Images for unidirectional and bidirectional TVS diodes are indicated below.
Unidirectional TVS diodes are used in the reverse bias state. Line voltages for protection are between the 0 to VFWM range.
Therefore, it is basically applied for DC power line protection.
Bidirectional TVS diodes are capable of supporting both positive and negative electrodes. Bidirectional TVS diodes are applied as protection lines if applied on both positive and negative such as data lines.
Bidirectional diodes can be applied on DC power lines.