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NAILING COMMON MISUNDERSTANDINGS ABOUT EMI FILTERS

NAILING COMMON MISUNDERSTANDINGS ABOUT EMI FILTERS
By Jan Nalborczyk, Technical Director, MPE Ltd

Despite EMC issues now usually being considered at an early stage during the product design process, unexpected problems still occur. In some cases, a simple approach to EMC and filtering is found to work but, more often than not, a more in-depth analysis is required to address the relevant EMC design considerations and subsequently identify the EMC solution. Misunderstandings are prevalent when selecting EMI filters in the course of equipment design, and here we explore some of the more common ones.

Filters will give their full quoted performance up to their full rated current. For instance, 30A filters will give full performance where used at a lower, 20A, current. The chief exception is for single-line filter designs where the magnetic core will progressively saturate and give lower inductance and lower performance as load current increases. A reputable filter manufacturer should allow for this and quote performance figures based on the worst case, full load condition.

It is not a good idea to use a 10A filter continuously at 12A, so that it is 20% over-rated. The heat dissipation within a filter is proportional to I2. Therefore a 20% overcurrent will represent 44% excess heat dissipation, which is unacceptable. Filters should never be used at above their rated current on a continuous basis.

You can use a 240V 50Hz mains filter on 120V 50Hz. Filters are normally suitable for operation at any voltage up to their rated voltage provided that their current rating is not exceeded, and the supply frequency is no greater than the rated frequency. It is also normally possible to use AC filters on DC supplies up to at least the same working voltage. Where filters are fitted with transient suppressors, it should be remembered that the level of transient suppression provided may no longer be optimum for the new working voltage.

It is sometimes possible to use a 240V 50Hz filter on a 115V 400Hz supply, especially when the filter capacitance values are low. The main problem lies in the heating effects of 400Hz supplies on the inductor cores and capacitors within the filter. In the case of the filter capacitors, leakage currents will be about four times higher on the 400Hz supply. Capacitor heating by the 400Hz supply can be significant on high-performance filters. This can be reduced by using low-loss capacitors in the filter design, but careful consideration needs to be given to harmonics on the 400Hz supply which will add to heating effects. The filter current rating may also have to be derated to take account of the additional heating within the inductor cores. Always contact the filter manufacturer for advice before proceeding.
Concerning filters used on higher frequency supplies and non-sinusoidal supplies, a check must always be made to ascertain the likely heating effect of a particular supply on any given filter. The heating effect will increase with frequency and will be even more pronounced for non-sinusoidal waveforms because of the high harmonic content. The filter manufacturer should be consulted for advice, as it is unlikely that a standard filter will be suitable, although a special design may well be practical.

You cannot generally utilise a powerline EMI filter for filtering out mains harmonics. Mains harmonics are most pronounced at the lower frequencies and have a very low source impedance. They will require very large values of capacitance and inductance to filter them out, and a purpose-designed harmonic filter is required to do this. A high-performance mains EMI filter will reduce some of the higher order harmonics which extend into its stop band, but its impedance is unlikely to be low enough and its capacitance and inductance values will not be high enough to have any great effect on the lower order harmonics.

If a high-performance powerline filter is placed on a supply known to contain high l

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