With the continuous development of automotive electronic control technology, the number of automotive electronic equipment has increased greatly, the operating frequency of the circuit has gradually increased, and the power has gradually increased, which makes the working environment of the automobile full of electromagnetic waves, which causes the electromagnetic interference problem to become increasingly prominent, and affects the electronic equipment. If it works normally, it will damage the corresponding electrical components.
Therefore, automotive application electronics will involve the common problem - automotive electromagnetic compatibility technology. Automotive EMC technology is concerned with the safety and reliability of the operation of automotive-specific electronic systems and their surrounding electronic systems, and is related to the safety and reliability of the operation of electronic control functions. Such as electronically controlled car brake system electronic control transmission system, electronically controlled steering system. It even affects the safety and reliability of the entire vehicle. Therefore, the electromagnetic compatibility of automotive electronic equipment is receiving more and more attention. At present, it is urgent to apply electromagnetic improvement technology for automotive sub-devices.
Various car manufacturers at home and abroad have developed a series of immunity test standards for this environment. At the same time, in order to ensure a good electromagnetic compatibility environment, a series of electromagnetic disturbance standards have been proposed for the products.
The following table is the standard that foreign auto electronics need to meet. Domestically, it is also based on foreign standards. At the same time, our domestic joint venture automakers and our domestic related automakers 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 can not be successfully tested and meet the requirements of the vehicle manufacturers. So how to improve the self-interference of our automotive electronics products and reduce their external electromagnetic emissions to meet the requirements of various vehicle manufacturers? Below we make some detailed analysis of the car DVD products.
Car DVDs generally have FM and AM functions, and DVD players must have a display system, so they are usually equipped with a 7-inch LCD screen. In order to increase the practicability of the manufacturers with higher design technology, the USB Host function and the GPS navigation function are provided on the DVD, and the car DVD eventually becomes the in-vehicle entertainment navigation system.
Due to the complex functions of the in-vehicle entertainment system, most of them use high-speed digital circuits. As we all know, because the rising edge of the digital circuit is steep, the harmonic components are rich, and eventually the device is very strong against the outside world; usually it can not meet the requirements of CISPR25. How to reduce the external interference of the entertainment system, we also need to classify the external interference. From the nature of the interference, the interference is mainly divided into two categories:
1. Conducted emission (transmitting along the external cable of the device)
For conductive emissions we generally have several ways to solve:
A. Source suppression: Find the source of the transmission, accurately locate a device, signal, and analyze and process it, mainly for clock and high-speed clock lines;
B. Perform filtering processing on the external cable interface. For the frequency of interference, the nature of the selected device is used for filtering. Commonly used devices include inductors and capacitors.
Second, space transmission (transmitting through the gap of the equipment or itself)
Our general approach to space launch is as follows:
A. It is a good method to suppress spatial emission by source, mainly through filter circuit, interface processing, PCB layout, wiring, and multi-layer board design.
B. Strengthen the shielding of the equipment and handle the structural gap of the equipment;
The above methods have advantages and disadvantages; for example, the consequences of reducing the source will lead to poor signal quality, we must first ensure the normal and stable function when reducing the source; increase the filter components, will not have a big impact on the signal quality (select device When there is a targeted choice and combined with the actual signal frequency for filtering selection), but it will lead to increased equipment costs; so we must use our own situation in the process of operation;
How to improve the immunity of electronic devices? We first introduce the common phenomena and solutions for the anti-interference experiment of the car DVD system:
1, the audio output speaker appears howling
For the audio output horn howling, we have to distinguish the essential cause of howling, the common horn whistle is mostly because the interference signal is coupled to the internal circuit through the device interface cable (very few is due to the unreasonable internal circuit design of the audio amplifier) Then, it is coupled to the input end of the audio power amplifier through various paths, which causes the power amplifier to amplify and output the useless 1KHz interference carrier signal, thereby causing howling; the solution generally has the following types:
A. Find and cut off the coupling path;
Finding the coupling path is a complicated analysis process. It should be combined with PCB layout, wiring, schematic design and other factors for comprehensive analysis. If necessary, it can be tested with an oscilloscope. Generally, CS (conduction sensitivity test) can be used for BCI. Simulation test
B. Filtering at the signal port of the device so that the interference signal cannot enter the device;
In the signal port filtering, the common mode filtering method is generally used for suppression (sometimes the capacitor is also used for filtering); when the common mode inductor is selected, it should be selected in a targeted manner; if the howling occurs at 60 MHz, it is necessary to select the effective filtering for 60 MHz. Device
2, screen garbled, jitter, system crash, reset;
For these kinds of situations, we must focus on identifying the specific circuit that is subject to interference, such as system reset. At this time, we should pay attention to the reset circuit, power supply circuit, control circuit, etc. If the screen is garbled, we should pay attention to the video processing circuit. The control signal, logic signal, etc. of the video processing circuit; specifically can be combined with the oscilloscope for observation, and finally find the sensitive source, generally according to the following two types of situations for analysis.
One type: interference injected from the signal port of the device or coupled to the signal port of the device;
The most cost-effective way to find such interference is to find out the sensitive points of our equipment and improve the anti-interference ability of this point, but it takes a lot of time and effort, and it must also have good knowledge of circuits and electromagnetic fields. Our current enterprises, when it comes to EMC testing, the time is very urgent, there is not enough time to carry out detailed analysis and positioning and testing of the equipment; so filtering in the equipment signal interface is a good choice; Since we are very clear that the interference is directly transmitted from the cable to the inside of the device, we can use the EMC device to filter the interference at the interface to avoid interference to other circuits on the board; as long as the interference is filtered out at the signal interface, our device appears The problem will also disappear; this method will often lead to a certain increase in cost; therefore, this method needs to be considered in combination with the actual situation;
Class 2: interference from the device gap to the inside of the device;
From the interference of the gap coupling, the gap can be directly blocked by the test; the interference problem caused by the gap is generally focused on the gap; if it is necessary to find the sensitive source, it is necessary to analyze the actual test phenomenon;
summary
For automotive electronic products, it is necessary to pass the strict requirements of automotive electronic electromagnetic compatibility standards, and it is necessary to consider all aspects of structure, interface, single board filter design and PCB design.
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