Transient Suppressors
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The document discusses the nature and sources of transients in electrical systems, emphasizing their detrimental effects on equipment, particularly in shipboard and aircraft power systems. It details various types of power systems and the types of voltage transients they experience, alongside common sources like lightning and load switching. Additionally, it illustrates methods for mitigating these transients, including the use of capacitive filters and transient suppressors, with a focus on bipolar transient voltage suppressors.
Transient Suppressors
- 1. Appendix F TE000-AB-GTP-010 Revision 2 F.1 General Transient Suppressors A transient is a short duration change, either positive or negative, in current or voltage. Positive transients are more of a concern because they are more likely to cause damage. Negative transients are a problem because they induce system malfunctions by disrupting logic systems, servo system control, etc. Transients can cause slow degradation, erratic operation, or catastrophic failure in electrical parts, insulation dielectrics, and electrical contacts used in switches and relays. The possible occurrence of transients mustbe considered in the overall electronic design. Required circuit performance and reliability must be assured both during and after thetransient. Transients originate from three major sources; normal switching operations (power supply turn-on and turn-off cycles), routine AC line fluctuations, and abrupt circuit disturbances (faults, load switching, voltage dips, magnetic coupling by electro-mechanical devices, lightning surges, etc.). Some examples of transient sources are shown in Table F- 1. Transients are a major cause of component failures in semiconductors. Random transients can permanently damage voltage sensitive devices and disrupt proper system Table F- 1: Transient Sources. Man-Made Switching (mechanical and solid state) of reactive loads; opening and closing of switches and relays. Fuse and circuit breaker interruptions and resettings. Generator and motor operation (overspeed and hunting startup, control and shutdown). Ignition system, arc welder, particle precipitator operation. Fluorescent light operation. Reflected waves. Electromagnetic pulse (EMP) from nuclear blasts or largechemical explosions. Current inrush. Thyristor switching power control. Natural Lightning F-1
- 2. F-2 operation. Normal shipboard power system on-off cycles have the potential of emitting spikes with enough energy to destroy an entire semiconductor device chain. Any surviving devices are also suspect. Trouble shooting, isolation, and replacing damaged devices is both time consuming and costly. This is especially true in the field. Transients are fairly common on shipboard power systems. As described inDOD-STD-1399, Section 300, shipboard power systems may encounter many types of voltage transients depending upon the type of powersystem. F.1.1 Power System Types F.1.1.1 Shipboard Power Systems There are three types of power systems used in shipboard applications: Type I - 400/115VRMS, 60 Hz, at either 3 phase or 1 phase. This is used mainly for ship’s servicing power and lighting distribution system. Voltage tolerance is 5 percent of the average of the three line to line voltages. Type II - 440 or 115VRMS, 400Hz, at either 1 phase or 3 phase. The voltage tolerance is not as precise as Type III systems. Type III - 440 or 115 VRMS, 400Hz, 3-wire, or 115VRMS 4-wire. This is similar to Type II except the average voltage tolerance between line to line is 1/2percent. F.1.1.2 Aircraft Power Systems: There are three types of aircraft power systems, characterized asfollows: AC power: AC Power is available as single phase or three phasewye-connected neutral or ground return system having a nominal voltage of 115/200VAC anda nominal frequency of 393/407Hz. The only alternate standard is anominal 230/400VAC when specifically authorized. The voltage unbalance will bea maximum of 3.0V, and the waveform distortion factor will be 0.05 maximum. The maximum frequency drift rate will be 15Hz per minute. In emergency situations, where the limited power available is necessary to operate flight controls and communications, the steady state voltage and frequency will be maintained between 104 and 122VAC and 360 to 440Hz. DC power: DC Power is available as two-wire or ground return systems at a nominal 28VDC. 270VDC is also available, but is unusual and must bespecifically authorized. These busses are maintained between 22 and 29VDC and between250 and 280VDC respectively. Because of the overall advantages of the 270V distribution system, it is expected this system will become increasingly common. Most aircraft provide two 28V and one 270Vbus. The distortion factor (the ratio of the rms value of the superimposed ac ripple to the average dc voltage) for the 28 volt bus is 0.035 maximum, and the distortionfactor for the 270 volt bus is 0.008 maximum. Maximum ripple voltage will be 1.5V on the 28 volt system and 6.0 volts on the 270 volt system. In emergency situations, where the limited power available is needed for weapons jettison and aircraft control, steady state voltages will be within the limits of 16 to 29VDC on the 28 volt bus, and 240 to 290VDC on the 270 volt bus.
