Line Filtering

The EMI filters shown in Figure 15 (U13 and U17), are used to suppress any noise riding on the &endash;48 Volts supply and/or any noise coming from the card supplies that may disturb the card itself or another card in the system. Often times these filters are required to meet conducted (and radiated) emissions standards.

The advantages of split EMI filtering (U13 and U17) may be undermined by the introduction of excessive inrush currents into the filter (U13) residing on the &endash;48V bus (see scope photo, Figure 5B and 5C).

The cause of the inrush current is the CX (line to line) capacitor, Figure 4. To minimize the inrush current choose a filter with the lowest possible CX capacitor value. To ensure the inrush current does not disturb the &endash;48V bus, either add a capacitor with a value 5 times that of the filter's CX capacitor nearby the system card on the motherboard or backplane. Another option is to remove U13 and only use U17. Figure 5A shows the Inrush current without the input EMI filter (U13 removed). Figure 5B shows the inrush current for a filter with a lower CX capacitor value.

Application Note 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

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