Introduction to TVS Overvoltage Protection Devices

Editor's Notes

• #2: Welcome to the training module on the Littlefuse TVS Overvoltage Protection Devices.
• #3: This module will introduce you to the various TVS device product offerings of overvoltage protection devices. You will become familiar with the key performance parameters of these devices and how they interact with the circuits they protect. We will then take a look at the Littelfuse portfolio of devices and discuss their selection methods.
• #4: The basic strategy for any overvoltage protection scheme is to place the protector in parallel with the protected circuit. Voltage in an electrical circuit is analogous to pressure in a plumbing system. The overvoltage protector then functions as a kind of high pressure safety valve. The protected circuit is not designed to handle voltages above a certain level. Forcing these circuits into conduction with high voltages can damage them permanently. The overvoltage protector, on the other hand, is specifically designed to breakdown at a certain voltage level and conduct large amounts of current without sustaining damage. The challenge is to insure that the overvoltage protector will break down before the protected circuit and safely redirect the unwanted energy away from the protected circuit. In a sense, there is a race between the overvoltage protector and the protected circuit to breakdown in the face of a rising voltage. We must be sure that the overvoltage protector wins this race every time!
• #5: Overvoltage protection devices can be divided into two classes. Clamping devices limit the voltage to a fixed level. In doing so, they absorb the excess energy of the overvoltage event as long as the event is present. TVS diodes are an example of clamping devices. Crowbar devices, once triggered, essentially short out the protected line, redirecting the excess energy away from the protected circuit. Once the overvoltage event has stopped, crowbar devices will “delatch” and allow the circuit to resume normal operation. Because they short out the protected line when activated, crowbar devices are used only in applications where the available power is limited – such as telephone circuits, data lines and signaling systems. TVS devices are all clamping devices.
• #6: The first parameter we will discuss is the Peak Pulse Power rating. This is one of the most misunderstood ratings for TVS or any other device. As you can see from the data sheet excerpt here, the Peak pulse power rating is 400 watts. However, on the next line down, you can see that the continuous power rating is, at best, 3.3 watts. Why such a large difference? The peak pulse power rating is measured during a very short energy pulse that simulates an inductively coupled lightning pulse. In this case, the pulse being used is the 10 by 1000 micro second double exponential waveform. This is a very common pulse used in the telecommunications industry. We will look at the actual pulse in more detail in the next page. What is key to understand here is that the peak pulse power rating will be different for every different type of pulse. The peak pulse power rating is primarily dependent on the size of the silicon chip used in the TVS diode and, to a lesser extent, the type of package used to contain the silicon chip. Because of this, peak pulse power is the common parameter across a part number series. Since the power is the product of the voltage and the current, higher voltage TVS devices in the same family will have lower peak current ratings so that the peak voltage times the peak current stays the same.
• #7: Peak pulse power ratings are measured by using laboratory generated pulses that are designed to simulate the pulses caused on cables by nearby lightning strikes. These waveforms are called double exponential waveforms. One classic waveform used in telephone circuits is a ten by one thousand waveform. That means the wave will take ten microseconds to achieve its peak value and will take one thousand microseconds to decay to half of the peak value. Specifying three things, the peak value, the rise time and decay time fully define the waveform. Most of the time the waveform is defined for the short-circuit current. Sometimes the open circuit voltage is also defined. The voltage and current waveforms may have different rise times and decay times. Note that these wave forms are not power wave forms. They are either voltage or current wave forms. That means that, in the laboratory, the voltage and the current must both be measured and the peaks multiplied together to get the peak pulse power. The most common current waveforms are ten by one thousand, which comes from the telecommunications industry, and eight by twenty, which comes from the AC power distribution industry. Both have uses in the Littelfuse TVS portfolio.
• #8: The next few parameters we will be discussing appear on what is known as a V-I curve. This characteristic curve displays the voltage and current relationships in a TVS diode. The voltage is shown horizontally and the resulting current is shown vertically. Most of the TVS devices sold are bidirectional devices. That is, they have the same characteristic curve in the positive and the negative directions, so it does not matter which way they are inserted into the circuit. As you can see from the graph on the left, the left side of the curve is the mirror image of the right side, except of course that negative voltages will drive negative currents. The curve on the right shows the characteristics of a unidirectional TVS diode. In one direction it behaves exactly like the bidirectional device, but in the other direction it looks more like a short circuit. Other asymmetric TVS curves include the low capacitance offerings which do not conduct at all in the reverse direction. This asymmetry often serves a useful purpose. They can offer specialized protection for asymmetric circuits including reverse polarity connection protection. AC power line applications will almost exclusively use the bidirectional TVS devices.
