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| resistor wattage for hotplugging snubber? |
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| k8943:
Thanks for that. Reply #8 did examine the fixed resistor case but thought I should give it a try. Discovered that 2.5ohms in series with the 24v Vin did a good job of limiting the inrush to the 2.2uFh input capacitor of the buck convertor at the moment the RJ12 is hotplugged on the linear bench supply (the most agressive testing environment as it turns out). 2ohms was too little. Seemed to go from 0->24v in 2.5uS so calculate the current as follows: I = Cdv/dt = 2.2uF * 24v / 2.5uS = 21amps Which implies wattage = I^2 R = 21 ^2 * 2.5 = circa 800W for 2.5uS Looking at the graph in Reply#3 and extrapolating back towards 2.5uS (not on the graph) have the impression a 1206 might not make it so instead searched on 2512 Digikey RMCF2512JT2R70CT-ND comes in at about 4cents per thousand so that would appear indeed to be the most cost effective solution excluding running costs. If the buck were run at maximum 0.5A and my selected 5v => current at 24v approx 0.5 * 5 / 24 = 0.1A Losses in resistor = 0.1 ^2 * 2.5 = 25mW Admittedly that's not a hill of beans. Intuitively and aesthetically it's not nice to put a series resistance in the power line but not sure what other critiques to bring to this low power situation? ************* Browsed TI's excellent webbench tool for power convertors with an increased input voltage range (10v-35v instead of 10v-28v - the 10v is in case project is running off backup 12v lead acid) and indeed this increases the BOM cost for the convertor by more than 50% as well as possibly the surface area whilst reducing the efficiency. So no slam dunk. ******** The DN2625 circuit sounds as though it will sound difficult situations very well. In this case of my humble application note that the RDS on is 3.5ohms. Thanks again for the input. Fascinating how even the most innocent question can big bang into a universe. |
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