Introduction to SIDACtor® Overvoltage Protection Devices

Introduction to SIDACtor® Overvoltage Protection Devices
This module will introduce you the SIDACtor family 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 look at the Littelfuse portfolio of devices and discuss selection methods. The estimated time to complete this module is 20 minutes.

Outline Overvoltage Protection Key Parameters Protected Circuit Issues
The race is on Clamp vs Crowbar Key Parameters Vdrm Vbo Vs Holding Current Surge Current Withstand Double Exponential Waveforms Power Cross Considerations Protected Circuit Issues Let-through voltage Turn-on time Capacitance Vbo Dependence Applied voltage dependence Surge Capability dependence Configurations Single Devices SLIC Devices 2-Chip Devices 3-chip “Y” Devices Package Options Selection Process We will begin with a brief discussion on overvoltage protection in general. That will be followed by a description of the key parameters that define SIDACtor protection devices as well as competitive thyristor-based overvoltage protection devices. Then we will look at some of the critical issues surrounding the protected circuit and possible interactions with the overvoltage protection device. We will look at the various configurations of the Littelfuse SIDACtor protector line, the available packages and finally, look at product selection guidlelines.

Overvoltage Protector
The Race is On! Overvoltage Protector Protected Circuit Overvoltage Event 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 analagous 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!

Clamping Devices vs Crowbar Devices
Vline Transient Event Vline Clamping Devices Clamp Voltage Vline Trigger Voltage Delatch Crowbar Devices 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. SIDACtor devices are all crowbar devices.

The race is on Clamp vs Crowbar Key Parameters Vdrm Vbo Vs Holding Current Surge Current Withstand Double Exponential Waveforms Power Cross Considerations Protected Circuit Issues Let-through voltage Turn-on time Capacitance Vbo Dependence Applied voltage dependence Surge Capability dependence Configurations Single Devices SLIC Devices 2-Chip Devices 3-chip “Y” Devices Package Options Selection Process Now let’s take a look at the key performance parameters of thyristor overvoltage protection devices. Understanding these parameters will help us zero in on selecting the right SIDACtor device for a particular application.

Key Parameters Voltage – Current (V-I) Curve
Voltage is displayed left to right Current is displayed up and down Most of the parameters we will be discussing appear on what is known as a VI curve. This is the image generated by a specialized oscilloscope called a curve tracer. The horizontal axis shows the voltage applied to the device with zero volts being where the two axis cross. The vertical axis shows the resulting current through the SIDACtor device with zero current also being where the two axis cross. So you can see that negative voltages create negative currents in the device in the lower left quadrant. Positive voltages force positive currents as shown in the upper right quadrant. Most SIDACtor devices are symmetric. This means that the upper right and lower left quadrants are mirror images of each other. This allows us to focus only on the upper right quadrant for this discussion.

Key Parameters: VDRM +I -V +V -I IT IS IH IDRM VDRM VBO VS VT
Vdrm is the maximum voltage that can be applied to the SIDACtor device without triggering the device into conduction. The threshold for conduction, called IDRM is usually defined as 5 microamps. This means that the Vdrm value should be larger than the highest expected operating voltage of the system. If operating voltages exceed the Vdrm, even for an instant, the SIDACtor device may interfere with the system operation. Because increasing temperatures will increase device leakage, the VDRM will drop with increasing temperature. VDRM VBO VS VT -I

Key Parameters: VBO +I -V +V -I IT IS IH IDRM VDRM VBO VS VT
Vbo is the breakdown voltage of the SIDACtor device measured at 50 or 60 hurts. When Vbo is applied to the device, it switches into the crowbar state. Between Vdrm and Vbo a SIDACtor device will behave much like a TVS diode clamping device. Vbo varies considerably from device to device and will increase slightly as the device’s temperature is increased. VDRM VBO VS VT -I

Key Parameters: VS +I -V +V -I IT IS IH IDRM VDRM VBO VS VT
Vs is a bit trickier. It is the peak voltage that will appear across the SIDACtor device during it’s transition into the crowbar state. During power cross and other slow overvoltage events, Vs will be the same as VBO. However, the thryistor structure takes some time to transition into the crowbar mode. This means that if the voltage is rising quickly, VS will be a few volts higher than VBO. Since a curve tracer only shows slow events, VS is not typically visible on a curve tracer for fast waveforms. The SIDACtor device must be selected so that Vs is below the voltage that could damage the protected circuit. Keep in mind that Vs is only present for a microsecond or two while many protected circuits are only rated for continuous exposure to high voltages. In some applications, VS is actually higher than the maximum voltage allowed by the datasheet for the protected circuit, but because the voltage is present for such a short time, the SIDACtor device will always win the race to breakdown. Vs is influenced by the size and speed of the SIDACtor device, the rate of rise of the applied overvoltage event and to a lesser degree, the source impedance of the overvoltage event. Because this more of a performance parameter than a device parameter, the exact conditions of the test circuit must be specified. Littlefuse uses a 100V/uS linear wavefront from a fairly high impedance source to define Vs. VS will also increase slightly as the device’s temperature is increased. VDRM VBO VS VT -I

