The PU6 is a plug-in power supply unit for 19" systems (e.g. VMEbus, CompactPCI®) and is especially designed for computer systems in public transport vehicles, being fully compliant with EN 50155. The PU6 has a nominal input voltage of 24 VDC with a wide input range of 16.8..30 V and a max. input voltage range of 14.4..33.6 V (according to EN50155). The otherwise identical PSU models PU5 and PU8 are available for the nominal input voltages of 110 VDC and 36 VDC respectively. It is controlled by a microprocessor which supervises the voltage and the temperature as well as one external input (for its key input function) and one output. Thanks to the onboard intelligence the PSU can also act as an SMBus slave and communicate with the CPU board via the backplane. This allows the PSU to power on the system at a programmed date and enables the CPU to make status polls with regard to, e.g., error messages, time-out or on/off. The PSU provides a DC/DC converter to generate the isolated 12 V from the input voltage. Two regulators generate 5 V and 3.3 V from the 12 V isolated side. The maximum power at full load is 100 W, minimum power is 5 W. The microprocessor is supplied with power independently and can turn off the main DC/DC converter to switch the unit into standby mode (power consumption less than 2 W). The subassembly is conformally coated, and components are secured against vibration. The PSU operates in -40 to +85°C environmental temperature. The thermal stress is extremely low due to integrated heat sinks and diversion of dissipated heat over the mounting surface.