Understanding Electromagnetic Compatibility Standards for Switch-Mode Power Supplies

By Ashok Bindra

Contributed By Electronic Products


By virtue of their inherent design characteristics, both AC/DC and DC/DC switch-mode power supplies (SMPS) generate electromagnetic interference (EMI) composed of radio frequency (RF) signals of multiple frequencies. Besides interfering internally with circuit components and functions in the system where these SMPS supplies are installed, such emissions can also interfere with other equipment in the vicinity of the source. To ensure that such emissions are kept to a minimum, international regulatory bodies have established rules and regulations; SMPS converter modules that comply with such standards are called electromagnetically compatible (EMC) power supplies.

Power supply maker CUI has published a white paper entitled “Electromagnetic Compatibility Considerations for Switched-Mode Power Supplies”¹ that investigates various international standards governing control of EMI, and describes them in greater detail. In the United States, the government agency responsible for regulating such interference is the Federal Communications Commission (FCC). Control of EMI interference is outlined in Part 15 of the FCC’s rules, which specifies that any spurious signal greater than 10 kHz is subject to regulation. Additionally, according to the type of emission, it also specifies the frequency bands in which these spurious emissions must be controlled.

There are two types of EMI emissions: radiated and conducted. While radiated emissions are those radiated and coupled through the air, conducted emissions are RF signals coupled by a direct path between the source and the receptor. Direct contact is provided by a transmission line, wire, cable, printed circuit board (PCB) trace or metal enclosure. It can appear in a common or differential mode on two conductors.

Also, per FCC Part 15, radiated emissions must be controlled between 30 MHz and 1000 MHz, and conducted emissions must be controlled in the frequency band between 0.45 MHz and 30 MHz.

Electronic equipment using SMPS power supplies has been classified as Class A and Class B by the FCC. Class A is designated for use in the commercial, business, and industrial environments, and Class B is labeled for use in the residential space. Class B devices include personal computers, calculators, and similar devices. The FCC emissions standard is more severe for Class B devices because they are more likely to be located close to radio and TV receivers in the home environment.¹

International standards

In Europe, the standards organization overseeing electromagnetic compatibility is the International Special Committee on Radio Interference, or CISPR. In 1997, the committee published the third edition of CISPR, Pub. 22, “Information Technology Equipment—Radio Disturbance Characteristics—Limits and Methods of Measurement,” commonly known as CISPR 22. The equivalent European reference number is EN55022. CISPR 22 has been adopted for use by most members of the European Community. Unlike FCC, CISPR 22 differentiates between Class A and Class B equipment and establishes conducted and radiated emissions for each class. In addition, CISPR 22 requires certification over the frequency range of 0.15 MHz to 30 MHz for conducted emissions.

To ensure that either standard, with minor exceptions, can be used to certify digital electronic equipment, FCC Part 15 and CISPR 22 have been made to be harmonious. However, harmonization demands that same standard must be utilized for both conducted and radiated emissions. According to the CUI white paper, conducted and radiated emission limits specified in FCC Part 15 and CISPR 22 are within a few dB of each other over the prescribed frequencies. Hence, using either set of limits does not compromise accuracy of the measurement and certification process. Nevertheless, measurements made above 1000 MHz must be in accordance with FCC rules and limits because the European CISPR 22 has no specified limits for frequencies above 1000 MHz¹. Also, since FCC limits are specified in μV while CISPR limits are provided in dBμV, it is necessary to convert the units of one to the other for direct comparison.

The specified limits for the FCC Part 15 are presented in Table 1. Table 2 provides similar limits for CISPR 22. Class B limits are more restrictive than Class A by a factor of 3 (~10 dB).

FCC Class A Conducted EMI Limit
Frequency of Emission (MHz) Conducted Limit (μV)
0.45 - 1.6 1000
1.6 - 30.0 3000
FCC Class B Conducted EMI Limit
Frequency of Emission (MHz) Conducted Limit (μV)
0.455 - 1.6 250
1.6 - 30.0 250
FCC Class B 3-Meter Radiated EMI Limit
Frequency of Emission (MHz) Field Strength Limit (μV/m)
30 - 88 100
88 - 216 150
216 - 1000 200
above 1000 200
FCC Class A 10-Meter Radiated EMI Limit
Frequency of Emission (MHz) Field Strength Limit (μV/m)
30 - 88 90
88 - 216 150
216 - 960 210
above 960 300

Table 1: Field strength limits for conducted and radiated emissions per FCC Part 15 regulations.


