An Engineer’s Guide to DC/DC Converter EMI – Part 1, Specifications and Measurements

“Most power supply applications must reduce electromagnetic interference (EMI) to meet the relevant requirements, and system designers must try various methods to reduce conducted and radiated emissions.

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Introduction

Most power supply applications must reduce electromagnetic interference (EMI) to meet the relevant requirements, and system designers must try various methods to reduce conducted and radiated emissions.

Compliance with Electromagnetic Compatibility (EMC) standards (eg, CISPR 32 for multimedia devices, CISPR 25 for automotive applications) is a very important task, which is closely related to product development costs and time-to-market.

For DC/DC converters, the switching voltage and current slew ratios (dv/dt and di/dt) that occur during switching transitions are improved, although faster switching power devices can increase switching frequency and reduce size , often causing increased EMI, causing problems throughout the system.

For example, Gallium Nitride (GaN) power devices switch extremely fast, resulting in a 10dB increase in EMI at high frequencies. EMI filters are an integral part of power Electronic systems and account for a relatively large proportion of the total volume and weight. Therefore, great attention must be paid to system EMI noise reduction and suppression, not only to meet EMC specifications, but also to reduce solution cost and increase system power density.

This article, the first in a series on EMI, reviews related standards and measurement techniques, with a primary focus on conducted emissions. Table 1 lists common abbreviations and nomenclature related to EMI.

IEC	International Electrotechnical Commission
CISPR 25	International Special Committee on Radio Interference, an IEC technical committee
EN 55022	European Standard, prepared by CENELEC and approved by the European Union (EU), is a modified version of the CISPR 22 derived standard
FCC Part 15	Federal Communications Commission; Part 15 Subsection B applies to unintentional radiators
ANSI C63.4	American National Standards Institute
CENELEC	European Electrotechnical Standards Committee
CE Mark	European Conformity Certification
ITE	Information Technology Equipment
EUT	equipment under test
OATS	open test field
ALSE	Shielded enclosure lined with absorber
SAC, FAR	Semi-anechoic room, fully anechoic room
LISN	Line Impedance Stabilization Network
AMN, AN	artificial power network, artificial network
AE	Auxiliary/Associated Equipment
CE, RE	Conducted Emissions, Radiated Emissions
CS, RS	Conducted susceptibility, radiation susceptibility
DM, CM	Differential Mode, Common Mode
RBW	Resolution bandwidth (of an EMI receiver/spectrum analyzer)
FFT	Fast Fourier Transform
PE, GW	Protective ground, green wire (both refer to ground or case ground)
dBmV, dBmA	0dBmV = 1mV, 20dBmA = 10mA

Table 1: Common acronyms, abbreviations, and units associated with EMI and EMC.

EMC Regulatory Specifications

EMC refers to the ability of a system or contained components to operate as required in its electromagnetic environment without causing out-of-tolerance electromagnetic interference to any equipment in the environment. Such interference can have serious consequences, so EMC provisions are established in various domestic and international regulatory codes.

In the EU region, power products sold in the communication market have generally adopted the EN 55022/CISPR 22 product standard for many years to meet compliance requirements in both conducted and radiated emissions. Power products outside the EU refer to this standard for CE compliance. Declaration (DoC) of compliance with EU EMC Directive 2014/30/EU.

Products designed for the North American market comply with FCC Part 15 limits. The IEC 61000-6-3 and IEC 61000-6-4 general EMC standards apply to light industrial and industrial environments, respectively.

However, the EN 55032 product standard has superseded EN 55022 (ITE), EN 55013 (broadcast receivers and related equipment) and EN 55103-1 (audio-visual equipment) with regard to emissions. This new standard officially became the harmonized radiation standard in compliance with the EMC Directive. More specifically, all products previously tested according to EN 55022 and shipped to the EU after March 2, 2017 must comply with the requirements of EN 55032.

With the EN 55022 standard withdrawn and replaced by EN 55032, power supply manufacturers and suppliers need to update their DoC certificates to the new standard to legally use the CE mark. Figure 1 shows EN 55022/32 Class A and Class B limits for conducted emissions using quasi-peak (QP) and average (AVG) signal detectors over the applicable frequency range from 150kHz to 30MHz.

Figure 1: EN 55022 Class A and Class B conducted emission limits using quasi-peak and average detectors.

For automotive end devices, the main driver for future EMC compliance will undoubtedly come from autonomous vehicles enabled by vehicle-to-vehicle communication. The CISPR 25 specification for “Onboard Receiver Protection” has set strict limits on conducted emissions, especially in the FM band (76MHz to 108MHz).

From a regulatory perspective, UNECE Regulation 10 replaced the European Union’s Automotive EMC Directive 2004/104/EC in November 2014, which requires manufacturers to obtain all vehicles, electronic components (ESAs), components and independent technical units type approval.

Conducted emissions for CISPR 25 tests are all measured in a specific frequency band within the frequency range of 150kHz to 108MHz. Specifically, the regulation frequency range is distributed among the AM broadcast, FM broadcast and mobile service frequency bands, as shown in the graph and table in Figure 2. Figure 2 also plots the relevant limit values ​​for CISPR 25 Class 5 (the most stringent requirement). Although the bandgap between frequency bands allows for higher noise spikes, automakers may choose to extend these frequency ranges based on their specific internal EMC requirements. These requirements are usually based on international IEC standards, changing only a few parameters for different tests or limits, the core content remains the same.

