Pass radiated emission test: compact isolation design without complicated EMI suppression technology

For various reasons, Electronic systems need to be isolated. Its function is to protect personnel and equipment from high voltages, or simply to eliminate unwanted ground loops on the PCB. It is a basic design element in a variety of applications, including factory and industrial automation, medical equipment, communications, and consumer products.

For various reasons, electronic systems need to be isolated. Its function is to protect personnel and equipment from high voltages, or simply to eliminate unwanted ground loops on the PCB. It is a basic design element in a variety of applications, including factory and industrial automation, medical equipment, communications, and consumer products.

Although isolation is essential, its design is also extremely complicated. When the control power and data signals pass through the isolation barrier, electromagnetic interference (EMI) is generated. These radiated emissions (RE) can negatively affect the performance of other electronic systems and networks.

For the circuit design with isolation, an important step is to transmit power across the isolation barrier and alleviate the generated RE. Although traditional methods may be effective, they often require trade-offs. This may include the use of discrete circuits and transformers to transmit power. This method is cumbersome and time-consuming, will take up valuable PCB space, and neither will increase the cost. A more cost-effective solution is to integrate the transformer and the required circuits into a smaller form factor, such as a chip package.

Although this can save board space and reduce design complexity and cost, it also makes the transformer smaller, with fewer windings, and requires a higher switching frequency (up to 200 MHz) to efficiently transmit the required power To the secondary side. At higher frequencies, parasitic common mode (CM) currents may capacitively couple from the primary side to the secondary side through the windings of the transformer. Due to the nature of the isolation barrier, there is no physical path for these CM currents to return to the primary side. The isolation barrier forms a dipole, radiating energy in the form of CM current and returning it to the primary side. This leads to another important consideration: compliance.

Electromagnetic compatibility (EMC) requirements

Before the product goes on the market, it must comply with EMC regulations. Integrating the transformer and the required circuits into a smaller package will generate EMI, so complex and costly RE suppression technologies are required to meet the requirements of electromagnetic compatibility (EMC) regulations.

EMC refers to the ability of an electronic system to work normally in its target environment without interfering with other systems. There are EMC regulations in different regions of the world to ensure that all products can work normally in the presence of other products. The radiation emission must be lower than the specified level corresponding to the target use environment and application. Therefore, EMC testing and certification has become an important part of the product launch process. Products sold in the EU need to have the CE mark, while products sold in the United States need to obtain FCC classification certification. In order to obtain these certifications, a set of EMC tests need to be performed on the system. In industrial, medical, communications and consumer environments, radiation emissions must generally comply with CISPR 11/EN 55011, CISPR 22/EN 55022 or FCC Part 15.

Pass radiated emission test: compact isolation design without complicated EMI suppression technology

Figure 1. Example of increased emission.

CISPR 11/EN 55011

This standard applies to equipment that generates radio frequency energy designed for industrial, scientific, and medical (ISM) purposes. Within the scope of the standard, the equipment may be divided into two groups. Group 2 contains all ISM RF devices that intentionally generate and locally use radio frequency energy. Group 1 includes all equipment that does not belong to Group 2 within the scope of this standard.

CISPR 22/EN 55022

This standard applies to information technology equipment (ITE) that meets the following conditions: The main function is to combine input, storage, Display, retrieval, transmission, processing, exchange or control of data and telecommunication information, and may be equipped with one or more terminal ports , Usually used to transmit information.

Under each standard, equipment is further classified, and each category is subject to a separate set of emission limits.

► Class A: equipment used in industrial applications and non-residential areas
► Category B: equipment used in residential environments

Since Class B restrictions cover residential (or light industrial) environments, and products in this environment are more likely to be very close to each other (within 10 meters of radio and television receivers), they are more stringent (10 dB lower than Class A) Many), so as not to cause interference problems.

Figure 2 shows the Class A and Class B limit lines related to CISPR 11/EN 55011 and CISPR 22/EN 55022. In this frequency range, compliance with CISPR 22/EN 55022 Class B standard means that it also meets CISPR 11/EN 55011 Class B standard.

Pass radiated emission test: compact isolation design without complicated EMI suppression technology

Figure 2. Radiated emission standards-limit lines.

Consider EMC at the beginning of the design cycle

According to reports, 50% of products fail in the first EMC test. This may be due to lack of relevant knowledge and failure to apply EMC design technology in the early stages of product design. If the EMC problem is ignored until the functional design is completed, it usually brings time-consuming and costly challenges. In addition, as the product development process continues to deepen, there are fewer and fewer technologies that can be used to solve EMC problems, because product changes will inevitably lead to planned overtime and increased costs.

To minimize design time and reduce project costs, EMC design at the beginning of the project is crucial. The selection and placement of components is also important. Incorporating devices that have already met industry standards into the selection and design can improve compliance.

EMI suppression technology: a better way is urgently needed

Compared with traditional methods using discrete transformers, integrating transformers and circuits into a chip-scale package can reduce the number of components, thereby greatly saving PCB space, but may introduce higher radiation emissions. Radiated emission suppression technology will make the design of the PCB more complicated or require additional components, so it may offset the cost and space saved by the integrated transformer.

For example, a common method to suppress radiated emissions at the PCB level is to form a low-impedance path from the secondary side to the primary side for the CM current, thereby reducing the RE level. To achieve this, you can use a bypass capacitor between the primary side and the secondary side. The bypass capacitor can be a discrete capacitor or an embedded mezzanine capacitor.

Discrete capacitors are the simplest solution and may be leaded or surface mount components. It also has the advantage of being suitable for 2-layer PCBs, but discrete capacitors are expensive and bulky, which will take up valuable PCB space, especially near isolation barriers where multiple components may be stacked.

