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EDN ACCESS ARTICLE: DESIGNING A NEBS-COMPLIANT POWER SYSTEM
A complete power system guarantees the problem-free operation of telecommunications equipment and that equipment's compliance with applicable standards. Designing the complete power system requires expertise in areas beyond power conversion. [HTML]

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Designing a NEBS-compliant Power System - SMH4804

PAGE 4

Pre-Testing for NEBS Compliance, Cont'd
 

Note the transient-voltage suppressors (TVS), gas discharge tubes (GDT), fuses and conducted EMI suppression filter.

The GDT is used for protection from lightning strikes. These devices act as a short when the breakdown voltage is exceeded. They are extremely rugged and are available in a number of different breakdown voltages.

The TVS are used for high voltage, short duration surges such as ESD. The TVS are available in many different breakdown voltages and power handling capabilities.

Fuses are used to protect the -48 Volt power supply from shutting down when the GDT or the TVS breakdown.

The EMI filters are used to suppress any noise coming from the -48 Volts supply and any noise coming from the DC-DC converter that may disturb the DUT 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 (U13and U17) may be undermined by the introduction of excessive inrush currents into the filter (U13) residing on the -48V bus (see scope photo, Figure 7B and 7C). The cause of the inrush current is the C_x (line to line) capacitor, Figure 6. To minimize the inrush current choose a filter with the lowest possible C_x capacitor value. To ensure the inrush does not disturb the -48V bus, either add a capacitor with a value 5 times that of the filter's C_x capacitor nearby the system card on the motherboard or backplane or remove U13 and only use U17. Figure 7A shows the Inrush current without the input EMI filter. Figure 7B shows the inrush current for a filter with a lower C_x capacitor value.

Note: Oscilloscope photos of the higher voltage tests were not taken because of the voltage limitation of the oscilloscope probes. If the protection circuit were to fail the applied test waveform might damage the oscilloscope probes or the oscilloscope.