When discussing the drive topologies in the previous section, the power stage was simplified to a single transistor in the examples. For a bipolar motor, the power stage is actually an H-bridge driving each coil with some options on how the chopping control to the bridge can be applied. The selection made will have an effect on the ripple current and, in some cases, the performance from the motor. The presentation will now look at the chopping modes available with an H-bridge. The diagram on the left shows the current path during the ON time of the PWM. One of the upper transistors and the lower transistor on the opposite side of the bridge are turned on so that current flows through the coil. When one or both of these transistors are turned-off, the inductive property of the coil will tend to keep the current flowing so the output will fly back. This current flows through a clamping diode or diodes and continues to flow in the coil until it reaches zero or the transistor is turned on again. If only the lower transistor is turned off, the current will flow through the upper transistor that is on, the coil, and the opposite diode on the upper side as shown in the center diagram. This is typically called slow decay mode or phase chopping. Since the rate of change of current in a coil is proportional to the voltage across the coil as given by the equation V = L di/dt, the current will decay slowly since the voltage across the coil is only the forward diode drop plus the drop across the one transistor. If both the lower and upper transistors are turned off, as shown in the figure on the right, then the current flows through 2 diodes, the coil and back to the supply. This is typically called fast decay mode or enable chopping. In this case, the current will decay much faster since the voltage across the coil is equal to the supply voltage plus the forward drop of the two diodes.