• #9: The first parameter for the V-I curve is V R . Taken from zener diode terminology, V R is the maximum continuous voltage that can be applied to the TVS diode that will not cause it to begin to conduct. V R should then be a higher number than the highest working voltage in the protected system. When designing for AC systems, remember that the peak voltages are much higher than the commonly used RMS voltages. You must also understand whether you are designing for phase to neutral or phase to phase protection as that also affects the peak voltages. Choosing a V R that is even slightly too low can lead to overheating of the TVS diode due to continuous power dissipation. V R is measured by running a very small test current, called I R through the device and measuring the resulting voltage. I R is only one micro ampere or one millionth of an ampere for most devices.
• #10: Our next parameter is V BR . This is the breakdown voltage of the TVS diode. This is the voltage at which real activity begins. V BR is measure by passing a test current called I T through the device and measuring the resulting voltage. That sounds a lot like V R and, at least on this simplified graph, I R and I T look like they may be very similar. In fact I T is at least ten times higher than I R and is usually a thousand times higher. Pull out any TVS datasheet and compare them. Note that I R is measured in micro amps but I T is measured in milliamps. For most products I T is one milliamp. Remember that the power dissipation is the voltage times the current, so V R times I T represents some real heating energy, especially on the higher voltage products. A 500 volt TVS diode will be dissipating 500 volts times one milliamp or a half a watt of power. So do not be tempted to use V BR as a substitute for V R as you may be surprised to hear of field problems due to TVS diode heating.
• #11: V C , sometimes referred to as the clamping voltage, represents the high end of the voltage characteristic curve. It is the peak voltage that will appear across the TVS device during the rated power pulse test. It is not the highest voltage that could ever appear across the TVS diode. A shorter pulse with more current will cause a higher voltage than V C to appear across the diode. Not all real world lightning wave forms will follow the industry specifications. Nature does not read specs! Similarly, the TVS diode could be destroyed of a voltage lower than V C but higher than V BR is applied to the device for a sustained period of time. The V C rating in the data sheet therefore is tied very closely to the peak pulse power test waveform. It is a good measure of comparing one TVS choice against another to get a good idea of their relative performance. You would expect a larger TVS diode to have a lower clamping voltage than a smaller TVS diode if they are measured using the exact same test wave form.
• #12: I PP is the peak pulse current that is associated with the clamping voltage. Like V C , it is very dependent on the peak pulse power wave form used to characterize the TVS diode. In fact, if you look at the data sheet and multiply the I PP times the V C , you should get a number that is very close to the peak pulse power rating for that device. As we saw with the V C rating, a TVS diode may survive a current higher than I PP as long as the duration of the pulse is shorter than the rated peak pulse power test waveform. An example of this is the 120 volt P6K-E TVS diode. This is a 600 watt device is rated to survive a 3.7 amp pulse using the standard 10 by 1000 waveform. However, this same device can survive a pulse in excess of 50 amps on an 8 by 20 waveform.
• #13: The part numbers used come from a variety of industries. Because of that there are some variations in the conventions. But all of the part numbers try to indicate some combination of peak pulse power, the package used and the voltage of the TVS device.
• #14: Shown are excerpts from two different TVS device families. The top excerpt is from the S-M-A-J family. The lower excerpt is from the P-4-S-M-A family. You may be surprised to learn that there is virtually no difference between these families aside from the part numbering system. Let us look at the top excerpt first. Note that the part number begins with S-M-A-J. This indicates the package is an S-M-A with a J bend lead. The next portion of the part number indicates the V R rating. The letter C in the suffix indicates a bidirectional part. In the bottom excerpt you can see that the part number starts with P-4. This indicates a peak pulse power rating of 400W. The next three letters are S-M-A which, like the example above, indicates the package. The number in the lower excerpt, however, does not indicate V R . Instead, it indicates a nominal value for V BR . A C in the suffix again indicates the bidirectional part. So comparing the two highlighted parts in the excerpt shows that an SMAJ10A is a higher voltage part than a P4SMA10A, even though their part numbers look to be very similar. In fact, the SMAJ8.5A is a much closer match to the P4SMA part. Take a moment to compare these excerpts.
