Onboard Charger EMI Compliance: Meeting CISPR 25 Class 5

What Is CISPR 25 Class 5?

CISPR 25 Class 5 represents the most stringent electromagnetic interference (EMI) standards for onboard chargers (OBCs) in electric vehicles (EVs). This standard, outlined in CISPR 25:2021 Edition 5.0, focuses on limiting both conducted and radiated disturbances over a frequency range from 150 kHz to 2.5 GHz. The primary objective is to protect vehicle receivers, including AM/FM radios and cellular bands, from interference.

How Do OBCs Typically Achieve CISPR 25 Class 5 Compliance?

OBCs, critical to charging EV traction batteries, must adhere to these standards while operating in various modes, as defined in IEC 61851-1:2017. These include Charging Mode 3 (AC Electric Vehicle Supply Equipment with communication) and Mode 4 (DC fast charging). Testing for compliance involves using Line Impedance Stabilization Networks (LISNs) or Artificial Networks (ANs) for conducted emissions, and semi-anechoic chambers for radiated tests.

Why Is EMI Compliance Crucial for Onboard Chargers?

High switching frequencies in compact OBCs can exacerbate EMI emissions, which, if unchecked, may interfere with critical vehicle systems. Therefore, achieving compliance with CISPR 25 Class 5 is crucial to ensure the proper functioning of on-board electronics and to prevent disruptions in vehicle communication systems.

What Are the Key CISPR 25 Class 5 Requirements for OBCs?

Frequency Range and Emission Types

CISPR 25 Class 5 covers a broad frequency range from 150 kHz to 5925 MHz, with particularly tight limits below 5 MHz. This ensures protection for sensitive systems such as radios and cellular devices.

Conducted Emissions

Conducted emissions are typically measured using voltage methods (AN/LISN) or current probes. OBCs often face challenges due to switching harmonics from high-frequency converters, such as those operating around 2.2 MHz.

Radiated Emissions

Radiated emissions are evaluated in semi-anechoic chambers, with stripline methods provided as alternatives. Proximity to sensitive systems requires stringent limits, with Class 5 demanding emissions to be 6 dB below the lowest measurable noise.

Limits Structure

Class 5 represents the strictest limits among the five classes defined, and is often necessary for OBCs located near antennas or radios, particularly during load transitions and various charging modes.

What Are Effective EMI Mitigation Strategies for Class 5 Compliance?

Filtering and Layout Optimization

Implementing multi-stage LC filters that are specifically tuned for conducted emission peaks is essential. For instance, Texas Instruments offers the TIDT203 reference design featuring the TPS25850-Q1 DC/DC converter, which demonstrates successful Class 5 compliance through optimized filtering and layout.

Strategic PCB layout is also crucial. Minimizing loop areas, employing symmetric routing, and shielding critical terminations such as CAN lines can prevent common EMI failures in high-voltage components like inverters and DC-DC converters.

Component Selection

Selecting components that inherently meet EMI standards can simplify the compliance process. For example, Diodes Incorporated's DC-DC converters are designed to limit radiated emissions, meeting automotive CISPR 25 standards.

How Can Manufacturers Support Compliance with Reference Designs?

Manufacturers like Texas Instruments and Diodes Incorporated provide reference designs to streamline the compliance process. The TIDT203 design, for example, has been tested to meet the full standards of CISPR 25, offering a practical example of successful EMI mitigation.

What Is the Typical Certification Process?

Pre-compliance testing is a critical step before pursuing formal certification at accredited labs, which typically takes 3 to 6 months. This process includes compiling test reports, construction files, and Declarations of Conformity across all operational modes. Non-compliance can result in significant issues, such as interference with CAN lines, leading to potential system shutdowns.

FAQ

What is the frequency range covered by CISPR 25 Class 5?

CISPR 25 Class 5 covers frequencies from 150 kHz to 5925 MHz, focusing on both conducted and radiated emissions.

How does shielding help in meeting EMI compliance?

Effective shielding, such as using PCB-level EMI shield cans, helps contain and mitigate emissions, thereby supporting compliance with stringent standards like CISPR 25 Class 5.

What are common challenges in achieving CISPR 25 Class 5 compliance?

Challenges typically include managing high-frequency switching harmonics and ensuring effective shielding and layout optimization to prevent emissions from crossing sensitive thresholds.

Why is pre-compliance testing important?

Pre-compliance testing helps identify and address potential EMI issues early in the design process, reducing the risk of costly redesigns and certification delays.

How can POCONS USA products assist in EMI compliance?

POCONS USA offers a range of PCB-level EMI shielding components, such as shield cans and spring contacts, that can help reduce emissions and support compliance with standards like CISPR 25.