The noise filter is a passive low-pass filter that transmits AC power to the power supply without attenuation and greatly attenuates the EMI noise introduced with the AC power; at the same time, it can effectively suppress the EMI noise generated by the power supply equipment and prevent them from entering. The AC power grid interferes with other electronic equipment.
Main design principles of noise filter:
The magnetic cores used in common mode inductor coils include ring, E-shaped and U-shaped. The material is generally made of ferrite. The ring-shaped core is suitable for large current and small inductance. Its magnetic circuit is longer than the E-shaped and U-shaped, and there is no gap. , a larger inductance can be obtained with a smaller number of turns. Due to these characteristics, it has better frequency characteristics. The coil leakage flux of the E-shaped magnetic core is small, so when the magnetic leakage of the inductor may affect other circuits or other circuits are magnetically coupled with the common mode inductor, and the required noise attenuation effect cannot be obtained, the E-shaped magnetic core should be considered. Common mode inductor. Differential mode inductor coils generally use metal powder-pressed magnetic cores. Since the applicable frequency range of powder-pressed magnetic cores is low, ranging from tens of kHz to several MHz, its DC overlap characteristics are good, and the inductance will not drop significantly when used in large current applications. Most suitable as a differential mode inductor.
The role of noise filter in power supply equipment:
The functions of using noise filters in power supply equipment are as follows:
(1) Prevent external electromagnetic noise from interfering with the work of the control circuit of the power supply equipment itself;
(2) Prevent external electromagnetic noise from interfering with the work of the load of the power supply;
(3) Suppress EMI generated by the power supply equipment itself;
(4) Suppress EMI generated by other equipment and propagated through the power supply.
When the switching power supply itself is working and when the electronic equipment is in the switching state, terminal noise will appear at the input end of the power supply equipment, causing radiation and conduction interference. It will also enter the AC power grid and interfere with other electronic equipment, so effective measures must be taken to suppress it. . In terms of suppressing the radiated interference of EMI noise, electromagnetic shielding is the best way. In terms of suppressing the conducted interference of EMI noise, the use of EMI filters is a very effective means, and of course it should be combined with good grounding measures.
Internationally, various countries have implemented strict electromagnetic noise restriction rules. For example, the United States has FCC, Germany has FTZ, VDE and other standards. If electronic equipment does not meet noise limit rules, the product cannot be sold and used.
Due to the above reasons, power supply equipment must be designed and used with grid noise filters that meet the requirements.
3EMI noise and filter types
There are two types of EMI noise on the input leads of power supply equipment: common mode noise and differential mode noise, as shown in Figure 1. The EMI noise that exists between the AC input lead and the ground is called its common mode noise. It can be regarded as an interference signal with equal potential and the same phase transmitted on the AC input line, that is, the voltages V1 and V2 in Figure 1. The EMI noise that exists between the AC input leads is called differential mode noise. It can be seen as an interference signal with a phase difference of 180° transmitted on the AC input line, that is, the voltage V3 in Figure 1. Common mode noise is transmitted from the AC input line Differential mode noise is the interference current that flows between the AC input lines. The conducted EMI noise on any power input line can be represented by common mode and differential mode noise, and these two types of EMI noise can be regarded as independent EMI sources and suppressed respectively.
When taking measures to suppress electromagnetic interference noise, we should mainly consider suppressing common mode noise, because common mode noise accounts for the main part in the full frequency domain, especially in the high frequency domain, while differential mode noise accounts for a larger proportion in the low frequency domain, so it should be based on This characteristic of EMI noise dictates the selection of appropriate EMI filters.
Noise filters for power supplies can be divided into integrated and discrete types according to their shapes. The integrated type encapsulates inductors, capacitors, etc. in metal or plastic casings; the discrete type installs inductors, capacitors, etc. on a printed board to form a noise suppression filter. Which form to use depends on cost, features, installation space, etc. The integrated type has high cost, good features and flexible installation; the discrete type has lower cost but poor shielding and can be freely distributed on the printed board.