Key Parameters: Holding Current (Ih)
IT IS IH IDRM -V +V As we have seen, the SIDACtor device is triggered by voltage. But once the device is in it’s crowbar state, how does it turn off? The answer to this question is simple. The current through the device must go below the holding current in order for the device to reset into its off state. From an application standpoint, this means that the holding current should be higher than the current that could be provided by the protected system so that the SIDACtor device. This is one reason we find SIDACtor devices used mostly in limited power systems such as telephone, data and signaling. VDRM VBO VS VT -I

Key Parameters: Surge Current Withstand
Double Exponential Waveforms Simulate induced lightning events Rise time & decay time in microseconds Surge current withstand ratings, also called surge capacity ratings, are divided into two categories. Those that simulate accidental contact with AC power lines and those that simulate the pulsed caused on cables by nearby lightning strikes. First we will look at the lightning simulation surges. The waveforms used to simulate lighting are call 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. The most common current waveforms are ten by one thousand, eight by twenty and two by ten. Others are ten by seven hundred, ten by five sixty and five by three twenty. 10 x 1000 8 x 20 2 x 10 10 x 700 10 x 560 5 x 320

Key Parameters: Power Cross Events
Accidental contact with power lines Construction Accidents Traffic Accidents Weather-related Accidents Inductive coupling Co-locating telephone cables with power cables Improper grounding Power cross events can have a number of causes. Some are accidental contact of the protected line with power lines. One example is construction crews digging up and rupturing telephone cables and power cables, causing them to be connected together. Traffic accidents involving power and telephone lines is another. Wind and ice storms have been know to break lines and have them fall into contact with others. Power cross events can also be caused by inductive coupling between power and telephone cables sharing a common duct. Improper grounding can also cause problems. Power cross events usually cause excessive current to flow in the protected equipment and the main protection used are overcurrent protectors such as fuses or PT seas. However, the overvoltage protector must survive long enough to allow the overcurrent device to operate. In some cases, the requirement is only that the overvoltage protector must not cause a fire. In other cases the requirement is that the overvoltage protector survive undamaged. To simulate a power cross event, the AC source open circuit voltage and short circuit current are defined as well as the duration of the test. Common tests are the 15 minute 600 volt 2.2 amp test found in GR-1089.

The race is on Clamp vs Crowbar Key Parameters Vdrm Vbo Vs Holding Current Surge Current Withstand Double Exponential Waveforms Power Cross Considerations Protected Circuit Issues Let-through voltage Turn-on time Capacitance Vbo Dependence Applied voltage dependence Surge Capability dependence Configurations Single Devices SLIC Devices 2-Chip Devices 3-chip “Y” Devices Package Options Selection Process Next we will take a few moments to look at some issues that arise in some real-world applications. These are topics that are frequently discusses in the part selection process. They are presented here from the perspective of the protected circuit.

Protected Circuit Issues: Let-Through Voltage
Let-through voltage: The peak voltage that the protected circuit will see in the real application. Similar to the datasheet Vs value: Vs is defined as the peak voltage measured directly across the SIDACtor at a particular voltage rate of rise. Dependent on: Actual incident voltage rate of rise Board layout: SIDACtor should be as near the entrance to the board as possible. SIDACtor should be “in-line” so that the surge must pass under the SIDACtor on the way to the protected circuit.. The let through voltage is the actual voltage the protected circuit will experience during a surge event. While similar to V-S, the peak value will depend on the actual rise time of the event as well as the board layout. It is always best to remember two things about the position of the SIDACtor on the board. Number 1, the SIDACtor should be placed as close as possible to the connector that brings the protected line to the board. This minimized the propagation of the signal to other traces on the board and maximized the board trace inductance between the SIDACtor and the protected circuit. Number 2, the SIDACtor should be situated so that the surge must pass directly under the SIDACtor on the way to the protected circuit. Placing the SIDACtor on a stub line will raise the let-through voltage and may introduce unwanted impedance mismatches on the signal line.