Another less than ideal solution is to use embedded bypass capacitors, which are formed when the two sides of the PCB overlap (Figure 3). This type of capacitor has some very useful characteristics because the inductance of the parallel plate capacitor is extremely low, so it is effective in a larger frequency range. It can improve the emission performance, but because the layer thickness needs to be customized to obtain the correct capacitance, and the PCB requires four or more layers, the design complexity and cost will increase. In addition, isolation must be used to ensure that the spacing of the internal overlapping layers meets the minimum distance standard specified by the relevant isolation standards.

Pass radiated emission test: compact isolation design without complicated EMI suppression technology

Figure 3. The internal PCB bypass capacitor formed between the center power supply and the ground plane.

Bypass capacitors also allow AC leakage and transients to couple from one ground plane to another across the isolation barrier. Although the bypass capacitor is generally small, high-voltage and high-speed transients can inject a large amount of current across the isolation barrier through this capacitor. If the application needs to withstand harsh electromagnetic transients, such as electrostatic discharge, electrical fast transients, and surges, this must also be taken into consideration.

Whether it is discrete or embedded, the use of bypass capacitors is not an ideal suppression technique. Although it can help reduce radiated emissions, it comes at the cost of adding components, using complex PCB layouts, and improving transient sensitivity. The ideal suppression technique does not require the use of bypass capacitors, so it can reduce the cost and complexity of PCB design.

Eliminate the need to use complex suppression techniques

Ideally, the integrated isolated power supply component should include measures to reduce the chip’s radiation emission, without the need for additional external complex measures to ensure that it passes the system-level radiation emission test. In this way, you only need to place the components on the 2-layer board to pass the rigorous radiation emission test without having to make the circuit board multiple times.

Low radiation emission isolator

The use of low radiation emission isolation design technology can avoid a large amount of radiation emission, even on 2-layer boards without bypass capacitors. In order to reduce radiation emission, excellent coil symmetry and coil drive circuit help to minimize the CM current transmission through the isolation barrier. Spread spectrum technology is also used to reduce the noise concentration of a specific frequency and spread the radiated emission energy to a wider frequency band. Using low-cost ferrite beads on the secondary side will further reduce radiation emissions. During RE compliance testing, these technologies can improve peak and quasi-peak measurement levels.

Pass radiated emission test: compact isolation design without complicated EMI suppression technology

Figure 4 Concept ADuM5020 and ferrite characteristic curve.

Figure 4 shows the ferrite beads placed on the secondary side close to the VISO and GNDISO pins. The ferrite used to collect the radiation emission pattern in the next paragraph is Murata BLM15HD182SN1. These ferrites have high impedance over a wide frequency range (1800 Ω at 100 MHz, 2700 Ω at 1 GHz). These ferrites reduce the effective radiation efficiency of the dipole. As shown in Figure 5, because of the impedance of the ferrite bead, the CM current loop is reduced, and the effective length of the dipole is significantly shortened, so that the dipole efficiency is reduced and the radiation emission is reduced.

Pass radiated emission test: compact isolation design without complicated EMI suppression technology

Figure 5. Ferrite beads are used to reduce the effective dipole.

This solution is cost-effective, low complexity, small footprint, and excellent RE performance. If it is incorporated into the product design at the beginning of the design cycle, it will help meet the requirements of EMC regulations.

Results from the test room

According to CISPR 22/EN 55022 test guidelines, the test is carried out in a 10 m half-wave anechoic chamber. Figure 6 shows a typical 10 m test chamber. According to the standard, the evaluation PCB is placed on a non-conductive workbench 10 m away from the antenna calibration point. Make sure that there are no other conductive surfaces near the DUT, as this will affect the test results. Figure 7 shows the peak sweep used to determine the high transmit frequency of the DUT. After these points are located, quasi-peak measurements can be performed. During the quasi-peak measurement, the table will rotate 360°, and the antenna height will increase from 1 m to 4 m. Record the worst-case quasi-peak measurement results and compare them with the limit line requirements.

Make sure that no external devices, metal planes or cables will interfere with the DUT’s radiated emission test. To test the evaluation board, a battery with an on-board low dropout regulator is used to maintain a small power supply current loop and eliminate unnecessary wiring.

Due to the use of spread spectrum technology, pay attention to the energy diffusion in a wide frequency range. From the worst-case quasi-peak measurement value and the margin of the CISPR 22/EN 55022 Class B limit line, when the output power is 5 V (500 mW) and the load is 100 mA, the margin exceeds 5 dB. The quantity has passed the CISPR 22/EN 55022 test. This margin is very beneficial, and it is recommended to achieve this margin, because in different test facilities, the quality of the test room, calibration and equipment accuracy may be different, which may cause fluctuations in the measurement results. This is essential if the final product needs to be tested in a different testing room and must comply with the CISPR 22/EN 55022 standard.

Pass radiated emission test: compact isolation design without complicated EMI suppression technology

Figure 6.10 m test room image and evaluation PCB.

Pass radiated emission test: compact isolation design without complicated EMI suppression technology

Figure 7. Peak plot.

Reduce complexity and contradictions in isolation design

Designing an isolated power supply may be the most challenging aspect of the design process. Building a solution requires weighing various design requirements and must comply with the regulatory requirements of many different regions around the world. The resulting sacrifices often have a negative impact on size, weight, and performance, or reduce the ability to meet EMC standards.

In order to successfully meet EMC standards, devices that have passed industry standard verification can be used early in the design phase. EMC should be incorporated into the design process, not considered after the fact. The use of suppression techniques such as bypass capacitors will reduce the ability of electronic systems to resist transients and increase cost and design complexity.

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