1、Filters’ structure
Type π、Type L、Type T
The power line filter should be properly connected in order to suppress noise effectively in practical application. Combined filter’s structure and parameters,and get a better EMI suppression effect. The input and output ends of the EMI signal are reflected when the output impedance and load impedance of the filter is not equal.The attenuation of EMI noise of power line filter is related to the inherent insertion loss and reflection loss of the filter, which can be used to suppress EMI noise more effectively.
When designing and choosing to use EMI filters, we should pay attention to the correct connection of the filters in order to cause as much reflection as possible, so that the filters can cause large impedance mismatch in a wide frequency range, so as to obtain a better EMI suppression performance.
2、Design of filter structure
According to the definition and principle of EMC, the EMC filter circuit should not only suppress the EMI, generated by the electronic device, but also suppress the external EMI signal. In order to improve the suppression ability of the EMC filter circuit, it is better to install a low-pass filter at the input end, and this low-pass filter also has a strong suppression ability to the EMI of the electronic device itself.
As the spectrum of the EMI signal is very wide, it is difficult to meet the requirements by using an filter alone. Since the inductance filter of each specification is good for a certain frequency band, so it is better to use the inductance filter with three different frequency filtering characteristics: high, medium and low. Like Figure 1, an additional L0 low-pass filter is designed to improve the ability to suppress foreign conduction. If only consider to improve the ability to suppress the electronic device, the connect position of the C1、C2 can be moved to the front of the power line. That is, more closer to the input,better to suppress the interference.
Common mode interference signals are mainly transmitted by the distributed capacitance of electronic devices to the ground, as shown in figure 1, the C5 is the distributed capacitance of interference devices to the ground. The capacity of the C5 is related to the volume of the interference equipment and the distance to the ground. If the distance from the electronic equipment to the ground is fixed, the capacity of the C5 is about a dozen uF to a few thousand uF. Since the capacity of the C5 is very small and the impedance to the low frequency signal is very large, the common mode interference signals that can pass through the C5 are basically high frequency signals.
