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The C-E diode is used in place of the diode that is normally connected across the inductive load. It allows current to decay in the inductor very rapidly, but still damps ringing and prevents damage to the transistor. The transistor must be a high voltage device (most have breakdown rated at several hundred volts). Some also have the base-emitter speedup diode incorporated.That drawing shows a commutation diode that would be placed across an inductive load to kill the back emf spike.
Look at this darlington:
http://www.datasheetcatalog.org/datasheet/motorola/MJ10012.pdf
It has a built in diode across C and E. This is the body diode, right? Is it there as a byproduct of how a darlington is manufactured (like a MOSFET's is) or is it intentionally made as part of the circuit? I don't think it does anything useful, but I'm not sure.
Read post #6.They spec the max forward voltage of the diode when its current is 10A but I don't know why the diode is there.
I always wondered what a very high voltage darlington is used for. It survives the high flyback voltage from an inductive load. So the parallel diode clamps ringing.Read post #6.