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From component selection to EMC test points, teach you how to design protection circuits?

With the continuous improvement of the integration level, processor speed, switching rate and interface rate of Electronic products, the ESD/EMI/EMC problems of electronic products are becoming increasingly prominent, especially when handheld electronic devices are developing in the direction of thinness and compactness and the functions of products are continuously increasing. The number of input/output ports has also increased, causing electrostatic discharge to enter the system and interfere with or damage the integrated circuit. Circuit protection is the most problematic part, and it is also an easily overlooked problem.

With the continuous improvement of electronic product integration, processor speed, switching rate and interface rate, ESD/EMI/EMC problems of electronic products are becoming more and more prominent, especially when handheld electronic devices are developing in the direction of thinness and compactness and product functions continue to increase, they The number of input/output ports has also increased, causing electrostatic discharge to enter the system and interfere or damage the integrated circuit. Circuit protection is the most problematic part, and it is also an easily overlooked problem.

In the fields of communication, consumption, military industry, aerospace and other fields, ESD is often the culprit of circuit failure, and the selection of overcurrent and overvoltage protection devices, elimination of conducted radiation electromagnetic interference, and EMC test environment have become difficulties for engineers in design. How to solve these problems?

1. Circuit protection starts from component selection

Circuit protection components usually include overvoltage protection devices and overcurrent protection devices. Engineers need to choose according to the characteristics of various components and different application types. In electronic products, the density of printed circuit boards continues to increase, and the operating voltage of semiconductor components and integrated circuits continues to decrease. Manufacturers use surface mount technology, chip multilayer ceramic technology, array technology and other new technologies to develop Products for small voltage and high current circuit protection requirements; it is foreseeable that in the future, electronic power technology will continue to develop, and domestic and foreign circuit protection component manufacturers will continue to vigorously develop new products and new technologies to provide suitable and safe circuit protection for various application fields. components.

Choosing an appropriate circuit protection device is the key to realizing efficient and reliable circuit protection design. When it comes to the selection of circuit protection devices, we must know the function of each circuit protection device. When choosing a circuit protection device, we need to know that the protection circuit should not interfere with the normal behavior of the protected circuit. In addition, it is necessary to prevent any voltage transients from causing repetitive or non-repetitive unstable behavior of the entire system. Perform multiple simulations test to achieve the reliability and practicability of the circuit protection scheme.

Second, the selection skills of circuit protection devices

In the face of the huge hazards caused by ESD, overvoltage, surge, overheating and other phenomena, the latest circuit protection devices need to consider many other issues in addition to the volt-ampere characteristics, protection level and other factors. For example, electronic devices are getting thinner and lighter. In order to meet size constraints and provide circuit protection in a smaller footprint, protection device manufacturers need to develop smaller components, which requires manufacturers to continuously increase the energy of components Density, when the interface speed of electronic equipment continues to increase, in order to ensure signal integrity, the capacitance of the protection device must be considered. The protection scheme must follow the development trend of the interface to ensure the reliability of the interface, and at the same time, it must also ensure the impact resistance of the protection components. times, earthquake resistance, moisture resistance and other factors.

3. Characteristics of overcurrent and overvoltage protection devices

Although there are many kinds of protection devices, functionally speaking, they can be divided into overcurrent protection and overvoltage protection. The most important overcurrent protection device is a fuse, also called a fuse. It is generally connected in series in the circuit, and its resistance is required to be small (low power consumption). When the circuit is working normally, it is only equivalent to a wire, which can stably conduct the circuit for a long time, and current fluctuations occur due to power supply or external interference. It should also be able to withstand a certain range of overload. Only when there is a large overload current (fault or short circuit) in the circuit, the fuse will act to protect the safety of the circuit by disconnecting the current to avoid the danger of product burning. .