Protected Circuit Issues: Turn-On Time
Time to clamp Time to crowbar Clamp to crowbar transition time Other real world application issues arise when the discussion involves the phrase turn on time. We have see at least three different, and yet valid, aspects of turn on time as it relates to the customer circuit. The scope trace shows the voltage across a 170 volt SIDACtor when a 100A 10x1000 surge waveform is applied. The time scale is 500 nano seconds or half a microsecond per each of the ten divisions so that the entire picture is five micro seconds wide. The voltage is fifty volts per division so that this waveform peaks very near the 170 volt rating of the part. Let us consider the time it takes to clamp the surge waveform. You can see that once the voltage achieves the 170 volt level, the SIDACtor is immediately clamping. This means that, at least on this time scale, the part clamps the surge waveform instantly. This is how we can claim that the SIDACtor turns on in nanoseconds. Next, let’s see how long the SIDACtor takes to crowbar. You can see that once the voltage is clamped, the voltage begins to fall after about 400 nanoseconds. This is typical of SIDACtor devices. Finally, notice the rather slow and graceful transition into the crowbar state. This has some advantages over other technologies. Gas Discharge Tubes, for instance, transition very quickly to their on state. This can cause excessive noise on customer circuits during a surge event– especially those that utilize blocking capacitors.

Protected Circuit Issues: Capacitance
Vbo Dependence In today’s high speed networks, the capacitive loading that the SIDACtor presents to the signal is also of some interest. The capacitance of a SIDACtor is dependant on three things. First off, the selected Vbo will determine the capacitance. This is a chart of the nominal capacitance of a SIDACtor in the popular C rated MC series. As you can see, as you select a SIDACtor with higher Vbo, the basic capacitance of the device decreases.

Applied Voltage Dependence Okay, so we’ve discussed the effect of the breakdown voltage on the capacitance of the SIDACtor. Next we will look at the effect of the applied voltage on the capacitance of the SIDACtor. The graph shown is for a popular C rated 260 volt SIDACtor. You can see that with no voltage applied, the capacitance is quite high compared with the capacitance with just a few volts applied. Other SIDACtor devices will have higher or lower capacitance, but they all follow the same basic curve. In telephone voice circuits, this has no real effect. However, in data circuits this characteristic can cause data errors. This is why we developed the MC line of SIDACtor devices. They have significantly less capacitance than the standard line. They are terrific in applications such as T1 E1, ADSL and ISDN. For very high speed data circuits such as VDSL or Ethernet, we recommend our biased bridge technology devices. These are the SDP and SEP protectors and they are specifically designed to eliminate this capacitance issue.

Surge Capability Dependence P2300xx A B C D RE ME Surge Rating (8x20) 150 250 400 1000 3000 5000 Capacitance 2V) 30 38 55 65 220 475 The third factor that determines the SIDACtor devices capacitance may be a bit more obvious. The higher the surge current rating, the larger the silicon die must be. That means the capacitance is larger too. This table shows the capacitance of various 230 volt SIDACtor devices.

The race is on Clamp vs Crowbar Key Parameters Vdrm Vbo Vs Holding Current Surge Current Withstand Double Exponential Waveforms Power Cross Considerations Protected Circuit Issues Let-through voltage Turn-on time Capacitance Vbo Dependence Applied voltage dependence Surge Capability dependence Configurations Single Devices SLIC Devices 2-Chip Devices 3-chip “Y” Devices Package Options Selection Process Now let’s quickly run through the various configurations that comprise the SIDACtor product line and note how each configuration is targeted to particular applications.

SIDACtor Device Configurations: Standard Single Devices
The Original Flexible Board Layout The two terminal standard single chip SIDACtor device is the original offering. It is still our most popular because many applications demand nothing more and nothing less. It gives the PC board designer maximum flexibility in their parts layout.

SIDACtor Device Configurations: SLIC Single Devices
SIDACtor Device for Negative Voltages Diode Device for Positive Voltages Subscriber Line Interface Circuit In SLICK SIDACtor devices there is a diode integrated into the design. This is perfect for slick circuits where positive voltages are damaging and must be eliminated. SLICK is an acronym for the subscriber line interface circuit. This is the device that creates the telephone line, including the voice signal, dial tone, busy signal, ringing and a host of other functions. One terminal, marked G in the diagram, is connected to ground. The other is connected to either the tip or ring line. Two of these protection devices are needed for every slick port, one for tip and one for ring.

