In the process of experimental testing, we often encounter a situation where, despite the PCB design engineer connecting the power supply filter to the equipment power line, the equipment still fails the “conducted disturbance voltage emission” test. The engineer might conclude that the filter’s performance is inadequate, and repeatedly replacing it does not yield the desired results.
Analyzing PCB equipment that exceeds standards generally involves two main aspects:
1. The disturbance generated by the equipment is too strong.
2. Insufficient filtering within the equipment.
For the first issue, we can address it by mitigating the disturbance at its source or enhancing the power supply filter’s order to improve its disturbance suppression capability. For the second issue, besides the filter’s inherent performance, the installation method of the filter significantly impacts its effectiveness—a detail often overlooked by design engineers.
In many cases, modifying the filter’s installation method can help the equipment pass the test successfully. Below are examples illustrating how common filter installation methods affect filter performance.
1. **Input Line is Too Long**
2. After the power cord of many PCB devices enters the chassis, it passes through an extended length of wire before connecting to the input end of the filter. For instance, the power cord might enter through the rear panel of the chassis, extend to the power switch on the front panel, and then return to the rear panel to connect to the filter. Alternatively, the filter may be located far from the power cord entrance, resulting in an excessively long lead wire.
3. Because the lead wire from the power inlet to the filter input is too long, electromagnetic disturbances generated by the device can be re-coupled to the power line through capacitive or inductive coupling. The higher the frequency of the disturbance signal, the stronger the coupling, which can lead to experimental failure.
4. **Flat Walking Line**
5. To achieve a neat appearance inside the case, some PCB engineers bundle cables together. However, this practice is not suitable for power cables. If the input and output lines of the power filter are routed in parallel or bundled together, the distributed capacitance between these parallel transmission lines effectively creates a capacitor between the input and output lines of the filter. This capacitor provides a path for disturbance signals to bypass the filter, significantly degrading the filter’s performance and potentially causing failure at high frequencies. The size of the equivalent capacitance is inversely proportional to the distance between wires and directly proportional to the length of the parallel traces. A larger equivalent capacitance has a greater negative impact on filter performance.
6. **Grounding and Housing**
7. This issue is also relatively common. When many PCB engineers install the filter, they often neglect the connection between the filter housing and the chassis, especially if insulating paint is present. Additionally, using a long grounding wire can lead to deterioration in the filter’s high-frequency characteristics and reduced filtering performance.
8. Due to the long grounding wire, the distributed inductance of the PCB wire becomes significant at high frequencies. If the filter is well-connected, interference signals can be grounded directly through the housing. However, if the connection between the filter shell and the chassis is poor, it acts like a distributed capacitance between the filter shell (ground) and the chassis. This can result in high ground impedance at high frequencies, particularly around the frequency where distributed capacitance resonates, causing the ground impedance to approach infinity.
9. The impact of poor PCB filter grounding on performance includes an increased ground impedance, allowing some disturbance signals to bypass the filter. To address poor PCB grounding, the insulating paint on the chassis should be removed to ensure a good electrical connection between the filter housing and the chassis.
10. In this installation mode, ensuring good contact between the PCB filter shell and the chassis can block openings for the power cord and enhance the chassis’s shielding performance. Additionally, having chassis shielding between the input and output lines of the PCB filter ensures phase isolation, eliminating disturbance coupling between the input and output lines, thereby maintaining the filtering performance of the PCB filter.
Analyzing PCB equipment that exceeds standards generally involves two main aspects:
1. The disturbance generated by the equipment is too strong.
2. Insufficient filtering within the equipment.
For the first issue, we can address it by mitigating the disturbance at its source or enhancing the power supply filter’s order to improve its disturbance suppression capability. For the second issue, besides the filter’s inherent performance, the installation method of the filter significantly impacts its effectiveness—a detail often overlooked by design engineers.
In many cases, modifying the filter’s installation method can help the equipment pass the test successfully. Below are examples illustrating how common filter installation methods affect filter performance.
1. **Input Line is Too Long**
2. After the power cord of many PCB devices enters the chassis, it passes through an extended length of wire before connecting to the input end of the filter. For instance, the power cord might enter through the rear panel of the chassis, extend to the power switch on the front panel, and then return to the rear panel to connect to the filter. Alternatively, the filter may be located far from the power cord entrance, resulting in an excessively long lead wire.
3. Because the lead wire from the power inlet to the filter input is too long, electromagnetic disturbances generated by the device can be re-coupled to the power line through capacitive or inductive coupling. The higher the frequency of the disturbance signal, the stronger the coupling, which can lead to experimental failure.
4. **Flat Walking Line**
5. To achieve a neat appearance inside the case, some PCB engineers bundle cables together. However, this practice is not suitable for power cables. If the input and output lines of the power filter are routed in parallel or bundled together, the distributed capacitance between these parallel transmission lines effectively creates a capacitor between the input and output lines of the filter. This capacitor provides a path for disturbance signals to bypass the filter, significantly degrading the filter’s performance and potentially causing failure at high frequencies. The size of the equivalent capacitance is inversely proportional to the distance between wires and directly proportional to the length of the parallel traces. A larger equivalent capacitance has a greater negative impact on filter performance.
6. **Grounding and Housing**
7. This issue is also relatively common. When many PCB engineers install the filter, they often neglect the connection between the filter housing and the chassis, especially if insulating paint is present. Additionally, using a long grounding wire can lead to deterioration in the filter’s high-frequency characteristics and reduced filtering performance.
8. Due to the long grounding wire, the distributed inductance of the PCB wire becomes significant at high frequencies. If the filter is well-connected, interference signals can be grounded directly through the housing. However, if the connection between the filter shell and the chassis is poor, it acts like a distributed capacitance between the filter shell (ground) and the chassis. This can result in high ground impedance at high frequencies, particularly around the frequency where distributed capacitance resonates, causing the ground impedance to approach infinity.
9. The impact of poor PCB filter grounding on performance includes an increased ground impedance, allowing some disturbance signals to bypass the filter. To address poor PCB grounding, the insulating paint on the chassis should be removed to ensure a good electrical connection between the filter housing and the chassis.
10. In this installation mode, ensuring good contact between the PCB filter shell and the chassis can block openings for the power cord and enhance the chassis’s shielding performance. Additionally, having chassis shielding between the input and output lines of the PCB filter ensures phase isolation, eliminating disturbance coupling between the input and output lines, thereby maintaining the filtering performance of the PCB filter.
