What techniques are used for the successful EMC test of traditional car companies?

Preface

The automobile industry and every automobile company must meet various electromagnetic compatibility (EMC) regulations and EMC tests. For example, there are two requirements (EMI) or noise to ensure that the electronic system will not generate too much electromagnetic interference. It must be able to prevent the noise generated by other software. This paper studies these requirements and discusses some techniques and methods to ensure that the equipment design meets these requirements.

Overview of EMC requirements

CISPR25 is a standard that proposes several types of recommended limit detection standards to evaluate the radiation transmission formed by components to be installed on the vehicle. In addition to providing guidance for manufacturers, most manufacturers also have their own requirements for CISPR25 to fill in the guidance standard. The purpose of CISPR25 test is to ensure that the components installed in the vehicle will not affect other software in the vehicle.

CISPR25 requires that the electromagnetic noise level in the test room must be at least 66dB. Because CISPR wants the noise level to be as low as 18dB（ μ V/m) Therefore, an area below 12 dB is required（ μ V/m) ambient noise level. As a reference, this may be equivalent to the broadcast field strength of a typical AM radio station with an antenna spacing of 1km.

In today's environment, the only way to achieve this is to test in a unique room designed to shield the test environment and external magnetic field. In addition, since the normal budget must limit the size of the detection room to a certain extent, it is very important to prevent the adverse impact of signal reflection in the detection environment of the detection room. Therefore, some non-reflecting electromagnetic (EM) wave materials must be placed on the wall of the test room (Fig. 1). The cost of the testing room is very expensive, and it is usually rented within one hour. In order to control the cost, it is best to properly design the difficulties of EMC/EMI evaluation, and then conduct a successful test in the test room.

1、 Maintain small loop

When there is a magnetic field, the circuit formed by the conductive material acts as an antenna and converts the magnetic field into a flowing current around the circuit. The current intensity is proportional to the closed loop area. Therefore, the occurrence of circuits should be avoided as much as possible and the necessary enclosed area should be kept as small as possible. For example, when there is a differential signal, there may be a circuit. A circuit is formed between the transmitter and receiver, and a differential line is selected.

Another common circuit appears in the area where two subsystems apply the same circuit, perhaps a display and a module control circuit to promote display (ECU). There is a common ground (GND) line in the chassis and the GND connection line from the overall ECU end. When the video stream is transmitted to the display with its own ground wire, it will form a huge circuit on the ground plane. In some places, this kind of circuit is inevitable. However, although the inductor or ferrite bead is introduced into the ground connection line, the DC loop will still exist, but from the perspective of RF radiation, this loop has long been broken.

In addition, when the signal is transmitted according to the twisted pair cable, each pair of differential control boards/receivers will form a circuit. Generally speaking, because twisted pairs are connected through gaps, the circuit area of connecting cables is very small. However, once the signal reaches the circuit board, the gap connection should be maintained to avoid expanding the circuit area.

2、 Bypass capacitance is necessary

CMOS circuit is very popular because of its fast and extremely low operation. When CMOS needs to be charged and discharged, the circuit only consumes power when it changes and node capacitance. From the perspective of power supply, CMOS with an average flow consumption of 10mA may absorb many times more current during clock conversion, but the flow consumption during clock conversion is very low or even zero. Therefore, the radiation limit focuses on the peak value of voltage and current rather than the average value.

In the clock conversion process, the current surge from the power supply to the chip power supply pin is the main radiation source. According to the bypass capacitor around each power pin, the capacitor will directly provide the current required by the chip during the clock pulse edge. Then, in the middle of the clock conversion cycle, the charge in the capacitor is applied to the low and stable current accumulation. Larger capacitors are suitable for the rapid increase of current, but have weak response to fast standards. In particular, small capacitors can respond quickly to requirements, but their total charge capacity is limited and consumption speed is fast. For most circuits, the best solution is to mix capacitors of different specifications (maybe 1 μ F and 0.01 μ F Capacitors in parallel). The smaller capacitor can be placed in the area very close to the power supply pin of the equipment, and the larger capacitor can be placed in the area far away from the power supply pin.

3、 Shielding

Good shielding methods should be used. There is no shortcut to this point. When the design is designed to avoid radiation as much as possible, it is necessary to shield a part of the difficult circuit. Although it can still radiate energy, better shielding can capture radiation and send it to the road before they escape the system. The figure below shows how to control EMI shielding.

Shielding can be used in many ways. It may be easy to close the system in the guide motor housing, or it may be a small customized plastic housing welded to the radiation source.

