Adding multiple DC power sources

[raff5184]

Hi,

what is the best (most energy efficient) way to add the powers from DC sources that have different voltages, at a fixed voltage value?

I have a harvesting circuit with multiple rectifiers. Each rectifier provides a voltage (and power) more or less different from the others and it is not always the same. I want to sum all the DC powers. The voltages are in the order of 10-20V and the power that the rectifiers can source about 200-800mW.
 Also, the voltage at which I want to add up all the powers is fixed and the single rectified voltages are randomly higher or lower than this value.

I did some tests, disregarding the "fixed" voltage level and tried to combine the powers by connecting the rectifiers in parallel on a resistive load. This doesn't work well, because only the rectifiers that have higher and comparable voltages (and powers) will add up, while I noticed that the sources with lower voltages don't contribute (even though they are capable of sourcing power when the other sources are off).

Do I need a DC-DC converter at the output of each rectifier? Is only one DC-DC converter enough, meaning would it force all the voltages at the same level?
Are there other simple ways to add DC powers (IC chips, simple circuits)? Even if they don't add at the fixed voltage level that I need them to combine, I can always use a DC converter after the sum

[penfold]

I suppose the 'correct' way to do it would be to have one DC/DC per power input. It could then be a question of control if they are all feeding, say, a battery that is a relatively stiff voltage, but whether or not it is a problem would need some knowledge of the actual sources from which you are scavenging. A discontinuous current boost converter gives you some level of "power" control, which with something like a PV cell, having a very soft IV characteristic, allows you to vary the duty cycle and pick up on the maximum-power-point. In contrast, if you have a source that is a stiff voltage or current source, you would need to be able to determine the operating current to control the boost and ensure you're making the most of the source.

You can connect the rectifiers in series, but then, similar to the voltage problem in parallel, the total output will be limited, by the lowest current. But again, there'll be solutions to it, but need to know the nature of the sources from which you're scavenging.

[mawyatt]

Currents can be "wired ORed" together and the result is the sum. Convert the individual sources into currents with SMPS techniques, then add the outputs.

Best,