Electromagnetic interference (EMI) and radio frequency interference (RFI) are critical challenges in the design of switchmode power supplies (SMPS). These forms of electrical noise can disrupt the proper functioning of the power supply and surrounding equipment. Understanding the causes, propagation forms, and mitigation techniques is essential for efficient and compliant SMPS design.
Typical Causes of Conducted and Radiated RFI Interference in SMPS
RFI interference in SMPS can be categorized into conducted and radiated forms:
Conducted RFI: This interference propagates through physical connections, such as power lines. Common causes include:
High-frequency switching transients in transistors.
Rectifier diodes generating voltage spikes during switching.
Parasitic capacitances in transformers and inductors.
Radiated RFI: This interference is emitted through electromagnetic waves and does not require physical connections. It often arises from:
Magnetic fields around transformers and inductors.
High-frequency switching components, such as MOSFETs or IGBTs, acting as antennas.
Forms of Electrical Noise Propagation in SMPS
For a power supply designer, two primary forms of electrical noise propagation are of concern:
Differential-Mode Interference:
Originates from the normal current flow in the power lines.
Appears as a voltage difference between the line and neutral conductors.
Example: Switching transients causing voltage spikes between live and neutral wires.
Common-Mode Interference:
Caused by currents flowing in the same direction through both power lines relative to the ground.
Induced by parasitic capacitance between components (e.g., heat sinks, transformer windings) and the chassis.
Example: Noise generated by the capacitive coupling of switching waveforms to the ground plane.
Importance of Reducing Interference Noise
Excessive EMI/RFI noise can:
Disrupt sensitive electronic circuits and communications.
Lead to regulatory non-compliance with standards such as FCC or CISPR.
Cause functional instability in the power supply and its load.
To ensure optimal performance and regulatory adherence, designers must minimize interference noise.
Best Position to Eliminate RFI Interference in SMPS
RFI interference is most effectively addressed at its source. Key positions include:
Switching Transistors: Screens or filters can prevent switching transients from coupling to the rest of the circuit.
Transformers: Proper shielding, such as Faraday screens, can reduce capacitive coupling between windings.
Input and Output Points: Placement of filters at these locations can prevent noise from entering or leaving the system.
Why Line Filters Are of Limited Value for Common-Mode Interference
Line filters, while effective for differential-mode interference, have limitations in addressing common-mode noise due to:
High Impedance Path: Common-mode currents often find alternate paths, bypassing the filter.
Parasitic Capacitance: Noise can couple through parasitic elements directly to the ground, evading the filter.
Frequency Range: Line filters are typically optimized for lower frequency ranges and may be less effective against high-frequency common-mode noise.
Mitigation Techniques
To minimize EMI and RFI, consider the following design strategies:
Use of Faraday Screens:
Shield sensitive areas to prevent capacitive coupling.
Ensure proper grounding of the screen to return noise currents to a "star" point.
Proper Layout and Component Placement:
Minimize loop areas in PCB designs to reduce radiated emissions.
Isolate high-frequency switching nodes from low-noise circuits.
Filtering and Shielding:
Implement differential and common-mode filters at input and output stages.
Use shielded transformers and components.
Controlled Switching Transients:
Optimize switching speeds to balance efficiency and noise generation.
Use snubber circuits to suppress voltage spikes.
By proactively addressing EMI and RFI challenges in SMPS design, engineers can create robust, efficient, and compliant power supply systems.
