SMD Power Supply Filter Design Tips

What is the purpose of using a filter in a circuit?A filter is a circuit capable of passing certain frequencies while attenuating (to reduce or block) other frequencies. Thus, a filter can extract important frequencies from signals that also contain undesirable or irrelevant frequencies. In the field of electronics, there are many practical applications for filters.Today qihe smt pick and place machine sharing some SMD Power Supply Filter Design Tips .

The purpose of the power supply filter is to suppress electromagnetic noise. The effects of noise can be divided into the following two types:

Emissions: It is necessary to reduce the noise generated by equipment and affecting power supplies or other equipment below the allowable value of regulations (such as FCC part 15), such as the noise generated by switching power supplies.

Immunity: It is to reduce the noise entering the equipment to a level that will not cause the equipment to operate abnormally, such as instruments used in broadcast station transmitting equipment.

Power filter installation

A. There must be no electromagnetic coupling path in the power supply filter.

①The power input cord is too long;

②The input line and output line of the power filter are too close.

Both of these are incorrect installation methods. The essence of the problem is that there is an obvious electromagnetic coupling path between the input wire of the filter and its output wire. In this way, the EMI signal existing at one end of the filter will escape its suppression by the filter and be directly coupled to the other end of the filter without being attenuated by the filter. Therefore, the filter input and output need to be effectively separated first.

In addition, if the above two types of power filters are installed inside the equipment shield, the EMI signals on the internal circuits and components of the equipment will be directly coupled to the outside of the equipment due to radiation generated on the (power) end leads of the filter. Therefore, the equipment shielding loses the ability to suppress EMI radiation generated by internal components and circuits. Of course, if there is an EMI signal on the filter (power supply), it will also be coupled to the components and circuits inside the device due to radiation, thereby destroying the suppression effect of the filter and shielding on the EMI signal. So it doesn’t work.

B.Do not bundle cables together

Generally speaking, when installing a power filter in an electronic device or system, you should pay attention to the fact that when bundling equipment cables, you must not bundle the wires at the (power) end and (load) end of the filter together, because this will undoubtedly aggravate the situation. The electromagnetic coupling between the input and output ends of the filter is eliminated, seriously damaging the ability of the filter and equipment shielding to suppress EMI signals.

C.Try to avoid using long grounding wires

The length of the wiring connecting the output end of the power filter to the inverter or motor should not exceed 30 cm.

Because an excessively long ground wire means a significant increase in ground inductance and resistance, it will seriously damage the filter’s common-mode suppression capability. A better method is to use metal screws and star-shaped spring washers to securely fasten the filter shield to the chassis at the power inlet of the equipment.

D.The input and output lines of the power filter must be kept apart.

The input and output lines of the power filter must be kept apart and should not be parallel to avoid reducing the filter efficiency.

E.The power filter shell and the chassis shell must be in good contact

The metal shell of the special filter for the frequency converter and the chassis shell must ensure good surface contact, and the grounding wire must be connected well.

F.The connection line of the power filter should be twisted pair.

It is better to use shielded twisted pair cables for the input and output connecting lines of the power filter, which can effectively eliminate some high-frequency interference signals.

Power filter parameters

The power filter is a passive device composed of filter inductors and capacitors. It allows power frequencies (50Hz, 60Hz, 400Hz) to pass through, while greatly attenuating electromagnetic interference signals. The power supply filter is bidirectionally reversible, which can not only prevent electromagnetic interference on the power grid from entering the equipment through the power supply, but also prevent electromagnetic interference generated by the equipment itself from polluting the power grid. It is an effective configuration to suppress conducted interference.

Insertion loss

Insertion loss is an important technical indicator of the filter. In a test system where the signal source output impedance and receiver impedance are both 50Ω, the signal source output voltage is U1 when the filter is not connected. When the filter is connected, the signal source voltage measured at the filter output is U2, then the insertion loss of the filter is: Il=20log (U1/U2). Since there are two types of interference: common mode interference and differential mode interference, insertion loss is divided into common mode insertion loss and differential mode insertion loss, which represent the ability to suppress common mode noise and differential mode noise respectively.

Leakage current

Leakage current (only meaningful for AC power line filters): Under rated AC voltage, the current from the filter housing to either end of the AC incoming line. Its value mainly depends on the leakage current of the common mode capacitor Cy. Depending on the use environment of the filter, various safety standards have different regulations on leakage current, with typical values ranging from 0.5 to 5.0mA.
Withstand voltage test method

According to the national standard: the voltage starts from zero, slowly increases to the test voltage at a speed of no more than 150V/s, lasts for 1 minute, and then slowly drops to zero. Application methods include between the filter lead-out terminals and between the lead-out terminals and the filter metal shell.

Read more: SMD Power Supply Filter Design Tips

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