Feedthrough filter selection method
With the rapid increase of the operating frequency of electronic devices, the frequency of electromagnetic interference is also getting higher and higher, and the interference frequency usually reaches hundreds of MHz or even GHz. Since the higher the frequency of the voltage or current, the easier it is to generate radiation, it is the interference signals with these high frequencies that cause the radiation interference to become more and more serious. Therefore, a basic requirement for a filter for solving radiated interference is to have a large attenuation of these high-frequency interference signals. This filter is a radio frequency interference filter. The effective filtering frequency of the common interference filter ranges from several kHz to several tens of MHz, and the effective filtering frequency of the RFI filter ranges from several kHz to above GHz.
A filter constructed in a conventional manner cannot be an RF filter. This is due to two reasons: The first reason is that the bypass capacitor in Figure 1 has a large parasitic inductance (resulting in series resonance and increased bypass impedance), resulting in a capacitor that does not have a lower impedance at higher frequencies. , can not play the role of bypass. The second reason is that the stray capacitance between the input and output of the filter causes high-frequency interference signal coupling, which makes the filter lose its effect on high-frequency interference. The solution to this problem is to use a feedthrough capacitor as a bypass capacitor. The feedthrough capacitor has very small parasitic inductance, very low bypass impedance, and eliminates high frequency coupling between the input and output due to the isolated mounting.
The various RF filters in this sample are based on feedthrough capacitors and are mounted in feedthrough mode (input and output are isolated by metal plates).
Although there are many varieties of RF filters in this sample, each model is designed with specific application requirements in mind, enabling designers to achieve satisfactory results in terms of performance, size, and cost. The factors to consider when choosing an RF filter are:
Cutoff frequency: The frequency at which the insertion loss of the filter is greater than 3dB is called the cutoff frequency of the filter. When the frequency exceeds the cutoff frequency, the filter enters the stop band. In the stop band, the interference signal is greatly attenuated. Depending on where the filter is used (signal cable filtering or power line filtering), two methods can be used to determine the cutoff frequency of the filter. When filtering the signal cable, it is determined according to the bandwidth of the effective signal that the cutoff frequency is greater than the bandwidth of the signal, so as to ensure that the useful signal is not attenuated. When filtering the power line or DC signal line, since the frequency of the effective signal is very low, the problem of signal distortion is not the main factor, so it is mainly determined according to the frequency of the interference signal, so that the interference frequency all falls on the stop band of the filter. Inside. The lower the cutoff frequency of the filter, the larger the size of the filter and the higher the price, so there is no need (when the frequency of the interference is not very low), do not blindly choose a filter with a low cutoff frequency.
Insertion loss: refers to the loss value (dB) of the filter in the stop band. Each filter has a table of insertion loss and frequency. When the filter is selected, the insertion is determined according to the frequency of the interference signal and the degree of attenuation required. Loss requirements. It should be noted that the insertion loss given on the product sample is measured in the 50 system. If the actual use condition is not 50, the insertion loss will be different.
Rated voltage: The voltage that the filter can withstand for a long time during normal operation. It is necessary to pay attention to the correct selection of DC and AC. In the AC application, the DC type must not be used. Otherwise, breakdown will occur easily. Since almost all electromagnetic compatibility tests have items of pulse interference, the effect of such high-voltage pulse interference should be considered when selecting a filter, and the withstand voltage value needs to have a certain margin.
Rated current: The current value that the filter can flow for a long time during normal operation. The rated current is determined by the lead diameter of the filter. The larger the wire diameter, the larger the rated current. For filter components, the rated current is also related to the saturation characteristics of the inductor. When the current exceeds the rated current, the performance of the filter will decrease.
Operating temperature range: The filter element can guarantee the predetermined performance and the ambient temperature in normal operation. The filter components in this sample have an operating temperature range of -55-+125C except for the special mark.
Filter volume: The volume of the filter is related to the filter's rated operating voltage, operating current, cutoff frequency, insertion loss, and manufacturing process. Filters with substantially the same electrical performance result in different volumes due to different manufacturing processes. When the electrical performance is close, the larger filter is less expensive (suitable for installations with large installation space).
The way the RF filter is installed has a large impact on the performance of the filter. First, the RFI filter must use a metal plate as the isolation board to isolate the input and output of the filter. Secondly, the filter should maintain a low impedance contact with the metal plate to ensure the bypass effect of the filter capacitor. It is best to install the filter on a tin or zinc plated aluminum plate or steel plate. In order to ensure a reliable connection, it is generally necessary to install an internal tooth gasket between the mounting flange of the filter and the insulation plate, and it is not possible to use a material such as a conductive adhesive to achieve a reliable connection. A problem to be aware of is that electrochemical corrosion occurs between the contact faces of different metals, resulting in an increase in contact resistance. After some devices are used for a period of time, the interference situation becomes severe due to the increased ground impedance of the filter. Especially when the low-frequency filtering effect of the filter is reduced, this factor should be considered.
Filter type
Type C feedthrough filter
It is a low self-induction device that avoids high-frequency ground-to-ground noise interference. It is low-cost and suitable for high-impedance sources and high-load applications.
L-type filter
This is a feedthrough filter with inductive and capacitive components that are typically used in circuits with low impedance sources and high impedance loads and vice versa. Note: The inductive component should face a low impedance source.
Pi filter
The Pi-type filter consists of two capacitive elements and an inductive element between the two capacitive elements, exhibiting low impedance to both the impedance source and the load. The Pi type filter provides better high frequency filtering performance than the C type and L type structure.
T-filter
The T-type filter consists of two inductive components and one capacitive component. Its circuit structure is characterized by high impedance from the input at either end. It is similar to the Pi-type filter structure, but no Pi-type filter is widely used in the field of switching conversion.
Simulation Giraffe Lights,LED Giraffe Lights,Simulation Garden Giraffe Lights,Outdoor Giraffe Lights of Garden
Sichuan Kangruntengpu Photoelectric Technology Co., Ltd. , https://www.tengpulighting.com