Why are extra flyback diodes necessary?

Barnhart Crane 6 years ago in Expansion modules updated by Gustav Widén (System support) 6 years ago 1

Why do the XA2 and XC10 manuals call for adding clamping (flyback) diodes to digital outputs?

From what I can gather, the modules are equipped with their own internal clamping diodes. Why would those be disabled for digital outputs?



The COUT:s on XA2 have internal circulation diodes, but not the DOUT:s. 

The XA2 highside drivers for the DOUT:s will actually cut the transient, but if the load is too large repeatitive stress like this will wear on the component. See "Maximum load inductance" in the XA2 user manual for the size of loads where this is necessary.

Note that the user manual states that it has to be a regular diode installed as flyback diode. 

Some have tried a solution with opposing zener diodes, but that normally has no effect, as connectors with opposing zener diodes are usually designed to cut at a high threshold that is higher than the level where the smart-FET cuts.