sidehack
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Curmudgeonly hardware guy
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December 23, 2014, 07:27:31 AM |
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This is the fourth string miner that I know of - the others being the OneString and Yazio (or whatever) boards with Bitfury chips, and the Prisma. I know plenty about how reliable the Prismas are, but not enough about the Bitfury-based boards. (A previous post mentions a fifth design I had not previously seen, so chalk that one up).
klondike_bar's assessment is not entirely correct; the active players in the circuit are voltage and also current. The capacitors at each string node will help balance out brief changes in current (which are natural occurrences with all the switching going on in the chips) to help maintain a constant node voltage, but won't really help much if one of the chips drops out entirely. Suddenly the current path at one node is cut in half, which means that, unless external provisions are made to compensate, the second chip in the node will have to push twice as much current as otherwise expected. This might result in the voltage across that node increasing until the chip is allowing that much current through, which will greatly increase the power dissipation (potentially quadrupling it) on that chip which very well might roast it. The best case would be the chip fails short, in which case you now have an unbroken current path, but with one of 15 nodes out, the rest of your chips' voltages increase by about 7%. If the chip fails open, you now have no current path and the entire board is down.
If, on the other hand, instead of the node voltage increasing until the current draw is in accordance with the rest of the two-chip banks, the current stays low and starves out the other banks, now you have a board which technically works but does not operate stably.
If each bank is provided with an active dummy impedance working alongside the capacitor, the two systems can effectively buffer the node voltages and chain currents to keep every chip functional. If one node's voltage starts to increase beyond what capacitor ripple can handle, this indicates it's not passing full system current and the dummy impedance can open up a bit to allow excess current through, keeping the system operating and the local node voltage at the expected value. If there's a stably controlled system like this in place, I'd have a lot fewer qualms about running one of these boards (we're actually toying with designing one of our own using that principle). I don't really trust unregulated strings as they are, but if there's some node-level regulation at least, it's certainly got more potential for stability and reliability than just trusting the chips to all operate within a narrow tolerance of "identically".
Another thing that's probably not something anyone wants to do, but could greatly assist the reliability of a chained design, is individually fused chips (alongside a controlled dummy impedance). If one fails short and overcurrents, the fuse blows and now that portion of the bank is an open circuit. Your dummy impedance will have to take its full share of the current, so with a chip like BM's where it's expected to draw around 9W you'll need a well-sinked dummy but that's not difficult. If an entire bank goes out, the dummy impedance will have to take the entire load, but it avoids the problem of overvolting the rest of the chain.
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