A typical mains filter includes components to attenuate both common-mode and differential-mode components. The common-mode choke L_CM consists of two identical windings on a single high permeability, usually toroidal, core, configured so that differential (line-to-neutral) currents cancel each other. This allows high inductance values, typically 1-10mH, in a small volume without fear of choke saturation caused by the mains frequency supply current. The full inductance of each winding (L_CM) is available to attenuate common-mode currents with respect to earth, but only the leakage inductance (L_LKG, which is determined by winding imbalance) will attenuate differential-mode interference.

Capacitors C_X1 and C_X2 attenuate differential-mode only but can have fairly high values, 0.1 to 0.47µF being typical. Either may be omitted depending on the detailed performance required, remembering that the source and/or load impedances may be too low for the capacitor to be useful. (If the filter is working into a standard CISPR LISN, then the differential impedance across C_X1 is 100Ω above a few hundred kHz; the differential impedance across C_X2 will be mainly determined by the impedance of the DC link capacitor, likely to be very low, and therefore C_X2 may well be redundant at least at low frequencies). Capacitors C_Y attenuate common-mode interference and if C_X2 is large, have no significant effect on differential mode.

Extra differential mode attenuation can be provided by adding a differential mode choke and/or a further X-capacitor. The differential choke is wound in the opposite direction to a common mode choke and therefore does not compensate the power current, so cannot have a high inductance value. Similarly, extra common mode attenuation is provided by adding a further set of Y capacitors and/or another common mode choke.

In many cases, the total circuit of power converter + filter + mains LISN can be simulated in e.g. Spice, taking care to include parasitic components, so as to predict and optimise with a fairly high degree of accuracy the required values of filter components for a given conducted emissions specification. An example can be found here.

Leakage currents and safety

Any filter with common mode Y capacitors to safety earth will pass current into this earth when used on a.c. mains power and a.c. signals. The amount of leakage current that is permitted in an equipment’s protective earthing conductor is limited by the relevant safety standards. There may also be a limit created by a system's reliance on ground current fault detection. Mains filters must therefore be designed with this limit in mind, so that there is a practical ceiling to the value of the common mode Y capacitors. For many medical applications, the limit on leakage current is so strict that it is already taken up by stray capacitance within the unit, and Y capacitors are effectively prohibited. In this case, common mode filtering can only be provided by a choke. Leakage current limits for some common safety standards are shown in Table 1.

Table 1 Safety earth leakage current limits

There are particular requirements for the voltage withstand performance of the capacitors that are used in mains filters; these are laid out in EN 132400/IEC 60384-14. Failure of “X class” capacitors would result in a fire hazard, while failure of “Y class” capacitors would result in both a fire and potential electric shock hazard. The properly rated components must be used in the appropriate positions. Table 2 shows the safety rating classifications.

Table 2 Safety-rated capacitor classifications

Many safety standards require a bleed resistor to discharge the capacitors when the connection to the supply is broken; if the equipment on/off switch is downstream of the filter, and the supply plug is removed from a live socket, a hazardous voltage can remain on the filter capacitors which is then accessible via the plug contacts. In general, a bleed resistor is needed if the total capacitance value is more than 0.1µF.

Insertion loss versus impedance

Off-the-shelf mains filters are universally specified between 50Ω source and load impedances. This does not reflect the real situation when they are installed in circuit.

The mains port HF impedance can be generalized for both common- and differential-mode by a 50Ω//50µH network as provided by a CISPR-16 LISN, which is used for all commercial standard mains emission tests (and many MIL-STD ones too). The equipment port impedance will vary substantially depending on load and on the HF characteristics of the input components. Common-mode impedance is a function of stray capacitance coupling to earth and can normally be approximated by a capacitive reactance whose value depends on the construction of key circuit components, such as the isolating transformer and any power switching heatsinks.

The effect of these load impedances differing from the nominal may be to enhance resonances within the filter and thus to achieve insertion gain (rather than attenuation) at some frequencies, typically below 150kHz. Some filter manufacturers provide extra curves for attenuation with 0.1/100Ω differential mode load impedances,  measured according to CISPR 17, and these figures are more realistic for real situations (see example graph, from Schaffner).

The problem of poor low frequency performance is usually worst for single-stage filters and can be dealt with by adding extra differential mode components, but this makes the filter more expensive. Understanding the total filter equivalent circuit can allow fairly straightforward modelling of the conducted emissions path, which will enable the circuit components to be optimized for a particular measurement result.

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