Electrostatic Discharge and PCB Design
How do I incorporate ESD safety into my designs?
The human body can easily generate 15 kV of electrostatic potential by simply walking across a carpeted floor, well more than enough energy to destroy many common sensors and microcontrollers. It is even possible (although not common) to accumulate static charge on the exterior of a data cable that is left to swing in the air/wind – perhaps between a computer and a remote device.
The solution is to incorporate ESD diodes in your off-board data lines. Wherever data enters or leaves your device via connectors or wires (I²C, SPI, USB, Antenna, Ethernet, VGA, etc…), the pcb-traces should connect to an ESD diode that is connected to ground. These diodes are placed between the interface connector and the sensitive microcontroller and are designed to provide a low-impedance path for high voltages to take to ground, so the energy reaching your microcontroller is kept to a minimum.
ESD Diodes are the electrical equivalent of a dam spillway. They don’t start conducting electricity until a certain threshold voltage is met, and then they provide a low-resistance path for charges to take to ground.
Here’s an example: A microcontroller data-line might be designed to operate at 5 V logic, and perhaps it can withstand 20 V for a short period of time, but certainly not >1000 V that might come in from an electro-static discharge. The ESD diode you place in your circuit will provide megaohms of resistance to potential differences less than 9 V, and almost no-resistance to potential differences over 9 V. When a high-voltage shock comes down the line, the diode becomes a short-circuit to any electric potential exceeding 9 V – the microcontroller is only exposed to low-voltage and hopefully survives the incident.
How do I select an ESD Diode?
ESD Diodes are specified by their working-voltage, Rating, Capacitance, Channels, and Clamping Voltage. The absolute easiest way to select a diode is to find the one recommended by the manufacturer of your integrated circuit and use it. Most manufacturers create reference designs for their devices that include a detailed bill-of-materials (BOM). Find the diode on the BOM and then find that diode at your favorite distributor’s website. Alternatively, find the datasheet for that diode, and then find a similar diode at a distributor or manufacturer’s website.
Working-voltage – this is the voltage of the data lines on the integrated circuit. The diode you select should have the same or slightly higher working-voltage rating as the digital lines in your circuit. Your diode’s working voltage cannot be lower than the digital lines in your circuit or it can cause data loss.
Rating – Diodes are rated based on their IEC 61000-4-2 Rating. The International Electrotechnical Commission’s immunity standard for Electrostatic Discharge. This details how to test a diode and rates it based on how well the diode protects a downstream device from the effects of electrostatic discharge. There are four levels (1-4) that describe the diode’s ability to protect against increasingly energetic discharges. You can choose any rating for your device – but a rating of 4 is the maximum protection
Capacitance – Diodes and their packages have an inherent amount of capacitance that is determined by their construction materials and package type/size. While you have a bit of leeway in your selection – you should generally try to choose a diode with as low a capacitance as possible.
Channels – this is the number of diode protection channels available, with one channel needed for each wire that you are trying to protect. 1, 2, 3, 4, 6, and 8 channel devices are available. You can use any combination of devices to protect your signal lines as long as you have enough channels for all of your signal lines.
Clamping Voltage – this is the greatest voltage that can exist on your line. Any voltage in excess of the clamp voltage will be short-circuited to ground. This is done to protect ICs that are on your PCB after the diode. You should pick a clamping voltage that is above the working voltage, but below the absolute maximum voltage for your device.
Use the search term “TVS Diode” at your favorite part supplier, then drill-down based upon application and channels needed.