As shown in Figure 1, it is usually used for the EMC front-end schematic of low-power power modules. FUSE is the fuse, MOV is the varistor, Cx is the X capacitor, LDM is the differential mode inductor, and Lcm iscommon mode inductor， Cy1 and Cy2 are Y capacitors and NTC thermistors. Among them, Y capacitor Although the main function of common mode inductors is not to improve the surge immunity of circuits, they indirectly affect the design of circuits.

Figure 1. Commonly used EMC front-end circuit

toACLThe surge voltage applied between ACN and ACN is called differential mode surge voltage, and the differential mode path is shown by 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 by the blue line in the figure.

Before designing surge resistant circuits, it is necessary to determine the corresponding circuit "electromagnetic compatibility standards", such as IEC/EN61000-4-5 (corresponding to GB/T17626.5) which specifies the requirements for surge immunity Experimental methods Test level, etc. Below, we will discuss the design of surge resistant circuits based on the provisions of this standard.

When the output is open circuited, the surge circuit generates a surge voltage of 1.2/50 μ s, and when short circuited, it generates a surge current of 8/20 μ s.

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

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

Introduction to related equipment

Case Analysis

Background: Taking a certain model of power module as an example, this module isZLGInput 85VAC~350VAC, and embed the front-end circuit in the EMC module. Anti surge requires 3 differential modules with a voltage of KV and a common mode voltage of 6KV. After replacing the larger fuse, it can withstand a differential mode voltage of 6KV. The front-end schematic and corresponding physical diagram are shown in Figure 2.

Figure 2. Example schematic diagram

1. Differential surge test

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

Don't worry, because the uncoupled part of the common mode inductor, as a differential mode inductor in the differential mode path, will allocate some voltage. In fact, after the common mode inductor, the circuit has already been protected. After testing and verification, commonly used rectifier diodes were selected1N4007Just enough.

2. Common mode surge test

When testing ACL-PE or ACN-PE for 6KV surge, i.e. common mode surge test, the common mode path is equivalent to an internal resistance of about 12 Ω, and the pulse voltage is 6KV voltage source and common mode inductor Y capacitors are connected in series. Due to the choice of Y1 level capacitor for Y capacitor, its voltage resistance is high, and the energy of 6KV common mode surge is not enough to damage it. Therefore, it is only necessary to ensure that the PE wiring maintains a certain degree of indirect connection with other wiring to easily pass the common mode surge test.

However, due to the high voltage and arcing generated at both ends of the common mode inductor during surge testing. If you approach the surrounding equipment, it may damage the surrounding equipment. Therefore, a discharge tube or varistor can be connected in parallel to limit its voltage, thereby serving to extinguish the arc. As shown in MOV2.

Another method of PCB design is to add discharge teeth at both ends of the common mode inductor, allowing the inductor to discharge through two discharges and avoiding discharge through other paths, thereby minimizing the impact on surrounding and rear equipment. As shown in Figure 3, the physical image of the PCB discharge teeth of the ZLG Zhiyuan Electronics model PA1HBxOD-10W power module with common mode inductance added.