Of course my circuit uses off the shelf components. I wasn't indicating my design suited their purposed, was only mentioned to illustrate doing better doesn't cost much time. As previously mentioned, I've got ~20 hours into my design, most of which was going through data sheets, setting up spice sims, and altering schematics. 80% is crap efficiency, especially at > $20 BOM cost for the module (modules are generally more expensive). All designs start as schematics, excluding trivial ones. Layout and board fabrication is trivial. Factoring in extraneous variables ahead of time is the difference between winning and losing.
Solution:
Get any one of the dozens of modules that have > 90% efficiency at the voltage and currents desired and internalize the power supply. Overall same heat inside the chassis, better performance per watt. Both things that are key factors in the market they are trying to sell into.
Lets not obsess about the efficiency. In the broad scheme of things it doesn't really matter. What matters is TTM (time-to-market or time-to-mine). Wise men know that a
birdVrm in the hand is better than two
birdsVrms in the
bushcatalog. The best Vrm module is probably the one that is the easiest to obtain and has the highest number of alternate suppliers and pin-compatible replacements.
Layout and board fabrication aren't trivial. Layout is actually extremely critical for a repetitive circuit that is simultaneous-switching-noise-limited. If you want to read a recent story take a look at the Enterpoint's Cairnsmore1 thread. They also apparently thought that they know how to put 4 chips on a PCB and look how much time it took for the developers to actually distribute the clock signal properly.
Exactly what you said is the answer. IC's are not typically designed to handle under current very well and typically things like enable signaling is left out of the design on mining hardware for the reason you indicated, they are always running. The slow rise times, as I'm inferring that you know from your previous knowledgeable posts, have adverse effects, especially on larger dies. I don't have any extra cards here, but one could test this by pulling the GPU BGA, attaching a sequence of increasingly sized load resistor, and measuring rise times. Following this, one could add rise delays to another card, which should be easy given the required pour patterns of most h-bridges, and measure GPU errors at different delays (going from idle to load is analogous to the problem we're discussing).
I don't share your concern about the importance of the rise time. I imagine that after the power-on all hashing engines are disabled. Then the SO-DIMM Linux computer will run some tests on the hasher cores and selectively enable and tune the PLL clock synthesizers. Thus the ramp-up could be quite gentle.
2112, what can you tell us about KNC's latest post/picture of the PCB....your opinion is highly regarded, thanks.
I do agree. I was waiting for a 2112's post on the PCM since they published news-30
Thank you very much, guys. The problem is that there isn't sufficient information to make a meaningfull comment. I don't want to be like Amy Woodward from HashFast and start making some half-assed and quarter-brained assumptions.
If you want a productive subject for the speculation about the PCB, here's a one.
The Vrm requires some way of setting the output voltage. Thus far most of the dedicated mining boards had that voltage set using a pair of surface-mount resistors. That's the cheapest way. The most flexible way would be to have some software-programmable Vrm that can change the output voltage on command. But that is a risky feature to offer when KnC is offering a warranty and everyone will try to overvolt and overclock.
There's also am old-fashioned way: use a trim-pot to set the voltage. During the factory testing tune the voltage with a screwdriver and seal the trim-pot using lacquer, glue or epoxy before shipping to the customers.
To protect against tampering, use some lacquer imported from Chernobyl and test it with a Geiger counter when somebody returns a burned-out miner under the warranty.
So what would you guys want to see in the shipping configuration:
A) surface-mounted pair of resistors
B) tamper-evident trim-pot
C) software-programmable voltage divider.