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Instead of buying in an expensive MTTP-Charger I want to buy a cheap module only. You get these small 2A/40V Buck-converters at eBay for 2-3 USD or the bigger 10-12A /40V from 9-11 USD.

Setup: Solarmodul 18V - Step-Down-Modul -(Skotty-diode)+(Fuse) - Battery.

Fuse is a should have, I know.

But I wonder if you need the Skotty-diode? What happends to the Step-Down Buck-converter-

  1. when there is no SolarInput? Then you have the 12.6V from the battery at the moduls OUTPUT.
  2. when the Buck-input(Solar) is short-circut, what will happen to the modul? Take all ampers from the battery to do the shortcircuit? Will it burn?

Perhaps the easy answer would bee to test it out in real, it iss only 2 USD.

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In a buck converter, the schottky diode (that you may be referring to) is absolutely needed. Don't even think about leaving it out - you'll be just wasting your time. When the transistor "opens" after pushing current into the inductor, that current needs to keep flowing to carry on passing energy into the output capacitor. Without the diode (schottky or otherwise) this current can't flow into the output cap so instead it creates a big voltage spike and destroys the switching transistor in the chip.

If you are referring to a diode placed in series with the power feed to the battery then it's likely this is also needed for two reasons: -

  1. With no power fed to the buck convertor, the battery terminal voltage would reverse bias everything and possibly cause circuit failure.
  2. Even if the switcher circuit self protected without the diode there is astill a good chance you'd drain the battery.
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