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I regularly see the use of Schottky diodes in order to protect microcontroller inputs from overvoltages (>5V). The Schottky diodes are connected to the 5V power supply (cathode) and the microcontroller input (anode).
I was wondering what happens when current flows through these diodes due to overvoltage? Where does this current flow into? It probably flows into the power supply unit...but what happens there? Isn't it so, in fact, that we are applying an overvoltage to the power supply output and that we expect the power supply to take care of it...?
Anyone with bad experiences with this solution? Are there any better solutions?
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(In fact, the ESD protection diodes in the microcontroller itself apply the same trick for short spikes...the spike current flows into the power supply...)
A Schottky barrier diode (SBD) is a type of diode that has a junction formed between a semiconductor (typically an n-type semiconductor) and a metal such as platinum (Pt), molybdenum (Mo), or titanium (Ti), instead of a pn junction.
SBDs are designed to exploit a Schottky barrier created by a difference between the work function of a metal (i.e., the energy needed to remove an electron from the surface of a metal) and the electron affinity of a semiconductor. (In the case of an n-type semiconductor, a Schottky barrier is created only when a metal has a higher work function than the n-type semiconductor.)
SBDs have lower forward voltage than pn junction diodes because SBDs use a metal-semiconductor junction with a lower potential barrier than the pn junction.
The pn junction diode is a bipolar device in which both electrons and holes act as donors whereas the SBD, which is typically composed of an n-type semiconductor and a metal, is a unipolar device in which only electrons act as donors. Therefore, SBDs have no reverse recovery charge due to residual minority carriers, which is a matter of concern when using pn junction diodes. However, due to the electrostatic capacitance that exists between terminals, there is a reverse recovery time, although it is slight compared to pn junction diodes.
The low forward voltage and short reverse recovery time of SBDs make them suitable for high-speed switching applications.
Silicon SBDs withstand only several tens of volts whereas some SiC SBDs using wide bandgap semiconductor provide a withstand voltage of more than 600 V.
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