For industrial applications there are basically three types of driver technologies. The essential difference is in the way they energize the motor winding. The name unipolar is derived from the fact that current flow is limited to one direction. As such, the switch set of a unipolar drive is fairly simple and inexpensive. The drawback to using a unipolar drive however, is it's limited capability to energize all the windings at any one time.
Unipolar drivers are good for applications that operate at relatively low step rates.
This process also produces an excessive amount of heat and must rely on a DC power supply for it's current source. Although they are typically more expensive to design, they offer high performance and high efficiency. Bipolar chopper drivers use an extra set of switching transistors to eliminate the need for two power sources. Additionally, these drivers use a four transistor bridge with recirculating diodes and a sense resistor that maintains a feedback voltage proportional to the motor current. The sense resistor monitors the linear rise in current until the required level is reached.