TVS Diodes

TVS diodes are used as a means of protecting electronic circuits from voltage spikes. The acronym itself stands for Transient Voltage Suppressor.

Basic Operation

A TVS diode is attached in parallel to an electronic circuit that needs to be protected. Because of the flow of electricity through the circuit, it receives the same amount of voltage as does the protected electric circuit.

When the circuit is functioning normally, a small trickle of electricity flows through the diode, but it is insignificant enough that it doesn’t have any effect on the circuit that is being protected. In the event that the voltage spikes, the TVS diode will go through a process called avalanche breakdown, described below, which permits a large amount of current to flow through the diode and shunts off excess voltage.

The TVS diode will be designed so that it regulates the voltage to a specific level. This is a method of voltage regulation referred to as clamping. The other option for voltage regulation would be crowbar voltage regulation, which has drawbacks for some applications.

With the TVS diode having gone through avalanche breakdown and for as long as the overvoltage condition that triggered that breakdown exists, the component will regulate voltage so that it never exceeds a specified level. Once the overvoltage condition passes, the TVS diode will return to normal operation, allowing the insignificant trickle of electricity to pass through it, and inducing a functionally irrelevant amount of capacitance and induction into the circuit. Other than that, it will operate completely transparently, not affecting the performance of the protected circuit to any degree, but being activated again, nearly instantaneously, if another voltage spike should travel through the circuit.

The Avalanche Breakdown

When avalanche breakdown occurs, it means that electricity is flowing across the component in the opposite direction that it would normally be flowing. This happens when the reverse bias voltage increases to a level that is given in the specifications of these components.

Within the component itself, a P and N type semiconductor normally permit current flow in one direction. When the reverse bias voltage gets above the specified level for the component, the electrons and holes in the semiconductor begin to move in the opposite direction. This flow of electrons and holes is referred to as avalanche current.

One feature of TVS diodes that distinguishes them from some other electronic components is that they are manufactured so that the exact avalanche voltage of the component can be specified. That voltage, and the precision that devices can be manufactured with which allow it to be specified so accurately, are a great deal of why these particular components are so important in modern electronics.

Variations

There are two major types of transient voltage suppression diodes. The first type, unidirectional, is similar to other types of avalanche diodes. It is differentiated from them principally by the fact that it will be designed to deal with a very significant avalanche current.

The other major type of transient voltage suppression diode is called a bidirectional TVS. These are sold as components, though they contain two avalanche diodes oriented in opposing directions. These diodes, wired in series, are both wired in parallel to the protected circuit.

Advantages

Aside from the aforementioned ability to absorb a significant amount of avalanche current without being damaged, transient voltage suppression diodes have other advantages that make them more suited for specific applications than are crowbar type voltage suppression components.

The transient voltage suppression diode reacts almost instantaneously to an overvoltage condition. As soon as too much voltage is present in the circuit, the device clamps that voltage down and can reliably sustain that voltage at that level for the duration of the overvoltage condition.

While being able to sustain the predictable amount of voltage is advantageous in and of itself, the instantaneous response of these devices is also significant in their usefulness.

In electronics applications, TVS diodes are quite often found protecting expensive equipment from being damaged by voltage surges caused by electrostatic discharge. In this they are similar to another type of TVS device, for which they are sometimes confused, but which are used in different applications.

Diode vs MOV

And MOV based transient voltage suppressor is sometimes confused for a TVS diode. They are different in both their manufacture and their applications.

MOV based transient voltage suppressors are used in a similar capacity to TVS diodes, but tending toward a larger scale. And MOV based TVS is usually used to keep equipment safe from voltage surges across power mains. These may be induced by lightning or by other forces. These devices can typically handle surges of as high as 50 kA, while a TVS diode would generally be designed to handle a full ampere in most applications, though they can sometimes range as high as 15 kA.

The TVS diodes are most useful for protecting the very sensitive electronic circuits from power surges in data lines. There is another type of diode, a Zener diode, that shares much in common with a TVS diode but, again, is different enough that it cannot replace a TVS diode.

The Zener Diode and TVS Diode

A Zener diode is, in terms of its construction, very similar to a TVS diode. Like a TVS diode, it is a semiconductor device and its method of operation is largely the same. Upon reaching a certain reverse bias current, it goes into an avalanche state, allowing excess energy to be shunted away from the circuit it is protecting. Also like a TVS diode, a Zener diode is wired in parallel to the circuit that it is protecting.

The Zener diode also shares the fast response time with the TVS diode. The principal difference is that the TVS diode is a specialized type of component that was made specifically for the purpose of protecting against voltage spikes. It is the preferred solution for protecting equipment against ESD.

