes the filtering mechanism from relying on fixed physical inductor parameters to using electronically controllable active circuit parameters. The operational amplifier circuit allows dynamic adjustment of the equivalent inductance value through feedback resistance, enabling the same filtering performance with much smaller physical components
2Object-affected harmful factors
If separate sense and injection points are used, then EMI filtering is achieved, but two bulky inductors are required
Solution Approach 1:
The patent merges the sense and injection functions into a single integrated circuit node. The operational amplifier simultaneously performs current sensing through its input and voltage injection through its output, eliminating the need for separate sense inductor and injection inductor. This consolidation reduces the total component count and physical volume while maintaining the differential filtering action
Solution Approach 2:
The operational amplifier serves multiple functions: it acts as a current-to-voltage converter for sensing, a voltage amplifier for injection, and an impedance transformer for matching. This multi-functionality allows a single component to replace what traditionally required multiple separate inductors and active components, reducing overall circuit volume
3Object-affected harmful factors
If inductors are used in active filters, then EMI reduction is achieved, but bandwidth limitations occur due to parasitic coupling and self-resonance
Solution Approach 1:
The patent replaces physical inductors with an active operational amplifier circuit that synthesizes inductive behavior through feedback. This electronic simulation of inductance eliminates the parasitic capacitance and self-resonance effects inherent in physical inductors, extending the usable bandwidth of the EMI filter into higher frequency ranges where traditional inductor-based filters become ineffective
Solution Approach 2:
The patent creates a dynamic, electronically controllable impedance that can adapt to different frequency conditions. The operational amplifier circuit with feedback allows the equivalent inductance value to be adjusted electronically, enabling the filter to maintain optimal performance across a broader frequency spectrum compared to fixed physical inductors
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 solution reduces EMI by up to 75% and minimizes the size and bulk of the filter, achieving better performance than passive filters with reduced insertion loss and adaptability to different frequency bands.
Implementation Method 1
connection capacitor that couples a input of a gain control circuit to a power line. Changes in a current flowing along the power line cause a sensed current to flow along the connection capacitor
Implementation Method 2
negative capacitance circuit that generates a voltage change that is applied to the connection capacitor. The voltage change generates a compensation current that is injected back along the power line through the connection capacitor
Data Source
AI summary
An active filter reduces Electro-Magnetic Interference (EMI) created by current flowing through a power line. The active filter connects to the power line at a single node through a connection capacitor. A sense current flows through the connection capacitor when the power line current changes. This sense current is applied to a non-inverting input of an op amp to drive a power amplifier circuit through a filter capacitor. The power amplifier circuit increases the current drive of the op amp to charge a transfer capacitor that converts the power amplifier output current to a transfer voltage. The transfer capacitor is connected to the connection capacitor so that the transfer voltage is injected back into the power line through the connection capacitor as an injected voltage that compensates for the sensed current. Op amp gain is adjustable by variable resistors that connect to the inverting input of the op amp.