>The per-port regulators would be non-isolated. Probably feeding off >an internal 48V bus.
Yes, so that will be 90 to 95% efficient, but it is fed from an isolated supply that at best will be 90%, probably less. Those numbers must be multiplied, giving 81 to 86% overall efficiency--and I am assuming best in class, which only a switch at a high price point would offer. >Well, the bulb has a peak permissible operation temperature of about >160 degrees C ... so likely no extra cooling required True in open air. However, inside a metal enclosure as I described to make it applicable to the situation, the enclosure would probably get warm enough to be a fire hazard. The light bulb might still be fine after the building burned down! :-) I've designed products and put them through UL, CSA and CE safety testing. >It'd still be a 400W PSU if it supplies 400W You can play various games with the numbers, such as choosing input versus output power to write in the spec. The label will have to indicate 500 W of power consumed, by law. No matter how one plays with the numbers, an 80% efficient supply that delivers 360 watts will consume 450 W, and turn 90 W into heat. If it is rated for 400 W output, it is probably in a portion of its range at 360 W where it is achieving near optimal efficiency and the above math would apply. In cases where half the loads drew less power and/or were not POE loads, the consumption would go down, and so would the heat. However the manufacturer can't design for that case and if they don't provide automatic fan control (which apparently most switches don't have), the fan must be designed for worst case, which in the above example is 100 W of heat. Also, at 25 to 50 % load, the efficiency will probably be lower because it was optimized for a larger load, and because some losses are fixed (not load dependent). >This whole thread is getting stupid If some increase in understanding occurs, then it isn't a waste. No it won't change the behavior of any switches, but it might help people understand why. Wilton
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