Transient Protection

by William D. Kimmel, P.E.

and Daryl D. Gerke, P.E.

Kimmel Gerke Associates, Ltd.

There are various ways of protecting against transients by using filters, transient protectors or a combination of both. Which one to use is dependent on the nature of the transient (ESD, EFT, surge and variations, including EMP) and on the particular application.

Let’s take a look at the requirements for the various transients and the types of protection that are appropriate for each.

Transient Characteristics

First, we need to look at the signatures of each type of transient.

Electrostatic Discharge (ESD) most commonly comes from human discharge directly to your equipment. ESD is characterized by a rise time of 1 nanosecond to a peak of perhaps 10 amperes. Appropriate transient protection needs to be fast enough to suppress the fast rise time and to handle the peak amperage, but energy is minimal.

Electrical Fast Transient (EFT) occurs when disconnecting an inductive load from the same circuit your equipment is connected to and results in a series of arcs. EFT is characterized by a rise time of 5nsec to a peak of perhaps 2kV. Appropriate transient protection needs to be fast enough to suppress the fast rise time, and to handle the peak currents arising from the discharge. Energy is higher than that of ESD but is still relatively low.

Surge arises largely from lightning discharge, although heavy equipment can be sources as well. The wave has about 1 microsecond rise time, much slower than ESD and EFT but with much more energy.

EMP has been largely associated with a nuclear burst, which produces an electromagnetic pulse with a rise time of about 5nsec.

Filters vs. Transient Protectors

Transient protectors operate on non-linearity to limit voltage to a safe levelby shunting excess current around the load. Clamps are threshold devices that limit the voltage to the specified threshold level. Crowbars are in a nonconductive state until the arc threshold is reached, and the impedance abruptly drops to a low value, shunting the current.

Filters operate by frequency discrimination. As most signals are broad band, the filter cutoff must be comfortably above signal bandpass.

With these factors in mind, let’s look at the limitations for each case.

Filters

Generally, we prefer to use filters over transient suppressers wherever feasible – this would include low frequency signal lines and all power input. The driving factors are bandwidth and charge capacity. For the high frequency transients (ESD, EFT, and EMP), we might be able to protect circuits and signals up to about 1MHz. Surge, being a much lower frequency transient and higher energy, the audio frequencies would be about the upper limit for filtering, and even that may be a stretch.

Power input, of course, has no significant bandwidth requirements, so filtering is usually feasible. The key is that the primary filter element, the capacitor, be fast enough and low impedance to shunt the transient and have enough capacity that the over voltage during the transient be minimal. Any series element between the source and capacitor must be able to withstand some over voltage.

For most electronic equipment, filters will serve adequately. The possible exceptions are those cases where the signal lines are long, as with telecom.

Crowbars

The crowbar, or thyristor, is typified by the gas discharge tube. When the voltage rises to a sufficiently high level, an arc is established and the impedance plummets. As the voltage across the tube is very low, it can carry copious amounts of current. The downside is that it is slow, with fast transient voltages rising to more than 1kV before firing. The primary purpose of a crowbar is for protection against high energy slow transients, specifically lightning surge.

Another condition is that once fired, voltage must be removed before the device is reset to a high impedance state. This means that it cannot be used for DC voltages unless provisions are made to interrupt the current.

Zener Diodes

Widely known by the trade name Tranzorb® is the first of the clamps we will discuss. Diodes are characterized by a low impedance in the forward direction and a high impedance in the reverse direction until the breakdown voltage is reached, at which time the diode abruptly begins to conduct. The breakdown voltage is quite abrupt, and the voltage remains virtually constant over a wide current range. The breakdown voltage can be controlled and can be from about 3 volts to hundreds of volts. The Tranzorb uses a large cross-section so that it can carry large currents.

Tranzorbs are very fast, so they can suppress the fastest of transients. There are two downsides. The first is that the diode, with a large cross-section, has lots of capacitance, from 20pF to hundreds of pF. The second is that when it breaks down, the voltage stays high, so that power absorption is high, so that these devices are limited in how much current they can carry.

As the threshold can be quite accurately defined, it is suitable for signal lines (providing the capacitance can be tolerated). If used for AC, you need to put the two diodes head-to-foot to provide bilateral protection. Consider a Tranzorb for low energy, high speed transients, not for high energy lightning surge.

Metal Oxide Varistor (MOV)

The MOV is a highly nonlinear resistor. At low voltages, the MOV has high resistance. At higher voltages, the resistance begins to fall, and the MOV starts to conduct, approximating a breakdown voltage of a Zener diode. The breakdown voltage is not as abrupt, however, so we say that it is a “soft” breakdown. The MOV is bilateral, approximating two Tranzorbs placed head to foot.

The traditional MOV, widely used in power protection is too slow for the fast transients so is useful only for slow surges. The newer SMT multilayer MOVs, as would be used on a circuit board, are very fast, fast enough for the fastest transients.

The breakdown knee is soft enough that the voltage can rise fairly high at maximum current. This is not a particular problem in power supplies, but may not provide adequate protection for signal line protection. The MOV, like the Tranzorb, is high capacitance, typically 100pF or more per line.

MOV parameters do drift with use, generally increasing impedance with age. Having said that, the MOV is the workhorse of power strips for computers, working very well for surge protection.

At the circuit board level, the MOV can handle more current than a similar size Tranzorb – an 0603 MOV can carry about as much as an 0805 Tranzorb. The MOV can be readily made into arrays to facilitate multipin connector protection. We have used these successfully for ESD protection.

Polymers. These are a fairly new technology, being organic.They are very fast, suitable for all fast transients. They are very low capacitance, typically a small fraction of a pF. Unfortunately, the breakdown voltage can be quite high, over one hundred volts, so you need to be sure the device you are protecting will handle that voltage.

Schottky Diodes. These diodes are characterized by a very low voltage breakdown, well under 0.5 volts, and have a very low capacitance, typically under one pF. As the breakdown voltage is very low, it can only be used for low level analog signals and RF inputs. As they are low voltage, they can handle considerable current without overheating.

On-chip diodes. Many chips have limited built-in protection against transients. This is commonly achieved by running the signal through two diodes, one of which is placed to shunt over voltages to Vcc and the other to shunt undervoltages to ground. These devices have limited capability and should not be relied upon to handle any transient. About the best we can say is that it might protect you against an occasional minimal transient that sneaks through your real protection.

Hybrids

We may need to combine the best features of the above devices to provide both high speed and high current protection. A combination of gas discharge tubes in parallel with Tranzorbs (MOVs can also be used) is commonly employed in power protection and telecom protection.

The Tranzorb (or MOV) is the fast element, suppressing the voltage until the crowbar has enough time to fire. At that time, the clamp element drops out of the picture, leaving the crowbar to handle the main current. Note the series element is needed to provide a back voltage for the crowbar – lacking that the clamp would suppress the voltage until it burned out, allowing a high voltage to exist until the crowbar had time to fire.

Summary

You need to protect your circuits against one or more common transients. There are several possibilities to consider. We prefer filters wherever feasible – make sure the components aren’t overstressed.

If you need transient protection, use Tranzorbs or multi-layer MOVs for fast transients. Use crowbars and large MOVs for slow surges. Use hybrids for broader protection.