Common mode Choke inductor’s Measurement Diagnosis 2

author: MagTop
common mode choke ,common mode choke filter,common-mode filter
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  1. The LISN principle is used to measure the saturation characteristics of common mode choke ,common mode choke filter.
  It is often difficult to measure the saturation characteristics of the core (whole or part) of a common mode coil. A simple test shows to what extent the attenuation of a common-mode filter isaffected by the inductance reduction caused by the 60Hz  scheduling current. An oscilloscope and a differential mode suppression network (DMRN) are required for this test. First, The oscilloscope is used to monitor the line voltage. Input signals from channel A of the oscilloscope in the following way, set the
time reference of the oscilloscope as 2ms/div, and then add the trigger signal to channel A. When the AC voltage reaches the peak, the wired current will be generated. At this time, the degradation of the filter efficiency is expected. The input of the DMRN is connected to the LISN, and the output is matched with an impedance of 50 and connected to the B-channel of the oscilloscope. When the common mode choke is operating in the linear region, during the input current fluctuation, the emission
increment detected in the B channel does not exceed 6-10dB.

During the peak in - line voltage, the bridge rectifier guides on and transmits the charging current.
If the common mode choke reaches saturation, then the emission will increase as the input surge increases. If the common mode choke reaches strong saturation, the emission intensity is the sameas it would be without the filter, that is, it is easy to reach morethan 40dB. These experimental data can be interpreted in other ways. The emission minimum (when the line current is 0) is the effect shown when the filter has no bias current. The ratio of peak emission to minimum emission, the degradation factor, is used to measure the effect of the line current offset on the actual effect of the filter. A large degradation factor indicates that the common mode choke core is not used properly at all, and the "inherent degradation factor" of a better filter is roughly 2-4. It iscaused by two phenomena: first, the decrease in inductance
caused by the 60Hz charging current (as described above); Second, the forward and reverse guide of the bridge rectifier. The Equivalent circuit of common-mode emission consists of a voltage source withan impedance of about 200pF, the diode impedance, and the common-mode impedance of the LISN. When the bridge Rectifier is forward biased, a voltage division occurs between the source impedance, 25, and the LISN common-mode impedance.

When thebridge rectifier is reverse-biased, there is a voltage division between the source impedance, the rectifier bridge reverse-biased capacitance, and the 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 biased forward, common-mode filtering is not affected.
Due to the partial pressure, the expected value of the inherent degradation factor is about 2. The actual value varies considerablydepending on the source impedance and the actual size of 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 ballast.

5. The method of measuring saturation characteristics of common mode choke by current principle If the tester is cautious, the saturation characteristics of a common mode choke can be tested with a device similar to the one in MIL-STD-461. The application of this principle is as follows: Two current probes are used in the test, the low frequency probe monitors the line current, and the high frequency probe only measures the common mode emission current. The line current monitor acts as the trigger source. However, one of the pitfalls of using current probes is that differential mode current attenuation is a function of the symmetry of the winding wires
in the tube core. If the winding layout is carefully and reasonablyarranged, the differential mode current attenuation of about 30dB can be obtained. Even if this attenuation value is reached, the measured differential mode component may exceed the expected common mode component value. Two techniques can be used to solve this problem: First, a high-order high-pass filter with a 6kHz turning frequency is connected to the oscilloscope in series (note that the terminal impedance of 50 is matched). Second, a wire is connected between each 10μF capacitor and the power bus. To measure common-mode radiation, the current probe should be clamped near these wires carrying minimal line currents.

Common mode Choke inductor’s Measurement Diagnosis 1#