Electrostatic Discharge and PCB Design

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Audience:

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.

This schematic subsection and PCB render shows a circuit with an 3-ESD Diode array used to protect the data and power lines between a micro-USB plug and an integrated circuit. Place the ESD Diodes as close to the plug as the design will allow.

This schematic subsection and PCB render shows a circuit with an 3-ESD Diode array used to protect the data and power lines between a micro-USB plug and an integrated circuit. Place the ESD Diodes as close to the plug as the design will allow.

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.

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