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I'm using a high voltage DC 84v ebike battery to power several functions on the bike that need considerably lower voltage. It's impossible to find high current and wide voltage DCDC down converters that are also very compact.

So I wondered

  1. Can I use three widely available compact 36v buck converters and wire the input in series to create a voltage divider. Then

  2. Can i connect the three outputs in parallel to increase the load current capability.

I'm using these:

https://www.ebay.co.uk/itm/5A-Dc-To-Dc-Cc-Cv-Lithium-Battery-Step-Down-Charging-Board-Led-Power-Converter/122658609704?ssPageName=STRK%3AMEBIDX%3AIT&var=423065381521&_trksid=p2057872.m2749.l2649

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    \$\begingroup\$ You can't wire the inputs to buck converters in series. Their current requirements vary hugely from one fraction of a second to the next. \$\endgroup\$ – Finbarr Dec 28 '17 at 9:57
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    \$\begingroup\$ And No, You cannot just connect outputs of voltage sources in parallel, unless you know exactly how they work. Ever. Especially not random eBay junk. \$\endgroup\$ – Asmyldof Dec 28 '17 at 10:22
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    \$\begingroup\$ It's not impossible to find these, you're just looking in the wrong place. 84V DC is below the range of input voltages for offline ( 90-260V AC) convertors but not very much, so tthat's where I'd look - for an IC and/or application notes that can be adapted to cover your range. (Yes - AC in - but the first component is a bridge rectifier to make DC, and that will work with DC in too) \$\endgroup\$ – Brian Drummond Dec 28 '17 at 11:45
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The most 'upper' buck-converter in the series connection will see the full input voltage as semiconductor blocking voltage in operation which will probably blow it up (84V input, specified for 36V).

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    \$\begingroup\$ It is in no way known or guaranteed that "the upper one" will see the full input voltage. This will depend on input capacitance and reference/trigger level tolerances. It is true, however, true that at least one, possibly all, will break spectacularly. \$\endgroup\$ – Asmyldof Dec 28 '17 at 10:26
  • \$\begingroup\$ @Asmyldof are you suggesting it is guaranteed that the “upper” converter will never see the full voltage or it is just unlikely? Do you always hope for the best or should we plan for the worst? \$\endgroup\$ – Solar Mike Dec 28 '17 at 11:02
  • \$\begingroup\$ @SolarMike Yes, you are right, at least one of them failing spectacularly is the best thing you can hope for. That's why I also added the word "all" there. If you would just read that comment again now, you may find that I at no point say what is guaranteed and in fact actually only talk about what is certainly not guaranteed. \$\endgroup\$ – Asmyldof Dec 28 '17 at 11:18

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