CISPR Class A Conducted EMI Limit
Frequency of Emission (MHz) Conducted Limit (dBμV)
Quasi-peak Average
0.15 - 0.50 79 66
0.50 - 30.0 73 60
CISPR Class B Conducted EMI Limit
Frequency of Emission (MHz) Conducted Limit (dBμV)
Quasi-peak Average
0.15 - 0.50 66 to 56* 56 to 46*
0.50 - 5.00 56 46
5.00 - 30.0 60 50
CISPR Class A 10-Meter Radiated EMI Limit
Frequency of Emission (MHz) Field Strength Limit (dBμV/m)
30 - 88 39
88 - 216 43.5
216 - 960 46.5
above 960 49.5
CISPR Class B 3-Meter Radiated EMI Limit
Frequency of Emission (MHz) Field Strength Limit (dBμV/m)
30 - 88 40
88 - 216 43.5
216 - 960 46.0
above 960 54.0
*Decreases with the logarithm of the frequency.

Table 2: Field strength limits for conducted and radiated emissions per CISPR 22 regulations.

As shown, FCC conducted emission limits are specified for frequency ranges of 0.45-1.6 MHz and 1.6-30 MHz. While FCC radiated emission limits are specified for frequency ranges of 30-88 MHz, 88-216 MHz, and 216-1000 MHz at a fixed measuring distance of 3 meters. These limits apply to both systems with SMPS installed and SMPS in stand-alone applications.

EMI compliance

To prevent SMPS generated noise and RF signals from reaching other components in the system or other equipment in the immediate vicinity, power supply manufacturers have been designing converters with built-in filters to minimize EMI interference so that they are in compliance with both regulatory agency’s standards.

When SMPS supplies, both AC/DC and DC/DC, meet FCC Part 15 or EN55022/CISPR 22 EMI standards, manufacturers ensure that design engineers are made aware of the EMC compatibility of the product by highlighting the fact in the compliance/safety section of the product datasheet. As an example, consider CUI’s single-output 120 W, universal input AC/DC converter VSBU-120 series with built-in EMI filtering. Since this series is compliant with both FCC Part 15 and CISPR 22 for Class B equipment, the maker clearly underscores this feature in the safety section of the products’ datasheets.

Specifically, if you open the datasheet of a member from this series, for example VSBU-120-48 with 48-V output, and go to the safety specifications section, you will notice that the manufacturer specifies EMI compliance with CISPR 22 at 220 VAC and FCC Part 15 at 110 VAC, for Class B equipment.

Many DC/DC converters require external EMI filtering to comply with these standards. Both active and passive filters are available. Vicor, for example, offers its QPI family of active EMI filters that provide conducted common-mode (CM) and differential-mode (DM) attenuation from 150 kHz to 30 MHz to comply with CISPR 22 specs.

Mitigation techniques

As noted in the white paper, other mitigation techniques can be applied. For example, feed and return loops should be kept close on wide bandwidth signal lines to minimize radiated emissions. Additionally, signal lines carrying RF or near-RF signals should be properly terminated to reduce reflection at the termination. Ringing and overshoot on these lines can also be minimized as a result of using the appropriate termination.

Reliable wiring connections should be implemented to and from the SMPS. Wiring must be of suitable size and be kept as short as possible, and wiring loops should be minimized. Avoid running input or output wirings near power devices to prevent noise pick up. Also ensure all grounding connections are made and properly secured. Earth ground wires should be kept as short as possible. If the circuit or system operations induce current transients, it is very important to have local decoupling capacitors to supply the pulsed current locally, instead of letting the pulsed current propagate up-stream to the supply.

In addition, PCB high impedance runs that contribute to EMI can be mitigated by using wide PCB metal stripes to decrease the impedance of power lines. Where possible, signal tracks should be designed considering their propagation delay vs. signal rise/fall time and include a ground and a power plane. Slit apertures in PCB layout should be strictly avoided, particularly in ground planes or near current paths to reduce unwanted antenna effects. Board metal stripes should be kept as short as is practical, and metal stubs, which can cause reflection and harmonics, should be avoided.

Further, floating conductor areas can act as a source of radiated emissions, so their use should be avoided except for overriding thermal considerations. And solid state components on the PCB should be decoupled close to chip supply lines to reduce component noise and power line transients.

In summary, switched mode power supplies can generate EMI because of their inherent design. Domestic and international regulatory bodies regulate these emissions through promulgation of regulations such as the FCC Part 15 rules and the CISPR 22 standard. This article has presented field strength limits for conducted and radiated emissions from both organizations.

To meet these standards, SMPS suppliers offer design guidelines for suppression of noise, including input/output filter circuits and system design considerations. Best practices for EMI mitigation in regard power supply, signal line, printed circuit board (PCB) and solid-state components are also readily available. For more information on the products discussed here use the links provided to access product pages on the Digi-Key website.

Reference:
  1. White paper: “Electromagnetic Compatibility Considerations for Switched-Mode Power Supplies”, by Don Li, Chief Technical Officer, and Jeff Schnabel, VP of Marketing, CUI. Inc.

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