Figure 2: CISPR 25 Class 5 conducted emission limits.

To address the challenges posed by the CISPR 25 limits, especially in the FM band, note that the noise current of 18dBµV from a 50Ω measurement resistor is only 159nA.

Measuring conducted EMI

LISN measures conducted emissions from the EUT. It is the interface that plugs into the measurement point between the EMI source and the power supply, ensuring repeatability and comparability of EMI measurement results. Figure 3 shows an example according to CISPR 16-1-2 [12] Or the functional equivalent circuit of a standard 50µH LISN as defined by the ANSI C63.4 standard (not a full schematic).

LISN provides:

• Produces a calibrated stable source impedance over a given frequency range.

• In this frequency range, isolate the EUT and measurement equipment from the input power supply.

• Establish a safe and suitable connection to the measuring device.

• Measure the total noise level of the two lines individually, denoted by L and N in Figure 3.

Figure 3: Conducted emission measurements using a V-LISN.

In short, repeatable results can be obtained using a predefined test plan with known source impedance. NOTE: A LISN may contain one or more independent LISN circuits.

The essence of LISN is a pi filter network. The EUT is connected to the input power lines L and N through a low-pass Inductor-capacitor (LC) filter, as shown in Figure 3. The LISN inductance value is based on the expected inductance of the power cord in the ideal installation of the product.

CISPR 16 and ANSI C63.4 specify an inductance of 50μH for LISN, which corresponds to the inductance of a distribution wiring system of approximately 50 meters in telecommunication equipment. In contrast, CISPR 25 specifies a 5µH LISN, which corresponds to the approximate inductance of an automotive wiring harness.

The LISN provides a well-defined impedance to the noisy emission signal. LISN manufacturers often provide calibration curves that indicate nominal impedance over a specific measurement frequency range. Per CISPR 16-1-2, the allowable tolerances are ±20% amplitude and ±11.5° phase.

For measurements with an EMI receiver or spectrum analyzer, the noise signal can be obtained through a high-pass filter network (shown in Figure 3) with a coupling capacitor of 0.1 μF, a discharge resistor of 1 kΩ, and a termination resistor at the measurement port is 50Ω. Figure 4 shows the simulated impedance plot of (50μH + 5Ω) || 50Ω LISN over the frequency range of 150kHz to 30MHz.

Figure 4: Measured 50Ω, 50μH LISN nominal impedance characteristics at the port over a regulated frequency range of 150kHz to 30MHz.

CISPR 25 test setup for automotive applications

Figure 5 shows the CISPR 25 recommended conducted emission test setup. The standard defines how the system under test is handled as well as the measurement scheme and equipment. According to the CISPR 25 specification, LISN is designated here as AN. When the automotive power return line exceeds 200mm, the EUT is remotely grounded, and two ANs are required: the two are used for the positive power line and the power return line respectively. Conversely, if the automotive power return line does not exceed 200mm, the EUT is locally grounded and only one AN needs to be applied to the positive supply.

The AN is mounted directly above the reference ground plane, and the AN enclosure is connected to the ground plane. The power return line is also connected to the ground plane between the power supply and AN. Connecting an EMI receiver to the corresponding AN’s measurement port ensures successful measurement of conducted emissions on each power line. At the same time, the measurement port of the AN plugged into the other power cord is terminated with a 50Ω load.

Figure 5: Overview of the CISPR 25 conducted EMI test scheme (voltage method).

Figure 6 shows the CISPR 25 conducted emissions test chamber used for pre-compliance testing. The LISN is the blue case on the right with the Li-Ion car battery behind it and the DUT on the insulation on the left. For testing at a specific supply voltage (eg 13.5V), a variable voltage source is used to feed through the bulkhead from outside the chamber. The results are obtained at the line side (hot loop) and return side (ground) via the respective LISNs.

Figure 6: CISPR 25 conducted EMI test setup using two single-pole LISNs and copper ground planes.

Figure 7 shows the results of a typical CISPR 25 conducted EMI scan, with peak and average measurements in yellow and blue, respectively. We can see that the DC/DC converter operates quietly with conducted emissions well below the stringent Category 5 limits. This measurement technique changes above 30MHz because the RBW of the EMI receiver is adjusted from 9kHz to 120kHz, which may result in a change in the measurement noise floor.

Figure 7: Typical CISPR 25 conducted EMI measurement.

Summarize

Electromagnetic energy generated intentionally or unintentionally can cause electromagnetic interference to other equipment. Commercial products are required to minimize electromagnetic energy generated during normal operation.

Many regulatory agencies around the world specify levels of allowable conducted and radiated EMI from end products. Such emissions can be quantitatively analyzed using applicable measurement techniques so that appropriate action can be taken to meet regulatory compliance.

EMC requirements are generally related to the overall condition of the system being measured on AC power lines (and signal lines), and DC/DC converters are sub-components and do not have specific EMC limits. However, users can perform pre-compliance testing to determine if EMI is causing adverse effects.