The simple explanation for phase switching:

When you turn on a phase you supply current to it until it reaches the
desired magnetic field strength. Then any current that still goes through
it becomes heat. When you wish to turn a phase off you have an interest to
remove the field asap (or not). The simple way is to turn the driver off.
The field will collapse and produce a terrific inductive kickback,
returning most of the energy put into it (Wl = L * I^2 / 2). A H bridge
driving a phase needs protection diodes to route this kickback current
safely (and to prevent it from becoming a destructive voltage).  They are
already built in and they cause the kickback to be rectified and to try to
charge the battery through the supply rails. This is good (it is also used
as energy recovery method in electric vehicles and some electric rail
engines, during regenerative braking). You can help the efficiency of the
process by driving the H bridge in the opposite direction, from when you
stop driving the phase, until the current through the coil goes to zero.
This will effectively help the rectifer bridge formed by the MOSFET bulk
diodes to be more efficient and to heat up less. There are more elaborate
schemes like this. The reverse forcing (one of the names by which this
scheme is known) of phases also speeds up the removal of the field and
helps achieve higher speeds.

Peter

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