The function of a power filter is to suppress electromagnetic noise. The function of a power filter is to obtain a specific frequency power signal or eliminate a specific frequency power signal rope by inserting a power filter into the power system. To prevent unnecessary losses, we need to understand the use of power filters. In order to use them, we also need to know how to choose a power filter, as follows:

1. Rated voltage
The rated voltage is the operating voltage of the power EMI filter when used at a specified power frequency, and it is also the allowable voltage value for the filter. If used in a 50Hz single-phase power supply filter, the rated voltage is 250V; The filter used in a 50Hz three-phase power supply has a rated voltage of 440V. If the input voltage of the filter is too high, it will damage the internal capacitor.
2. Rated current
Rated current (Ir) is the continuous operating current allowed under rated voltage and specified ambient temperature conditions.
As the ambient temperature increases, or due to copper losses in the inductance wire, magnetic core losses, and ambient temperature, the operating temperature is higher than room temperature, it is difficult to ensure the performance of insertion loss. We should choose the rated current of the filter based on the actual possible working current and working environment temperature.
Unless otherwise specified, the rated current given in the EMI filter manual is at room temperature+25 ℃ (nominal temperature), and the typical insertion loss or curve given also refers to a value of+25 ℃. The relationship between high working current (Imax), rated current, and temperature is as follows: in Equation 1.0 of String 3: Imax is the high working current, Ir is the rated working current at room temperature, Tmax is the high working temperature+85 ℃, Ta is the actual working temperature, and Tr is the room temperature+25 ℃. According to equation 2.0, an example of the relationship between Imax/Ir and Ta is given:+25 ℃, Imax=Ir+ 45℃，Imax=0.816Ir;+ 85 ℃, Imax=0. In addition, some foreign filter companies stipulate that+40 ℃ (nominal temperature) is the working current value Ir
When the IEC climate class is 25/085 and the specified ambient temperature is+40 ℃ (nominal temperature), check the allowable working current curve for other ambient temperatures.
One of the main reasons affecting the relationship between working current and environment is the soft magnetic material in the filter. EMI filters generally use a high permeability soft magnetic material, manganese zinc ferrite, with an initial magnetic permeability μ I=7000~10000, but its Curie point temperature is not high, only around 130 ℃. The higher the magnetic permeability, the lower the Curie point temperature. After passing through the Curie point, the magnetic permeability rapidly decreases, leading to a decrease in the inductance value in the filter and seriously affecting the filtering effect. Therefore, it is necessary to correctly select the rated current of the power filter based on the working temperature, or improve the heat dissipation conditions (working environment) of the filter to ensure its installation and use.
3. Insertion loss
(1) Insertion loss definition: Insertion loss is one of the important technical parameters for EMI power filters. The central issue considered by designers and engineering applications is to achieve the highest possible insertion loss while ensuring that the filter's safety, environmental, mechanical, and reliable performance meet relevant standards. The insertion loss of a filter is a function of frequency, expressed in dB (decibels). When the circuit is not connected to a filter, the voltage (power) of the signal source at the receiving circuit end is U (P), and the input voltage (power) at the receiving end is U (P) after the filter is connected. The definition of insertion power consumption I.L (InsertionLoss) can be derived using the following equation: assuming that the actual load impedance remains unchanged before and after the filter is inserted, Therefore, each power in equation 1.1 can be replaced by its corresponding expression for load voltage and impedance: the insertion loss represented in the equation needs to be measured at frequency by removing and inserting a filter.
(2) The insertion losses of EMI power filters include common mode (represented as CM) insertion losses and differential mode (represented as DM) insertion losses. The specific testing methods for them have been described in CISPR Publication No. 17, and will not be explained here. For example, the insertion loss of a DNF05-H-6AEMI filter produced by a certain manufacturer measured according to relevant standards.
