Analysis of surge immunity circuits in EMS

Everyone knows that EMC describes the performance of products, namely, electromagnetic emission/interference EMI and electromagnetic immunity EMS. EMI also includes conduction and radiation; EMS also contains static electricity, pulse group, surge, etc. This paper will analyze and design the front circuit of the power supply from the perspective of EMS surge immunity.

Analysis of anti-surge circuit

As shown in Figure 1, it is usually used in the EMC front schematic diagram of small power supply module. FUSE is fuse, MOV varistor, Cx is X capacitance, LDM is differential mode inductance, Lcm is common mode inductance, Cy1 and Cy2 are Y capacitance, NTC thermistor. The main function of the common mode inductance is not to improve the surge immunity of the circuit, but they indirectly affect the design of the circuit.

Figure 1. Common EMC front-end circuit

The surge voltage applied between ACL and ACN is called differential mode surge voltage, and the differential mode path is shown in the red line in the figure; Yes, the voltage applied between ACL (or ACN) and PE is called common-mode surge voltage, and the common-mode path is shown in the blue line in the figure.

Before designing the anti-surge circuit, the corresponding circuit "electromagnetic compatibility standard" must be determined. For example, IEC/EN61000-4-5 (corresponding to GB/T17626.5) specifies the requirements, test methods and test levels of surge immunity. Next, we will discuss the design of anti-surge circuit according to the provisions of this standard.

When the output is open, the surge circuit generates 1.2/50 μ S surge voltage, 8/20 will be generated in case of short circuit μ S Surge current.

The effective output impedance is 2 Ω, so the peak open-circuit voltage is XKV and the peak short-circuit current is (X/2) KA.

During the anti-surge test of ACL (or ACN) and PE, the coupling circuit is connected in series with 10 Ω. If the resistance ignores the influence of the series coupling capacitance, the short-circuit peak current will become about (X/12) KA.

Introduction to relevant equipment

1. Varistor

The most important parameters for the selection of varistor are: maximum allowable voltage, maximum clamping voltage, and can withstand surge current.

First of all, the maximum allowable voltage resistance voltage is greater than the maximum value of the maximum voltage of the power supply output voltage. Second, we should ensure that the maximum clamping voltage does not exceed the maximum allowable surge voltage of the subsequent circuit; Finally, we should ensure that the surge current through the varistor does not exceed the surge current it can withstand.

Other parameters such as rated power can be determined by simple calculation or experiment.

2. Y capacitance

When conducting the common mode surge test, if the cost and other factors are taken into account, and there is no added varistor or other equipment for clamping voltage in the common mode path, the Y capacitor voltage resistance shall be higher than the test voltage.

3. Input rectifier diode

If the maximum clamping voltage is greater than the maximum reverse voltage that the input rectifier diode can withstand, the diode may be damaged. Therefore, the diode with reverse resistance greater than the maximum clamping voltage should be selected as the input rectifier diode.

4. Common mode inductance

Theoretically, the common-mode inductance only works in the common-mode path, but because the two windings of the common-mode inductance are not fully coupled, the uncoupled part will be used as the differential-mode inductance in the differential-mode path, affecting EMC characteristics.

Case analysis

Background: Take a power module of a certain model as an example. The module is ZLG input 85VAC~350VAC, and the EMC module is embedded with the front circuit circuit. Anti-surge requires three differential modules with voltage of KV and common mode voltage of 6KV. After replacing a larger fuse, it can withstand 6KV differential mode voltage. The schematic diagram and corresponding physical diagram of the front stage are shown in Figure 2.

Figure 2. Example schematic diagram

1. Differential mode surge test

When selecting the varistor, the maximum allowable voltage should be slightly greater than 350V, and the maximum clamping voltage of this voltage level varistor is about 10000V (50A test current). Secondly, on the differential mode path, the internal resistance is equal to 2 Ω. The pulse voltage is 6KV. If the voltage source is connected in series with the varistor, the peak current is about (6) KV-1KV)/2 Ω=2500A. Finally, 681KD14 was selected as the varistor. Its peak current is 4500A, the maximum allowable working voltage is 385VAC, and the maximum clamping voltage is 1120V.

Don't worry, because the uncoupled part of the common mode inductor, as the differential mode inductor in the differential mode path, will distribute part of the voltage. In fact, after the common mode inductor, the circuit has been protected. After test and verification, the common rectifier diode 1N4007 can be selected.

2. Common mode surge test

When testing 6KV surge for ACL-PE or ACN-PE, that is, common-mode surge test, the common-mode path is equivalent to internal resistance of about 12 Ω, and the pulse voltage is 6KV voltage source and common-mode inductance. Y capacitor in series. Because Y1 capacitor is selected as Y capacitor, its withstand voltage is high, and the energy of 6KV common-mode surge is not enough to damage it, so PE can easily pass the common-mode surge test if the wiring is kept indirect with other wiring.

However, high voltage and flashover will occur at both ends of the common-mode inductor during the surge test. If it is close to the surrounding equipment, it may damage the surrounding equipment. Therefore, a discharge tube or varistor can be connected in parallel to limit its voltage, thus playing the role of arc extinguishing. As shown in the figure MOV2.

Another method is to add discharge teeth at both ends of the common-mode inductor during PCB design to make the inductor discharge through two discharge tips and avoid discharging through other paths, so as to minimize the impact on the surrounding and rear equipment. As shown in Figure 3, ZLG Zhiyuan Electronics model is PA1HBxOD-10W power module PCB discharge tooth, and common mode inductance is added.

Figure 3. Physical picture of discharge tooth

summary

EMC experiments are usually very practical, but if we master some basic principles in the design, EMC will have more directions to test in the previous circuit to shorten the project development time. In this paper, a simple example is used to introduce the selection of equipment and typical circuits in the front stage from the perspective of surge test. In future articles, we will continue to discuss the relevant content of anti-surge circuit from the perspective of EMC design performance index EMC pre-stage circuit.

Power supply module with stable performance and perfect surge protection circuit will ensure the stability and reliability of system power supply to the maximum extent.