Introduction
It is common that engineers choose bidirectional TVS over unidirectional TVS to avoid the issue of directionality. However, choosing a bidirectional TVS without predetermining whether the protection target signal is unidirectional or bidirectional may reduce the protective effects. The differences between unidirectional and bidirectional signals will be explained to further understand whether unidirectional or bidirectional TVS devices can provide a better ESD suppression effect.
1. Uni- and bidirectional signals
Signals in general can be categorized as unidirectional or bidirectional. A unidirectional signal or power supply means that only positive or negative voltage will appear on the signal line; i.e., a signal of single polarity application is a unidirectional signal, as shown in Fig. 1 and 2. A bidirectional signal indicates that the signal voltage consists of both positive and negative voltages, as shown in Fig. 3.
Figure 1: unidirectional signal (consists of only positive voltages)
Figure 2: unidirectional signal (consists of only negative voltages)
Figure 3: bidirectional signal (consists of both positive and negative voltages)
2. Unidirectional and bidirectional TVSs
Since a signal that needs TVS protection can be unidirectional or bidirectional ,it is necessary to determine if the signal level contains positive voltage, negative voltage, or both, when choosing a TVS device. For unidirectional signals, a unidirectional TVS can provide better ESD suppression. The unidirectional TVS device symbol is shown in Fig. 4. Note that the voltage level of this signal must not be greater than the breakdown voltage (VBR) of the TVS or the signal will trigger the TVS breakdown in normal working condition. That’s why it is necessary to evaluate the signal voltage level and the DC parameters (VBR, VF) of a TVS device before choosing the optimal TVS device. For example, if a unidirectional signal contains positive voltage and is coupled with a unidirectional TVS, there will not be any forward conduction since the unidirectional signal does not carry negative voltage. The DC parameters (VBR, VF) of a unidirectional TVS device are shown in Fig. 5.
Figure 4: unidirectional TVS symbol
Figure 5: unidirectional TVS DC parameters (VBR, VF)
If the signal to be protected is bidirectional, meaning that it carries both positive and negative voltages, a bidirectional TVS device is needed to prevent signal distortion as the misuse of unidirectional TVS would cause forward conduction of negative voltage signal. The symbol of bidirectional TVS and its I-V curve (DC properties) are provided in Fig. 6 and 7.
Figure 6: bidirectional TVS symbol
Figure 7: bidirectional TVS DC parameter (VBR)
3. Differences in application between unidirectional and bidirectional TVS devices
A common misconception is that a unidirectional TVS can only provide protection for positive voltage surges, which could not be more wrong. The reality is that both unidirectional TVS and bidirectional can provide protection against surges of both positive and negative voltages. It’s just that careful selection is necessary as unidirectional TVS devices do not work on a signal line where both positive and negative voltages exist. Common examples of bidirectional voltage applications are: audio, RS485, RS232, etc. For instance, if a unidirectional TVS is used for an RS232 bidirectional signal cable, the negative signal (-12V) will be clamped at -0.7V due to forward conduction of the unidirectional TVS (at roughly 0.7V). The red dotted line shows the waveform when the negative voltage is clamped at -0.7V. This will lead to signal distortion or even system crash, as shown in Fig. 8 and 9.
Figure 8: unidirectional TVS implemented on a bidirectional signal
Figure 9: negative signal clamped to -0.7V
On the other hand, if a bidirectional TVS is used for a unidirectional voltage signal, distortion caused by signals being clamped at an inappropriate level will not appear. However, a bidirectional TVS provides far less protection than a unidirectional TVS device against negative voltage surge, meaning that optimized protection cannot be achieved. The core chip is highly vulnerable due to its advanced manufacturing process, and the chip’s I/O protection circuit is usually a unidirectional ESD device. Therefore, if a bidirectional TVS is used in an external circuit, the unidirectional ESD device in the core chip may initiate conduction when there is a system-level negative voltage surge, ultimately resulting in the core chip being fried as shown in Fig. 10.
A bidirectional TVS initiates conduction only when both positive and negative voltages reach breakdown voltage. Therefore, the positive voltage conduction curve is similar in both unidirectional and bidirectional TVSs. However, when there is a negative voltage, the voltage passing through a unidirectional TVS device would be clamped at -0.7V therefore only initiating forward conduction for protection; whereas a bidirectional TVS device needs to reach its negative breakdown (approximately -6V for 5V TVS VBR) under negative voltage, thus resulting in the difference in negative surge voltage protection. TLP curve comparison is shown in Fig. 11.
Additionally, if we compare a unidirectional TVS and a bidirectional TVS in the same packaging, it can be observed that the clamping voltage provided by the unidirectional TVS is better than that of the bidirectional device despite the same breakdown conduction under positive voltage. This is because implementing bidirectional TVS devices requires more elements and features more complicated structure under the same chip footprint; whereas a unidirectional TVS can maximize the size of the protection device and lower the clamping voltage and dynamic conduction resistance to provide better protection.
Figure 10: a bidirectional TVS used for unidirectional signals (0V ~ +5V)
Figure 11: TLP curve comparison between unidirectional TVSand bidirectional TVS
In short, a unidirectional or bidirectional TVS can be identified by the device wiring diagrams in the electrical characteristic specifications (Fig. 4 and 6). Either unidirectional or bidirectional TVS can provide effective protection against positive or negative voltage surge. If the target signal is unidirectional (with only positive or negative voltage), a unidirectional TVS device is definitely better than a bidirectional one in terms of ESD protection. However, in cases where the target signal is bidirectional, such as audio, RS485, and RS232 (with positive voltage), only bidirectional TVS devices can be chosen. This is to prevent communication errors or even system crashes if a normal signal gets clamped at -0.7V by the unidirectional TVS under negative voltage due to caused by the forward conduction of diodes.