![]() ![]() The situation gets worse when these power cables are run alongside other cables, so there are adjacent sources of heat in addition to cutting off of air flow. In fact, the National Electrical Code (NEC) in the US, and similar codes around the world, define maximum free-air allowable dissipation for these cables-and then add derating factors for cables with little or no air flow. In many commercial and industrial settings, the numerous AC cables go through risers and plenums and so have almost no air flow. This situation is changing as data cables and even DC power cables are increasingly routed alongside other heat-dissipating cables, often in locations with little or no convection cooling aspects. This power is dissipated as heat, of course, and is not problem in most designs, where the power loss and resultant heat in the cables is negligible compared to the overall system dissipation. However, the same laws of physics that characterize voltage drop also call out power dissipation P = I 2R. Noise is attenuated via ferrite beads on the power lines as well as bypass capacitors placed close to the load. Among these are use of heavier-gauge wire, employing four-wire Kelvin sensing at the load (although it brings its own potential problems due to creation of a feedback loop oscillation), or going to a more distributed architecture with an intermediate bus converter supplying multiple point-of-load (PoL) DC/DC converters situated in close proximity to the loads. ![]() The IR-drop problem can be addressed by various strategies. When designers worry about the distance between power source and load, it’s usually related to losses due to IR drop (V = IR) or noise pickup. ![]()
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January 2023
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