In EMI testing, how to measure the saturation of common mode coil cores using the principle of LISN

In EMI testing, it is often difficult to measure the saturation characteristics (overall or partial) of the common mode coil core. Through a simple test, we can see that the attenuation of the common mode filter is affected by the inductance reduction caused by the 60 degree influence Hz current. This test requires an oscilloscope and a differential mode rejection network (DMRN). First, monitor the line voltage with an oscilloscope. According to the following method, input a signal from channel A of the oscilloscope, set the time reference of the oscilloscope to 2ms/div, and then add a trigger signal to channel A. When the AC voltage reaches the peak, a wired current will be generated. At this time, the reduction in filter efficiency is expected. The differential mode rejection network (DMRN) is connected to the input LISN, connected to the B-channel of the oscilloscope, and the output terminal uses a 50 impedance. When operating in a linear region, the emission increase monitored by the B-channel during input current fluctuations should not exceed 6-10 dB. Figure 1 shows the test results on the oscilloscope, with the curve above showing common mode emission; The following curve is the line voltage. During the peak online voltage, the bridge rectifier conducts in the forward direction and the charging current is transmitted.

Figure 1. The degradation of the common mode choke caused by a 60Hz charging current displayed on the oscilloscope

If the common mode choke reaches saturation, as the input wave increases, the emission will increase. If the common mode choke reaches strong saturation, the emission intensity is the same as without adding a filter, which can easily reach over 40dB.

Other methods can explain these experimental data. When the filter has no bias current, it emits the effect of a minimum value (when the line current is 0). The ratio of peak emission to minimum emission, or degradation factor, is used to measure the impact of line current offset on the actual performance of the filter. The large degradation factor indicates that the magnetic core of the common mode choke is not properly used, and the "inherent degradation factor" of the filter is almost between 2-4. It arises from two phenomena: first, a decrease in inductance caused by a charging current of 60 Hz (as described above); The other is the positive and negative conduction of the bridge rectifier. The equivalent circuit of common mode emission consists of a voltage source with an impedance of about 200PF, a diode impedance, and a LISN, as shown in Figure 2. The common mode impedance is composed of. When the bridge rectifier is positively biased, a voltage divider occurs between the source impedance, 25, and the LISN common mode impedance. When the bridge rectifier is reversely biased, voltage splitting occurs at the source impedance, rectifier bridge reverse bias capacitance, and LISN. When the reverse bias capacitance of the diode rectifier bridge is small, common mode filtering has a certain effect. When the rectifier bridge is positively biased, common mode filtering is not affected.

Figure 2. Common mode radiation equivalent circuit

Due to partial pressure, the expected value of the intrinsic degradation factor is approximately 2. According to the actual size of the source impedance and the reverse bias capacitance of the diode rectifier bridge, the actual value varies greatly. Flugan utilizes this principle to reduce the conduction and emission of the ballast, which is an invented circuit.