Switch mode power supplies require EMI filtering to meet the most common EMI requirements. As discussed in the EMI Filter help topic PI Expert provides a starting base for a conducted EMI filter. This topic gives you a better understanding of the different EMI components and their use.
Y capacitors are used in line to ground applications where failure of the capacitor can cause a shock hazard. There are different sub classes of Y capacitors. The two most commonly used are Y1 and Y2 which are used differently. When a Y1 capacitor (250 VAC safety rated) is used, it is connected between the primary (input) and the secondary (output) across the safety isolation barrier. The Y1 capacitor provides a return path for common mode currents that are induced in the secondary and would otherwise be forced to return to the primary via the AC input lines. To avoid coupling the noise being shunted by the Y1 capacitor into the source pin of the PI device, the Y1 capacitor is connected from the high voltage DC bus side of the primary to the stable node on the secondary (which is the return, when the output rectifier feeds the positive side of the output capacitor). For this Y1 capacitor to work most effectively, the length of the PCB traces between it and the leads of the transformer must be kept as short and as direct as possible. For cases where the Y capacitor needs to be returned to the DC negative rail, make sure that the Y capacitor trace is routed such that the common mode currents flow directly into the bulk capacitor negative node and they do not share trace between the Source pin and the bulk capacitor negative node.
Increasing the Y1 capacitor size has the side effect of increasing earth leakage current while decreasing common mode EMI noise. The absolute limits for the fault current that earth ground is allowed to conduct are reiterated in AN-15 (see table 1 and the text on page 9).
It is important to never substitute one rating of Y capacitor for another Y capacitor and especially not a different type of capacitor all together. The different ratings of Y capacitors have very specific ratings that directly apply to safety. Failure to use the appropriate Y type capacitor could lead to potential electric shock.
X capacitors are used between line and neutral or line and line thus failure of the X capacitor can not cause a shock hazard. X capacitors are divided into three subclasses. X1 capacitors are designed for absorbing periodic high-powered pulses, which makes them expensive. Therefore they are not regularly used in power supply EMI filters. X3 capacitors are also not normally used in power supplies. X2 capacitors are most commonly used and are available in values that range from 1 nF to 1 µF and usually meet most safety agency flammability requirements.
Inductors designed for EMI filter usage have three important parametric specifications: effective impedance at the target frequency, steady state current rating, and surge current capacity.
DM chokes are usually wound on powdered iron or ferrite cores that are solenoidal or toroidal in shape. DM chokes that are limited to a single winding layer will have the lowest inter-winding capacitance and the highest self-resonant frequency.
CM chokes are a matched pair of coupled inductors. A CM choke has two separate windings that are wound on the same core (either toroid or Bobbin style). The windings are wound in the same direction, so that the magnetic flux from differential mode signals (such as the line frequency) cancels, in the core, and passes through unimpeded. However they appear as a high impedance to common-mode current thereby reducing common-mode EMI. The two windings are usually wound on separate portions of the core, so that winding-to-winding capacitance is minimized.
For more information on EMI components and reduction techniques please see AN-15.