Measurement and Diagnosis of Common Mode Inductors 1

The design of power filters can usually be considered from both common mode and differential mode perspectives. The most important part of a common mode filter is the common mode choke coil. Compared with the differential mode choke coil, a significant advantage of the common mode choke coil is its extremely high inductance value and small size. An important issue to consider when designing a common mode choke coil is its leakage inductance, which is the differential mode inductance. Usually, the method of calculating leakage inductance is to assume it is 1% of the common mode inductance, but in reality, the leakage inductance is between 0.5% and 4% of the common mode inductance. The impact of this error may not be ignored when designing the optimal performance choke coil.  diode

The importance of leakage sensation

How is leakage sensation formed? A tightly wound and fully wound circular coil, even without a magnetic core, where all magnetic flux is concentrated within the core of the coil. However, if the circular coil is not fully wound or wound loosely, the magnetic flux will leak out of the core. This effect is proportional to the relative distance between wire turns and the magnetic permeability of the helical core. The common mode choke coil has two windings designed so that the current flowing through them is conducted in opposite directions along the core of the coil, resulting in a zero magnetic field. If, for safety reasons, the coils on the core are not double wound, there will be a considerable gap between the two windings, which will naturally cause magnetic flux leakage. This means that the magnetic field is not truly zero at the points of concern. The leakage inductance of a common mode choke is differential mode inductance. In fact, the magnetic flux related to the differential mode must leave the core at some point, in other words, the magnetic flux forms a closed loop outside the core, not just limited to the annular core.

If the core has differential mode inductance, the differential mode current will cause the magnetic flux inside the core to deviate from zero. If the deviation is too large, the core will experience magnetic saturation, making the common mode inductance basically the same as the inductance without a magnetic core. As a result, the intensity of common mode radiation is as strong as if there were no choke coils in the circuit. The magnetic flux deviation caused by differential current in a common mode toroidal coil can be obtained by the following equation:

In the formula, is the change in magnetic flux in the core, Ldm is the measured differential inductance, is the peak current of the differential mode, and n is the number of turns of the common mode coil.

Since it is possible to prevent magnetic saturation of the core by controlling the total B to be less than B saturation, there are the following rules: diode

Oscilloscope and a differential mode suppression network (DMRN). Firstly, use an oscilloscope to monitor the line voltage. Input the signal from channel A of the oscilloscope as follows, set the oscilloscope's time reference to 2ms/div, and then apply the trigger signal to channel A. When the AC voltage reaches its peak, a wired current will be generated, and the degradation of the filter performance is expected. The input of the differential mode suppression network (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 operates in the linear region, the increase in emission detected by the B channel during input current fluctuations does not exceed 6-10dB. Figure 1 shows the results of this test on the oscilloscope, and the curve above represents common mode emission; The curve below represents the line voltage. During the peak period of online voltage, the bridge rectifier conducts in the forward direction and transmits charging current. diode