Active Shield Transformer Winding for Common-Mode Noise Reduction
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Solution Overview
Problem
Conventional methods for reducing electromagnetic interference (EMI) in switch mode power supplies, such as those using Y capacitors and electrostatic shields, face limitations in effectively canceling common-mode noise due to increased parasitic capacitance and earth leakage current, and often require additional complexity and cost to achieve compliance with emission standards.
Innovation Solution
The implementation of an active shield winding between the primary and secondary windings in a transformer, wound in the same direction as the secondary winding and connected to a quiet termination, which eliminates displacement currents by carrying alternating voltages of the same polarity and amplitude, thereby reducing common-mode noise without increasing earth leakage current.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If Y capacitors are used to bypass noise between primary and secondary sides, 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, allowing noise bypass while limiting earth leakage current through the capacitor's impedance characteristics.
Solution Approach 2:
The harmful displacement currents are extracted from the main noise coupling path by providing a separate dedicated path through the electrostatic shield. By removing these currents from the primary-secondary coupling path and directing them through the shield to ground, the noise reduction effect is achieved without requiring large Y capacitors that would increase earth leakage.
2Object-affected harmful factors
If electrostatic shields are used to block noise via capacitive displacement currents, then noise transmission is reduced, but parasitic capacitance across windings increases
Solution Approach 1:
The electrostatic shield acts as an intermediary that blocks capacitive coupling between primary and secondary windings. By placing the shield between the windings and connecting it to ground through a capacitor, displacement currents are redirected through the shield rather than coupling directly between windings, reducing noise transmission while the ground connection path manages the parasitic capacitance effect.
3Object-affected harmful factors
If conventional shields are used to reduce common-mode noise, then noise cancellation is improved, but additional complexity and cost are introduced
Solution Approach 1:
The electrostatic shield serves multiple functions simultaneously: it blocks capacitive coupling between windings, provides a controlled path for displacement currents to ground, and reduces common-mode noise. This multi-functionality is achieved with a single component rather than multiple separate shielding structures, reducing overall complexity.
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 common-mode noise emissions, improving compliance with EMI standards while minimizing additional complexity and cost, by eliminating displacement currents through the active shield winding, thus enhancing the effectiveness of noise cancellation in switch mode power supplies.
Implementation Method 1
an active shield winding placed between the primary and secondary windings... is wound in a same direction as the secondary winding... 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
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.