Active Shield Winding for Common-Mode Noise Reduction
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Solution Overview
Problem
Existing power converters using isolated transformers generate significant common-mode noise due to parasitic capacitances between windings, which affects nearby components and circuits, and existing noise reduction methods either increase earth leakage current, add complexity, or are costly.
Innovation Solution
The implementation of an active shield winding between the primary and secondary windings in a switch mode power supply, where the active shield winding has the same or opposite number of turns as the secondary winding, wound in the same direction, and connected to a quiet termination to prevent displacement currents and induce a voltage opposite to residual common-mode noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If Y capacitors are used between primary and secondary sides to bypass noise, then common mode noise is reduced, but earth leakage current increases
Solution Approach 1:
An electrostatic shield is introduced as an intermediary component between the primary and secondary windings. This shield provides a low-impedance path for displacement currents, preventing them from coupling directly between windings. The shield is connected to primary ground through a capacitor, which blocks DC leakage current while allowing AC noise signals to be shunted to ground, thus reducing common mode noise without increasing earth leakage current.
Solution Approach 2:
The transformer structure is segmented by inserting an electrostatic shield that divides the magnetic path and electrical fields into distinct regions. This segmentation creates separate capacitive coupling paths: one between primary winding and shield, another between shield and secondary winding. By controlling the capacitance values in these segmented paths, noise reduction is achieved while managing leakage current through proper grounding and capacitor selection.
2Object-affected harmful factors
If electrostatic shields are used to block noise via capacitive displacement currents, then common mode noise coupling is reduced, but parasitic capacitance across primary and secondary windings increases
Solution Approach 1:
The electrostatic shield acts as an intermediary that introduces controlled parasitic capacitances in series between the primary and secondary windings. Instead of a direct capacitive coupling path, the displacement current must pass through multiple capacitive interfaces (primary-shield and shield-secondary), which reduces the overall noise coupling effect despite the presence of additional parasitic capacitances.
Solution Approach 2:
The parasitic capacitances inherent in the electrostatic shield structure are converted into a beneficial feature. These capacitances create multiple high-impedance paths for noise signals while the shield's connection to primary ground provides a low-impedance reference. The parasitic capacitances between shield and windings, when properly managed, help filter high-frequency noise while the overall structure reduces common mode coupling.
3Object-affected harmful factors
If conventional shields are used to reduce noise, then electromagnetic interference is decreased, but leakage inductance increases and cost and complexity increase
Solution Approach 1:
The electrostatic shield serves as a mediator that reduces electromagnetic interference through capacitive coupling mechanisms rather than relying on magnetic shielding materials. This approach avoids the increased leakage inductance associated with conventional magnetic shields and ferrite materials, while still providing effective noise reduction through the capacitive displacement current path to ground.
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 configuration effectively reduces common-mode noise by eliminating displacement currents between the active shield and secondary windings, minimizing earth leakage, and improving compliance with electromagnetic interference (EMI) standards without increasing complexity or cost.
Implementation Method 1
an active shield winding placed between the primary and secondary windings... induce a voltage opposite to residual common-mode noise
Implementation Method 2
parasitic capacitances between windings... displacement currents through the stray capacitance coupling in the transformer
Data Source
AI summary
A system for reducing common-mode noise includes a switch mode power supply having primary and secondary sides, primary and secondary side grounds, an input voltage source, a primary switch, a transformer, a core, and a power output. The primary and secondary sides each have a quiet termination. The transformer includes a primary winding, a secondary winding, and an active shield winding between the primary and secondary windings. The active shield winding has two terminations, is wound in a same direction as the secondary winding, and occupies a same axial position on the core as the secondary winding. One of the terminations of the active shield winding is connected to the quiet termination of the primary side, so that the terminations of the secondary winding and the active shield winding that are adjacent each other carry alternating voltages of a same polarity and a same amplitude.