The PCB layout shown above includes multiple passives that perform different functions, but only one of these is ideal for electrostatic discharge (ESD) protection. This is a TVS diode, which is a component that can divert an incoming ESD pulse away from a sensitive component. They are often placed on data lines coming off of a connector, but they can be used in other places where there is a risk of ESD reaching a data line and its connected component.
In this article, we’ll show where to place a TVS diode in a PCB layout, particularly near connectors mounted along a board edge. Another point that is often not considered in TVS diode usage is the PCB stackup, and specifically how the PCB stackup should be designed to provide the required connections on the anode of a TVS diode.
TVS Diodes in a PCB Layout
Before showing placement examples in a real PCB layout, it’s important to note how TVS diodes work and the required connections. This will then inform how the PCB stackup should be constructed so that the correct connections can be made to the TVS diode terminals.
How Do TVS Diodes Work?
TVS diodes work like any other diode; they conduct strongly in one direction while they conduct very weakly in the other direction (known as rectification). TVS diodes are also bidirectional, being equivalent to back-to-back diodes, so they are always in reverse bias when placed between a circuit and ground. TVS diodes are one of many options for ESD protection that can be used in an electronic device.
TVS diodes are two-terminal devices that have simple placement guidelines: one terminal (the cathode) is placed on a conductor that needs ESD protection, and the other terminal (the anode) is connected to GND. An example is shown in the schematic below.
When there is a signal traveling on the conductor, the TVS diode will be in reverse bias at the signal’s voltage level, so it will not enter breakdown and it will be insulating. There will be a small amount of leakage current in this situation, but it is minimal and can be ignored. When high-voltage ESD occurs and a current is driven onto the protected line, the diode will be driven deep into reverse bias and will enter the breakdown region. This causes the TVS diode to become conductive. Instead of the ESD current flowing into the protected component, the current will pass through the diode and through its anode connection into ground.
PCB Layout Example
As was mentioned above, a ground connection is needed when using a TVS diode. The anode connection needs to be low impedance so that the current can pass into ground, and the current flow in the ground net also needs to be low impedance. The best way to do this is to have a ground plane below the surface layer, and then make a direct connection to the ground plane through a via.
An example showing the placement location of TVS diodes in an example PCB layout is shown below. Two connectors are coming onto a PCB with chassis ground connections on their housings. The arrows mark the location on two data lines where we could add TVS diodes for protection.
Ideal TVS diode placements near two connectors.
The best place to put the TVS diodes is closer to the point where ESD is likely to occur. In this case, it would be near the connector rather than near the protected component. This is because the component will be concealed inside an enclosure, while the connector will poke out the edge of the enclosure. In this particular layout, the stackup uses a ground plane on the next layer to provide low-impedance ground connections for all components. This also provide a low impedance path in the ground plane
Not all electronic devices will need TVS diodes in the PCB layout. If there is some reason a system might experience ESD, even if it is a small surge, then it makes sense to put TVS diodes on the vulnerable signal lines. Some examples include:
- On data lines for hot-swappable cables or devices that will be brought up to high voltage
- On data lines coming out of a connector when the conductors are exposed to open air
- When a system or portion of a system will be deployed near other high voltage equipment
- When there are high-voltage elements on the board placed near data lines needing protection, and isolation is not possible
There are other instances where one can conceive ESD might occur on data lines. If there is the possibility of ESD occuring, then consider using TVS diodes as part of a comprehensive ESD protection strategy.
Placement of TVS diodes for ESD protection and PCB stackup design can be difficult if you aren’t using the complete set of PCB design tools in OrCAD from Cadence. OrCAD includes the industry’s best PCB design and analysis software with a complete set of PCB project management tools for managing design data. OrCAD users can access a complete set of schematic capture features, mixed-signal simulations in PSpice, and powerful CAD features, and much more.