SIDACtor Device Configurations: 2-Chip Devices
TwinChip 2-in-1 Packaging Matched Chips TwinSLIC 2-in-1 Packaging 1 pkg per port Broadband Optimized Half the capacitance Better linearity Combining two SIDACtor chips in a single package has some advantages: The twin chip design give board designers twice the density of protection. That plays well in todays telecom market where product density is a key selling feature. Another advantage of the twin chip series is that, because the two SIDACtor chips are cut from the same silicon wafer, they are perfect matched pair. Twin slick devices offer the same advantages to the slick card designer. Telecom designers appreciate the one package per port simplicity of these designs. Our broadband optimized devices have two chips in series inside the device, but the center terminal is not available. This is used as a single device but it has the advantages of half the capacitance of a single device and better capacitance linearity. These devices are often used in VDSL applications where these advantages are valued.

SIDACtor Device Configurations: 3-Chip “Y” Devices
Lower Capacitance Better Capacitance Balance Better Voltage Balance Better Surge Balance 3-for-1 Placement Advantage The Littelfuse three chip wye configuration exhibits lower capacitance because the devices are placed in series. Circuit analysis also proves that the capacitance unbalance to ground is also significantly better than a two chip design. Most designs use three chips with the same breakdown voltage. That means the breakdown voltage between any two pins is the same. Because the center chip is common to the wing chips, breaking down either side will cause the other side to break down as well. This is very useful in transformer coupled circuits. And, of course, placing a single component instead of three is a cost advantage for our customers.

SIDACtor Device Configurations: Tracking Devices
BATTRAX Trademark Sophisticated SLIC protection Retains diodes for positive surges Negative breakdown voltage determined by reference Positive voltage tracking also available Many modern SLIC circuits require a protector with a breakdown voltage that can be modified or programmed. Littelfuse trademarked the name Battrax for their line of these sophisticated devices. They aren’t actually SIDACtor devices at all, but specially designed SCR devices in a diode array. The SCR is triggered when a voltage more negative than the reference voltage is detected. This means that the protection voltage is determined by the voltage on the reference pin. Most SLIC circuits still require the protection diodes for positive surges. However, there are some SLIC devices that now ring the telephone line with positive voltages. For these applications Littelfuse also makes positive voltage tracking devices.

SIDACtor Device Configurations: Bridge Devices
For Broadband Applications And more! Patented Bridge Design Low Capacitance Great Capacitance Balance Perfect Voltage Balance Perfect Surge Balance Biasing provides flat lower and flatter capacitance The newest configuration for the SIDACtor line is the SDP series. These use a patented bridge circuit to surround the center SIDACtor chip with a network of steering diodes. Originally conceived to address issues in the broadband market, these designs are also finding application in voice circuits as well as T1 E1. The bridge makes for a very low capacitance design which is highly prized by VDSL and Ethernet designers. The bridge also provides great capacitance balance – meaning the capacitance on the tip line will be very close to the capacitance on the ring line under all operating conditions. Perfect voltage balance is assured because there is only one breakdown device in the circuit. And when it breaks over, the tip and ring lines both go to ground simultaneously, assuring perfect surge balance. Some versions of the bridge also have biasing terminals. This allows the designer to preload the center SIDACtor chip with a fixed voltage. This drives the capacitance of the solution down and also makes the capacitance independent of the line voltage.

The race is on Clamp vs Crowbar Key Parameters Vdrm Vbo Vs Holding Current Surge Current Withstand Double Exponential Waveforms Power Cross Considerations Protected Circuit Issues Let-through voltage Turn-on time Capacitance Vbo Dependence Applied voltage dependence Surge Capability dependence Configurations Single Devices SLIC Devices 2-Chip Devices 3-chip “Y” Devices Package Options Selection Process Next we will survey the wide variety of package options available for the SIDACtor product line. Packages can be divided into two types, through hole and surface mount. But first, lets take a look at the part numbering scheme used in the SIDACtor product line.

SIDACtor Part Numbering
A: ITU K.20 & K.21 B: TIA-968-A Most SIDACtor devices use a very straightforward part numbering format. Lets take a moment to look at this numbering scheme. Beginning with the letter P the following three numerical digits are a nominal breakdown voltage. For voltages less than 100 volts, there will be leading zeros. For example, an 8 volt part will have as the three number code. Next comes construction variable. Usually this is the number of chips actually contained in the device. A number one in this position indicates that this device has the reverse polarity diode found in the slick protection devices. The next position is a letter that defines the package used. One exception is that QFN devices will have the letter Q followed by two numerical digits. Next comes the general surge rating. A rated devices are suitable for most European and Chinese telecom applications as they are usually paired with PTC devices for overcurrent protection. B rated devices are often used in customer premises equipment. They are designed to meet the TIA dash 9 68 A requirements. C rated devices will comply with GR interbuilding standards. This is a key North American requirement for telephone company equipment. D and E rated devices are premium surge capable devices for special applications where GR-1089 is not enough. We have only a handful of competitors at the D level and no real competition at the E level. Back to the part numbering scheme, an L after the package designator indicates roas compliance. Finally the part number may have lead forming options or packing option suffixes. C: GR-1089

