When you specify a power supply, input voltage, output voltage and power rating are only part of the picture. Electromagnetic compatibility (EMC) matters too, particularly when the PSU will sit inside a larger electronic or industrial system.

A power supply can generate electromagnetic noise that affects other equipment, and it can be affected by disturbances from its surroundings. Poor EMC performance can lead to interference, unreliable operation, failed compliance testing and the cost of redesign.

Considering EMC requirements early reduces the possibility of these problems. It helps you select a PSU that suits the complete application, not just the electrical specification.

What is an EMC power supply?

Electromagnetic compatibility is the ability of electrical and electronic equipment to operate correctly in its intended environment without causing unacceptable disturbance to other devices.

For a power supply, EMC has two sides:

• Emissions: electromagnetic energy generated by the PSU that could interfere with other equipment.
• Immunity: the PSU’s ability to continue to function correctly when exposed to disturbances from other sources.

Both matter. A unit with low emissions may still be vulnerable to external disturbances, while a highly immune PSU could still produce excessive noise.

In Great Britain, the Electromagnetic Compatibility Regulations 2016 cover both emissions and immunity. Equipment sold in the EU must meet the equivalent EMC Directive (2014/30/EU).

Why does a power supply generate electromagnetic interference?

A switch-mode power supply converts energy efficiently by switching current and voltage on and off at high frequency. Those fast switching edges produce unwanted electrical noise across a wide range of frequencies.

That noise can leave the PSU in two ways:

• Conducted noise travels along connected cables, including the AC mains input, DC output and ground connections.
• Radiated noise travels through the air and can couple into nearby cables, circuit boards or other equipment.

From an EMC perspective, the PSU is never isolated. Its input and output connections, enclosure, cables, grounding and neighbouring components all influence the system’s final EMC performance.

Common-mode and differential-mode noise

Conducted noise is often referred to as either differential-mode or common-mode.

Differential-mode noise flows out on one conductor and returns on another, such as line and neutral. Common-mode noise flows in the same direction on both conductors and returns through ground, often via stray capacitance.

Each type is attenuated differently. X capacitors mainly address differential-mode noise. Common-mode chokes and Y capacitors target common-mode noise.

Power supply EMI vs EMC: what’s the difference?

Electromagnetic interference (EMI) is the unwanted disturbance itself.

Electromagnetic compatibility (EMC) is the wider requirement that equipment can coexist with other devices. It covers both what the equipment emits and what it can withstand.

EMI is the problem. EMC is the requirement that addresses both the noise a PSU produces and its resistance to noise from elsewhere.

What are the main power supply EMC requirements?

The exact requirements depend on the equipment type, application, market and the relevant standards and certifications. Most designs need to address four areas.

Conducted emissions

Conducted emissions are high-frequency currents that flow from the power supply into connected wiring. On an AC-input PSU, this includes noise travelling back into the AC mains.

Input filtering reduces these emissions so they stay within the defined emission limits. Conducted emissions can also be relevant on DC input and output connections, depending on the design and standard.

Radiated emissions

Radiated emissions are electromagnetic energy released into the surrounding environment.

The PSU can be a direct source, but connected cables often radiate more than the unit itself because they act as unintended antennas. Switching frequency, PCB layout, enclosure design, grounding and cable routing all affect radiated performance.

Harmonics, flicker and voltage fluctuations

AC-input power supplies also affect the quality of the mains supply shared with other equipment.

A switching PSU without power factor correction draws current in short peaks rather than a smooth sine wave. This introduces harmonics onto the line. IEC/EN 61000-3-2 sets harmonic current limits for equipment drawing up to 16 A per phase, and many AC/DC power supplies above 75 W use active power factor correction (PFC) to meet them.

Sudden changes in current draw can cause voltage fluctuations on the supply. These may produce visible flicker in lighting located on the same circuit. IEC/EN 61000-3-3 sets limits for these effects.

Immunity

An EMC-compliant power supply must also withstand disturbances present in its operating environment. Common examples include:

• Electrostatic discharge (ESD)
• Electrical fast transients and bursts
• Surge voltages
• Voltage dips, short interruptions and variations
• Conducted RF disturbances
• Radiated electromagnetic fields

What does immunity testing involve?

Immunity testing follows the IEC 61000-4 series of international standards. Each one defines a reproducible method to simulate a particular disturbance:

• IEC 61000-4-2: electrostatic discharge, simulating a discharge from a person touching the equipment
• IEC 61000-4-3: radiated RF electromagnetic fields
• IEC 61000-4-4: electrical fast transients and bursts on supply, signal, control and earth ports
• IEC 61000-4-5: surge voltages caused by lightning or switching transients
• IEC 61000-4-6: conducted disturbances induced by RF fields
• IEC 61000-4-11: voltage dips, short interruptions and variations on AC inputs (IEC 61000-4-29 covers DC inputs)

These standards set out how each test is performed. The test levels and pass criteria come from the standard that applies to the finished equipment. An industrial environment usually calls for higher immunity levels than a residential one.

How are conducted and radiated emissions measured?

Conducted emissions testing

Conducted emissions are measured using a line impedance stabilisation network (LISN), also called an artificial mains network. The LISN presents a defined, repeatable impedance to the equipment and passes the noise to the measuring receiver.