- 3. F-3 F.1.2 Power System Transients For Type I and Type II power supply systems, voltage transients of 10% or lessmay occur several times an hour, and transients of 10 to 16% may occur several times a day (percentage based on nominal user voltage). On Type I systems, the time to reach the transient maximum varies from 0.001 to 0.03 second, and to reach the transient minimum from 0.001 to 0.06 seconds. This depends on the rating of the generator and the type of regulator and excitation systemused. For Type III power supply system, voltage transients of 1 percent or less may occur several times an hour. The time to reach the transient maximum may vary from 0.001 to 0.1 second. Aircraft power transients are required to tolerate transient variations in both voltage and frequency values. Limiting voltage-time and frequency-time envelopes can be found in MIL-STD-704. F.1.3 Shipboard Equipment Transient Protection Some system protection is normally built into shipboard equipment, but this protection will not always prevent damage. The equipment is still susceptible to high frequency, high voltage transients. These can occur from electromagnetic pulses, lightning, static discharges, switching of low factor loads, switching from normal to alternatepower supplies, or active ground detector tests (an active ground detector superimposes500V DC on an AC system). F.2 Common Sources of High Frequency Transients F.2.1 Lightning A single lightning strike can have a length of over 2Km and a peak current upto 400KA. Lightning usually occurs in two or three strokes, but can vary anywhere between one to twelve. The rise time for the first return stroke is about 1.5s with decay to 1/2 crest value of 40s. The highest peak currents (i.e. 400KA) occur in the tropical regions of the world, because of the greater height of the thundercloud. Maximum peak currents in the temperate zones are about 250KA. The initial phase of a lightning discharge begins with a downward moving stepped leader. This is when the electric field between the thundercloud and the earth is sufficient to cause dielectric breakdown of the intervening air. The stepped leader lowers the cloud to ground charge in incremental steps. It ionizes a channel which becomes the path of the lightning stroke. After the channel is generated, the return stroke travels from ground up to the base of the cloud. Although the stroke travels from earth to cloud, thecharge transfer of electrons is normally from cloud toearth. Lightning can affect electrical equipment by: (1) a direct strike on the secondary system, (2) a direct strike on a primary conductor, or (3) a strike on a nearby object causing an induced voltage in an overhead line by the magnetic field produced.
- 4. F-4 A model of a voltage transient on a 120VAC system is waveform that rises to a peak in 500ns, and then decays in a sinusoidal waveform at a frequency of 100kHz. DOD-STD-1399 (NAVY), Section 300 gives a rise time of1.5s. F.2.2 Load Switching Transients F.2.2.1 Power Systems Switching transients tend to have lower frequencies than the "spikes," which are of primary concern here. Their levels, at least in the case of restrike-fastswitching operations, are generally less than twice normalvoltage. F.2.2.2 Electromechanical Switching Switching operations involving restrikes, such as those produced by air contactors or switches, can produce voltage escalations reaching several times the system voltage. The worst case is generally found on the load side of the switch. Therefore, it usually involves only the device being switched, and the prime responsibility rests with the end user. However, transients can also affect the line side of the switch. They canbe reflected back onto the line and be passed on to other equipment, possible damaging it. F.2.2.3 Low/High Inductive Sources Fast rise time voltage transients can cause secondary effects due to inductance in the circuit. This secondary voltage transient is described by the relationship: V(t) L di dt Where: L = Inductance (H) di/dt = time rate change of current. Induced voltages have been recognized for a long time, but because of their short durations, they were not a problem until the introduction of semiconductors. Many power semiconductors are relatively immune to most transients. However, small geometry semiconductors have been damaged with transient voltages of only 25ns duration. The waveform of an induced voltage is usually oscillatory in nature. This is because the switch gap alternately sparks over and extinguishes itself. The followingsequence occurs: (1) The switch opens and interrupts the line current. (2) The current in the inductor needs to dissipate so it charges the capacitance in the line andinterwinding capacitance of the inductor. (3) This raises the voltage until it reaches the spark-over voltage of the switch. (4) At this point the contacts spark over, and the currentis dissipated to the load. (5) However, the spark-over occurs very rapidly. Whenit extinguishes itself it effectively opens the switch, which is a repeat of step (1). The entire process then repeats itself until there is insufficient stored energy left in the inductor to cause the spark-over.
