“With the continuous development of automotive Electronic control technology, the number of automotive electronic devices has greatly increased, the circuit operating frequency has gradually increased, and the power has gradually increased, which makes the automotive working environment flooded with electromagnetic waves, resulting in increasingly prominent electromagnetic interference problems. If it works normally, it will damage the corresponding electrical components.
With the continuous development of automotive electronic control technology, the number of automotive electronic devices has greatly increased, the circuit operating frequency has gradually increased, and the power has gradually increased, which makes the automotive working environment flooded with electromagnetic waves, resulting in increasingly prominent electromagnetic interference problems. If it works normally, it will damage the corresponding electrical components.
Therefore, automotive electronic products will involve a common problem – automotive electromagnetic compatibility technology. The automotive electromagnetic compatibility technology is related to the safety and reliability of the operation of the specific electronic system of the vehicle and its surrounding electronic systems, as well as the safety and reliability of the operation of the electronic control function. Such as electronically controlled automobile braking system, electronically controlled transmission system, electronically controlled steering system. Even affect the safety and reliability of the vehicle. Therefore, more and more attention is paid to the electromagnetic compatibility of automotive electronic equipment, and it is urgent to widely apply electromagnetic improvement technology for automotive electronic equipment.
Various automakers at home and abroad have formulated a series of immunity test standards for this environment. At the same time, in order to ensure a good electromagnetic compatibility environment, they also put forward a series of electromagnetic disturbance standards for products.
The following tables are the standards that foreign automotive electronics need to meet. The domestic ones are generally formulated according to foreign standards. At the same time, our domestic joint venture car manufacturers and our domestic related car manufacturers also follow the following standards, but there are slight differences in specific requirements.
The above standards can be divided into two categories, one is the external radio disturbance of the equipment, and the other is the external interference to our equipment, which requires our equipment to have certain anti-interference characteristics. Most of the products in these two aspects cannot pass the test smoothly and meet the requirements of vehicle manufacturers. So how to improve the immunity of our automotive electronic products and reduce their own external electromagnetic emissions to meet the requirements of various vehicle manufacturers? Below we make some detailed analysis on car DVD products.
Car DVD generally has FM and AM functions, and the DVD player must have a Display system, so it is generally equipped with a 7-inch LCD screen. In order to increase the practicality, manufacturers with relatively high design technology have equipped the DVD with the USB Host function and GPS navigation function, and the car DVD has finally become a car entertainment navigation system.
Due to the complex functions of the in-vehicle entertainment system, high-speed digital circuits are mostly used. We all know that due to the steep rising edge of the digital circuit and the rich harmonic components, the equipment will eventually cause strong radiation to the outside world; usually it cannot meet the requirements of CISPR25. How to reduce the external interference of the entertainment system, we also need to classify the external interference. In terms of the nature of the interference, the interference is mainly divided into two categories:
1. Conducted emission (emission along the external cable of the device)
For conducted emission, we generally have several ways to solve it:
A. Source suppression: Find the source of the emission, accurately locate a certain device and signal, and analyze and process it, mainly for clocks and high-speed clock lines;
B. Filtering is performed at the interface of the external cable. According to the frequency and nature of the interference, the device is selected for filtering. The commonly used devices are inductors, capacitors, etc.
2. Space launch (launch through equipment gaps or itself)
Our general approach to space launch is as follows:
A. It is a good method to suppress the space emission through the source, mainly through the filter circuit, interface processing, PCB layout, wiring, and multi-layer board design;
B. Strengthen the shielding of the equipment, and deal with the structural gaps of the equipment;
The above methods have advantages and disadvantages; for example, the consequences of reducing the source will lead to poor signal quality. When reducing the source, we must first ensure the normal and stable functions; adding filter components will not have a great impact on the signal quality (select components When it is necessary to select it in a targeted manner and filter it according to the actual signal frequency), it will lead to an increase in the cost of the equipment; therefore, we should use it flexibly according to our own situation in the process of operation;
How to improve the immunity of electronic equipment? Let’s first introduce the common phenomena and solutions of the vehicle DVD system immunity experiment:
1. The audio output speakers whistle
For the howling of the audio output speakers, we must distinguish the essential reasons of the howling. Most of the common speaker howling is because the interference signal is coupled to the internal circuit through the device interface cable (very few are due to the unreasonable design of the internal circuit of the audio power amplifier) , and then coupled to the input end of the audio power amplifier through various paths, causing the power amplifier to amplify and output the useless 1KHz interference carrier signal, resulting in howling; the solutions are generally as follows:
A. Find and cut the coupling path;
Finding the coupling path is a complex analysis process. It is necessary to comprehensively analyze various factors such as PCB layout, wiring, and schematic design. When necessary, it can be tested with an oscilloscope; generally, CS (conduction sensitivity test) can be used for BCI. simulation test;
B. Filter the signal port of the device so that the interference signal cannot enter the device;
For filtering at the signal port, common mode filtering is generally used for suppression (sometimes capacitors are used for filtering); when selecting common mode inductors, targeted selections must be made; if howling occurs at 60MHz, it is necessary to select effective filtering for 60MHz device;
2. The screen is garbled, shakes, the system crashes, and resets;
For these cases, we must focus on finding out the specific circuits that are disturbed, such as system reset. At this time, we must pay attention to the reset circuit, power circuit, control circuit, etc.; if the screen is garbled, we must pay attention to the video processing circuit, The control signal, logic signal, etc. of the video processing circuit; it can be observed with an oscilloscope, and finally the sensitive source can be found, which is generally analyzed according to the following two situations.
Class 1: interference injected from or coupled to the signal port of the equipment;
For this kind of interference, the most cost-effective way is to find the sensitive point of our equipment and improve the anti-interference ability of this point, but it takes a lot of time and energy, and it is necessary to have good theoretical knowledge of circuits and electromagnetic fields; In our current enterprises, when it comes to the EMC test, the time is usually very urgent, and there is not enough time for detailed analysis, positioning and testing of the equipment; therefore, it is a good choice to filter the signal interface of the equipment; Since we are very clear that the interference is directly conducted from the cable to the inside of the device, we can use EMC devices to filter the interference at the interface to prevent the interference from being transmitted to other circuits on the board; as long as the interference is filtered out at the signal interface, our device will appear. The problem will also disappear; this method often leads to a certain increase in cost; so this method needs to be considered in combination with the actual situation;
Category 2: Interference coupled from the gap of the device to the inside of the device;
From the interference of the gap coupling, the gap can be directly “blocked” for testing; generally, the immunity problem caused by the gap can focus on the gap. If you need to find the sensitive source, you need to analyze the actual test phenomenon;
For automotive electronic products, to pass the strict requirements of automotive electronic electromagnetic compatibility standards, it is necessary to consider all aspects of structure, interface, single-board filter design, and PCB design.
The three elements of the EMC problem and the understanding of the three laws of EMC will make the EMC problem more regulated. Adhering to the law of EMC will save time and effort in solving the EMC problem, and achieve twice the result with half the effort.