4、 Better matching resistance can minimize resistance EMI

When a fast signal is transmitted according to the transmission line and the characteristic impedance changes on the transmission line, part of the signal should be reflected back to the signal source, and part of the signal will continue to transmit along the original direction. Reflection will cause radiation, which will not change at all. To complete low-level EMI, it is necessary to follow appropriate rapid design practices. There are more excellent resources to provide you with relevant transmission route customization information. The following are some preventive measures proposed when planning transmission routes:

Remember that there is a signal between the ground plane and the signal wiring. The radiation may be caused by the final fracture of the ground plane of the signal wiring, so attention should be paid to the damage or final fracture of the ground plane in the signal wiring. Try to reduce obtuse angle in signal wiring sequence. Exquisite angles are much better than oblique ones.

Generally, FPD-Link signal will allow part tapping; For example: coaxial power supply, power connection, AC lotus capacitor, this. In order to avoid reflection in the component as much as possible, you can try to use the widget of the 0402 model, and set the total width of the wire connection to the same total width as the welding layer of the 0402 component. In addition, the characteristic impedance of the wiring shall be set according to the dielectric thickness in the operation layer.

5、 Short ground wire

All current introduced into the chip will also be discharged from the chip again. Several methods recommended in the article show that the connection line of the chip must be short. For example, the bypass capacitor should be close to the IC, maintain a small circuit, etc. However, when the grounding current returns to its origin, the path that needs to be passed is often abandoned. Ideally, one layer of the circuit board is specially used for grounding. Until GND, this method is not much longer than the buried hole. However, some circuit boards have a wound on the ground plane, which pushes the ground current back to the power supply from the chip. GND When this method transmits current, it is used as an antenna to push or receive noise.

6、 The speed shall not exceed the specified level

There is a trend in the industry to worry about the time limit and use the logical device as fast as possible to obtain the best time limit. Unfortunately, very fast logic devices have steep pulse edges and high-frequency components, which will generate EMI. One measurement method to reduce system EMI is a logic device. Its speed is as low as possible, but it will still reach the time standard. One way for FPGA to reduce the edge rate is to allow the drive strength to be set at a lower level. In some places, the series resistance on the logic line can be used to reduce the signal conversion rate in the system.

7、 Power line inductance

In the second method, we discuss that the bypass capacitor can be used as a means to reduce current surge damage. The inductance in the power line is another level of the same problem. By arranging inductance or ferrite beads on the power line, the circuit that is forced to transmit to the power supply can meet the dynamic power requirements of the capacitor (not the power supply).

8、 Pay close attention to resonance

For various interference sources, inductors and capacitors need to be used to reduce the dv/dt and di/dt problems of EMI. However, inductors and/or capacitors have adverse characteristics independent of self-resonance. This can usually be corrected by increasing the resistance connected to the inductor. This resistance can absorb the kinetic energy formed by vibration, thus increasing it to a sufficient level, causing problems. When a series inductor leads to a component including a bypass capacitor (separate component or parasitic inductor from the power line), it will cause another hidden problem. The resulting L-C may have impact on the resonant frequency. Similarly, this situation can also be solved by the resistance (often in parallel with the inductance).

9、 The capacitor is arranged at the input end of the switching power supply

Automotive designers worry about the impact of AM electromagnetic band. Most vehicles are equipped with a high-gain amplifier whose tunable operating frequency band is 500kHz to 1.5MHz, which is particularly sensitive. If a component pushes a signal in this frequency band, it is probably AM I heard in the recorder. Many switching frequencies used for switching power supply are in this frequency band, which leads to problems in automotive applications. Therefore, the switching frequency of the switching power supply of most vehicles is higher than this frequency band - usually 2MHz or higher. If the input or output of the switching power supply is not worried enough, the switching noise will enter other subsystems sensitive to the fundamental frequency or sub-harmonic frequency.

10、 Peak radiation can be reduced by spread spectrum recording

For FPD-Link serializer or deserializer (SerDes), for other components, there are generally data bus and clock, and spread spectrum timing options. In spread spectrum recording, allocate clock signal. Therefore, due to the edge of the clock and signal pulse, the energy propagates in a wider working frequency band than the frequency band that must be occupied. Since the EMI specification is set to limit the peak radiation of all frequencies in a frequency band, spreading noise in a wider frequency band is conducive to greatly reducing the noise peak. DS90UB914A-Q1 is a good example of a string resolver, which is often used with DS90UB913A-Q serializer. This kind of equipment is used for excellent driving assistance system (ADAS) video link between camera and processor. The decoder is responsible for the clock provided by the image sensor in the camera to the serializer, and exports the clock Cpu with the data for use. 10 or 12 fast data lines and fast clock simultaneously change one source of operation EMI. To reduce this EMI, the DS90UB914A has the option of using the spread spectrum clock and exporting data (instead of the low vibration clock provided by the image sensor). The spread spectrum clock operates according to the memory in the demultiplexer.

Epilogue

As vehicles increasingly rely on electronic products to achieve not limited to entertainment and comfort, but not limited to entertainment and comfort, it is necessary to complete the correct operation and the impact of other software. According to the technology and method briefly described in this paper, select the appropriate components for EMC testing, and engineers can design a stable system to make the vehicle system not affected EMI affected by problems, and work reliably.