- 5. F-5 The stored energy in the coil is expressed by the followingequation: E 1 LI 2 2 Where: E = energy in joules L = inductance in henries I = current through the inductor inamperes The maximum inductive switching transient for shipboard 110VAC systems is defined as a peak voltage of 2,500V and a waveform as shown in Figure F-1.1 This waveform has 100 90 50 30 1.5s 2,500 V Peak been adopted as the worst case switching transient which would be generated by a large inductive source. An example of such a worst case transient 0 Time 40s Figure F-1: Maximum Inductive Switching Transientfor Shipboard 110VAC System. generator could be an elevator motor on an aircraftcarrier. F.2.2.4 Electromagnetic Pulse (EMP) During a high altitude nuclear detonation, gamma rays are released and sethigh energy electrons into motion. These electrons are subsequently deflected bythe electromagnetic belt surrounding the earth and an electromagnetic pulse is created. This Table F-2: Comparison of EMP, Lighting, and Static Discharge Pulses. Event Filed Density or Magnitude Rise Time EMP (from Nuclear Blast) 50KV/m @ 500Km 5KV/ns Lightning 3V/m @ 10Km 600KV/s Static Discharge (Human) 20KV at Impact 2KV/ns 1 As defined in DOD-STD-1399, Section 300 Voltageasa%ofPeakVoltage
- 6. F-6 deflection can generate a voltage pulse of 50,000V/m at a point 300 miles fromthe detonation, with a rise time of approximately 5,000V/s. This is much more severe than a lightning strike, which has a field density of 3V/m, 6 miles from point of discharge, and a rest time of 600V/s (see Table F-2). Because of the large magnitude of the voltage and frequency spectrum of an EMP, there are basically no "off-the-shelf" R-C or L-C filters that can effectively reduce or eliminate such an EMP. Metal Oxide Semiconductor (MOS) circuits and small area geometry semiconductors are especially vulnerable to the EMPs. Because of this vulnerability,effective suppression techniques and protective devices must be used to protect against EMP. F.2.2.5 Electrostatic Discharge Electrostatic Discharge (ESD) is a high voltage, low current pulse. Its rise time is in the range of nanoseconds (ns), compared to microseconds (s) for a lighting orswitching transient. The rate of voltage increase is about 2 KV/ns and reaches a maximum of about 35,000 V. Some can be damaged by less than 100V. For additional information on ESD, see Appendix E. A comparison between EMP, lightning, and static discharge pulses is presented in Table F-. F.3 Reducing the Transient Problem F.3.1 Capacitive Filters The primary method of suppressing transients is a low pass filter. The simplest form of a filter is a capacitor in parallel with the DC input. This will suppress any transient to which the capacitor presents an impedance lower than the transient source. Acapacitor can be an effective filter as long as it: (1) does not load down the system and create any current in-rush problems (a resistor in series with the capacitor will reduce the in-rush problems but also reduces the effectiveness of the capacitor), (2) does not haveany parasitic inductance which will degrade the high frequency admittance of the device, and (3) does not degrade with time or ripple current. If the transient has high DC components, a capacitor filter can become ineffective. Also, the inductance associated with the filter resistance will reduce its suppression effectiveness. In addition, transient oscillations (ringing) can develop. When this happens,the capacitor can have the effect of increasing the transient voltage if the transient sourceis inductive. Filters are often used in conjunction with transient suppressors. These will be discussed in the next section. F.3.2 Transient Suppressors A second method of suppressing voltage spikes is to use a transient suppressor. Basically, this is a device which switches a low impedance load into the circuit when the current or voltage goes over a set limit. The low impedance load then absorbs the excess power, effectively leveling off the spike. To be effective, a transient suppressor must have a fast response time and be capable of handling high energy pulses. It also must be
