With the continuous improvement of electronic product integration, processor speed, switching speed and interface speed, electronic products ESD/EMI/EMC problems have become increasingly prominent, especially when handheld electronic devices are developing in the direction of light, thin and compact, and product functions continue to increase. 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 a problem that is easily overlooked.

In the fields of communications, consumer, military, aerospace, etc., ESD is often the culprit that causes circuit failures. The selection of overcurrent and overvoltage protection devices, the elimination of conducted radiation electromagnetic interference, and the EMC test environment have become difficult points for engineers in design. How to solve these problems?Yuhuaxin Electronics Editor：

1. Circuit protection starts from the selection of components

Circuit protection components usually include overvoltage protection devices and overcurrent protection devices. Engineers need to make selections based on the characteristics of various components and different application types. In electronic products, the density of printed circuit boards continues to increase, and the working 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 small sizes and meet the Products requiring low voltage and high current circuit protection; 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 technologies to provide suitable and safe circuit protection for various application fields Components.

Choosing an appropriate circuit protection device is the key to high-efficiency and reliable circuit protection design. When it comes to the selection of circuit protection devices, we must know the role of each circuit protection device. When selecting circuit protection devices, we must know that the protection circuit should not interfere with the normal behavior of the protected circuit. In addition, we must prevent any voltage transients from causing repetitive or non-repetitive unstable behaviors of the entire system, and perform multiple simulations. Test, so as to realize 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 factors such as volt-ampere characteristics and protection levels. For example, electronic equipment is becoming thinner and lighter. In order to meet size restrictions 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 rate of electronic equipment continues to increase, in order to ensure the integrity of the signal, the capacitance of the protection device must be considered. The protection scheme must follow the trend of interface development to ensure the reliability of the interface and the impact resistance of the protection components. Factors such as frequency, earthquake resistance, moisture resistance, etc.

Third, the characteristics of overcurrent and overvoltage protection devices

Although there are many types of protection devices, they can be divided into overcurrent protection and overvoltage protection in terms of function. The most important overcurrent protection device is a fuse, also called a fuse. It is generally connected in series in a 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 conduct the circuit stably 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 overloads. Only when a large overload current (fault or short circuit) occurs in the circuit, the fuse will act, and the safety of the circuit will be protected by disconnecting the current to avoid the risk of product burns. .

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

Therefore, you need to pay attention to the basic conditions of its fusing characteristics and rated current when choosing; in addition, consider the environment around the fuse when installing. 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.

Four, EMC test points

Electromagnetic compatibility (EMC) refers to the state in which the performance of a device or system is not degraded 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 in the normal operation process cannot exceed a certain limit; on the other hand, it requires the equipment or the system itself to have good anti-electromagnetic interference. The appliance has a certain degree of immunity to electromagnetic interference in the environment, that is, electromagnetic sensitivity.

EMC includes two parts: EMI (electromagnetic interference) and EMS (electromagnetic immunity):

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

2. EMS refers to the ability of a machine to not be affected by the surrounding electromagnetic environment in the process of performing its 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, thereby causing interference to other parts of the equipment or other external equipment. For the system to have electromagnetic compatibility problems, namely electromagnetic interference, three factors must be possessed, namely electromagnetic interference source, coupling path, and sensitive equipment. Therefore, when solving the problem of electromagnetic interference, it is necessary to prescribe the right medicine from these three factors and eliminate one of them to solve the electromagnetic compatibility problem. Commonly used 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.

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

In communication bureaus (stations), 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, which is designed and installed by the construction department, and the other It is a lightning protection grounding device buried in order to prevent the destruction of communication equipment or power supply equipment by lightning overvoltage.

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

2. Shielding technology

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

The shielding technology is mainly to use a complete metal shield to enclose the dotted 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 be grounded for shielding.

3. Filtering technology

The filter is one of the indispensable key components in the radio frequency system. It is mainly used for frequency selection, allowing the required frequency signal to pass and reflecting the unwanted interference frequency signal.

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

When the signal filter has known input and output impedances, it can attenuate out-of-band interference while ensuring extremely low insertion loss in the passband.

EMI filtering is within the scope of the expected working frequency bandwidth, the change 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.