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Suppose I used a single DC power source as an input to multiple isolated DC-DC converters (i.e. flyback converters) and series connected their outputs to obtain a larger overall output voltage. If each converter circuit was configured the same, could I expect that the load would divide evenly among them (i.e. each converter would output roughly the same power and current)? My thought is that, since they aren't switching in-sync, there would be differences in stored energy among each converter's output cap? If so how different or how would I find the max expected current draw from any converter?

The reason I ask is because I was looking into building a flyback converter and I was following this app note: http://www.ti.com/lit/an/snva866/snva866.pdf and when calculating parameters relevant to choosing a transformer, I ended up calculating a large peak current for the primary side of the transformer. From equation 8 in that app note, this is primarily dependent on max output power and minimum input supply voltage. Anyway, it seems that a large primary side current limits me to a small selection of relatively large [in size] transformers. So I'm thinking if I can bring down the power requirement by my idea above, then it will ease in transformer selection.

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  • \$\begingroup\$ If you connect the DCDC converter outputs in series to add up the voltages then the current through all converters would be the same so assuming they are all able to output their rated voltage then they would share the load equally. However usually these DCDC converters are not designed to be used like that so you might run into issues. Switching "in sync" or not is totally irrelevant. Generally you should just use a DCDC converter which outputs the required total voltage. \$\endgroup\$ – Bimpelrekkie Mar 2 at 8:03
  • \$\begingroup\$ You mention flyback converter design and the transformer being too large. You do not ask what options you have to use a smaller transformer. How about an increased switching frequency? A different IC might be needed though. Using isolated DCDC converters in series is quite unusual. That fact alone should indicate that maybe it's not such a good solution. \$\endgroup\$ – Bimpelrekkie Mar 2 at 8:13
  • \$\begingroup\$ I once used a 24 volt output converter in series with a 3.3 volt output to get 27.3 volts but recognized that the current capability was only that of the 24 volt converter. You can use them subtractively too. This isn't generally the case but is likely for most isolated flyback converters. \$\endgroup\$ – Andy aka Mar 2 at 8:17
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Connected in series the current through the output of each converter will be equal: this is a consequence of kirchoff's node law.

If you want more power however you should consider a forward converter topology in preference to the simpler flyback topology. above about 150W a forwards converter is more practical because the transfromer used in a forwards converter is more compact than the one in a flyback.

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  • \$\begingroup\$ I think I overthought this a little. Thank you for your answer. You have given me another topology to look into as well. \$\endgroup\$ – eeMoo Mar 8 at 17:55

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