- 7. F-7 able to clip the transient at a safe voltage level and then dissipate the energy before any damage occurs. Good engineering judgement must be used when selecting a suppressor. Voltage and current spikes are not precisely known and not very predictable, butthey obey the laws of probability. Therefore, the statistical distribution of the spikes can be taken into account. Also, the power rating, maximum operating temperature,size, parasitic leakage, and capacitance of the device should all be considered. The basic requirements of a transient supressor are asfollows: a. A response time less than the rise time of the transient. b. A clamping voltage level less than the maximum voltage the equipmentcan withstand. Also, it should not interfere with the normal operation of the equipment. c. Self-restoring. d. Maintenance free. To avoid interference with normal equipment operation, zener or varistor clamping voltages should be more than 20% higher than the maximum operating voltage of the equipment. On the other hand, the surge protection voltage level (P) should be lower than the equipment withstanding voltage(W). Although various schemes have been used for transient suppression, such aszener diodes, avalanche diodes, varistors, RC filters, spark gap arrestors, etc, this discussion will focus on Bipolar Transient Suppressors, avalanche diodes, and metal oxide varistors. Use of circuit breakers, fast fuses, and thermistors is intended to protect against extended overload conditions and these types of devices are not recommended for suppression of fast transients. F.3.3 The Bipolar Transient Voltage Suppressor (TVS)2 F.3.3.1 TVS Operation The TVS consists of two special construction zener diodes oriented back-to-back. It is characterized by extremely rapid response time, low series resistance, and highsurge voltage handling capacity. The volt-ampere characteristic for a typical TVS given in Figure F-2. It shows the device can conduct current in both directions. It also shows the TVS is useful forbi-directional transient suppression. A zener diode is primarily used as a voltage regulator. Therefore, its selection is concerned with the dynamic slope of breakdown, breakdown voltageminimum VBR(Nom) Vm(Wkg) I IL IT ISM V VSM and maximum within set limits, and wattage. It is usually selected for the average power that must bedissipated Figure F-2: Voltage-Current Characeristic Curve for a TVS 2 Reference MIL-HDBK-978(NASA)
- 8. F-8 while regulating. In contrast, TVS diodes are used as voltage suppression agents. They are selected for the instantaneous power that must be dissipated. They pay less attention to the maximum applied voltage of the transient when at low current values, as long as the device does not exceed its maximum temperature rating. A TVS is required to display minimal leakage currents since the clamping voltage is usually set marginally above the maximum operating voltage of the circuit. The zener diode is not as concerned with leakage current since regulation occurs most often within the avalanche region. The functional parameters that determine the performance of the TVS are defined as follows: a. Working peak voltage (VM, WKG) or stand-off voltage (VR): This parameteris the maximum permissible dc working voltage. It is the highest reverse voltage at which the TVS will be nonconducting. c. Maximum peak surge voltage (VSM) or maximum clamping voltage (VCmax): This is the maximum voltage drop across the TVS while subjected to the peak pulse current, usually for 1 ms. d. Minimum breakdown voltage (BVmin): The breakdown voltage is thereverse voltage at which the TVS conducts. This is the point where the TVS becomes a low impedance path for the transient. e. Test current (IT): The current is the zener current at which the nominal breakdown voltage is measured. f. Maximum leakage current (IL): This current is the current leakage measured atthe maximum dc working voltage (VM or VR). g. Maximum peak surge current (ISM): This current is the maximum permissible surge current. F.3.3.2 TVS Construction The TVS diode is generally made using two passivated diffused, planar or diffused planar junctions on an epitaxial substrate process die placed back-to-back in a single glass-to-metal or double-slug package. The three basic die processes arepassivated diffused, planar, and diffused planar junction on epitaxial substrate. F.3.3.3 TVS Applications When choosing a TVS for a particular application, the following important factors should be considered: a. The maximum clamping voltage (VC) should be determined in order toprovide adequate protection for a circuit or component. Once VC has been determined, it will be used to calculate the power for worst case designs for a givenapplication. b. The TVS selected should display a reverse stand-off voltage (VR) equal to or greater than the circuit operating voltage (maximum ac or dc peak voltage with tolerances).