In the process of the fuse breaking the circuit, due to the existence of the circuit voltage, an arc will occur at the moment when the melt is disconnected. A high-quality fuse should try to avoid this arcing; after breaking the circuit, the fuse should be able to withstand The circuit voltage applied to both ends. The pulse damage of the fuse will gradually reduce the ability to withstand the pulse, and the necessary safety margin should be considered when selecting; this safety margin refers to the total fuse (action) time of the fuse, which is the pre-f Sum of arc time and arcing time.

Therefore, it is necessary to pay attention to the basic conditions of its fusing characteristics and rated current when choosing; in addition, the surrounding environment of the fuse should be considered during installation. The fuse will only fuse when it reaches its own melting heat value. If it is in a cold environment Under the circumstance, its fusing time will change, which must be paid attention to when using it.

Fourth, the main points of EMC test

Electromagnetic compatibility (EMC) refers to the state in which the performance of a device or system does not degrade in an electromagnetic environment. EMC, on the one hand, requires that there is no serious interference source in the system, that is, the electromagnetic interference generated by the equipment to the environment in the normal operation process cannot exceed a certain limit; Appliances have a certain degree of immunity to electromagnetic interference in their environment, that is, electromagnetic susceptibility.

EMC includes two parts: EMI (Electromagnetic Interference) and EMS (Electromagnetic Immunity):

1. EMI electromagnetic interference is the electromagnetic noise generated by the machine itself in the process of performing its due functions, which is not conducive to other systems.

2. EMS refers to the ability of the machine to not be affected by the surrounding electromagnetic environment in the process of performing its due functions.

5. How to eliminate electromagnetic interference?

Electromagnetic interference refers to the electromagnetic waves generated during the working process of electronic equipment, which are emitted to the outside world, thereby causing interference to other parts of the equipment or other external equipment. For the electromagnetic compatibility problem or electromagnetic interference to occur in the system, there must be three factors, namely the electromagnetic interference source, the coupling path, and the sensitive equipment. Therefore, when solving the problem of electromagnetic interference, it is necessary to solve the problem of electromagnetic compatibility by manpowering from these three factors, and eliminating one of the factors. Commonly used and effective methods are: grounding technology, shielding technology, filtering technology.

1. Grounding technology

Grounding technology can be divided into working grounding, lightning protection grounding and protective grounding.

The working grounding is to directly ground the neutral point of the three-phase power supply in the low-voltage AC power grid.

In the communication bureau (station), there are usually two kinds of lightning protection grounding: one is a lightning rod lightning protection grounding device specially designed to protect buildings or antennas from lightning strikes, which is designed and installed by the building department, and the other It is a lightning protection grounding device that needs to be installed to prevent lightning overvoltage from damaging communication equipment or power supply equipment.

Protective grounding is to make a good electrical connection between the metal shell part of the power receiving equipment and the live part insulated under normal conditions and the grounding device.

2. Shielding technology

The electromagnetic shielding technology in the circuit is mainly to survive in the common electromagnetic environment. By using the electromagnetic interference suppression technology to prevent the interference of other factors in the actual work, the application of the technology is unreasonable.

The shielding technology mainly uses a complete metal shield to surround the point conductors to improve the inductive ability of the shield and ensure that the same charge as the charged conductor can appear on the outside. If the charge on the outside flows into the ground, there will be no leakage on the outside. , and the better the conductivity of the metal shield, the better the shielding effect of static electricity, the shielding technology needs to play a shielding role through grounding.

3. Filtering technology

The filter is one of the essential key components in the radio frequency system. It is mainly used for frequency selection, allowing the desired frequency signal to pass through and reflecting the unwanted interfering frequency signal.

Filtering technology is mainly divided into signal filtering and EMI filtering:

Signal filtering is when the input and output impedances are known, while attenuating out-of-band interference, it ensures extremely low insertion loss in the passband.

EMI filtering is within the range of the expected operating frequency bandwidth, the variation range of the termination impedance will increase, and the insertion loss affected by the loading current is determined by the ability to suppress external interference signals.