When I was a teenager, I started having access to the internet via the age-old dial-up modem. If you have no idea what it was, it's a piece of plug-in card that slots right into the computer motherboard. I was warned that I'd need to unplug the telephone line that connects to the internet.
As you could've guessed, I took no heed to the advice, until my only way of access to the internet is fried by lighting. That is the start of a few torturous weeks when I was cut off from chatrooms, which were popular before the age of Facebook.
In PCB design, you'll be having your precious designs destroyed by lighting if you forgot to place any form of transient voltage suppressors at external interfaces. The surge fromform lighting strikes have a habit to couple into cables and goes right into the electronics.
What is a Transient Voltage Suppressor
A Transient Voltage Suppressor (TVS) is a generic term given to electronics components module that mitigate the sudden rise of voltage or transient. Transient voltage, especially from lightning surges, can hit thousands of volts almost instantly and with a high amount of current flowing through power or communication lines.
Other instances of transient voltage include inductive load switching and electrostatic discharge (ESD) from the human's body, where the latter increases the voltage level to hundreds or thousands of volts but releasing minimal current.
Transient voltage, regardless of the causes, can damage other electronics components in a PCB. A TVS component is helpful in safely channelling the excessive voltage, and in some cases, current from the components it is meant to protect.
Common Types Of Transient Voltage Suppressor
Unless you're designing a self-contained PCB that does not require any form of human interaction, you'll need to familiarize yourself with these common types of transient voltage suppressors.
1. TVS Diode
A TVS diode shares the same fundamentals of a Zener diode. It clamps voltage to a certain level when placed in reverse-bias. However, a TVS diode is designed to absorb and discharge a large amount of energy, making it ideal for transient protection.
2. Metal Oxide Varistor
The metal oxide varistor or MOV is an excellent surge suppression device made of ceramic material with zinc oxide grains pressed into it. In usual operation, the MOV is non-conductive, but when the voltage exceeds the clamping voltage value, it becomes a conductive path to channel the excessive current.
MOV is commonly used for transient protection.
3. Gas Discharge Tube
As the name implies, the gas discharge tube or GDT is a component where a couple of electrodes are housed in a metal tube containing a special type of gas. When the voltage reaches the sparkover value, the GDT goes into the arc mode, where it becomes conductive and is used to channel high current from transient events.
The polyfuse is also known as a polyswitch or polymeric positive temperature coefficient resettable fuse, works by altering the resistance of its polymer element according to the current passing through. In normal condition, the polyfuse is in a conductive state. However, an increasing amount of current, which also increases the polyfuse temperature will result in an open circuit state. The polyfuse resets when the temperature eventually drops.
Choosing Transient Voltage Suppressors For PCB Design
Transient voltage suppressors are mandatory when there's a risk of a transient event affecting the PCB. However, there isn't a single solution that fits every application. Each type of TVS components mentioned has its pros and cons.
For example, the TVS diode has a quick reaction time to transient spikes, but it has rather low energy handling capacity. In such situations, you'll want to add a polyfuse in series to the incoming power or communication line, to break off excessive current from flowing through.
You'll also want to get the values of the TVS components right to ensure that circuits are protected accordingly. Parameters like clamping voltage, response time, energy absorption capacity are some of the basic considerations when choosing TVS components.
TVS components placement is crucial for protection.
Your routing strategies also influence the effectiveness of the TVS components. You'll want to have the TVS components close to the connector so that incoming transient are channeled through safely to the ground. Avoid other critical signals near the TVS components.
The impedance of the ground connection of the TVS components is also crucial, as it determines how well the excessive current is diverted to ground. To be safe, test it out with a SPICE simulator to ensure the TVS protections are working as expected.
Simulation and Modeling Transient Voltage Suppressors
TVS components are prominently used for their low cost and low clamping voltage, however, determining their exact interactions with devices that require precise noise measurements can still be difficult. Working with TVS components in SPICE can provide you with key parameters such as forward current and forward voltage, resistances for forward and reverse-bias regions, and relevant surges.
Further, though, SPICE tools are capable of measuring impedance for TVS components alongside inductance and capacitance. What’s notable, too, is that SPICE tools such as PSpice can simulate both for small signal analysis and for macro-models to determine what is necessary for larger projects like system design and analysis. PSpice has all that you need for any TVS additions in your designs.
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