PoE is the ideal solution to power remote loads which are connected to LAN but mounted away from the AC mains or any other power source. Installing a LAN cable is easier than a mains cable as voltages are inherently safe and within the SELV (Safety Extra-Low Voltage) limits. Depending on the size and length of the cable, the voltages used, and the classification of the devices, the PoE standards allow remote-powered device (PD) loads like WLAN antennas or cameras with up to 71W of consumption. This power needs to be coupled onto the LAN cable wiring without affecting the data line transmission and, in many cases, is done by a separate
AC/DC power supply, which takes space and requires AC power. But quite often, customers see this as the only way to add PoE functionality to an existing system.
Having the power injection directly integrated into the hub or the Ethernet switch makes life much easier. This customer wanted to add high-power PoE to its existing industrial DIN rail mounting Ethernet switch to remote power loads. As this device is not only used for new installations but also as a retrofit option in existing racks, the big challenge was to fit this additional feature into the existing enclosure. No additional space for the necessary PoE power supply was available, and the power solution had to be extremely slim with as few losses as possible.
Thermal conditions in racks can vary from having good cooling and only a few units mounted on the DIN rail up to environments with fully-populated racks, convection cooling only, and heat-dissipating devices like power supplies mounted directly below the Ethernet switch. The higher the voltage injected into the cable, the lower the I
²R losses and voltage drop on the cable (power loss is proportional to the square of the current), and therefore, more power is available for the load at the end of the cable. Therefore, the customer wanted to have a higher voltage than the standard 48V output from typical
DC/DC converters, with a minimum requirement of 52V.
The PoE standard specifies a DC supply with a 2250VDC / 60s isolation barrier to the ground and Basic grade insulation. With these special requirements, the customer contacted RECOM asking for a solution, which was found to be based on a standard 180W DC/DC converter with a 24V input and the output trimmed up to 52.8V. The only change needed was a reduced pin length.
The low-profile, half-brick housing with baseplate cooling was offered with threaded inserts to allow mounting to a small heatsink to enhance the thermal performance. The flat baseplate does not require any gap-filling material between the baseplate and heatsink, which keeps thermal resistance as low as possible. The constant, high efficiency of up to 91% guaranteed low losses over the full operating range, significantly reducing thermal stress in the system, which also increased overall reliability. This configuration and its low losses enabled operating ambient temperatures of up to +60°C with convection cooling only when loaded with 120W. The converter module pin length was reduced from the standard 5.08mm to 2.8mm to avoid short circuits to another adjacent PCB. The standard mounting holes in the housing were replaced by threaded ones, allowing a solid mechanical connection between the PCB, module, and heatsink.
The
RP180H has Basic grade 2250VDC / 60s isolation as standard, a showstopper for many other manufacturers who can only offer functional isolation or lower withstand voltage. In addition, a regulated 12VDC, 1A auxiliary supply was needed to power the Ethernet switch. RECOM offered a standard part, the
RS12-2412SZ, in a compact SIP8 package to fulfil this function. The small footprint of only 22 x 9.6mm has a mounting height of only 12.6mm, so it could be placed on the same PCB as the RP180H, which has a mounting height of 12.7mm. With a few modifications, RECOM was able to solve all the challenges of this customer while simultaneously providing them with a simplified development path on their roadmap.
