Filter related knowledge

Everyone has different opinions on the use of filters. Now I will talk about my knowledge about filters so that everyone can learn from it. It may seem a bit like showing off one's skills in front of an expert, but I still believe that writing so much will be useful, and it can be regarded as a small contribution to the forum! I also hope that experts will criticize, supplement and give advice.

First of all, let's talk about why we use filters. As the name implies, filters are a kind of control of electromagnetic noise. Power supply noise is a kind of electromagnetic interference. The spectrum of its conducted noise is roughly 10kHz~30MHz, and can reach up to 150MHz. Therefore, this high-frequency interference will seriously affect the equipment's ability to capture details, making low noise louder, high-frequency burrs, and low-frequency contours blurred. In severe cases, it will even affect the irritability of the listening experience!

According to the different propagation directions, power supply noise can be divided into two categories: one is the external interference introduced from the power supply line, and the other is the noise generated by electronic equipment and transmitted through the power line.

This shows that noise is a two-way interference signal, and electronic equipment is both the object of noise interference and a noise source. From the perspective of formation characteristics, noise interference can be divided into common mode interference and series mode interference.

Series mode interference is the noise between two power lines (referred to as line-to-line), and common mode interference is the noise between two power lines and the earth (referred to as line-to-ground). Therefore, the influence of electrical noise interference on audio equipment is serious!

In turn, the electromagnetic interference filter should meet the requirements of electromagnetic compatibility and must be a bidirectional RF filter. On the one hand, it should filter out the external electromagnetic interference introduced from the AC power line, and on the other hand, it can also filter out the noise interference emitted by the device itself to the outside, so as not to affect the normal operation of other electronic equipment in the same electromagnetic environment.

In addition, electromagnetic interference filters can suppress both series mode and common mode interference.

The basic components of an electromagnetic interference filter are two input terminals, two output terminals and a ground terminal. When in use, the casing should be connected to the ground.

The circuit includes common mode choke (called common mode inductor) and filter capacitor. L has no effect on series mode interference, but when common mode interference occurs, since the magnetic flux of the two coils is the same, the total inductance increases rapidly after coupling, so it presents a large inductive reactance to the common mode signal, making it difficult to pass, so it is called a common mode choke coil. Its two coils are respectively wound on low-loss, high-permeability ferrite magnetic rings. When current passes through, the magnetic fields on the two coils will strengthen each other.

The inductance of L is related to the rated current I of the EMI filter. It should be pointed out that when the rated current is large, the diameter of the common mode choke coil should also be increased accordingly so that it can withstand a larger current.

In addition, appropriately increasing the inductance can change the low-frequency attenuation characteristics.

The filter capacitor uses high-quality thin-film capacitors with a capacity range of approximately 0.01.uF~0.47uF, which is mainly used to filter out cross-mode interference.

The filter capacitor should be connected across the output end and the midpoint of the capacitor should be grounded to effectively suppress common-mode interference.

The filter capacitor can also be connected in parallel at the input end. Ceramic capacitors can be used. The capacity range is 2200pF~0.1uF. To reduce leakage current, the capacitance should not exceed 0.1Uf, and the midpoint of the capacitor should be connected to the ground. The withstand voltage of the filter capacitor is 630VDC or 250VAC.

There is a two-pole composite filter, which has a better effect of filtering out noise because it uses two-pole filtering.

The following is some theoretical knowledge about filters:

The main technical parameters of a filter are: rated voltage, rated current, leakage current, test voltage, insulation resistance, DC resistance, operating temperature range, operating temperature rise Tr, insertion loss Adb, dimensions, weight, etc. The most important of these parameters is insertion loss (also called insertion attenuation), which is the most important indicator for evaluating the performance of a filter!

Insertion loss is a function of frequency and is expressed in dB. Assuming that the noise power transmitted to the load before and after the electromagnetic interference filter is inserted is represented by P1 and P2, the following formula is obtained: Adb=10lg(P1/P2)

Assuming that the load impedance remains unchanged before and after insertion, P1=(V1)^2 / Z, P2=(V2)^2 / Z. V1 is the voltage directly loaded on the load, and V2 is the noise voltage on the load after the electromagnetic interference filter is inserted between the noise source and the load.

Combined, we get Adb=20lg(V1/V2)

Insertion loss is expressed in decibels. The larger the decibel value, the stronger the noise suppression capability! Theoretical calculations are complicated and the error is large. Usually, actual measurements are performed based on the theory.

Let's talk about the formula of the filter's ground leakage current: Ild=2πfCVc (lld is the leakage current, f is the frequency of the power grid.)