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Is there a circuit or device that can facilitate load sharing of different power supplies? For example, suppose I have a single 100 Watt Load that operates at 12VDC, a 9VDC source capable of delivering 75 Watts, and a 12VDC source capable of delivering 75 Watts. I need help from the 9VDC source to deliver enough power, but the voltage is wrong.

What are some options? I'm not afraid of answers that go outside of the electrical domain (ex: flywheels), but I would prefer a circuits/electronics solution. Is there such a thing as a multiple input, single output SMPS? Can I throw a boost converter on the 9VDC up to 12VDC and then put it in parallel with the 12VDC?

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Put the voltage sources in series and use a buck regulator to reduce the total voltage to 12 volts. You will likely have to build an isolating flyback converter to allow you to add one supply in series with the other but, if one is already isolating then it becomes easier.

The above works with supplies of the same voltage and power because the current to the load (or buck converter that drives the load) will be identical. When you have a 9 volt and 12 volt supply, you need to raise or lower one of those voltages to ensure that power delivered to the load is spread proportionally to the power outputs of each supply. If both are 75 watt supplies then you adjust (for instance) the 9 volt supply (using a flyback circuit) to be more like 11 volts.

Why 11 volts and not 12 volts you might ask - well the flyback converter that raises the voltage from 9 volts to 11 volts burns power and leaves you with slightly less than 75 watts. Say it burns 6 watts - this now leaves you with 69 watts to power the load (plus the 75 watts from the 12 volt supply).

69 watts and 11 volts is a current of 6.27 amps and very similar to the 6.25 amps from the 12 volt supply running a power of 75 watts.

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Well, yeah, that's possible.

If you think about it, that's close to what a power grid is – a lot of different power sources feeding one network.

Luckily, this works on smaller scales, too, and slightly easier, without having to operate all the power plants synchronously:

In a DC network, you'd "agree" on a fixed voltage, like you'd suggested. Typically, you'd even buffer that voltage with very "agile" power storages – in your dimensions, large capacitors, probably – and feed the load from that.

Now, the question is which voltage to agree on. Intuitively, yes, stepping up the 9 V to 12 V would allow you to directly cooperate with the 12 V source and feed the 12 V load.

That works well if the 12 V load isn't very sensitive to slight voltage variations – because there's little to no "headroom" for the power sources to cooperate in case of a load increase.

So, if your 12 V load isn't very sensitive, or you can significantly overdimension your boost converter to handle a load increase e.g. from 10 W to 50 W without dropping much on the 12 V output, then go directly.

In many cases, but that really depends on the characteristics of your power sources and loads, to have the multi-source voltage (let's call it intermediate circuit) "far enough" from the output voltage to allow for an effective SMPS to react to a changing sources voltage without significant effect on the load voltage.

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