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How can surge requirements be considered in PCB design?
How can surge requirements be considered in PCB design?

How can surge requirements be considered in PCB design?

2020-09-11 17:10:24
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Author:pcb factoty

In the case of engineering PCB design, only the multifunctional design scheme of the system software is considered. For example, when the system software is in specific work, it only needs to bear the current of 1a, and the design scheme is based on this. However, it is possible that the surge design scheme required by the system software should be 3KA (1.2 / 50uS & 8 / 20us), So now, if I design the scheme according to the specific work flow of 1a, can I achieve the working ability of the transient surge?


For example: 0.36mm wide 1oz copper foil, 35um thick line in a 40us rectangular current surge, the maximum surge current is about 580A. If we want to make a 5ka (8 / 20us) protection design, the reasonable front-end PCB wiring should be 2 oz copper foil 0.9mm width. For safety devices, the width can be appropriately relaxed..


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Filter considerations in PCB design

Surge voltage, ringing voltage, spark discharge and other instantaneous interference signals are characterized by extremely short action time, but high voltage amplitude and large transient energy. Transient interference will cause fluctuations in the output voltage of the single-chip switching power supply; when transient When the voltage is superimposed on the rectified and filtered DC input voltage VI, and the VI exceeds the drain-source breakdown voltage V(BR)DS of the internal power switch tube, it will also damage the TOPSwitch chip, so suppression measures must be taken. Generally, electrostatic discharge (ESD) and Electrical Fast Transient (EFT) are more harmful to digital circuits than to analog circuits. Electrostatic discharge produces strong radio frequency radiation in the frequency range of 5-200MHz. The peak of this radiated energy often appears Self-oscillation occurs between 35MHz and 45MHz. The resonant frequency of many I/O cables is usually within this frequency range. As a result, a large amount of electrostatic discharge radiation energy is stringed into the cable. When the cable is exposed to 4-8kV static electricity When in a discharge environment, the induced voltage that can be measured on the I/O cable terminal load can reach 600V. This voltage far exceeds the typical digital threshold voltage value of 0.4V. The typical induction pulse duration is about 400 nanoseconds. The I/O cable is shielded, and both ends of it are grounded, so that all the internal signal leads are in the shielding layer, which can reduce the interference by 60-70dB, and the induced voltage on the load is only 0.3V or lower. Electrical fast transient pulse The group also produces quite strong radiation emission, which is coupled to the cable and the chassis line. The power line filter can protect the power supply. The common mode capacitance between the line and the ground is an effective device to suppress this transient interference. The interference is bypassed to the chassis and far away from the internal circuit. When the capacitance of this capacitor is limited by the leakage current and cannot be too large, the common mode choke must provide greater protection. This usually requires the use of a dedicated center tap Common mode choke coil, the center tap is connected to the chassis through a capacitor (capacity is determined by the leakage current). Common mode choke coils are usually wound on high permeability ferrite cores, and their typical inductance is 15 ~ 20mH .

The filter cuts off the path of electromagnetic interference propagating along the signal line or power line. Together with the shield, it can form a complete electromagnetic interference protection. Whether it is to suppress the interference source, eliminate the coupling or improve the immunity of the receiving circuit, filtering technology can be used. Different interferences should adopt different suppression techniques, from simple line cleaning, to interference suppressors, filters and transformers of individual components, to more complex voltage stabilizers and purifying power supplies, and expensive and well-performing non-products. Discontinuous power supply, the following is a brief description:


Transient interference suppressors include gas discharge tubes, metal oxide varistors, silicon transient absorption diodes, and solid discharge tubes. Among them, metal oxide varistors and silicon transient absorption diodes work a bit like ordinary The zener tube is a clamp-type interference absorption device; while the gas discharge tube and solid discharge tube are energy transfer type interference absorption devices (take the gas discharge tube as an example, when the voltage appearing at both ends of the discharge tube exceeds the ignition of the discharge tube When the voltage is applied, the gas in the tube is ionized and an arc is generated between the two electrodes. Because the voltage drop of the arc is very low, most of the transient energy can be transferred, thereby protecting the equipment from the transient voltage damage). Transient interference suppressor and The protected equipment is used in parallel.


The gas discharge tube is also called the lightning arrester, and is currently commonly used in program-controlled switches. The lightning arrester has a strong surge absorption capacity, high insulation resistance and small parasitic capacitance, and will not bring any harmful effects to the normal working equipment But there is a big difference between its arcing response to surge and the arcing response to DC voltage. For example, the arcing voltage of a 90V gas discharge tube for DC is 90V, and the arcing for a surge of 5kV/μs The maximum voltage may reach 1000V. This indicates that the gas discharge tube has a low response speed to the surge voltage. Therefore, it is more suitable as a primary protection for lines and equipment. In addition, the voltage level of the gas discharge tube is very small.


Varistors are currently widely used transient interference absorption devices. The main parameters describing the performance of varistors are the nominal voltage and current capacity of the varistor, that is, the surge current absorption capacity. The former is often confused by users. A parameter. The nominal voltage of a varistor refers to the voltage drop that appears across the varistor under constant current conditions (for a varistor with an outer diameter of 7mm or less, take 0.1mA; for a varistor with an outer diameter of 7mm or more, take 1mA). There is a large dynamic resistance, and the voltage that appears across the varistor (also known as the maximum limit voltage) under the impulse current of the specified shape (usually the standard impulse current of 8/20μs) is about the nominal voltage of the varistor 1.82 times (this value is also called residual voltage ratio). This requires the user to make an estimate in advance when choosing a varistor. For occasions that are likely to encounter a larger inrush current, they should choose to use a larger size device (The current absorption capacity of a varistor is proportional to the flow area of the device, the withstand voltage is proportional to the thickness of the device, and the absorbed energy is proportional to the volume of the device). When using a varistor, pay attention to its inherent capacitance. According to the size and nominal The voltage is different, and the capacitance is between thousands to hundreds of pF, which means that the varistor is not suitable for use in high frequency applications, and is more suitable for power frequency applications, such as thyristors and power inlets for protection. Special attention should be paid to the high-speed performance (up to ns) of the varistor when it absorbs transient interference. Therefore, when installing the varistor, you must pay attention to the inductive reactance of its lead. Excessive lead will introduce the inductance of the lead. Induced voltage (on the oscilloscope, the induced voltage is spike-like). The longer the lead, the greater the induced voltage. In order to achieve a satisfactory interference suppression effect, the lead should be shortened as much as possible. Regarding the voltage selection of the varistor, it is necessary to consider the The possible voltage fluctuations of the protection circuit (usually taken as 1.21.4 times). If it is an AC circuit, also pay attention to the relationship between the effective value of the voltage and the peak value. Therefore, for the 220V line, the nominal voltage of the selected varistor should be 220×1.4×1.4≈430V. In addition, as far as the current absorption capacity of the varistor is concerned, 1kA (for 8/20μs current wave) is used for thyristor protection, 3kA is used for surge absorption of electrical equipment; 5kA is used In lightning strikes and overvoltage absorption of electronic equipment; 10kA is used for lightning strike protection. Varistors have many voltage levels and are suitable for primary or secondary protection of equipment.


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