In order to supply the components of the system with their required DC voltages, power supply units (PSUs) are built into CompactPCI systems. The tasks of the PSUs include the following: to provide a secure galvanic isolation from the public mains, to reduce and rectify the voltage and to smooth out voltage fluctuations and interference. Also important for the system is an output current limiter for safety purposes. This prevents, for example, a damagingly high current flow in the event of a short circuit on a board. Other technical requirements for PSUs are overvoltage protection, lightning protection and electromagnetic compatibility (EMC). This concerns interference emissions and interference immunity. The characteristics stated are regulated by law (the EMC Directive and the Low Voltage Directive) and are documented in Europe by the CE mark and declaration of conformity. In CompactPCI systems, output voltages of 5 V and 3.3 V (for processors) and ±12 V (e.g. for hard disk drives and fans) are required as a rule. The specifications for the individual bus technologies vary considerably in the requirements they set out for system power supply.
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