26/09/2022

Components and selection details, do you really understand?

By micohuang

Do you know how to select components? The correctness of component selection is very important, and different test environments will lead to different results. This article teaches you a trick to correctly select components, in the environment of EMC peripheral circuits and the selection details, it is recommended that you collect this article!

Do you know how to select components? The correctness of component selection is very important, and different test environments will lead to different results. This article teaches you a trick to correctly select components, in the environment of EMC peripheral circuits and the selection details, it is recommended that you collect this article!

1. Varistor

Several important parameters for the selection of varistors are: maximum allowable voltage, maximum clamping voltage, and surge current that can be withstood. First of all, it should be ensured that the maximum allowable voltage of the varistor is greater than the maximum value of the output voltage of the power supply; secondly, it should be ensured that the maximum clamping voltage will not exceed the maximum surge voltage allowed by the subsequent circuit; then the surge current flowing through the varistor should be ensured will not exceed the surge current it can withstand.

Other parameters, such as rated power, high-energy pulses that can be tolerated, etc., can be determined through simple verification or experiments. It should be noted that varistors suffer from performance degradation issues.

2. Gas discharge tube

Gas discharge tube is a switching device. Compared with varistor, it has some different characteristics, such as long conduction delay, freewheeling after conduction, small inter-electrode capacitance, high insulation resistance, small leakage current, etc. Used in series and parallel with varistor. For example, when connected in series, it can solve the problems of large leakage current of the varistor, degradation or failure of long-term use performance; when connected in parallel, the response time of the protection circuit is accelerated, and most of the current is divided after the gas discharge tube breaks down.

3. TVS

Also as a protection device, compared with varistors and gas discharge tubes, TVS has a faster response speed and poorer surge resistance. It is a clamping device and has a more stable clamping voltage. Often used as an electrostatic protection device, it can also be used in conjunction with a varistor and a gas discharge tube to release surge energy as a graded protection.

4. X capacitor

As a safety capacitor, the X capacitor is connected between the L and N lines to filter out the differential mode interference of the power supply. It is larger in size, but allows higher ripple current and higher withstand voltage. X1, X2 or X3 capacitors can be selected according to different applications. For example, the commonly used X2 capacitor can be used in places where the transient voltage of the grid is less than or equal to 2.5KV.

5. Y capacitor

The Y capacitor is usually connected across the primary circuit and the secondary circuit or between the primary circuit and the protective ground to filter out common mode noise. Its capacity is usually small to meet leakage current requirements. Y capacitors can be divided into Y1, Y2, Y3, and Y4 grades. Different grades can withstand different pulse voltages, and require sufficient margin in electrical and mechanical properties to avoid breakdown and short circuit, which is critical to personal safety.

6. Differential Mode Inductance

Usually used to filter out low frequency interference. During differential mode surge testing, a portion of the energy is stored and then released. In the electrostatic test at the output end, it will also have the same effect. If the differential mode Inductor is placed behind the rectifier bridge, be careful that the high voltage generated when its energy is released will damage the rectifier bridge.

7. Common mode inductance

Common mode inductors are often used to filter out high frequency interference. During the common mode surge test, clamping devices can be connected in parallel on the windings or discharge teeth can be added to prevent arcing from affecting the normal operation of the circuit. In addition, the incomplete coupling between the two windings will form a differential mode inductance.

Eight, thermistor NTC

In order to prevent cold start and the problem of excessive inrush current, NTC is usually added to the front-end circuit. If the NTC is placed between the clamping device and the fuse, the differential mode surge test may burn it. If it is placed behind the clamping device, the fuse may be blown. Therefore, the fuse with too fast blowing time and too small current cannot be used.

9. Examples

Taking the AC-DC switching power supply surge test as an example, when a common mode voltage of 6KV is applied to ACL-PE or ACN-PE, the path is equivalent to a voltage source with an internal resistance of about 12Ω connected in series with the common mode inductor and Y capacitor. Because the Y capacitor is of Y1 grade, its withstand voltage is high, and the surge energy is not enough to damage it, so it is only necessary to ensure that the PE wiring and other wirings maintain a certain distance.

However, during testing, the high voltage at both ends of the common mode inductor may cause arcing, which may be affected if other devices are close to each other. Therefore, a varistor is connected in parallel on it to limit its voltage, thereby playing the role of arc extinguishing. If the cost is considered, the use of discharge teeth can also be considered. In addition, it can also be considered to design anti-surge circuit with gas discharge tube and varistor.

10. Summary

Above, a brief introduction to the common components of switching power supply EMC peripheral circuits. According to the needs of the product, the EMC peripheral circuit may also be modified accordingly, and the corresponding test should be carried out after confirming the selection. Of course, the basic selection basis still has to be followed, otherwise it may occur that only the test sample meets the test requirements, and various problems will occur once the product is mass-produced. The above are some methods of selecting components, I hope to help you.

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