ESD Protection in Circuit Design

How a Discharge Reaches the Circuit

An electrostatic discharge arrives at a connector, a button, a seam or a cable, and from there it travels through the product to find a return. The current is short in duration but very high, and its path through the board is decided by the layout, not by the schematic. The damage it causes may be immediate, such as a punctured gate oxide or a melted trace, or it may be latent, leaving a component that works but that fails later. Because the path is decided by the layout, the protection is a layout problem as well as a component choice.

Where the Devices Go

A protection device works by clamping the voltage and diverting the current, and it must be placed so that the discharge current reaches it before it reaches the circuit it protects. That means the device goes at the connector or the point of entry, before any other component, and its connection to the protected line must be shorter than the path to the circuit. A device placed after a series resistor protects the resistor rather than the device beyond it, unless the resistor’s value and rating are part of the design. The device’s return must be a low impedance path to the ground that the discharge will use.

The Return Path

The discharge current has to return to its source, and the return path is the part of the design that is most often neglected. The device clamps the line to the ground, and the current then flows through the ground to the point where the discharge entered, which is usually the chassis or the connector’s shield. If that path has a high impedance, the voltage at the device rises and the clamp does not protect the circuit. The ground connection from the protection device should be short and wide, and the connector’s ground should be bonded to the same reference as the protection device’s return.

Series Elements and Coordination

Where a series element, such as a resistor, a ferrite or an inductor, is placed between the connector and the circuit, the protection should be on the connector side of it. The series element then limits the current that reaches the circuit and allows a lower clamping voltage to be used. The protection’s own rating and the series element have to be coordinated so that the discharge is absorbed by the device rather than by the circuit, and the coordination should be checked against the standard’s waveform rather than assumed. Where a device with a low capacitance is required, such as on a high speed line, its clamping voltage may be higher, and the series element becomes more important.

Layout Details That Decide the Result

The details decide whether the protection works. The trace from the connector to the device should be direct and short, with no branch to the circuit before the device. The device’s return should be a via to the ground plane placed at the device rather than at the end of a trace. The protected trace beyond the device should not run back past the connector or parallel to the unprotected one. Where several lines are protected, their returns should be grouped so that the current does not flow through the sensitive area. And the connector’s shield, where there is one, should be bonded rather than left floating.

Selecting a Device

The selection considers the working voltage, the clamping voltage, the capacitance, the leakage and the response time. A device that clamps at a voltage above the circuit’s rating does not protect it, and one whose capacitance is too high will distort a high speed signal. The leakage matters where the line is a battery or a high impedance input. The standard that the product must meet defines the waveform and the level, and the device should be selected against that waveform rather than against a general description. Where the product’s port is exposed, the requirement is usually a specific level and a number of discharges.

Verification

The protection should be verified by a test rather than by an inspection. The applicable standard defines the waveform, the voltage and the number of discharges, and the product must survive them and still function. A test on the board alone is a useful check on the layout, and the full test on the product is what demonstrates compliance. Where a failure occurs, the analysis should identify the path that the current took, which is usually a gap in the return or a device that was placed too far from the connector. The remedy is a layout change rather than a bigger device.

Protection and the Assembly Process

The protection is only as good as the assembly. A component that is damaged by a discharge during handling may pass the test and fail later, so the handling procedures are part of the protection strategy. The protective devices should be placed before the board is exposed to a connector, and the assembly line should be controlled for static as well as for the components. Where a device is added late in the design, its footprint may not be in the position that makes the layout effective, which is why the protection belongs in the first layout rather than in the last revision. Designing it in early costs a few millimetres and saves a redesign.

FAQ

Where should an ESD device be placed? At the point of entry, before any other component on the line.

Why does the return path matter? The clamp only holds if the current can return by a low impedance path; a poor return raises the voltage at the device.

What does a series element do? It limits the current that reaches the circuit, which lets a lower clamping voltage protect it.

Why is a low capacitance device needed on a high speed line? Because the device’s capacitance loads the line and distorts the signal.

How is protection verified? By applying the standard’s waveform and level to the product and confirming that it survives and still functions.

Conclusion

ESD protection is a component and a layout, so place the device at the entry, give it a short return and coordinate it with the series element. Then test the product. Protection layout belongs to PCB design and layout, the assembly sits in PCB assembly, and the verification is part of PCBA testing. Protected designs are first built during prototype PCB assembly in 2026.