(3) The factors that affect the insertion loss of power EMI filters include impedance matching and installation. In practical applications, the impedance of the input and output terminals of the EMI filter is no longer 50 Ω as measured in the curve in Figure 2.3, so its attenuation of the interference signal will not be equal to the insertion loss given in the product standard or manual. If the network structure and parameters of the EMI filter are reasonable and installed properly, it is possible to achieve insertion losses better than those specified in the standard. On the contrary, if the network configuration and parameter selection are improper, and there are problems with installation, it may not achieve good application results, but instead will have the opposite effect, as shown in Figure 2.5, where insertion loss gain occurs. Another influencing factor is the operating temperature and rated operating current of the filter. The insertion loss measurement standard for EMI filters, as specified in CISPR Publication No. 17, MIL-STD-220A and GB7343-87, all emphasize the need to measure its insertion loss under rated current loading conditions. As mentioned earlier, this is because the inductor L in the filter uses ferrite or other magnetic materials, and under high current operation, the magnetic saturation state causes performance degradation. As shown in Figure 2.6, the test situation of a problematic EMI filter is shown. Curve ① shows the insertion loss curve tested under a normal 50 Ω system, while Curve ② shows the insertion loss curve tested under a 50 Ω system and a rated current of 30A. The difference between the two is quite significant.
4. Impedance matching
(1) When choosing a filter for impedance matching reasons, the first step is to choose the filter circuit and insertion loss performance that are suitable for your use. The reason for choosing the filtering circuit first is different from the traditional concept of filters operating under matching conditions. The so-called matching means that the filter needs to perform the expected processing or transformation of a portion of the spectrum while maintaining the amplitude of the input/output signal unchanged (or a fixed proportion). Unlike EMI power filters, which are low-pass filters with power frequency as the conduction object and operate under mismatched conditions, Because matching cannot be achieved in practical applications, such as the input impedance of the filter RI - the source impedance of the power grid changing with the size of electricity consumption, and the output impedance of the filter Rl (load impedance) - the source impedance changing with the size of the power load. To achieve ideal suppression effect, the correct impedance matching should be followed. No matter how complex the power EMI filter is, its common mode and differential mode filtering networks can be abstracted.
(2) Impedance mismatch analysis can analyze that in EMI power filter circuit networks, the inductance L is generally regarded as a high resistance component and the capacitance C is regarded as a low resistance component. In order to achieve better filtering results, according to the mismatch principle of the filter: if the actual load is inductive high resistance, choose a filter with an output load of capacitive low resistance; If the actual load is capacitive low resistance, choose a filter with an output load of inductive high resistance. Similarly, for the input impedance and grid source impedance of the filter, the principle of impedance mismatch should also be followed to select the filter.
According to Equation 1.4, the greater the difference between Zo and Rl, ρ The larger the size, the greater the reflection generated by the port. When the impedance of both ends of the EMI filter is in a mismatch state for the controlled interference signal, the EMI signal will generate strong reflections on its input and output ports. In this way, the attenuation of the EMI signal by the filter is equal to the inherent insertion loss of the filter plus the reflection loss. In the practical use of EMI filters, impedance mismatch can be used to achieve more effective suppression of EMI signals. This is why when choosing an EMI filter, it is important to carefully analyze the correct combination of its port impedance to generate as much reflection as possible and achieve effective control of the EMI signal.
The ability of EMI filters to suppress EMI signals not only depends on the insertion loss measured by the filter in a 50 Ω system, but also on the correct termination of the filtering network with the EMI signal source and load. Therefore, when selecting filters, special attention should be paid to the label content on the EMI filter to see if it accurately identifies the parameters and network structure of the filtering network. Obviously, EMI filters that neither provide network parameters nor network structure bring trouble to correct termination and optimization applications.
In addition, some EMI filter labels indicate power and load termination, which may be set for a specific electronic device and have no universal significance, and can only be one of the recommended termination methods by manufacturers. When applying EMI filters, it is necessary to analyze the network structure of EMI filters and the equivalent impedance of the access circuit in detail. Terminating according to the above impedance matching principles can correctly achieve the expected purpose.
From the above, we understand that when choosing, we should also pay attention to the rated voltage, rated current, insertion loss, and other related aspects. Nowadays, electronic products aim to meet the requirements of miniaturization, high performance, high precision, high reliability, and high responsiveness, resulting in high distribution density of circuit components and greatly reduced circuit volume. However, the more exquisite the circuit, the more components will be squeezed into a small space, increasing the chances of interference, among which electromagnetic interference and noise are troublesome. The emergence of power filters solves this problem.