Embedded system design is a complex process and often, overlooking ESD protection can result in serious post-deployment issues. A design that’s missing a Transient Voltage Suppressor Diode can potentially result in a barrage of complaints from angry customers and product recalls. Such a scenario is practically a nightmare for any embedded systems design engineers.
In this article, you’ll learn the importance of Transient Voltage Suppressor Diodes (or TVS diodes) for ESD protection and how to protect critical components effectively in your design.
How ESD Affects Embedded Systems
Embedded systems are electronic circuits driven by a microcontroller. Besides the microcontroller at its core, the circuit often consists of other types of ICs, communication, and I/O interface.
Each of the components has a degree of tolerance against ESD. Unfortunately, it doesn’t take much static discharge for the components to fail. For example, logic ICs will fail upon contact of transient discharge of as low as 250V.
MOSFET, which is commonly used in various embedded system applications, is very susceptible to ESD. Exposure to transient discharge is likely to render the component unusable.
Some components, such as the LM 317 voltage regulator, can withstand up to 2,500V of static discharge. However, it’s still comparatively low compared to the possible ESD coupled from a human body, which is in the range of kilovolts.
When many kV of ESD are coupled into an unprotected circuit, some of the input may fail. The microcontroller’s input can be damaged and record all states of the push button as a logic high. A transceiver IC could stop receiving data packets because of ESD damages.
Considering that an embedded system circuit is made up of hundreds, if not more, ESD-susceptible components, it makes sense to make ESD protection part of the design strategy.
How Transient Voltage Suppressor Diodes Protect Embedded Systems Against ESD
ESD protection is best realized with TVS diodes. TVS stands for Transient Voltage Suppression which is exactly what the diode does. Like a regular diode, a TVS diode is a semiconductor with a P-N junction. When forward-biased, the TVS diode conducts as a normal diode.
However, the TVS diode enters into the avalanche mode when driven above a certain voltage in negative bias. It then becomes conductive and any excessive current is shunted off. The voltage across the TVS diode is clamped down when it’s in avalanche mode.
TVS diodes are ideal for ESD protection for their quick response and low capacitance. These components are also known to have low clamping voltage and leakage current.
The idea of protecting embedded systems with a Transient Voltage Suppressor Diode is to prevent ESD from having a direct propagation path to critical components. This is achieved by placing TVS diodes strategically along with circuits that could potentially be exposed to ESD.
For example, it’s quite common for ESD to be coupled into a circuit via a push button or input pin. Therefore, it’s mandatory for a TVS diode to be placed on the circuit and prevent ESD from damaging the corresponding input pin.
Similarly, TVS diodes are a must when you’re designing wired communication modules like RS485, CAN-bus, or Ethernet. As these communications require manual wiring, there’s a chance of ESD being coupled from the human body when the technician connects the conductors of cable.
Choosing a Transient Voltage Suppressor Diode For Embedded Systems Design
It’s agreeable that using TVS diodes in embedded systems design helps protect critical components from ESD. The question is, how do you choose the right TVS diode.
To do that, you’ll need to ensure that several parameters of the TVS diode meet the design requirement.
Reverse Standoff Voltage
The reverse standoff voltage, (VRM) is the threshold before the Transient Voltage Suppressor Diode starts entering the avalanche mode. You’ll want to choose a TVS diode with a higher VRM than the operating voltage of the component it protects.
Clamping Voltage
As the TVS diode shunts off transient current, it will clamp the voltage to a certain level. The clamping level, VCL, must be higher than the maximum operating voltage of the circuit it protects but remain small enough to ensure the safety of the connected components.
Capacitance
Ideally, you’ll want to choose TVS diodes with low capacitance or configure the TVS to have low capacitance. This is particularly true when you’re protecting high-speed signals like USB from ESD. Additional capacitance on the electrical trace may affect the signal integrity.
Peak Pulse Current
Peak pulse current, or IPP, indicates the amount of current that the TVS diode can safely handle, within a specific timeframe. This parameter is usually based on an 8/20µs or 10/1000µs waveform according to the IEC 61000-4-5 standard. Ensure that the TVS diode has a higher IPP than the expected transient current.
PCB Design & Layout Tips For Transient Voltage Suppressor Diodes
The next step, and an equally crucial one, is how you’re designing with the TVS diodes on the PCB. If you don’t adhere to the best practice, you’ll find that the protection offered is degraded.
Here are some PCB layout tips to help you out.
Positioning
Place the Transient Voltage Suppressor Diode as close as possible to the point of human contact. For example, you’ll want to place TVS diodes near the USB connector instead of the transceiver IC. The reason is, you’ll want to prevent the ESD from coupling to adjacent components. Any transient current should be suppressed near the ESD source.
Ground Return Path
It’s also important to keep the ground return path from the TVS diode as short as possible. A short return path keeps parasitic inductance to the minimum and prevents delay in the TVS diode’s response time.
Also, a short ground return minimizes interference to other components by keeping the transient current from passing through them. Ideally, you’ll want to have separate ground planes for the TVS diode and the protected components.
Avoid Vias
The trace connecting the Transient Voltage Suppressor Diode with the corresponding signal and ground should be free of vias. Having vias introduces parasitic capacitance to the circuit and this could cause propagation delay and switching losses, particularly on high-speed circuits.
Summary
With the demand for smaller, more compact electronics products, ESD is an increasingly prevalent issue in embedded system design. It’s imperative to have a proper ESD protection strategy and that means making TVS diodes part of the design.
Don’t leave ESD protection to chance as a single human interaction could result in irreversible hardware failure.
Explore our range of Transient Voltage Suppressor Diodes to protect your design.
