Foreword
It is common that engineers choose bidirectional TVS over unidirectional to avoid the issue of directionality. However, choosing a bidirectional TVS without determining the signal to protect is unidirectional or bidirectional may compromise the protective effects. The following will show you the unidirectional or bidirectional TVS will provide the best ESD suppression for these signals. But, first, let’s talk about the difference between a unidirectional signal and a bidirectional one.
1. Uni- and bidirectional signals
For signals in general, there are unidirectional and bidirectional signals. A unidirectional signal or power supply means that there is only positive or negative 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 (the signal with positive voltage)
Figure 2: unidirectional signal (the signal with negative voltage)
Figure 3: bidirectional signal (the signal with both positive and negative voltages)
2. Unidirectional and bidirectional TVSs
A signal that needs TVS protection can be unidirectional or bidirectional. That’s why it is necessary to determine if the signal level contains positive or negative voltage, or both, when we are choosing a TVS. For a unidirectional signal, a unidirectional TVS provides better ESD suppression. The unidirectional symbol looks like the one in Fig. 4. Note that the voltage level of this signal must not be greater than the breakdown voltage (VBR) of the TVS. If it is, 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 TVS for a comparison before choosing the right TVS. 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 TVS DC parameters (VBR, VF) are shown in Fig. 5.
Figure 4: unidirectional TVS symbol
Figure 5: unidirectional TVS DC parameters (VBR, VF)
When the signal to protect is bidirectional, meaning that it carries both positive and negative voltages, a bidirectional TVS is needed to prevent signal distortion as the misuse of unidirectional TVS causes 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 the application of unidirectional and bidirectional TVSs
There has been a misunderstanding that a unidirectional TVS provides protection only for positive voltage surge, which could not be more wrong. The correct way to say it is that both unidirectional TVS and bidirectional protect against the surges of both positive and negative voltages. It’s just that careful selection is required, as a unidirectional TVS does not work on a signal line where there are both positive and negative voltages. Examples of common bidirectional voltage applications are: audio, RS485, RS232, and so on. For example, if a unidirectional TVS is used for an RS232 bidirectional signal cable, the negative voltage of a negative signal (-12V) will be clamped at -0.7V due to unidirectional TVS forward conduction (at roughly 0.7V). The red dotted line is the waveform with the negative voltage clamped at -0.7V, which will lead to signal distortion or even system crash, as shown in Fig. 8 and 9.
Figure 8: a unidirectional TVS used for a bidirectional signal
Figure 9: negative signal clamped to -0.7V
If, on the other hand, a bidirectional TVS is used for a unidirectional voltage signal, there will not be distortion due to signal being clamped at an inappropriate level. However, a bidirectional TVS provides far less protection than a unidirectional one against negative voltage surge, and that prevents optimized protection. A core chip is highly vulnerable in an advanced process, and the chip’s I/O protection circuit is usually a unidirectional ESD device. Therefore, if a bidirectional TVS is added in an external circuit, the unidirectional ESD device in the core chip may initiate the conduction when there is a system-level negative voltage surge, and the core chip is 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 unidirectional TVS clamps its voltage at -0.7V and initiates only forward conduction for protection, whereas the bidirectional TVS needs to reach negative breakdown (-6V for 5V TVS VBR, for example) for negative voltage, thus the difference in the negative surge voltage protection. See the TLP curves in Fig. 11.
In addition, to compare a unidirectional TVS and a bidirectional TVS both in the same packaging, you will find the clamping voltage is better in unidirectional than in bidirectional TVS, despite of the same breakdown conduction at positive voltage. This is because the bidirectional TVS requires more elements and features more complicated structure for the same chip footprint; whereas the unidirectional TVS maximizes the size of protection device and lowers the clamping voltage and dynamic conduction resistance for better protection.
Figure 10: a bidirectional TVS used for unidirectional signals (0V ~ +5V)
Figure 11: TLP curves, unidirectional TVS vs. bidirectional TVS
In short, a unidirectional or bidirectional TVS can be identified by the device wiring diagrams in the electric specifications (Fig. 4 and 6). Either unidirectional or bidirectional TVS provides effective protection against positive or negative voltage surge. If the signal to protect is unidirectional (with only positive or negative voltage), the unidirectional TVS is definitely better than the bidirectional one in terms of ESD protection. It is necessary to use only a bidirectional TVS device if the signal to protect is bidirectional, such as audio, RS485, and RS232 (with positive voltage), as to prevent communication errors or even system crash if the unidirectional TVS clamps the normal signal at -0.7V at negative voltage due to forward conduction of diodes.