Package Options: Thru-Hole
TO-218 M R First we will look at the through hole packages. The T-O 92 package is normally a three terminal plastic transistor package. However, Littelfuse modifies this package by trimming the center lead short. As such, it serves as a stand off to assure the T-O 92 is inserted to the correct depth on the printed circuit board. T-O 92 devices all contain a single SIDACtor chip and use the package designation E in the part number. The D-O 15 axial package may contain one SIDACtor chip or a stack of two SIDACtor chips. This package is footprint compatible with many leaded gas tube devices. All axial SIDACtor devices use package code G in the part number. The T-O 2 20 package, with its mounting tab, is only used for the 3000 Amp SIDACtor device family. Note that the mounting tab is connected directly to the center lead. We call this the R package. The modified T-O 2 20, which we call the A pack, can contain one, two or three SIDACtor chips. The modification refers to the missing mounting tab so that the body of the package is completely covered in epoxy. The T-O 2 18 is the largest package and is only used to house the 5000 Amp SIDACtor family. Like the T-O 2 20 package, the mounting tab is connected to the center pin. The third pin, however, is not connected to anything inside the package, which is designated M. A

Package Options: Surface Mount
Q12 3x3 QFN 3.3x3.3 QFN C Q22 S On the surface mount package side there are even more options. The D-O 2 14 package, also known as an SMB package and is designated in the part numbering scheme with the letter S. It is the most popular package for single chip SIDACtor devices. A, B, C, and D rated SIDACtor devices are offered in the D-O 2 14. Splitting one lead of the D-O 2 14 package creates the three leaded modified D-O 2 14 package. This package supports the TwinChip, TwinSLIC and BATTRAX product lines. The Littelfuse name for this package is the compact D-O 2 14, which uses the letter C in the part number. The smallest SIDACtor packages are the 3 by 3 QFN and the 3.3 by 3.3 QFN packages. These are single chip packages for the A, B and C rated SIDACtor devices. In the part number, they will use the Q 12 and Q 22 designations respectively. The six pin modified M-S Oh 13 package is very flexible. It can contain three, four or even six SIDACtor chips. This U designated package is also used for the dual and quad BATTRAX devices. The 5 by 6 QFN package is the home for the SDP and SEP bridge devices. It fits the same printed circuit board footprint as the very popular S-O 8 package. It uses the package designation code Q 38. Finally, the T-O 2 63 or D squared pack is the surface mount version of the through hole T-O With the mounting tab trimmed, pin two trimmed and pins 1 and 3 formed, the T-O 2 20 becomes a surface mount device. This N package is used only to support the 3000A SIDACtor family. U Q38 N 5x6 QFN

The race is on Clamp vs Crowbar Key Parameters Vdrm Vbo Vs Holding Current Surge Current Withstand Double Exponential Waveforms Power Cross Considerations Protected Circuit Issues Let-through voltage Turn-on time Capacitance Vbo Dependence Applied voltage dependence Surge Capability dependence Configurations Single Devices SLIC Devices 2-Chip Devices 3-chip “Y” Devices Package Options Selection Process Finally, lets take a look at a few simple steps to guide you through the selection process for a particular application.

Selection Methods Datasheet Parameters Configuration Selection
Vdrm, Vbo, Vs Ih Surge Current Withstand Capacitance & Linearity Configuration Selection Package Selection To properly select a device for a new application, it is usually best to first identify the datasheet parameters required. It is sometimes useful to visualize where the application lies with respect to a V-I curve. The green area shows the normal operating voltages of the system to be protected. Vdrm or peak off-state voltage parameter of the Sidactor must be above all of the green voltages. The red area represents voltages that will damage the protected system. Vs or switching voltage of the Sidactor must be below all of the red voltages. So the resulting yellow area is the range of voltage where the Sidactor device maybe active. Once the Voltages are determined, select the correct holding current and surge withstand capability. Next the capacitance and linearity issues need to be addressed. With this data, you should be able to select an appropriate configuration for the application. Finally, a suitable package is selected.

Thank You! Thank you. This completes the Introduction to SIDACtor Devices module.

Introduction to SIDACtor® Overvoltage Protection Devices

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