The concept of safety and how it impacts the filter section are as follows
1. Any exposed metal (conducting) part (e.g. the chassis or output cables) is capable of causing an electrical shock to the user.
2. To prevent a shock, such parts must be Earthed and isolated from the high voltage parts of the power supply.
3. No single point failure anywhere in the equipment should lead the user to be exposed to an electrical shock. There should be two levels of protection, so that if one gives way, there is still a level of protection present.
4. Levels of protection that are considered essentially equivalent are a) Earthing of the exposed metal surface b) a certain physical distance (typically 4mm) maintained between any exposed metal and any part of the circuit containing high voltage c) a layer of approved insulator (minimum dielectric withstand capability of 1500VAC or 2121VDC) between any exposed metal and any part of the circuit containing high voltage.
5. So for example we could connect the enclosure to Earth. That gives us is one level of protection. But if the Earth connection failed, maybe due to something as simple as a loose contact, we would need to rely on one more level of protection. This could be simply the stipulated 4mm of separation. But what if we wanted to mount the Fet on to the enclosure for better heatsinking? The separation is now obviously going to be insufficient. We could then place one layer of approved insulator between the Fet and the enclosure. In this position, the insulator would serve as 'basic insulation'.
6. What if we have an exposed conductor which is not, or cannot be connected to Earth? This could be the case where we have floating output cables for example. Then we would need at least two levels of protection between the output and the high voltage Primary side. Besides the basic insulation, we would need another layer (with identical withstand properties) called the 'supplementary insulation'. Together these two layers are said to constitute 'double insulation'. We could also use a single layer of insulation, with dielectric withstand properties equivalent to double insulation (3000VAC or 4242VDC). That would be called 'reinforced insulation'.
7. If the equipment is by design, meant only for a two-wire AC cord, there is no Earth protection even present. In that case two layers of approved insulators will always be required from Primary side to any exposed metal.
8. Now we must understand why we connect the enclosure to Earth in the first place. Hypothetically, we could just use any two levels of protection, not necessarily including Earth. A prime reason for using an Earthed metal enclosure is that we want to prevent radiation from inside the equipment from spilling out. Without a metal enclosure, whether Earthed or not, there is little chance that an off-line switching power supply can ever comply with the radiated emission limits. That is especially true when the switching speeds are within tens of nanoseconds. But the metal enclosure is naturally eyed as an excellent fortuitous heatsink by engineers. So power semiconductors are often going to be mounted on it (with insulation). However by doing this, we also create leakage paths (resistive/capacitive) from the internal subsystems/circuitry to the metal chassis. Though these leakage currents are small enough not to constitute a safety hazard, they can present a major EMI problem. If these tiny leakage currents are not 'drained out', the enclosure will charge up to some unpredictable/indeterminate voltage, and will start radiating. That would clearly be contrary to the very purpose of using a metal enclosure. So we need to connect the enclosure to Earth. We note that even if we didn't have power devices mounted on the enclosure, there could be other leakage paths present. Besides that, an unearthed enclosure would also inductively pick up and re-radiate the strong internal electric/magnetic fields.
9. Therefore, a) providing a good metal enclosure, and b) properly connecting it to Earth, is the most effective method to prevent radiated EMI. But by creating this galvanic connection to Earth, we also provide a multi-lane freeway for the conducted (common mode) noise to flow merrily into the wiring of the building. Now, to be able to stay within the applicable conducted emission limits, we would need to provide a common mode filter somewhere.
10. Generally, if the equipment is designed not to have any Earth connection (i.e. a two wire AC cord), there will usually be no metal enclosure present either. Keeping the issue of radiation limits aside for now, the good news here is that no significant common mode (CM) noise can be created, simply because CM noise needs an Earth connection by its very definition. Therefore a CM filter is superfluous and can be omitted. But conducted noise limits include not only common mode, but differential mode (DM) noise too. So irrespective of the type of enclosure and Earthing, DM filters are always going to be required.
11. One of the simplest ways of suppressing any noise is to provide decoupling between the nodes involved. For CM noise this means connecting high frequency ceramic capacitors between the L and E wires and also between the N and E wires, possibly at several points along the filter. But each of these CM line filter capacitors also unintentionally pass some of the AC line current into the chassis (besides the CM noise). To reduce the chances of a fatal electrical shock, safety laws restrict the total amount of current that can be injected into the Earth/enclosure. This in turn means that we have to limit the total CM filter capacitance. If the capacitance of an LC filter is restricted, we may need to increase the L to compensate. Therefore, the inductance used for the CM filter stage in off-line applications is usually fairly large (several mH).
Practical Line Filters
We can now look at a typical power supply line filter shown in Figure 2_1. Its overall purpose is to control conducted emissions, and therefore it has two stages (as highlighted) --- one for differential mode and one for common mode. Let us make some observations
Both the CM and DM stages are usually pi-filters (LCL or CLC) especially in higher power converters, as they provide the desired attenuation characteristics. Sometimes T filters may be used. We could also just use a simple LC filter, or even a plain decoupling capacitor for low power applications. Tuned filter stages are occasionally seen. But under line transients or applied input surge waveforms as used for immunity testing, they can display severe unexpected oscillations, and so they are generally avoided.
The filter is usually placed before the input bridge, because in that position it also suppresses the noise originating from the bridge diodes. Diodes have been known to produce a significant amount of medium to high frequency noise, especially at the moment they are just turning OFF. Small R-C snubbers (or just a C) are often placed across each diode of the input bridge. Alternatively, we can look for diodes with softer recovery characteristics. Input bridge packs, using ultrafast diodes are often peddled as offering a significant reduction in EMI. In practice they don't really make much difference, at least not enough to justify their additional cost. In fact typically, the faster a diode, the greater the reverse current and forward voltage spikes it produces at turn-off and turn-on. So very fast bridges may in fact produce worse results.
Typical practical values of the CM choke inductance are 10mH to 50mH (per leg) in high power converters. The DM choke is always much smaller (in inductance, not in size!). Typical values are 500uH to 1mH.
We have shown both the CM and DM filter stages as being symmetrical. We have for example, identical DM chokes on each of the L and N lines. We will later see that the DM choke is also part of the CM filter equivalent circuit. In general we try to maintain balanced impedances because any imbalance basically causes some of the CM noise to get converted to DM noise. When this happens the resulting EMI spectrum may be rather confusing to analyze and fix, except perhaps for really low power equipment.
As regards the CM noise, it is very important that we perform the attenuation equally in both the L and N lines. Otherwise we would have a leftover DM noise component. But if we try to achieve this by actually matching the CM filter inductance present on each line, our production is not going to like it. But what if we use the same core for both lines, i.e. a coupled inductor? That would automatically assure us a good inductance match (assuming of course that we have placed an equal number of windings per leg). If we are winding the CM choke ourselves, we must be clear that the relative direction of the windings is as indicated in Figure 3_2a. We can visualize easily that with this arrangement, the magnetic field inside the core cancels out for DM noise (as also for the input AC line current), but not so for the CM noise. The CM choke as shown in the figure, therefore presents an inductive impedance to the CM noise, but none (in theory) to the DM noise.
Note: The reader is cautioned that there are several widely used but confusing symbols for the CM choke in some schematics in related literature. Whatever the symbol, the direction of the windings must be as shown in Figure 3_2a.