An Introduction to Transient Voltage Suppressors (TVS)

This article is an intro to transients and the devices used to suppress them. In it, you'll learn about a variety of devices used to suppress transients, including transient voltage suppressor diodes, metal oxide varistors, PolySwitches, and avalanche diodes.

Transients are temporary spikes or surges in voltage or current that can potentially impact circuits in ways ranging from minor glitches to catastrophic failure. A voltage transient can be anywhere from a few millivolts to thousands of volts, and they can last from nanoseconds to hundreds of milliseconds. Some transients are repetitive, such as those caused by inductive ringing in a motor, while other transients are more sporadic, such as ESD events.

Current transients can be caused, for example, by inrush current. The figure below shows a device being hot-plugged.

Inrush Current Spike/Transient.

The applied voltage is 5V (cyan waveform) while the current—technically known as the "inrush current"— (yellow waveform) surges very high: its measured value is 26.2 Amps with a time duration of 21.6µs. In this instance, the device, itself, is able to withstand the current transient. However, depending on the current-sourcing capabilities of the connected power supply, it may become current-limited and, as a result, may starve the device of current. This can result in an undesired device reset or worse.

Origins of Transients

Transients can be generated from internal or external connections to a circuit. Examples of internally generated transients include:

• Inductive load switching:
  • relay coils
  • motors
  • solenoid coils
  • transformers
• Arcing:
  • faulty contacts in breakers, switches, and connectors
  • motors with faulty windings or insulation
  • poor electrical wiring
  • poor grounding
• IC logic switching:​
  • TTL
  • CMOS

Externally generated transients enter a circuit (or system) by pathways including:

• Power input lines:
  • lightning strikes
  • inductive switching caused by the turning on of other equipment connected to the same power source
• Data/signal input and/or output lines:
  • I²C
  • serial communication
  • Ethernet
• Other attached wires/cables, such as grounds

ESD (electrostatic discharge) is another common form of externally generated voltage transient. ESD events can cause either immediate damage or, arguably even worse, latent damage—sometimes referred to as the "walking wounded." The term walking wounded is used because an ESD damaged component may continue to work normally for hours, days, or even months before a catastrophic failure occurs.

Devices Used to Suppress Transients

There is a multitude of devices that can be used to help suppress voltage transients. Such a device is referred to as a Transient Voltage Suppressor (TVS). A few of the more popular TVS devices are listed below.

Bypass Capacitor

Bypass capacitors—used for suppressing voltage transients—are also referred to as decoupling capacitors. Such capacitors, usually in sets of two or three with values of one or two orders of magnitude between them, are often placed at each power source as well as at each analog component to ensure that power supplies are as stable and noise-free as possible.

Capacitor symbol

Typical values:

• For logic applications: 0.01 - 0.22µF
• For power applications: 0.1µF and greater

 Applications:

• Low-power applications
• RC snubbers
• Decoupling of digital logic power rails (for cleaner power)

 Advantages:

• Low cost
• Readily available
• Simple to apply
• Fast acting

 Disadvantages:

• Uneven suppression
• Many may be required: one or two for each individual device

Zener Diodes

A Zener diode is a specially designed diode that has a reduced breakdown voltage called the Zener voltage. These diodes exhibit a controlled breakdown which allows the current to keep the voltage across the Zener diode close to the zener breakdown voltage. As a side note, this characteristic is what makes zener diodes useful for generating reference voltages.

Zener diodes are also often used to protect circuits from overvoltage transients, such as ESD events.

Zener diode symbol

Applications:

• Diversion/clamping in low-energy circuits
• Good for high-frequency circuits
• Good for high-speed data lines

Advantages:

• Low cost
• Fast acting
• Easy to use
• Readily available
• Standard ratings
• Bidirectional
• Calibrated clamping voltage
• Usually fail open (as opposed to fail short)

Disadvantages:

• Limited to low-energy handling

Transient Voltage Suppressor Diodes

Transient voltage suppressor diodes are very popular devices used to instantaneously clamp transient voltages (e.g., ESD events) to safe levels before they can damage a circuit. Although standard diodes and Zener diodes can both be used for transient protection, they are actually designed for rectification and voltage regulation, and, therefore, are not as reliable or robust as transient voltage suppressor diodes.

