HI > On Aug 28, 2015, at 4:46 PM, Angus <[email protected]> wrote: > > On Sun, 23 Aug 2015 23:36:27 -0400, you wrote: > >> Hi Angus, >> Thanks for your reply. My original suspicion is the +6.5V rail. That needs >> to come from somewhere, and you need headroom if it's an unregulated input. >> Pumping 2A through a regulator like that is no easy feat. Although rated >> for 3A, you need to keep the junction temperature below 125C. The part has a >> 30C/W thermal resistance (Tja) with a 1" square copper pad. So 6.5V and 3A >> calculate as follows: (6.5V-5V)*3A=4.5W. 4.5W*30C/W=135C RISE, add that to >> ambient, 25C, and you are 35C over max junction. In my world we have a 70C >> ambient, and that leaves me (125C-70C)/30C/W=1.83W, or a maximum of >> 1.83W/1.5V=1.22A. Generally a tab mounted TO-220 (a D2Pak) can have a Tjc >> (junction to case) or less than 1C/W. It's all the reest of the mounting >> that piles on the thermal resistance. >> >> The Trimble board has a top layer pad, some far side pad, and probably >> multiple layer of ground plane over the rest of the board. The mounting >> post is mostly likely part of the thermal resistance calculation. There is >> also a time constant involved, it probably can't take the 2A forever, just >> long enough to get the oven up to temp. I suspect they may have gotten the >> thermal impedance down as low as 10C/W. I will test it with 12V and let >> everyone know if I fry the board. >> >> Bob >> > > Hi Bob, > > There's not really any need for a 6V or so rail - a DC-DC converter > (and probably some filtering) can provide the right voltage just where > it's needed. > > What looks like a Symmetricom variant is in listing #271483752431. > It does appear to have a DC-DC converter, etc., on the board. > > Anyway, I dug out my Trimble to have a look at it, and I see that > there's a 10V tantalum capacitor on the input power, so it's certainly > not meant to run on 12V.
If they derated that cap 20%, it is not meant to run on anything over 8V. It’s rare to see a voltage between 6.3 and 10 on those parts. Obviously 6.3 is to close to run on a 6V line and get any decent MTBF. Bob > > Angus. > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
