Train displays for passenger information and advertisements are used across trains and metros. Size depends on the space available, and displays may include features like embedded computers with Ethernet connectivity, radio or Wi-Fi units for wireless data transmission, and outputs for audio systems for announcements or background music.
This customer offers very flat thin-film transistor (TFT) displays with sizes up to 40”. Depending on the individual needs of an application, customers can tailor their system with the aforementioned features. With all the different screen sizes and available options, it became difficult to manage the various combinations of power supply solutions. The idea was to have one common design for all systems, which was flexible enough to be adapted to the individual application.
The basic system needs were always the same, in which the source can be a 24V or a 110V rail power grid and the display driver and control units need 24V with power levels up to 70W. The power supply also needed protection against high transients on these source voltage rails. Interfaces, wireless units, audio systems, backlights, etc. need either 12V or 5V with maximum currents of 2A. Electronics within the display unit may not need isolation from the 24V, but units connected to externally facing systems in the train require an isolated supply. All the mixing-and-matching of system components with the already-more-robust
railway application seemed to be a formidable task right up to the moment when RECOM got involved. Standard (footprint-compatible) modules were used as building blocks on common PCB layouts to create a solution matching the individual needs of an application.
An isolated, EN50155-compliant DC/DC converter generated a regulated 24V / 140W output. This converter is available in both 24V and 110V input versions in a common, industry-standard, half brick pinout and housing. This gave enough power to generate the 12V and 5V rails from the 24V bus with either
isolated or
non-isolated DC/DC converters.
This solution offered the following key benefits:
- There was only a single point of isolation between the 24V or 110V rail and the rest of the system
- Only one EMI filter on the high-voltage side was needed for this high-power module
- Less PCB area was wasted in meeting spacing requirements for high voltages, which made the design easier and more compact
- A full family of solutions were designed with industry-standard, half-brick footprints.
The standard series of non-isolated, step-down converters used in this design is available as 12V / 2A and 5V / 2A versions, both in the same small package and pinout. Thanks to their low-loss technology, no heatsinks are needed. Depending on the load, either the 12V or the 5V version is mounted. As most loads do not need isolation, this reduces size and loss of the power solution, while gaining in efficiency.
For the few externally facing applications requiring isolation, tiny 24V to 12V / 30W or 5V / 10W isolated converters can be used instead. Transient protection at the input was solved with a standard protection circuit readily available from RECOM. The height of all components was below the 25 mm target. With one single PCB and the modular building blocks from RECOM, the customer was enabled to build very flexible custom power configurations for high, low, or even single-piece quantities.
