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Feeding a SMPS with DC instead of mains

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  • #16
    Solar power is generated in about half-volt units. The photovoltaic junctions are stacked in series to get you to battery charging voltages, generally 20V or so open circuit to charge 12V batteries, and higher to charge 24 or 48V batteries. Your friend is probably stacking a bunch of solar cell arrays to get up to 220V, or floating many of the arrays to charge each battery from a floating solar cell, but provide the power out as high voltage DC.

    The question keeps coming back - why? If the charging scheme is built on the idea of running switching power supplies from high voltage DC, you get all of the quirks mentioned above. Some SMPS use doublers, some use active power factor correction (which isn't generally a problem, but there are different variants to take into account) and others will happily run on DC fed in. Pretty much, you're going to have to know the schematic and operation of every SMPS you connect to this setup. Each one will have a potentially different set of quirks.

    The solar power industry is built on the idea of putting solar-collected energy into the cheapest mass electrical storage available, then converting that to usable standard AC as efficiently as possible. That has amounted to arrays of lead-acid batteries for availability and cost reasons, although advancing lithium battery tech is getting there. This approach uses power inverters to make standard AC power from variable DC power. It trades off some inefficiency in converting twice, once to AC standard power, and again to the thing being powered, for ease and low cost of storage.

    If you're going with stacked, high voltage battery storage, you get the same problems unless you can eliminate a conversion step, and you take on the additional problem of now having to tinker and select for things that accidentally can be powered by high voltage DC. It gets a lot more special-cased.
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    Oh, wait! That sounds familiar, somehow.


    • #17
      With the special case argument presented, I wonder what 'special case' you are solving for by using a SMPS?

      If your solar-power friend is looking to get 110 or 220 VAC line power to run his home/camper/mars rover then maybe an "off the shelf" variable frequency drive would work. Most modern units have AC line inputs to the rectifier, but also DC bus connections specifically designed for load sharing. You still need to size it for the expected max load and minimum expected DC voltage, but that's minor compared to the minutia discussed above. Try looking at VFDs made by Yaskawa for starters. Generally low-cost and available.
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      • #18
        There are plenty of off the shelf solar -> battery -> AC inverters on the market for off-grid applications. That's the way I would go. Once you've got a standard distribution voltage and frequency it's easy to hook to other things.
        Experience is something you get, just after you really needed it.