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Power Over Ethernet Interoperability Guide by Sanjaya Maniktala

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Recommendations for DC Disconnect
Assuming we have easy access to the bulk capacitor of the PSU, we can calculate how much energy is being delivered by the surge, and how much bulk capacitance is available to absorb this. A sample calculation is presented in Fig. 11.10. We have basically assumed a triangular current waveshape of height Imax (numerical values provided in the figure) and 120 μs (extrapolated average duration as shown). Then, based on an average current of Imax/2, lasting for 120 μs, we can equate the energy delivered by the surge event, to the change in stored energy based on ½ × CV2. We thus get 524 μF for 3 kV. The closed-form equation we can also use is
Here Vpse is the normal operating PSE voltage (say 51 V) and Vf is the ...

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