• #15: Here are the Littelfuse TVS diode families sorted by numbering conventions. The left-hand column shows the families that have the V R embedded in their part numbers. The right-hand side column shows the TVS diode families with V BR embedded in their part number. Now lets go through these in groups of devices with similar power ratings.
• #16: Here are photos of the six axial packages used by Littelfuse for their TVS diode products. The photos show each of the packages roughly to scale with each other. The lower two photos show the AK devices. Their leads are pre-formed at the factory.
• #17: The only TVS diodes available in the DO-41 package is the P4KE family. These 400 watt diodes often are chosen to replace low power zener diodes that are failing in current designs due to overheating or surge failures.
• #18: The DO-15 package is home to three TVS diode families. The P6KE is a 600 watt family. The SA is a 500 watt family and the SAC is a very special low capacitance design. In most applications the capacitance of the TVS diode is of no real interest to the circuit designer. However, some communications circuits will use TVS diodes for surge protection. If the data rate is high, the TVS capacitance can become a real issue. The SAC achieves its low capacitance by having a relatively high voltage isolation diode in series with it. This combination nicely lowers the overall capacitance but does make a bidirectional device impossible, so all of the SAC line is unidirectional.
• #19: Two device families are found in the DO-2O1 package. Both are 15 hundred watt device families. The one point five KE family is a standard product while the LCE family is the big brother of the SAC family we just saw in the DO-15 package.
• #20: The P-600 package is used for the higher power TVS devices. These range in power from the 3000 watt 3-K-P family all the way up to the 30KPA family. The SLD family is a special device designed for the automotive industry. It is rated on the very long 10 by 150 thousand micro second wave form. Because the wave form lasts over a hundred times longer than the standard 10 by one thousand wave form, the peak pulse power is much lower.
• #21: The AK6 and AK10 devices are unique in the TVS diode industry. These are specifically designed to compete with metal oxide varistors in protecting AC power lines. These have a peak pulse current rating – not a peak pulse power rating. And because varistors are rated using an 8 by 20 wave form, so are the AK devices. The peak pulse power ratings would be very impressive indeed. The 430 volt AK10 would have a peak pulse power rating in excess of 6 megawatts!
• #22: Here are photos of the three surface mount packages used by Littelfuse for their TVS diode products. The photos show each of the packages roughly to scale with each other. It is a cruel reality that the last letter in each DO-214 package name does not match the corresponding SM designation. Because the SM designations are more logical – the package size increases with each succeeding letter - they are easier to remember and hence, are more commonly used.
• #23: The SMA package is home to the 400W devices. Notice how prominently the package type appears in the part number family name. As you may have guessed, the SMAJ and the P4SMA devices are virtually identical. The only difference is the part numbering system used.
• #24: The SMB package hosts the 600 watt standard TVS devices and a couple unique offerings. The SACB is the surface mount version of the axial SAC device. Both are low capacitance designs that are useful on communication lines where high capacitance can cause problems. This is a great choice for customers who have run into performance issues with a 600 watt device and don’t want to redesign their PC boards for a larger device. They can simply upgrade to the thousand watt device and keep their existing board footprint.
• #25: The SMC package is where the high power surface mount TVS devices are found. Originally used to house the standard 15 hundred watt devices, Littelfuse stretched the SMC package to 3000 watts and now has managed to pack a full 5000 watts into the same footprint.
• #26: Here is a table that summarizes the Littelfuse Axial TVS diode offerings. The first column indicates the type of naming convention used. As we have seen, some devices are named for their nominal breakdown voltage while others are named for their reverse standoff voltage. The second column indicates the polarity configurations available. Most devices are available as either unidirectional or bidirectional devcies. The low capacitance devices are a special case of a unidirectional device and are not available as bidirectional devices. The remaining columns indicate the various packages available.
• #27: Here is a similar table for the surface mount devices. You may find it useful to print out these two charts to use as a handy quick reference for the Littelfuse TVS diode family.
• #28: Thank you for taking the time to view this presentation on “ Introduction to TVS Overvoltage Protection Devices ” . If you would like to learn more or go on to purchase some of these devices, you may either click on the part list link, or simply call our sales hotline. For more technical information you may either visit the Littelfuse site, or if you would prefer to speak to someone live, please call our hotline number, or even use our ‘live chat’ online facility. You may visit Element 14 e-community to post your questions.