Engineers must know: Use and selection tips of power supply filters

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Engineers must know: Use and selection tips of power supply filters [Copy link]

During the experimental test, we often encounter such a situation: although the design engineer connected a power filter to the power line of the equipment, the equipment still cannot pass the "conducted interference voltage emission" test. The engineer suspects that the filtering effect of the filter is not good. He keeps changing the filter, but still cannot get the ideal effect.
The reasons for the equipment exceeding the standard are nothing more than the following two aspects:
1) The interference generated by the equipment is too strong
2) The filtering of the equipment is insufficient

For the first case, we can solve it by taking measures at the source of disturbance to reduce the intensity of disturbance, or increase the order of the power filter to improve the filter's ability to suppress disturbance. For the second case, in addition to the poor performance of the filter itself, the installation method of the filter also has a great impact on its performance. This is often overlooked by design engineers.
In many tests, we can make the equipment pass the test smoothly by changing the installation method of the filter. The following are some common examples of the impact of incorrect filter installation methods on filter performance.

Input line is too long

After the power cord of many devices enters the chassis, it is connected to the input end of the filter through a long wire. For example, the power cord enters from the rear panel of the chassis, runs to the power switch on the front panel, and then returns to the rear panel to connect to the filter. Or the installation location of the filter is far from the power cord entrance, causing the lead to be too long (as shown in Figure 1).

Because the lead from the power inlet to the filter input is too long, the electromagnetic interference generated by the equipment is re-coupled to the power line through capacitive or inductive coupling. The higher the frequency of the interference signal, the stronger the coupling, causing the experiment to fail.

Parallel routing
Some engineers often bundle cables together to make the wiring inside the chassis beautiful, which is not allowed for power cables. If the input and output lines of the power filter are routed in parallel or bundled together, due to the distributed capacitance between the parallel transmission lines, this routing method is equivalent to connecting a capacitor in parallel between the input and output lines of the filter, providing a path for the interference signal to bypass the filter, resulting in a significant decrease in the performance of the filter, and even failure at high frequencies (as shown in Figure 2). The size of the equivalent capacitance is inversely proportional to the distance between the wires and directly proportional to the length of the parallel routing. The larger the equivalent capacitance, the greater the impact on the filter performance.

This situation of grounding and shell
is also quite common. When many engineers install filters, the shell and chassis of the filter are poorly overlapped (with insulating paint); at the same time, the grounding wire used is long, which will cause the high-frequency characteristics of the filter to deteriorate and reduce the filtering performance.
Because the grounding wire is long, the distributed inductance of the wire cannot be ignored at high frequencies. If the filter is well overlapped, the interference signal can be directly grounded through the shell. If the shell and chassis of the filter are poorly overlapped, it is equivalent to a distributed capacitance between the shell (ground) of the filter and the chassis, which will cause the grounding impedance of the filter to be large at high frequencies, especially near the frequency of the resonance of the distributed inductance and distributed capacitance, the grounding impedance tends to infinity.

The impact of poor filter grounding on filter performance: Due to poor filter grounding and large grounding impedance, some interference signals can pass through the filter (as shown in Figure 3). In order to solve the poor overlap, the insulating paint on the chassis should be scraped off to ensure that the filter housing and the chassis have a good electrical connection.
In this installation method, the filter housing and the chassis have good contact, which can block the opening of the power cord on the chassis and improve the shielding performance of the chassis; in addition, the input and output lines of the filter are isolated by the chassis shield, eliminating the interference coupling between the input and output lines and ensuring the filtering performance of the filter.