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EMI filters;

The above is the basic block diagram of the EMI filter.

C1C2C3C4 are Y capacitors, and L1 is a common mode choke (common mode inductor): mainly for common mode noise.

For Y capacitors, the selection surface Y is too narrow, and basically there is no selection. Generally, there are 2 or 4. There are not many choices for capacitance value, mainly considering the leakage current of the power supply. The larger the leakage current is, the better the EMI is.

Common mode choke: Using a high magnetic permeability value can make the volume smaller and the number of turns less. Now the material with a magnetic permeability of 10000 is very mature. This value is at the mH level. For a power supply of 100 watts, a few mH, or about 10mH.

The value of the common mode choke can refer to Sha Zhanyou's book, but it is only experience. "Switching Power Supply Design - Introduction and Example Analysis"

Differential mode inductor L2L3: Because it is easy to saturate, it is generally not used. If used, it is only at the uH level.

X capacitor: Generally, two are selected, at the front and end of the EMI filter. The value range is also very narrow. It can be debugged in the laboratory.

Differential mode inductors are generally not used, and differential mode interference is mainly filtered out by X capacitors.

Common mode inductor empirical value: relationship with rated current.


For low-power models (such as more than 10W):


After the rectifier bridge, you can use the CLC (two electrolytic capacitors and one inductor) structure, and the input end does not need to connect the X capacitor.

The above is a comparison of EMI countermeasures for low-power models. The above is the CLC mode, and the following is the traditional pre-rectifier bridge filtering. The CLC mode can save X capacitors.

High-power models:

Generally, pre-rectifier bridge filtering is used. If the first-level EMI filtering is not feasible, the second-level EMI filtering can be used.

The selection of Y capacitors mainly considers a leakage current, which is I=2*3.14*F*C*Uc. F is the grid frequency, C is the sum of the Y capacitor capacitance, and Uc is the voltage drop on the Y capacitor. As long as the safety regulations can be passed, the larger the Y capacitor, the better the EMI.

The common mode inductor will not be repeated here.

Transformer shielding:

Transformers have two types of shielding: electric field and magnetic field.

There are two types of electric fields: primary shielding and secondary shielding.

Generally, the shielding layer of the transformer is between the primary and secondary windings. A section of enameled wire is welded to a section of copper foil, and one end of the enameled wire is welded to the primary ground of the transformer, or called the hot ground! Secondary shielding is to connect the shielding winding to the secondary ground (also called cold ground).

Magnetic field shielding is to add a layer of copper shielding on the outermost of the transformer. This can shield the transformer from external interference and external interference.

In practical applications, we generally use primary shielding and magnetic field shielding. In high-power models, magnetic field shielding is a must. As for

the impact of drive design on EMI,

we still don’t know whether it has a greater impact on EMI when it is turned off or when it is turned on?

Most of the forum believe that the impact on EMI is greater when it is turned off; while the moderator Bing believes that it has a greater impact on EMI when it is turned on. (Please indicate the source when reprinting the original article from Power Network)