Schottky diodes, yes, your primary go-to when you want to protect a potentially battery-powered system from polarity inversion, right? Yes, indeed, if you have one power supply voltage input, just use one Schottky diode and you’re done. But what if you have a board with two power supply inputs? For example, you might have both 5 V and 3.3 V logic components on your board, and assuming you do not use an on-board switched-mode power supply, that means you’ll have two power connectors. So, just use two Schottky diodes.
Hold on. Now, this is where you need to scrutinize your specific wiring. Often times in logic circuits with both 5 V and 3.3 V power supplies, a “common ground” return path is used for both voltage potentials. Assuming the worst in a prototyping environment, you might connect a voltage source across the two different potential levels. So, how is this problematic? Well, if your circuit design is botched, you might have been assuming that current would never be able to flow across that path and somehow have some short across that path. Like, “Hey, I don’t care whether this circuit gets 5 V or 3.3 V, just give me either power supply that is available.” So you wire the 5 V and 3.3 V together just like a “Y” connection, and, just like that, you’ve got a short.
In any case of uncertainty about this, adding two more Schottky diodes to enforce polarity on every pin in your two power connectors will eliminate this possibility, even when using a common ground. You must still also use two separate Schottky diodes for each ground return since otherwise you have another short circuit path on ground for the same reason.
In my particular case, it’s literally that I’m designing a “debug” board module that can connect with any random board that I may not be immediately aware of such wiring implications, so I figured it’s better to be safe than sorry.
What if you are designing a complete system where all extremes are known, and you want to minimize the component count and cost as much as possible? If these assumptions are held, you can progressively reduce the number of short circuit and polarity protection Schottky diodes you have in your circuit.
-
You have no short circuit connections between the voltage supply and ground return of any voltage supply, of course!
-
You do not ever have any two different voltage supplies wired directly together. If two voltage supplies must be wired together to provide an “either voltage X or Y” function, they are always connected with two diodes.
-
You use separate ground return paths for each voltage supply, and the ground return paths are never bridged together anywhere in your circuit.
-
Alternatively, if you use a common ground return path, you must have polarity protection Schottky diodes on each power supply’s ground connection.
-
Finally, the ultimate omission. If you use a mechanically polarized power connectors on your board, and the final connection to the ultimate power source is polarized, such as a diode bridge rectifier connection to AC line power, then you can omit the final polarity protection Schottky diode from each power supply connector on your board. That being said, it is worth reiterating that instead of using a Schottky diode to enforce polarity on your board, you could instead use a diode bridge rectifier… but as I’ve mentioned previously, the motivation of using a simple Schottky diode in place of a diode bridge rectifier is that it is cheaper.