EMI Testing: Peak, Quasi-Peak and Average

To comply with safety regulations, all consumer electronic products must undergo electromagnetic interference (EMI) testing. Ensuring that EMI tests meet regulatory standards is crucial for bringing power converters to market. Designers conduct preliminary EMI measurements of power converters in their own laboratories, allowing them to detect potential issues early and reduce the costs associated with third-party certification testing. This article introduces three waveform measurements defined by standards: Quasi-Peak (QP), Average (AV), and Peak (PK), and explains their significance to help users better understand EMI test results.

 

Introduction

Electromagnetic compatibility (EMC) refers to a device or system's ability to operate in its electromagnetic environment without being affected or causing unacceptable electromagnetic interference to other devices. EMC testing evaluates EMI generated by a device during operation, including conducted emissions (CE) and radiated emissions (RE). CE testing requires an EMI receiver, a line impedance stabilization network (LISN), and an EMI filter. RE testing, on the other hand, must be conducted in a semi-anechoic chamber (SAC) free from electrical interference. The SAC consists of absorbing materials, a non-conductive turntable for the device under test (DUT), a preamplifier, a spectrum analyzer, an EMI receiver, and a broadband antenna. A schematic diagram of the RE laboratory setup is shown in Figure 1.

Additionally, EMC includes a device's ability to resist interference in an EMI environment, known as electromagnetic susceptibility (EMS). EMS tests include electrostatic discharge (ESD) immunity, electrical fast transient (EFT) immunity, and surge immunity.

To meet market demands, power converters are becoming increasingly compact. However, reducing the size of magnetic components (such as transformers and inductors) requires increasing the switching frequency. Higher frequencies lead to greater conduction, switching, and drive losses, potentially causing severe EMI issues.

 

Safety Regulations for CE and RE

The International Special Committee on Radio Interference (CISPR) has introduced CISPR 32/EN 55032, replacing the previous EN 55022 standard. It specifies CE and RE limits for Class A and Class B devices. The figure below illustrates CE limits for Class A and Class B devices in the frequency range of 150 kHz to 30 MHz under EN 55032.

EMI can degrade or even damage surrounding electronic systems. For example, it may disrupt automotive electronics, potentially leading to the failure of anti-lock braking systems (ABS). Therefore, during the design phase of power converters, engineers must adhere to regulatory guidelines to ensure that their products do not interfere with other equipment.

RE testing involves three key requirements: test distances of 3 meters and 10 meters for the 30 MHz to 1000 MHz frequency range, defined distances between the DUT and the test setup, and insulated mounting on a ground plane. The figure below shows RE limits under EN 55032 for Class A and Class B devices using QP detectors in the 30 MHz to 1000 MHz frequency range.

EN 55032 applies to multimedia equipment with a rated RMS power voltage not exceeding 600 V. CE and RE limits vary based on the device's intended environment: Class B devices (blue) for residential use and Class A devices (red) for other environments. Both must comply with QP and AV limits.

 

PK, QP and AV features

EMI receivers measure EMI levels using three standard parameters: PK, QP, and AV. These parameters are chosen based on different application needs. The figure below illustrates these three measurement values.

For EMI testing, regulatory limits are selected based on product applications. The following diagram uses Class B QP and AV limits as reference lines to determine compliance.

(1) Peak Detection (PK):

PK represents the highest electromagnetic emission level recorded within a unit of time for each harmonic frequency. It is used to measure the most severe EMI signal emitted by a device. PK values are typically higher than QP and AV values, and PK-based regulations are the strictest. PK detection is particularly useful for evaluating EMI under extreme conditions. For example, military equipment testing primarily relies on PK measurements because transient or single-pulse interference could cause digital system malfunctions.

(2) Quasi-Peak Detection (QP):

QP represents the weighted average of EMI over a unit of time, applying a specific weighting factor based on amplitude and repetition rate. If a signal at a particular frequency appears frequently, its QP value will be higher. In the following figure, the pulse signal has a higher repetition rate compared to Figure 4, resulting in a higher QP value.

QP values closely approximate real-world EMI conditions and generally fall between AV and PK values. QP is commonly used for EMI testing of electronic products. For example, in industrial motor EMI testing, PK may exceed regulatory limits due to its sensitivity to transient interference. However, since such interference occurs infrequently, QP detection—factoring in signal repetition rates—often remains within legal limits. Therefore, even if a product fails PK testing, it may still pass compliance requirements based on QP and AV values.

(3) Average Detection (AV):

AV represents the average EMI level over a frequency range, emphasizing frequently occurring interference signals while minimizing the impact of sporadic peak disturbances. AV measurements require a longer sampling period to determine an average level of EMI noise. Since AV values are typically lower than QP values, AV detection is suitable for assessing EMI in continuous operation or stable conditions, such as evaluating a device’s normal working state.

 

Practical Testing Scenarios

In practical EMI tests, PK detection is used to identify the worst-case interference scenarios, while QP and AV measurements help assess the overall EMI performance of a power converter.

To ensure compliance, measurement uncertainties should be considered, and a safety margin of at least 6 dB below regulatory limits is recommended. The power converter specifications used in this test are listed in the table below.

Table 1. Power converter specifications

	DC-DC Converter
Output Power	30 W
Input Voltage	24 V
Output Voltage	3.3 V

Figure 7(a) shows the CE test results for the power converter. The highest PK value (blue line) is approximately 30 dB below the QP limit, and the highest AV value (green line) is also about 30 dB below its limit. In Figure 7(b), the RE test results indicate that the PK value is approximately 10 dB below the regulatory limit. These results suggest a high probability of passing compliance testing on the first attempt.

Among the three detection methods, PK produces the highest measurement values. If PK measurements fall below the regulatory limits, the product is considered compliant. However, even if a product barely meets PK limits, conducting additional QP and AV tests can help identify potential issues and ensure EMC compliance under various conditions.

 

Conclusion

For EMI testing, using a spectrum analyzer with software that supports QP, PK, and AV detection is essential. QP detection accounts for noise pulse energy and frequency, AV detection provides an average value over multiple pulse measurements, and PK detection captures the highest peak interference level. This approach ensures that the worst-case EMI scenarios are considered.

When evaluating EMC, all three detection methods should be considered to ensure compliance across different conditions and to provide an accurate assessment of a product’s regulatory performance.

 

 

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