Application of Conducted Noise Analysis Technology in Filters

Conducted emission is one of the important issues in electromagnetic compatibility design. In order to meet the requirements of the standards for conducted emission limits, EMI filters are usually used to suppress the conducted noise generated by electronic products. Quickly selecting or designing a filter that meets the needs is the key to solving the problem. Conducted noise analysis techniques include common mode noise, differential mode noise analysis, common mode impedance, and differential mode impedance analysis, which are the basis of filter design.

Common-mode noise and differential-mode noise

Conducted noise is divided into two categories based on transmission characteristics: differential mode noise and common mode noise. Differential mode noise occurs when the current directions of two power lines are opposite to each other, while common mode noise occurs when the current directions of all power lines are the same, as shown in Figure 1. In general, common mode is the biggest problem, which is caused by improper grounding of stray capacitance .

Figure 1 Common mode noise and differential mode noise

If there are unequal load or line impedances, the common-mode current will be converted into part common-mode current and part differential-mode current. When the power system supplies power to the circuit , if the circuit has unequal impedances and there is common-mode noise at the output of the power supply, the common-mode noise will act on the circuit in a differential manner, and the circuit may fail. Therefore, when generating common-mode current, the common-mode noise must be reduced first, and the impedance must be balanced second. In addition, due to the characteristics of common mode and differential mode, the frequency of common-mode current is greater than that of differential mode. Therefore, common-mode current will generate a large amount of RF radiation and will be inductively and capacitively coupled with adjacent components and circuits. In actual power circuits, differential-mode noise is more like a voltage source, and common-mode noise is more like a current source, which makes common-mode noise more difficult to eliminate. Common-mode noise, like all current sources, requires a flow path. Because its path includes the chassis, the housing may become a large high-frequency antenna.

Common-mode noise and differential-mode noise analysis

In the electromagnetic compatibility laboratory, people use LISN and receiver to complete the conducted emission test, and the test results will give the total noise characteristics on the power line.

Figure 2 is a schematic diagram of using LISN to test power supply noise. Since the LISN output uses a standard 50Ω impedance, the noise voltages obtained by the two LISNs are:

Figure 2 LISN test diagram

VL=25xIcm+50xIdm

VN=25xIcm－50xIdm

It is not possible to separate common mode and differential mode noise using a standard LISN, but it is possible with the help of some special devices. For example, LISN UP, CM/DM separator, ESA2000 and PREMIPRO can all accomplish the noise separation task. Figure 3 shows the schematic diagram of the CM/DM separator, which is a transformer-based device that uses the principle that common mode voltage cannot make the transformer work.

Figure 3 Schematic diagram of CM/DM separator

At this point, we have learned whether the product's conducted emission meets the standard requirements, and analyzed the characteristics of differential mode noise and common mode noise (see Figure 4 (a) and Figure 4 (b)). The next step is to select or design a filter to solve the conducted emission problem.

Figure 4 (a) Total noise and differential mode noise Figure 4 (b) Total noise and common mode noise

Analysis of Power Supply Input Impedance Characteristics

The filter insertion loss given by the filter manufacturer is the performance in a 50Ω standard impedance system. It is well known that the input impedance of the power supply has discontinuity as the frequency changes, and the insertion loss characteristics of the filter also vary greatly as the impedance changes. The attenuation brought by a 100?H inductor and a 100nF capacitor under ideal conditions are shown in Figure 5(a) and Figure 5(b) respectively.

Figure 5 (a) Attenuation of a 100µH inductor (ideal) Figure 5 (b) Attenuation of a 100nF capacitor (ideal)

In order to give full play to the performance of the filter, before selecting or designing the filter, it is necessary to analyze the input impedance of the power port, including common mode impedance, differential mode impedance, common mode noise phase angle, and differential mode noise phase angle. Impedance testing can be done with the help of a dedicated impedance tester or a conduction analyzer.

Power filter

There are usually four techniques for power supply filtering to curb interfering noise. In actual use, two or even more of them are mixed. They are:

Adding a capacitor between the positive and negative power lines is called an X capacitor.

The capacitor added between each power line and the ground line is called Y capacitor.

Common mode containment (the containment coils on the two power lines are wound in the same direction).

Differential mode containment (each power line has its own containment coil).

The filter test template can be used to illustrate the filtering effect of each component. The filter model is shown in Figure 6. The analysis results are shown in Figures 7, 8, 9, and 10.

Figure 6 Power filter model

Figure 7 (a) Differential mode noise before and after using only 100µF differential mode capacitors Figure 7 (b) Common mode noise before and after using only 100µF differential mode capacitors

Figure 8 (a) Differential mode noise before and after using only 0.01µF common mode capacitors Figure 8 (b) Common mode noise before and after using only 0.01µF common mode capacitors

Figure 9 (a) Differential mode noise before and after adding 227μH differential mode inductor Figure 9 (b) Differential mode noise before and after adding 227μH differential mode inductor

Figure 10 (a) Differential mode noise before and after full filtering Figure 10 (b) Common mode noise before and after full filtering

Solution

The common mode noise, differential mode noise analysis, common mode impedance, differential mode impedance analysis, and noise phase angle test in the conducted noise analysis are all preparations for the final solution. The electronic products meet the limit requirements of conducted emission, which is ultimately achieved through power supply filters. However, how to quickly and accurately determine the parameters in the filter based on the above analysis results is not a simple task. Using special filter design software, the test data can be imported into the dedicated software, which can help engineers quickly get customized filters for the product. The application scheme is shown in Figure 11.

Figure 11 Automatic filter design

Conclusion

Usually, the limits on conducted emissions are for the input end of the power cord, and do not include the output end of the power supply. The noise analysis method introduced in this article can refer to the common-mode noise and differential-mode noise tests at the output end of the power supply. For example, using two current probes and a CM/DM separator (or LISN UP) can quickly diagnose the common-mode noise at the output end of the power supply. If the requirements are not high, an inverted LISN and CM/DM separator can also be used to test the common-mode noise at the output end of the power supply.