Measurement and Diagnosis of Common-Mode Inductors 1

The design of power filters can generally be considered from two aspects: common mode and differential mode. The most critical component of a common mode filter is the common mode choke. Compared to differential mode chokes, a notable advantage of common mode chokes is their extremely high inductance value while maintaining a compact size. An important consideration in designing common mode chokes is their leakage inductance, also known as differential mode inductance. Typically, the leakage inductance is assumed to be 1% of the common mode inductance, though in reality it ranges between 0.5% to 4%. When designing chokes for optimal performance, this error margin may not be negligible.

The Importance of Leakage Inductance  diode

How is leakage inductance formed? In a tightly wound and fully circumferential toroidal coil, even without a magnetic core, all the magnetic flux is concentrated within the coil's "core." However, if the toroidal coil is not fully wound or is not tightly wound, the magnetic flux will leak from the core. This effect is proportional to the relative distance between turns and the permeability of the solenoid core. A common-mode choke has two windings, which are designed to carry currents in opposite directions through the core, resulting in a net magnetic field of zero. For safety reasons, if the coil on the core is not wound with double strands, there will be a significant gap between the two windings, naturally causing magnetic flux "leakage." This means the magnetic field is not truly zero at all concerned points. The leakage inductance of a common-mode choke is differential-mode inductance. In fact, the flux associated with differential mode must leave the core at some point—in other words, the flux forms a closed loop outside the core, not confined solely within the toroidal core. diode

If the core has a differential-mode inductance, then the differential-mode current will cause the magnetic flux within the core to deviate from zero. If the deviation is too large, the core will experience magnetic saturation, making the common-mode inductance essentially the same as that of an inductor without a magnetic core. As a result, the intensity of common-mode radiation will be the same as if there were no choke in the circuit. The magnetic flux deviation caused by differential-mode current in the common-mode toroidal coil can be derived by the following formula:

In the equation, ΔΦ represents the change in magnetic flux within the core, Ldm is the measured differential-mode inductance, Idm is the differential-mode peak current, and n denotes the number of turns in the common-mode coil.

By controlling B to keep it below the saturation level, the magnetic saturation of the core can be prevented, following the following principles:

The oscilloscope and a differential mode rejection network (DMRN). First, use the oscilloscope to monitor the line voltage. Input the signal to the A channel of the oscilloscope as follows: set the time base to 2ms/div, then apply the trigger signal to the A channel. When the AC voltage reaches its peak, line current is generated, and the degradation of filter performance is expected. The input of the DMRN is connected to the LISN, while the output is matched with a 50-ohm impedance and connected to the B channel of the oscilloscope. When the common-mode choke operates in the linear region, the measured emission increase on the B channel during input current fluctuations does not exceed 6–10dB. Figure 1 shows the oscilloscope display results of this test, with the upper curve representing common-mode emission and the lower curve representing line voltage. During the peak of line voltage, the bridge rectifier conducts in the forward direction and transmits charging current.diode