On Aug 21, 2008, at 12:59 PM, Terrance wrote:
> --- In Ilya Goldberg <> wrote:
> Do you attempt to limit the inrush current, or is momentary (but HUGE)
> load not a problem for your powersupply? My switchers would refuse to
> power up with a setup like that :-(
The inrush current is limited by the ESR of the capacitors, so its
far from a dead short. If the ESR is too low, that can be increased
with resistors, but this has to always be less than the load unless
you use a switch, and the whole point is KISS. I've never used
additional resistors, and had no problems with the switchers I've used.
Initially this will be well above their rated current, but good
switchers should limit their output current by dropping the output
voltage (rather than just shut-down).
One of the ones I've used was an IDEC PS5R-SC12 (12V DC 2.5A,
universal AC input).
The one on-board the ts7260 (5 V) as well as the TS-12W had no
problems with this either. It did take them a good long time to come
up to voltage.
Currently I'm working with a substantial 12 V battery (used run a a
fairly large pump in a solar application), so its even less of a
problem.
-I
>
>
>> I solved the graceful-shutdown problem a different way.
>> I used a TS7260 because it has an on-board power supply that allows a
>> wide input voltage range. Its on-board ADC is conveniently connected
>> to Vin with a voltage divider, which lets me monitor Vin in software.
>> I then put 3x 2.5F/5.4V super-caps (in series) across Vin, which gave
>> me a UPS from 12 V (nominal, lead-acid battery+charging system) down
>> to 4.5 V or so when the power-supply would give out. You need low-
>> ESR supercaps for this to handle the board's entire load.
>> This gave me several seconds of running time depending on what was
>> plugged into the board (just a USB flash drive in my case).
>> This cost around $15. I used EMHSR-0002C5–005R4 from Nesscap, but I
>> see digikey doesn't sell them anymore, so I'll have to find something
>> equivalent next time.
>>
>> I wrote a polling-program (userland, not kernel) to poll the ADC
>> every 100mS, and do a shutdown if Vin dipped below a configured
>> voltage. Since there was no convenient way to halt the CPU and bring
>> it back up if the voltage never got low enough to actually shut it
>> down, I just shutdown my application (separate program), dismounted
>> all the filesystems and left the polling program running. This let
>> me re-mount everything from within this program (and restart my
>> application) if the voltage climbed back up to a preset "turn-on"
>> level.
>> The TSBAT3 scared me away because it just seemed way too complicated
>> (and expensive).
>> Supercaps don't last forever either, but they last a good long time
>> if they don't get too hot.
>> No charging programs or circuits, just dump the upstream power supply
>> straight into the supercaps (which is a very big load if they're
>> fully discharged).
>> Works great.
>>
>> -Ilya
>>
>
>
>
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