DM choke

Occasionally, unbalanced filters may be tolerable — for example a single DM choke (i.e. on one line only). Or sometimes, in very low-power applications, just a plain decoupling capacitor (e.g. Cl) may suffice. Sometimes tuned filter stages are seen in... 

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. 

In Figure 10-1, we have shown both the CM and DM filter stages as being symmetrical (balanced). So for example, we have placed identical DM chokes on each of the L and N lines. In Fig. 10-1 we see that in fact the DM choke is also a part of the CM equivalent circuit (and vice versa). And since line impedance imbalance can cause CM noise to get converted into DM noise, it is always advisable to keep both the CM and DM stages symmetrical (balanced). 

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. 

But what is the peak current This is the term that is usually completely underestimated by most designers, and that is why they often don t realize that their DM choke is ineffective because it is saturating. 

The temptation to wind a few more turns on the DM choke (if present of course) in an effort to raise its inductance may just produce enough ampere-turns to saturate it. What happens... 

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. 

Therefore, we have to ensure that our DM choke does not saturate with a peak instantaneous current of 8 A. Both the CM and DM chokes must have copper thick enough to handle 1.3 ARMS. 

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... 

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

Schardl CL, Leuchtmann A, Tsai H-F, Collett MA, Watt DM, Scott DB. Origin of a fungal symbiont of perennial ryegrass by interspecific hybridization of a mutualist with the ryegrass choke pathogen, Epichloe typhina. Genetics 136 1307-1317, 1994. 

Experiments were cazvied out with a set-up of short-time action (the time of outflow from 5 to 20 s) with a cylindrical channel (the inner diameter 0 5 ima the relative length t/dm = 1.4) with a sharp entrance edge. Channels of somewhat different dimensions were also used. The liquid outflow into the atmosphere was accompanied with a sharp decrease in the flow rate at f> m const when the initial temperature was about or exceeded 0.9 rlier choking does not occur because of a short time of the liquid stay in the channel ( vio mcs in our experiments). Bat intensive volume boiling on fluctuation centres ensures a vaporization rate necessairy for the choking of the flow. Wg. 2 gives the results of eaperiments with several liquids the values of have been calculated by formula... 

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