off. A state diagram for the commutation process for a simple two-step commutation
sequence between two bi-directional switches from Fig. 2.18a is shown in Fig. 2.21c.
This commutation method has practical limitations. During the current reversal
period the current direction is unknown, because the current reversal switch is subject
to hysteresis. During this time period the commutation cannot take place. Since the
direction of current is unknown, the correct device that will conduct the current cannot
be determined. The second disadvantage is that current direction can be difficult to
determine, especially in high power drives when the levels of current are low and
when the load current has to be within a threshold level. Then the threshold level
may also be relatively large. This may result in a distorted current waveform.
For current commutation techniques it is required to know the load phase current
direction. High precision determination of the direction of current is a key
issue. Any inaccuracies cause errors, which result in switching losses and the possibility
of destroying switch cells. To solve this problem a new technique has been
developed [33]. This technique uses the voltage across the bi-directional switch to
determine the current direction. This conception is based on an intelligent gate drive
circuit. In addition, to control the IGBT this gate driver can also detect the current
direction and enables the exchange of information between other gate driver devices.
This process ensures that all gate drivers can operate with safe commutation.