Way back in undergrad circuits class, our TA loved to give us little “black box” circuits. We’d measure the voltage and current characteristics, then have to determine what components made up the circuit. Our TA particularly loved to throw diodes in there. At the time I couldn’t figure out why, as I didn’t see much purpose in diodes. However, I did come to appreciate diodes when I was looking for ways to protect my PCBs from electrostatic discharge. Diodes are a crucial component for managing voltage levels, which is a critical effect of electrostatic discharge (ESD).
PCB ESD protection can come from many sources, even just an inadvertent touch, and cause a voltage discharge that wreaks havoc on your board. One of the most common sources of ESD is at inputs, like cables being plugged in or buttons being pressed. Here, you’ll want to minimize the voltage spike that will reach your sensitive components.
Despite the underwhelming appreciation of diodes from my misspent youth, they are a critical component to a lot of circuit design and protection.
How to apply ESD protection on inputs
The first line of defense in ESD protection is to minimize the impedance of the path to ground. It’s a way to minimize the voltage (VESD) that the board will experience during a discharge. When you consider Ohm’s law, VESD=IESDR, IESD isn’t something you can control, so the only way to minimize VESD is to minimize R. The problem is that you can’t just short the path to ground, because the ground plane will provide a direct path to all of your sensitive components.
Instead, you can add a protection circuit at the input in the form of a transient voltage suppressor. Transient voltage suppressors, or TVS, consist of two diodes combined with an avalanche diode, and they also come as a single component. This significantly reduces the chances of your components being damaged without significantly increasing the cost or complexity of your design.
The TVS subcircuit basically creates a current divider between the TVS and the IC or components that you want to protect. It presents a high input impedance to the input and thus doesn’t interfere with normal operation. However, when the input current spikes dramatically under an ESD event, the TVS reaches its breakdown voltage and shunts the current to ground, instead of the sensitive components you are trying to protect.
Each time something is plugged into your device, it introduces an opportunity for an electrostatic discharge.
How to use TVS protection effectively
When you add a TVS to your design, you want to smooth out your traces as much as possible to minimize any EMI generated at corners and propagating across the board. The Texas Instruments recommendation is to use large radius curves instead of corners. If you can’t because of the PCB assembly technology, then use a maximum corner angle of 45°. To me, it looks like a 135° angle, where instead of a right angle, you cut off the corner, like a stop sign, and have two very obtuse angles instead. Also, I highly recommend TI’s Technical Resources section when you really need to get down in the weeds to solve a problem.
You should use wide angle turns in your traces near ESD protection circuits. Sharper corners are more likely to generate EMI as the voltage spike dissipates.
While we’re on the subject of traces - you should avoid using a VIA to connect the ESD source (your input) to the TVS if it is at all possible. Stick to traces for the TVS, since VIAs can really complicate ESD protection. When you’re so close to a likely source of ESD, it’s a mess you don’t want to get into.
Finally, you should also include a buffer resistor in your protection circuit. You add it in series between the possible ESD source and the IC you’re trying to protect. That helps decrease the peak current that will reach the IC from the current divider you’ve added at the input. Some TVS diodes can “reset” themselves from a massive voltage spike, but not all. If they’ve been cooked from a discharge, they might short to ground and eliminate any protection you were getting from the TVS. Diodes are mighty, but it’s good to have a second line of defense.
Choosing the right TVS for your application requires careful consideration of the inductance on your board and the voltage range you need to protect against. Texas Instruments goes into exquisite detail if you’re ready to start your design. While you’re designing, you can minimize the work you’ll need to repeat in your next product by using modular designs and internally managed requirements. A great tool for this is with Altium Vault, which can be used in conjunction with PCB software, Altium Designer. While it won’t pick the components for you (or do your circuits labs), it makes your design flow so much easier. Altium representatives can help you get started today!
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