Common-Mode EMI Filter Using Tuned Choke and Capacitance Multiplication
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Solution Overview
Problem
Offline power converters face challenges in effectively filtering common mode electromagnetic interference (EMI) noise due to the large size and cost of components required, as well as safety concerns related to capacitance values needed for differential mode filtering.
Innovation Solution
The implementation of a circuit and method that utilize a filter circuit with capacitors and inductors to provide a common mode noise cancellation signal, including a capacitance multiplication circuit to effectively increase capacitance while maintaining safety, and the use of a chassis-connected inductor to reduce the physical size and cost of common mode filtering components.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If large capacitors and inductors are used to filter common mode EMI, then EMI filtering effectiveness is improved, but device size and cost increase
Solution Approach 1:
The patent changes the electrical parameters of the circuit by introducing a resonant frequency matching mechanism. The resonant frequency of the LC circuit is designed to match the switching frequency of the power converter, creating a notch filter effect that provides high impedance at the EMI frequency. This allows smaller component values to achieve the same filtering effectiveness that would otherwise require large components.
Solution Approach 2:
The patent makes the filtering circuit multi-functional by having it simultaneously perform common mode EMI filtering and provide a reference voltage to the controller. The third capacitor couples the reference voltage terminal to the housing, allowing the same circuit nodes to serve both filtering and voltage reference functions, thereby reducing the need for separate components.
2Object-affected harmful factors
If large capacitors are used for differential mode filtering, then filtering effectiveness is improved, but safety risks increase
Solution Approach 1:
The patent changes the filtering approach by using frequency-selective parameter optimization. Instead of using large capacitors that provide broadband filtering, the circuit uses tuned LC resonance that provides high impedance specifically at the switching frequency. This allows much smaller capacitor values to achieve effective filtering, thereby reducing safety risks associated with large capacitance values.
3Object-affected harmful factors
If traditional common mode filtering components are used, then EMI suppression is improved, but cost increases
Solution Approach 1:
The patent achieves cost reduction through parameter optimization. By tuning the resonant frequency of the LC circuit to match the switching frequency, the design achieves effective common mode rejection with smaller, less expensive components. The reactive impedance created by the resonant circuit provides superior filtering performance per unit cost compared to traditional resistive or broadband capacitive filtering approaches.
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 reduces common mode EMI noise while minimizing the size and cost of filtering components, ensuring safety by allowing smaller capacitors and inductors, thus optimizing space and reducing the risk of electrical hazards.
Implementation Method 1
An inductive choke has first and second coils, the first and second coils being magnetically coupled
Implementation Method 2
A first capacitor is coupled between the first power terminal and the first filter input, and a second capacitor is coupled between the second power terminal and the second filter input
Implementation Method 3
An inductor is coupled between the housing and a ground terminal
Data Source
AI summary
Described embodiments include a circuit for filtering electromagnetic interference (EMI) that includes an electrically conductive housing enclosing the circuit, a first power terminal providing a first signal, and a second power terminal providing a second signal, the first and second signals forming a differential power input. A filter circuit provides a common mode noise cancelling signal at an output responsive to first and second inputs. An inductive choke has first and second coils that are magnetically coupled. The first coil is coupled between the first power terminal and a first converter input. The second coil is coupled between the second power terminal and a second converter input. A third capacitor is coupled between the filter output and the second power terminal. A fourth capacitor is coupled between the first power terminal and the second power terminal, and an inductor is coupled between the housing and a ground terminal.