DM noise

Looking at Figure 9-1, L stands for Live (or Line or Phase ), N for Neutral, and E is the Safety Ground or simply, earth. EUT stands for Equipment Under Test. Note that the earth is shown represented by the IEC symbol for Protective Earth (ground with a circle around it) and is occasionally labeled PE in literature. The DM noise generator is across the L and N pair. It tries to push/pull a current Idm through these two wires. No current flows through the earth connection on account of this noise source. 

Note There is nothing special about the DM noise current direction indicated in Figure 9-1. It can well be the other way around — that is, going in through either L or N, and coming out of the other. In off-line power supplies, we will see that in fact, the direction reverses every ac half-cycle. 

Figure 9-3 Impedance Presented to the CM and DM Noise Generators by the LISN...

The transformer-based device shown in Figure 9-5 exploits the fact that common mode voltages cannot cause transformer action — because transformer action requires that a differential voltage be applied, so as to produce current in the windings, and thereby causes the flux to swing within the core. Unlike the LISN MATE, in this case both CM and DM noise components are outputted. This device used to be available from AEMC in France, at www.aemc.fr. 

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). 

This high-frequency voltage ripple shown in Figure 11-1 is in effect the DM noise generator. It is essentially a voltage source (VESR hf), but producing noise in the form of a noise current Idm-... 

Therefore, the DM noise generator is modeled as a voltage source during the times when the diodes are ON, but as a current source during the times when the diodes are all OFF. 

Since about 1971 the phenomena of input oscillations or input instability has received quite a lot of attention. It has been shown that instability can occur if the output impedance of the filter is not within a certain safe window, as related to the input impedance of the converter (we are talking about the impedances presented to the power flow now — not the CM or DM noise). So, with the modern trend of low-impedance all-ceramic solutions in dc-dc converters, the possibility of this particular type of instability is becoming more and more real. 

Example What is the DM noise spectrum measured at the LISN for a 5 V 15 A flyback at an input of265 VAC, with a transformer turns ratio of 20 We are using an aluminum electrolytic bulk capacitor whose datasheet states that it has a capacitance of270 p,F, a dissipation factor (tangent of loss angle) of tan S = 0.15 as measured at 120 Hz, and a frequency multiplier factor of 1.5 at 100 kHz. 

Note We can ask — since the break point associated with the rise and fall times didn t enter the picture here, does that mean that it doesn t matter how fast we turn-on and turn-off the mosfet Yes from the DM noise viewpoint it really doesn t matter much. However there are parasitics that we have ignored (chiefly the ESL and trace inductances). And since, unlike the ESR, these will produce frequency-dependent voltage spikes, it is in our interest not to keep the mosfet crossover (transition) times too small. 

We realize that, as for the DM noise calculation, from the viewpoint of the noise envelope and its required attenuation, only the fundamental harmonic really counts. The current caused by this is... 

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