Transient Voltage Suppressor (TVS) Diode Symbols: Unidirectional (left) and bidirectional (right).

Applications:

• Diversion/clamping in low-energy circuits and systems
• Good for moderate-frequency applications

 Advantages:

• Fast acting
• Easy to use
• Readily available
• Bidirectional or unidirectional
• Calibrated low clamping voltage
• Fails short-circuited 

Disadvantages:

• High capacitance limits frequency
• Low energy handling
• More expensive than Zener diodes, or MOVs

MOVs (Metal Oxide Varistors)

A metal oxide varistor (MOV) is a bidirectional semiconductor voltage transient suppressor. MOVs behave as voltage-sensitive variable resistors. The voltage at which the device conducts (i.e., switches) is a function of the number of grains between its electrode leads. This value can be varied, during the manufacturing process, to create any desired voltage breakdown threshold.

MOVs (Metal Oxide Varistors) symbol

Applications:

• Diversion/clamping in most low- to moderate-frequency circuits at all voltage and current levels.
• Can be used in AC or DC applications.

Advantages:

• Low cost
• Fast acting
• Readily available
• Calibrated low clamping voltage
• Easy to use
• Standard ratings
• Bidirectional
• Fails short-circuited

Disadvantages:

• Moderate- to high-capacitance limits high-frequency performance.
• Can only dissipate a relatively small amount of power, and, therefore, are unsuited for applications that demand continuous power dissipation.

Avalanche Diode

Avalanche diodes, like Zener diodes, are designed to break down and conduct very high currents at a specific reverse-bias voltage. This behavior is referred to as the avalanche effect. Similar to Zener diodes, which are somewhat restricted in the maximum breakdown voltage range, avalanche diodes are available with breakdown voltages over 4000 V.

Avalanche diodes are connected in a reverse-biased configuration, i.e., the cathode is connected to the more positive voltage. Thus, during normal conditions the diode has minimal effect on the circuit, but when the voltage across its terminals exceeds the specified threshold, it begins to conduct.

Avalanche diode symbol

Applications:

• Used to protect circuits against damaging high-voltage transients.

Advantages:

• Specified with a clamping voltage V_BR (breakdown voltage).
• Specified with a maximum-sized transient that it can handle.
• The avalanche breakdown event is not destructive—assuming the diode isn't overheated.

Disadvantages:

• A known side effect is RF noise generation.

PolySwitch

A PolySwitch (also referred to as polyfuse or resettable fuse) is a positive temperature coefficient (PTC) resistor that is constructed from a conductive polymer mixture. During normal operating conditions (i.e., during normal temperature conditions), the conductive elements within the PolySwitch form low-resistance "chains" which allow for the current to flow rather easily. However, when the current flowing through these chains increases to a point where their temperature rises above some critical threshold (a point called the "trip current"), the crystalline structure of the conductive polymer abruptly changes into a stretched out and shapeless state which, as a result, increases the resistance of the PolySwitch, which causes a sudden drop in current flow. Once tripped, the PolySwitch will remain in the tripped state until the the fault is removed and the temperature of the PolySwitch returns to a safe level.

PolySwitch symbol

Applications:

• Overcurrent protection for speakers, motors, power supplies, and battery packs
• Where a self-resetting fuse (i.e., a fuse that does not require replacement) is needed

Advantages:

• Low cost
• Easy to use
• Self-resetting

Disadvantages:

• Requires a cooling-down period to reset

TVS Summary

Although many options and devices exist for transient voltage suppression, different TVS devices are more appropriate for certain types of transients or certain operating conditions. It's wise to first understand your circuit's needs and requirements, and then begin your search for the best TVS solution.