I. Structure of a Power Supply Filter
Power filters are generally designed as passive filters consisting only of resistors, capacitors, and inductors, without active components like transistors. The right figure shows an example of a power filter. The power supply is connected to the top of the filter, and there is a common-mode inductor at the power supply terminal. This means the two power supply wires are wound in the same direction on the iron core. If there is a common-mode signal on the power supply wires, the magnetic fields generated by the signal in the common-mode inductor will add together, resulting in a larger impedance. Conversely, the magnetic fields generated by the differential-mode signal in the common-mode inductor will cancel each other out, allowing the signal to flow through the common-mode inductor. The current flowing through the power supply is primarily common-mode, but noise can also appear in differential-mode form. To suppress differential-mode noise, a separate differential-mode inductor is needed, or individual inductors for each phase.
Special safety decoupling capacitors are used in power filters, which are divided into two categories: X capacitors and Y capacitors.
X capacitor: Suppresses differential mode interference (interference between power lines).
Y capacitor: Suppresses common-mode interference (interference between each power line and ground).
Because increasing the Y capacitor will increase the leakage current of the appliance, and the leakage current of the appliance has a specified range, the Y capacitor cannot be too large and is generally smaller than the X capacitor.
X and Y capacitors are safety capacitors, meaning that their failure will not cause electric shock or endanger personal safety. Both have a self-healing function, which will restore the partially short-circuited portion to its original insulating state.
II. How to correctly install a power filter
1. The power supply filter must not have an electromagnetic coupling path.
① The power input cable is too long;
② The input and output lines of the power supply filter are too close together.
Both of these installation methods are incorrect. The root of the problem lies in the significant electromagnetic coupling path between the filter's input and output wires. As a result, EMI signals present at one end of the filter can escape its suppression and couple directly to the other end without being attenuated by the filter. Therefore, the filter's input and output must be effectively separated.
2. Do not bundle cables together.
Generally speaking, when installing power filters in electronic devices or systems, it is important to note that the power supply and load cables should never be bundled together. This would undoubtedly increase the electromagnetic coupling between the filter's input and output terminals, severely compromising the filter's and equipment's shielding ability to suppress EMI signals.
3. Avoid using long grounding wires as much as possible.
The wiring length connecting the output of the power filter to the frequency converter or motor should not exceed 30 cm.
Because excessively long grounding wires significantly increase grounding inductance and resistance, they severely compromise the filter's common-mode rejection capability. A better approach is to securely fasten the filter's shield to the device's power input housing using metal screws and star-shaped spring washers.
4. The input and output lines of the power filter must be kept at a distance.
The input and output lines of the power filter must be spaced apart and should never be parallel, in order to avoid reducing the filter's efficiency.
5. The power filter housing must have good contact with the chassis housing.
The metal casing of the inverter-specific filter must have good surface contact with the chassis, and the grounding wire must be properly connected.
6. Twisted-pair cables should be used for connecting power filters.
Shielded twisted-pair cables are preferred for the input and output connections of the power filter, as they can effectively eliminate some high-frequency interference signals.
