ESD Protection Basics: Design and Simulation

It’s possible your next electronic device could be deployed in an area where it is exposed to high voltages or static electricity. In these cases, a system’s vulnerability to ESD should be determined through testing and simulation. There are also components that may be needed to protect a system from ESD and to ensure the system can withstand high voltage pulses.

In this article we’ll look at the two most basic aspects of ESD protection: using ESD protection components and shielding. ESD components are the most common route to building a system that can withstand high voltage pulses, but in some cases, additional protection can be provided by shielding on the board or in the enclosure.

How ESD Protection Components Work

ESD protection can be provided by specific components or connections to shielding in a system. The most common approach is placement of a component that can suppress the current/voltage induced in a circuit by ESD. These protection components and circuits operate in two possible ways, depending on where the suppressing elements are placed in a schematic:

• The component can divert a high voltage/high current pulse from a vulnerable component to ground.
• The component could be designed to absorb or withstand the ESD event. The simplest example is a resistor, which converts the pulse to heat, although resistors will generally burn up from an ESD pulse.

There are many commercially available options for ESD suppression, which can be used to target specific components that might be exposed to ESD. These measures can provide several kV worth of protection with pulse times reaching as low as nanoseconds. For now, we’ll look at common ESD protection components that can be used to protect against ESD events, although shielding and grounding are two important elements of protecting a system against ESD.

Circuit Protection Diodes

The most common circuit protection device is a transient voltage suppression (TVS) diode. These can be bidirectional (back-to-back) diodes, or they can be unidirectional with current flow only allowed in one direction. A Zener diode can also be used as an ESD protection component and it provides an equivalent function as a unidirectional TVS diode.

The schematic examples below show where to place diodes for circuit protection and their polarities. Because these diodes are nonlinear and rectifying, they only conduct strongly in one direction. In the positions where they are placed in the circuit below, these diodes are effectively insulating and will not allow any current to flow across the diode.

When an ESD pulse is received, the high voltage of the pulse quickly drives a protection diode deep into reverse bias until reverse breakdown occurs. The diode becomes conducting and there is now a short circuit across the terminals of the protected component. This diverts the received pulse to ground and prevents current from reaching the protected component.

Note that this protection mechanism requires that protected component have moderate to high impedance input. This is generally the case for most components that will require some circuit protection. Note that many integrated circuits can have some ESD protection built into them, usually on the order of a few kV. If higher levels of protection are needed, then you should add these diodes.

Simulating ESD Events in a System

ESD events inject a pulse into a system at a vulnerable point, which will typically be some area of the system that is exposed to the external environment. As was mentioned above, this is typically near connectors or any exposed metal elements in an enclosure, both of which are points where a large discharge event can inject current into the system. If you want to simulate an ESD event and its effects on your system, then you need to guess where the ESD event might occur.

This requires a bit of forethought as a designer, because you will need to know which elements might be exposed to a user and thus might experience an ESD event. To simulate an ESD event, you can place a pulse source somewhere in a circuit and run a transient analysis simulation. As the input pulse voltage amplitude is varied, you can generate results in your circuits that show how the protection diode diverts the pulse and reduces the voltage seen by the protected components.

Example results generated with Sigrity showing the effect of adding a TVS diode to a signal trace. Notice that the TVS diode provides significant damping thanks to its nonlinear rectification properties.

For Extra ESD Protection: Add Shielding

Once a system has been tested for survival from expected ESD events or the system has been evaluated against an industry standard, it’s worth questioning whether additional protection might be necessary. The most common cause of ESD is discharge through the human hand, which can accumulate static charge through triboelectrification. The result is discharge into a device that can cause equipment failure if not suppressed.

Because users normally interact with a device through the enclosure, it makes sense to explore integration of ESD protection through enclosure shielding. One measure that will provide additional protection from ESD (and RFI noise in some cases) is shielding added to the board or the enclosure. Some of the shielding options for specifically addressing ESD are:

• Component shielding cans
• Shielded enclosures
• EMI gaskets (for use on very large enclosures)

These EMI gaskets can be used to shield openings in an enclosure and provide a conductive path for ESD from the external environment.

When ESD could enter the system from an exposed bit of metal on the enclosure, then it is best to use an enclosure that provides chassis ground. Metalized enclosures can be used as a safety ground for any ESD pulse or any short circuit fault in the system. This will nicely divert the current injected by the ESD into a large conductor and will prevent damage to the electronics on the PCB. Make sure that the connection to the chassis ground for any shielded connector and the mounting elements on the PCB is of low impedance. Typically a direct connection to the chassis with screws is sufficient to provide ESD shielding.

When you’re ready to design and simulate your ESD protection circuit, make sure you use the industry’s best circuit design and simulation tools in PSpice from Cadence. PSpice users can access a powerful SPICE simulator as well as specialty design capabilities like model creation, graphing and analysis tools, and much more.

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