In this article we will discuss typical electrostatic discharge (ESD) problems and common solutions. Electrostatic discharge (ESD) is a common phenomenon that is encountered on an everyday basis by many of us. It is the sudden flow of electrical energy between two objects of differing electrical potential (or charge). ESD is essentially a lightening bolt on a very small scale and, like a lightening bolt, the electrical energy will try to find a low impedance path to ground in order to equalise the electrical potential. Anybody who has walked across a carpeted floor and touched a metal doorknob may have felt or even seen a small spark jump from their hand to the metal doorknob.
Another very common occurrence is getting out of your car and touching a grounded path. It is worth noting that the human body is only sensitive to electrostatic discharges greater than around 2000-3000 Volts but higher voltages can easily be generated in real life. While in most cases this kind of phenomenon is not hazardous to humans aside from a surprise jolt and a little discomfort, our electronic devices can be affected in more severe ways. The purpose of electrostatic discharge testing of electrical products is to evaluate their ability to withstand these events.
When an electrostatic discharge current travels through an electronic device it will try to find a low impedance path to ground. While in some cases this may be through the chassis of the device, it is not at all uncommon for the current to travel through sensitive electronic circuitry with enough energy to permanently damage components such as Integrated Circuits (ICs), transistors, diodes in some cases passive components such as high precision resistors. An electrostatic discharge can also create local but intense electromagnetic fields which may couple into nearby circuitry and disrupt signals. Some common ways to mitigate the effects of electrostatic discharge phenomena include:
- Proper grounding
- Galvanic isolation
A common and effective way to minimise the affects of ESD on a device is to stop discharges from occurring in the first place. Using plastic with a high breakdown voltage and sufficient spacing between touchable points and conductors may provide enough insulation to prevent an ESD event from occurring. Insulation is effective on weak points such as switches, LEDs, rotary controllers, displays and connector shields usually where the enclosure integrity is jeopardized. In some cases entire circuits or parts of circuits can be encapsulated in a potting compound such as a resin or silicone.
Unlike insulation, which provides no path for ESD currents, proper grounding allows for a low impedance path to ground. Metallic connector shields and screws should have a low impedance connection to the metallic chassis which in turn should be connected to the protective earth or functional earth via a low impedance connection. This allows the electrical energy to find a path to ground without travelling through sensitive circuitry. This can be complex if the product is double insulated or uses no earth arrangement but is in a system with multiple other earth paths to ground i.e. via coaxial shields to other equipment.
Spark Gap/Gas Discharge Tubes (GDT)
Gas discharge tubes and other spark gap devices act as transient suppression devices by conducting electrical currents to ground, effectively creating a short circuit. In the case of a high-voltage spike, the normally non-conductive gasses (or air in the case of a simple spark gap with exposed electrodes) become ionized allowing for electrical current to be conducted through the gap between the terminals of the device. GDTs take a relatively long time to trigger compared to other transient voltage suppressors. It is not uncommon for a GDT or spark gap to allow pulses of 500V or more to pass through unsuppressed before current is conducted to ground via the ionized gas/air between electrodes. Gas Discharge Tubes are more commonly used in slower rise time surge transients such as an AC mains surge.
ESD events can be mitigated using transients suppression components and/or filter networks. Transient suppression components include, but are not limited to: Transient voltage suppression (TVS) diodes, capacitors, variable resistors/voltage-dependant resistors (varistors/VDR) and filter networks. These components work by reacting to sudden overvoltage conditions and should be placed as close as possible to the point of ESD current entry rather than the circuit (or part of the circuit) that is being protected.
Galvanic isolation is the separation of electrical circuits that allow for signals to pass through but stray currents are eliminated. Common methods of achieving galvanic isolation include but are not limited to: Transformers (coupled inductively/magnetically), opto-isolators (coupled photo-electrically), capacitors (blocks DC but allows AC to pass) and hall-effect sensors (coupled inductively/magnetically).
In some cases a change in the firmware of a device is sufficient enough to allow it to self-recover after an ESD event. Where a devices processor crashes (due to an ESD event) a watch-dog timer (WDT) will reset the processor. This can essentially bring it back to an original state making the malfunction in operation self-recoverable. In other cases the firmware may help stop the device from malfunctioning. A parasitic reset (when the reset pin of the processor erroneously reads high due to the ESD event) or blocked status can be managed by the firmware. Often this kind of solution is used in conjunction with filters which differentiate valid signals from short ESD events.
When designing a circuit it is important to keep these different techniques in mind. Different kinds of devices and circuit layouts will benefit from different techniques and often there is more than one solution to the problem. More than often, multiple electrostatic mitigation techniques will be required in order to achieve EMC compliance during electrostatic discharge testing.