CM choke

Typical practical values for the inductance of a CM choke in medium-power converters range from 10 to 50 mH (per leg). The DM choke is always much smaller (in inductance, but not in size as we will see). Typical values for the DM choke are 500 iH to 1 mH. 

Note The reader is cautioned that there are several widely used but confusing symbols for the CM choke found in schematics in related literature. But whatever the symbol, as long as it is meant to serve as a common mode choke, the direction of the windings must be as shown for the toroid in Figure 10-1. 

If we reverse the current direction in one of the windings of a CM choke, then it becomes a DM choke (for both lines). However, now it is also subject to the flux produced by the ac line input current (no cancellation occurs). DM chokes, in general, should always be put through a saturation check — because of the impedance they present to the line current. 

Theoretically, there is no need for any air gap in a common mode choke, because the flux due to the line current is expected to cancel out completely. In practice, it doesn t fully, mainly due to slight differences in the individual winding arrangement (despite the equal number of turns). At a minimum, this causes the core to get dc-biased in one direction, and thereby cause an imbalance in the inductance it presents to the two lines. This would expectedly degrade the EMI performance, but in extreme cases, the core may even saturate. Note that core saturation in the filter is clearly not a catastrophic event (like the saturation of the main inductor/transformer of the converter can be), but since it is accompanied by severely worsening EMI-suppression efficacy, we need to prevent that too. Therefore, as in a forward converter transformer, a small air gap is usually present, even in a CM choke. 

The leakage inductance of the CM choke appears as a DM filter element too. 

In another well-documented EMI problem at a leading power supply manufacturing house, it was discovered that the CM choke had to be rotated by 90° (not 180°) to comply. That clearly spells bad news if the unit is already in production, because it means the PCB layout has to be redesigned (and perhaps the power supply needs to be requalified too). 

So in DM chokes, the major concern is core saturation. But in CM chokes, underestimating the current will make it run excessively hot, due to higher copper losses. 

Knowledge of the input RMS current is necessary to correctly estimate the copper losses in both the CM and DM chokes, whereas knowledge of the peak current is necessary to correctly estimate the DM core volume (its energy handling capability). In a CM choke, the input ac line current effectively cancels out, so its saturation is usually not considered either a possibility or a concern. That means it is chopped out of a vertically offset sine wave of that time period. 

Y-caps are also present on output rails Y-caps may be required to be rated > 1500V Electrolytic (bulk) capacitor to damp out input instability Leakage Inductance of CM choke is DM choke... 

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