Electromagnetic Interference (EMI) filters (also called power line filters) are passive, frequency selective, bi-directional (two-port) networks comprised of inductors, capacitors, resistors, and ferrites.
These low pass filter devices are often thought of as just components whose sole purpose is to suppress unwanted emissions emanating from the electronic devices they’re installed in, so these products can pass regulatory compliance emissions tests.
Don’t overlook the fact that EMI filters can also be used to prevent unwanted RF noise (such as electrical fast transient burst, conducted RF, and some surges) from entering susceptible devices, especially if the EMI filter is combined with proper device shielding in a kind of “belt and suspenders” approach to electromagnetic compatibility. In this case, EMI filters help make RF susceptible devices more rugged in their surrounding environments, if these environments happen to also be heavily polluted with RF noise. EMI filters come in handy when the electronic products they are paired with must comply with national and international standards for both emissions and immunity (susceptibility).
Pro Tip: Not always, but typically, common-mode (CM) noise predominates up until ~ 30 MHz, whereas differential-mode (DM) noise prevails up until ~ 1 MHz. EMI filters help suppress both types of noise (CM and DM).
Placement
EMI filters for power line filtering are placed between the AC (or DC) power supplied to the end-product and its power supply input (i.e., typically placed ahead of noisy switched-mode power supply inputs).
Ideal EMI Filter
If installed correctly, an ideal (perfect) EMI filter should be able to substantially reduce the amplitude of all noise frequencies greater than the filter’s cutoff frequency (the stopband or reject band) and pass all low frequencies signals (the passband).
Source and Load Impedances
For the EMI filter to do its job, it’s important to understand both the source and load impedances (Z) it will be connected to. The wrong configuration of EMI filter will only work if there is a Z mismatch between source and load.
See Table 1 where:
L = Inductor
R = Resistor
F = Ferrite
C = Capacitor
| Source Z | Filter Configuration | Load Z | Analysis |
| Low (<100Ω) |  |
High (>100Ω) | Shunt element faces High Z load and series element faces the Low Z source. |
| >Low (<100Ω) |  |
Low (<100Ω) | Use inductive series element or T filter for greater roll-off. |
| High (>100Ω) |  |
Low (<100Ω) | Shunt element faces High Z source and series element faces the Low Z load. |
| High (>100Ω) |  |
High (>100Ω) |
Use shunt capacitive element or π filter for greater roll-off. |
Table 1
Pro Tip: The number of passive components (elements) in an EMI filter will impact the amount of insertion loss (dB) it can provide. One-element filters provide roughly 20 dB/decade of roll‑off or attenuation, two-element filters provide ~ 40 dB/decade, and three-element filters provide ~ 60 dB/decade of attenuation. If more attenuation is required, select a multi-element filter.
Choosing an EMI Filter
When considering an EMI filter, several important specifications come into play. These specifications include the following:
- End-use voltage rating: operating voltage, nominal voltage, rated voltage, etc.;
- End-use current rating: operating current, nominal current, rated current, etc.;
- Rated frequency: typical frequencies are 50 or 60 Hz and 400 Hz for military applications, and DC.
- Insertion Loss (dB) or IL (dB): The filter’s attenuation – specified in dB;
- Size and structure: many shapes and sizes available;
- Environment: temperature range – operation and storage;
- Safety certification standards: required by most end-product applications;
- Dielectric Strength Voltage: Hi-pot or the high potential insulation test voltage;
- Leakage current: critical in medical device applications where patient safety is paramount.
Example EMI Filter Specifications (Source: Schaffner FN 2090 Datasheet)
- Rated Voltage: 250 VAC (50/60 Hz), or 250 VDC
- Rated Current: 1-30A @ 40°C Maximum
- Insertion Loss (dB): Per CISPR 17; CM=50 Ω/50 Ω sym; DM=50 Ω/50 Ω asym (provided on manufacturer’s datasheet in the form of bode plots with attenuation listed on the vertical axis and frequency on the horizontal axis).
- Size and structure: Unique to each filter. See manufacturer’s datasheet.
- Temperature range (operation and storage): -25°C to +100°C (25/100/21)
- Certified to: UL 1283, CSA 22.2 No. 8 1986, IEC/EN 60939 (applies to AC and DC applications)
- High potential test voltage:
- P –> PE 2000 VAC for 2 sec (equiv. cap <88 nF)
- P –> PE 2550 VDC for 2 sec (equiv. cap >88 nF)
- P –> PE 2500 VAC for 2 sec (B types)
- P –> N 1100 VDC for 2 sec
- Leakage current: @ 250V AC/50 Hz = 0.45 mA; @ 120V AC/60 Hz = 0.26 mA
Pro-Tip: Insertion Loss (dB) is one of the most important parameters to select when specifying an EMI filter for any given application. See Reference 7 for more on this important topic.
Leakage Current
The amount of leakage current present on the power supply lines to ground is highly dependent on the voltage on the conductor, capacitance reactance (XC) between the conductor and ground, and the resistance between the conductor to earth. Be sure to thoroughly review the manufacturer’s specifications for this critical parameter, especially if the filter is to be installed into a medical device.
Summary
- EMI filters are passive, low pass devices.
- They help suppress RF noise emanating from or to any electronic device.
- They are comprised of inductors, capacitors, resistors, and ferrites.
- EMI filters help suppress both common-mode and differential-mode noise.
- They are placed between the AC (or DC) power supplied to the end-product and its power supply input.
- A perfect EMI filter will substantially reduce the amplitude of all noise frequencies greater than the filter’s cutoff frequency and pass all wanted low frequencies signals.
- For the EMI filter to best do its job, it’s important to understand both the source and load impendances (Z) it will be connected to.
- A multi-element filter will provide more IL (dB) than a single-element filter.
- The following specifications are important should be considered when specifying an EMI filter:
- Rated Voltage
- Rated Current
- Rated Frequency
- Insertion Loss (dB)
- Size and structure
- Operating environment
- Safety certifications
- Dielectric Strength Test Voltage
- Leakage Current
References and Further Reading
- “Three Essential Parameters to Check While Selecting Power Line EMI Filters,” Croydon EMC Solutions YouTube Channel, May 2, 2020.
- “Most Important Criteria to Select EMI EMC (EMV) Filter,” Croydon EMC Solutions YouTube Channel, March 29, 2020.
- “Multi-stage AC/DC EMI Filter with Excellent Attenuation Performance,” Schaffner Two-Stage Filters FN 2090 Datasheet.
- Montrose, M., EMC Made Simple, Printed Circuit Board and System Design, Montrose Compliance Services, Inc., 2014
- MacArthur, D., “What Every Electronics Engineer Needs to Know About: Filters,” In Compliance Product Insights, November 2018.
- MacArthur, D., “Let’s Talk About Why Filters Fail,” In Compliance Product Insights, November 2019.
- MacArthur, D., “An Alternative Approach to Specifying an EMI Filter,” In Compliance Product Insights, November 2020.
