Measurement and Diagnosis of Common Mode Inductors 2

If the common mode choke reaches saturation, the emission will increase as the input surge increases. If the common mode choke reaches strong saturation, the emission intensity will be the same as without a filter, which means it is easy to reach above 40dB.diode

These experimental data can be explained using other methods. The minimum emission value (when the line current is 0) is the effect exhibited by the filter without bias current. The ratio of peak emission to minimum emission, also known as the degradation factor, is used to measure the impact of line current offset on the actual performance of the filter. A large degradation factor indicates that the common mode choke core has not been properly used, and the "inherent degradation factor" of a good filter is roughly between 2-4. It is caused by two phenomena: first, the decrease in inductance caused by 60Hz charging current (as described above); Secondly, the forward and reverse conduction of the bridge rectifier. The equivalent circuit of common mode emission consists of a voltage source with an impedance of approximately 200pF, a diode impedance, and the common mode impedance of LISN, as shown in Figure 2. When the bridge rectifier is forward biased, voltage division occurs between the source impedance, 25, and LISN common mode impedance. When the bridge rectifier is reverse biased, voltage division occurs between the source impedance, rectifier bridge reverse bias capacitor, and LISN. When the reverse bias capacitance of the diode rectifier bridge is small, it has a certain effect on common mode filtering. When the rectifier bridge is forward biased, it has no effect on common mode filtering.diode

Figure 2 Equivalent circuit of common mode radiation

Due to the generation of partial pressure, the expected value of the intrinsic degradation factor is around 2. The actual value varies significantly, mainly depending on the actual size of the source impedance and the reverse bias capacitance of the diode rectifier bridge. In a circuit invented by Flugan, this principle is applied to reduce the conduction emission of the thyristor.

If the tester is quite cautious, a testing device similar to MIL-STD-461 can be used to detect the saturation characteristics of the common mode choke. The application of this principle is as follows: two current probes are used during testing, with the low-frequency probe monitoring the line current and the high-frequency probe only measuring the common mode emission current. The line current monitor serves as the trigger source. However, a potential hazard of using current probes is that the decay of differential current is a function of the symmetry of the winding wires inside the core. If the winding layout is carefully and reasonably arranged, a differential mode current attenuation of about 30dB can be obtained. Even if this attenuation value is reached, the measured differential mode component may still exceed the expected common mode component value. The following two techniques can be used to solve this problem: first, connect a high-order high pass filter with a turning frequency of 6kHz in series with the oscilloscope (note to match the terminal impedance with 50). Secondly, connect a wire between each 10 μ F capacitor and the power bus. In order to measure common mode radiation, the current probe should be clamped near these wires carrying extremely small line currents. In order to quickly and easily introduce the function of common mode choke, the following statement can be considered: "The magnetic fields on both sides of the common mode choke core cancel each other out, so there is no magnetic flux that saturates the core." Although this statement concretizes the intuitive description of the function of common mode choke, it is not actually the case.diode