Optimising SMPS filters for EMC, safety and protection

February 23, 2017 // By EDN Europe
Adam Chidley, European Product Manager, Avnet Abacus
SMPS input filters control EMI and protect against surges and transients. Their design is linked with international standards for EMC and safety and an optimum solution has to take a ‘holistic’ approach.

The days are long gone when a power converter specification started with volts and amps. These days, you’re more likely to see EMC and safety compliance requirements first with functional specifications many pages in. Standards specified will depend on the application but common ones are UL/EN 60950-1 for safety (soon to be replaced by EN 62368-1) and the EN 61000 series for EMC. Others for specific applications might be ANSI/AAMI ES 60601-1 for medical safety or EN 61800-5-2 for motor drives.

Safety and EMC requirements are linked because, for example, transients could induce failures affecting safety. Standards can also conflict; reducing leakage currents for safety makes controlling common mode EMI emissions more difficult so optimum input EMC filter design is a compromise.

Although the EMC standards do not put a limit on differential mode (DM) noise as such, it is always best to minimise it, as it can convert into common mode (CM) noise. In fact, the standard test method for measuring CM noise using a Line Impedance Stabilisation Network (LISN) to CISPR-22 indicates half of the value of the DM noise, even if no CM noise is present.

You could take every EMC specification requirement in sequence and build up a filter ending up with the classic circuit shown in Figure 1.

You might see this filter in a high-power converter but not in a cell-phone charger. Although they’re both switching supplies, meeting the standards depends heavily on the converter design and power level. It’s best to minimise EMI from the start by using soft-switching technologies and with careful layout. For example, a source of CM noise is through the capacitance of a switching semiconductor tab to a grounded heatsink, so select a device whose tab is not a switching node or avoid grounding the heatsink. If it must be grounded, an electrostatic screen between the tab and heatsink is effective. An ungrounded heatsink also reduces mains leakage current, an example of safety and EMC interacting.

In Figure 1, C1, C2, L1 and L2 attenuate converter DM emissions and help absorb transients. L1 and L2 do also provide some additional CM noise attenuation. C1 and C2 are rated for the continuous line voltage and transients according to the application ‘overvoltage category’, requiring ‘X1’ or ‘X2’ types.

C3, C4 and T1 reduce CM noise. The capacitors have to withstand line to earth voltages and transients, according to the ‘overvoltage category’, so are ‘Y1’ or ‘Y2’ types. There are limits to the capacitance values; if a chassis earth connection opens, line current can leak through these capacitors to metalwork that a user could touch. The maximum current allowed can be as low as 10 µA in some medical applications.

T1 is a ‘common mode’ choke attenuating CM noise. Normal running-current magnetic field cancels in the core, so high inductance can be designed in without core saturation. Field from CM noise currents does not cancel, so CM noise ‘sees’ a high impedance and is attenuated. T1 is often a toroid with separated windings for the line voltage.

FS1 is normally a ‘fast blow’ type