Electromagnetic Interference (EMI)
Electromagnetic Interference (EMI) is unwanted electrical noise that interferes with the normal operation of electronic equipment. EMI may be generated by switching power supplies, electric motors, wireless transmitters, relay contacts and many other sources. Inductors, ferrite beads and common-mode chokes are widely used to suppress EMI by reducing high-frequency noise while allowing the desired signals or power to pass with minimal loss.
What Is EMI?
EMI is unwanted electromagnetic energy that disrupts electronic circuits. It can travel through electrical wiring (conducted EMI) or through the air (radiated EMI).
| Type | Description |
|---|---|
| Conducted EMI | Travels through power or signal cables. |
| Radiated EMI | Travels through electromagnetic waves in free space. |
Common Sources of EMI
| Source | Typical Example |
|---|---|
| Switch-Mode Power Supplies | Phone chargers, laptop adapters and ATX power supplies. |
| Electric Motors | Drills, pumps, fans and industrial machinery. |
| Relay Contacts | Arcing during switching. |
| Radio Transmitters | Wi-Fi, Bluetooth and mobile networks. |
| Class-D Amplifiers | High-frequency switching outputs. |
| Automotive Ignition Systems | Spark generation. |
Effects of EMI
- Distorted audio.
- Communication errors.
- Reduced wireless range.
- Unstable microcontroller operation.
- Television or radio interference.
- Measurement errors.
- Unexpected equipment resets.
How Inductors Reduce EMI
An inductor presents very little opposition to DC and low-frequency current, but its reactance increases with frequency.
XL = 2πfL
High-frequency noise therefore encounters much greater opposition than the desired low-frequency current, reducing EMI.
EMI Suppression Components
| Component | Purpose |
|---|---|
| Ferrite Bead | Suppresses high-frequency noise on signal and power lines. |
| Common-Mode Choke | Blocks common-mode interference while passing normal current. |
| Power Choke | Reduces ripple and conducted noise. |
| LC Filter | Removes switching noise from power supplies. |
| EMI Line Filter | Prevents noise entering or leaving equipment. |
Applications
| Equipment | Purpose |
|---|---|
| Computer Power Supplies | Reduce conducted EMI. |
| LED Drivers | Suppress switching noise. |
| Industrial Controllers | Improve electrical reliability. |
| Audio Equipment | Reduce hum and high-frequency interference. |
| Wi-Fi Equipment | Improve signal quality. |
| Medical Equipment | Minimise electrical interference. |
Design Considerations
- Use shielded inductors where appropriate.
- Keep high-current loops as short as possible.
- Place EMI filters close to connectors or power inputs.
- Use proper PCB grounding techniques.
- Select ferrite materials suitable for the operating frequency.
- Minimise loop area to reduce radiated emissions.
Testing EMI Problems
- Inspect EMI filters for damage.
- Measure power supply ripple using an oscilloscope.
- Use a spectrum analyser for RF measurements.
- Check grounding and cable routing.
- Look for loose ferrite beads or damaged common-mode chokes.
Common Faults
| Fault | Possible Cause |
|---|---|
| Excessive Noise | Missing or failed EMI filter. |
| Radio Interference | Poor shielding or filtering. |
| Buzzing Audio | Switching noise entering audio circuits. |
| Equipment Resetting | Conducted EMI affecting digital electronics. |
| High Ripple | Faulty inductor or filter capacitor. |
Interesting Facts
- Every commercial switch-mode power supply contains an EMI filter near its AC input.
- Ferrite beads appear as small cylinders on many USB, HDMI and monitor cables.
- Common-mode chokes can greatly reduce noise without significantly affecting normal power transmission.
- Good PCB layout is often as important as the filter components themselves.
- Most countries require electronic products to meet electromagnetic compatibility (EMC) standards before they can be sold.
Key Points
- EMI is unwanted electromagnetic noise.
- EMI may be conducted through wires or radiated through the air.
- Inductors, ferrite beads and common-mode chokes are widely used to suppress EMI.
- Proper filtering and PCB layout greatly improve EMC performance.
- Effective EMI suppression improves reliability and reduces interference with other equipment.