Under CISPR 32 and EN 55032, the measured frequency range is 150 kHz to 30 MHz. Results are compared against the emission limits for the relevant class.

Radiated emissions testing

Radiated emissions are measured with an antenna, usually from 30 MHz upwards, in a semi-anechoic chamber or on an open-area test site. The chamber minimises reflections and blocks external signals, so the equipment’s own emissions can be measured accurately.

A PSU can pass one test and fail the other, so both need checking.

EMC Class A vs Class B: what’s the difference?

Class A and Class B define different emission limits for different environments. CISPR 32 and EN 55032 use these classes for multimedia equipment. EN 55011 applies them to industrial, scientific and medical (ISM) equipment.

Class B limits are stricter and apply to equipment used in residential environments. Class A limits apply to equipment intended for commercial or industrial settings. Class A equipment must carry a warning that it may cause radio interference in a residential environment.

The same two classes appear outside Europe. In the US, FCC Part 15 uses Class A and Class B for digital devices, with Class B covering equipment marketed for residential use. In the UK and EU, meeting the relevant EMC requirements forms part of UKCA and CE marking.

A Class A power supply isn’t automatically unsuitable. What matters is whether its classification matches the finished equipment and where it will be used. If you’re not sure, establish the applicable standard and class before you select the PSU.

Which EMC standards apply to power supplies?

Several EMC standards can apply, depending on the product and its market:

• EN 61204-3: EMC requirements for low-voltage power supplies
• EN 55032 / CISPR 32: emissions for multimedia equipment
• EN 55011 / CISPR 11: emissions for ISM equipment
• EN 61000-6-1 to -6-4: generic immunity and emission standards for residential and industrial environments
• EN 60601-1-2: EMC for medical electrical equipment

The standard that matters most is the one that applies to your finished product. The PSU should support it, not replace it.

What EMC information should you look for on a PSU datasheet?

A power supply datasheet should show which EMC tests the unit has passed. Check:

• Emission standards and class: for example, EN 55032 Class B for conducted and radiated emissions
• Immunity standards and test levels: which IEC 61000-4 tests were performed, and at what level
• Performance criteria: whether the PSU kept operating normally during the test or recovered afterwards
• Test conditions: input voltage, load, grounding and any external filtering used during testing

The absence of a test on the datasheet doesn’t mean the PSU would fail it, but it does mean you can’t assume it passes.

Ideal Power datasheets include EMC information within the technical specifications. Some AC/DC PSU ranges specify EN 55032 Class B emissions alongside immunity to IEC/EN 61000-4-2, -4, -5, -6 and -11.

Can a power supply affect the EMC performance of the whole system?

Yes. A PSU’s own EMC compliance doesn’t guarantee that the finished product will pass.

Conditions during system testing can differ significantly from those used to assess the PSU alone. Cables can carry common-mode noise or radiate energy. Poor grounding creates unwanted current paths. PCB layout can increase coupling between noisy and sensitive circuits.

Load matters too. A power supply may behave differently at light, typical and full load, because its switching frequency, current levels or control mode can change.

Treat the PSU’s EMC specification as one part of the overall system design, not as cover for the finished product.

Why consider EMC early in the design?

EMC issues are harder and more expensive to fix once a design is finished. A PSU may perform well in isolation and still cause problems once integrated.

When selecting a power supply, review:

• Input and output voltage
• Maximum output current and power
• Applicable EMC standards and emission class
• Conducted and radiated emissions
• Harmonic current requirements
• Immunity performance and test levels
• Operating environment
• Cable, grounding and enclosure arrangements

Pre-compliance testing during development can reveal emissions or immunity problems while there’s still time to change layout, cable routing, filtering or grounding.

What happens if EMC problems are found?

Start by identifying the source and the path the noise is taking. A spectrum analyser with a near-field probe can help locate problem frequencies and determine whether the issue is mainly conducted or radiated. Formal compliance testing uses an EMI receiver with defined detectors, but a spectrum analyser is well suited to diagnosis.

Possible fixes include extra filtering, better cable routing, changes to grounding or shielding, or PCB layout adjustments. Sometimes selecting a power supply with stronger EMC performance is the most effective option.

Finding these issues during development reduces the risk and cost of late changes.

How can engineers reduce EMC issues?

Address EMC at the source and select components that support the required performance:

• Choose a PSU with suitable emission limits and immunity levels
• Follow the manufacturer’s recommended EMC circuit where additional filtering is needed
• Route cables to minimise coupling, and keep input and output wiring apart
• Use appropriate grounding and shielding
• Keep switching circuits away from sensitive components
• Add filters or suppression components where required
• Test early, before final compliance testing

Some Ideal Power power supplies include a recommended EMC circuit on the datasheet for applications with higher requirements. Follow the manufacturer’s guidance for the specific product.

Why choose Ideal Power?

Ideal Power supplies a wide range of power supplies and power conversion products for applications where electrical performance and compliance both matter.

We help you look beyond the headline power rating by considering input and output requirements, EMC performance, relevant standards, operating environment and mounting arrangement.

That support is particularly useful when you’re designing a PSU into a new product, or replacing an existing unit without compromising EMC performance.

If you’re not sure which power supply suits your application, speak to Ideal Power about your requirements.