Zener diodes regulate voltage, but they don’t have the clamping feature that a TVS diode does. That clamping feature, again, is not only what differentiates TVS diodes from many other types of voltage regulation, but what also makes them so useful in industrial applications. No matter how long or short a voltage transient across the circuit it is protecting might be, a TVS diode can be relied upon to provide consistent voltage regulation for the duration of that transient event.

Specifications

As is the case with any electronic component, transient voltage suppression diodes are not perfect, which means that they do, at least to a small degree, change the way energy flows through the circuit that they are integrated with, even when they are not performing their intended function.

A transient voltage diode will leaks current during normal circuit operation. This amount of current is generally insignificant to the operation of the circuit and, for all intents and purposes, it is transparent to the circuitry. This is referred to as the leakage current. The top end of that leakage current is referred to as the Maximum Reverse Standoff Voltage. Below that voltage, the component will not be conducting any significant amount of current through it.

Should there be a voltage spike, however and it exceeds what is referred to as the Breakdown Voltage of the component, the component will begin conducting electricity, performing the function of relieving the electrical pressure on the circuit.

This conduction will only take place to the level of the Clamping Voltage, which is the voltage that the TVS diode can be reliably expected to maintain for the duration of the transient voltage spike. The transient voltage can equate to a significant amount of current flowing through the device, and the ability of these devices to endure that sort of current is one of their major advantages.

These devices are designed to deal with electrostatic discharge so, when the overcurrent condition does come, it’s going to come in the form of a pulse. The device will have a specification that indicates the highest level of amperage it can sustain without taking damage during that pulse. This is referred to as the maximum peak pulse current and it’s important that this is selected carefully to ensure that the device can hold up to its intended usage.

These are the most important specifications of the device but, because electrical engineering is a precise business, some specifications that are generally not significant to the operation of the device or the circuit need to be given to builders.

Effects on Circuits

Incorporating a transient voltage diode into the circuit will introduce some changes. The operation of the device will involve Parasitic Capacitance, which is generally not an issue in today’s high precision components, but which is consistently being engineered to be lower, so that the effects on circuits are less. Parasitic Capacitance can negatively impact the performance of the circuit when high-speed signals are present.

Parasitic Inductance is related to the switching speed of the device, and is an important specification for builders, as well. Once the device does switch, it’s important to know exactly how much power the TVS diode can shunt from the circuit it’s protecting without breaking down itself.

These devices are designed to protect against very short, transient events. This affects how much time they have to disperse the energy they absorb, which converts to heat with in the device. Because high amounts of current may be involved, it’s important that these devices can handle a great deal of energy flowing through them.

This is generally solved by increasing the size of the device. These devices aren’t as easily cooled as most other electronic components, as the heat generated within them is generated in a very short amount of time. While a heat sink may be used to cool the device off, this will really only have any significant effect after the event has passed and the device cools down somewhat more quickly because of the addition of the heat dispersing element.

Getting Specifications

Because these components do serve such a critical role in electronics, it’s necessary to have a great deal of information on them before selecting a component. In some cases, this has to be gleaned from the data sheets for the device and not all manufacturers include the same information in their data sheets.

Some of the most important specifications of TVS diodes have to be calculated, as well. For example, the Clamping Factor and the Voltage Clamping Ratio both need to be derived from the specifications given by manufacturers. To calculate these values, divide the clamping voltage by the breakdown voltage to derive the clamping factor. The Voltage Clamping Ratio is the ratio of the Clamping Voltage to the Working Peak Voltage.

The TVS diode should have a lower clamping ratio than what you would see on a MOV.

Buying Components

Despite the vital role they serve in electronics, these components have a very low price. Transient Voltage Suppression diodes also come in a huge number of different configurations, making it a fairly easy process to find a design suitable for any given build.

In addition to the bidirectional and unidirectional types of transient voltage suppression diodes, there are combination devices available, which offer bidirectional and unidirectional function in one component.

When selecting these devices, the maximum clamping value, the peak pulse current and the maximum breakdown voltage are all very important specifications to take into account. Because these devices are manufactured with such precision these days, these components can be counted on to reliably perform as specified.

Because these devices may need to be larger than other components to handle the heat generated by the current flowing through them, it’s important to also check the physical specifications of the component, ensuring that it can fit into the build desired. There are many different variations available, and there are many compact designs.

These products are made by the vast majority of the most well known manufacturers, including Fairchild Semiconductor, Hitachi, Maxim, Panasonic, Sanyo, Texas instruments, Toshiba and many others.

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