- 9. F-9 c. To select the appropriate TVS one must also determine transient pulse power (Pp). This can be accomplished by using the simple definition; transient pulse power (Pp) equals the peak pulse current (Ipp) multiplied by the clamping voltage (VC),or: PP VC IPP F.3.3.4 Examples of TVS Applications Microprocessor System TVS Application: The TVS is placed on the signal and input power lines to prevent system failures caused by the effect of switching power supplies, AC power surges and transients, such as electrostatic discharges. A TVS acrossthe signal line to ground will prevent transients from entering the data and control buses. TVSs shunted across the power lines ensure transient-free operating voltage. DC Line TVS Applications: A TVS in the output of a voltage regulator can replace many components used as protection circuits such as the crowbar circuit illustrated by Figure F- 3. It may also be used to protect the bypass transistor from voltage spikes across the Unregulated DC Voltage Regulator Regulated DC Figure F-3: DC Line TVS Application. collector-to-emitter terminals. Protecting Switching Power Supplies. A designer needs to protect against three types of transients: load transients, line transients, and internally generated transients. Because transients have high energy levels they cause improper operation and component failure. The following are typical components that need to be protected in a switching power supply: High Voltage Switching Transistors Rectifiers Output Rectifiers Control Circuitry Figure F-4 shows a typical switching power supply with TVS devices used for protection in voltage sensitive areas.
- 10. F-10 O Figure F-4: Typical Switching PowerSupply. Relay and Solenoid TVS Applications: The coil inductance of a solenoid or relay can release energy that can damage contacts or drive-transistors. A TVS used as shownin Figure F-5 would provide adequateprotection. The proper TVS can be selected by determining peak pulse power (Pp) and pulsetime (tp). Knowing the values of VCC, L, and, RL the following equations can be used to determine Pp and tp. I VCC RL PP IPVC VCC t1 t t1 P 2 F.3.3.5 Failure Mechanism and Data In extensive tests of transient voltage suppressors, thethree most critical parameters were peak pulse power, peaksurge voltage and reverse leakage current. Because of its inherent use as a transient suppressor, the TVS must sustain high voltage pulse power. Opens or shorts caused by cracking of the die are main-failure modes if peak reverse power rating is exceeded. Long term life tests have Figure F-5: Coil and Contacts, DC. TVS TVS Output to External Circuits Control Circuits TVS TVS VCC RL VC L
- 11. F-11 yielded a significant number of failures in these areas. Failure mechanisms of TVSsare similar to those of other diode types. F.3.4 Avalanche Diode A second type of semiconductor transient suppressor is the avalanche diode. The avalanche diode was first developed to protect telephone circuits from lightning. This device suppresses surges by limiting the peak voltages through avalanche breakdown. It is especially effective for short duration pulses (i.e., in the order of 10 ms). It can absorb relatively large transients because of its large junction area. It can also quicklydissipate relatively large amounts of heat because it has silver heat sinks which are metallurgically bonded to the silicon chip. The design and structure of this device provides inherently lower impedance compared to zener diodes having the same steady-state voltage ratings. The clamping speed of the avalanche diode is in the order of microseconds. This gives them the ability to protect very sensitive devices from fast rise time pulses. They Avalance Diodes Low Capacitance Diodes Avalanche Diode Bipolar Avalanche Diode Low Capacitance Varistor Low Capacitance Diode Figure F-6: Low CapacitanceProtectors. are also available in special low inductanceconfigurations. One disadvantage of avalanche diodes is their large capacitance. This is due to their large junction area. When used on DC or low frequency AC lines, the capacitance will not attenuate or alter the signal. However, in high frequency applications insertion loss occurs. To compensate, a low capacitance diode can be added, as shown in FigureF-6. However, it must have a reverse breakdown voltage greater than that of the suppressor, and must be able to withstand the maximum peak pulse current of the suppressor with the minimum voltage drop across it. The low capacitance diode will also reduce the response time of the suppressor by the very nature of its construction. Under pulse conditions the low capacitance diode, which is essentially a high voltage rectifier, will conduct in the forward conduction mode only. This mode is slower than the avalanche modeof conduction.
- 12. F-12 F.3.5 Metal Oxide Varistor The Metal Oxide Varistors (MOV) is a very nonlinear device. At normal voltages, it has a very high resistance, about 160 k. However, when a surge occurs, its resistance decreases by several orders of magnitude. The device is made from a ceramic-like material composed of small granules of zinc oxide suspended in a matrix ofbismuth oxide. At normal voltages, a steady-state current (standby current) flows through the device which is less than 1 mA peak. Once the transient voltage exceeds the normal voltage by about 100 V, the resistance decreases. One disadvantage of the varistor is thata relatively large voltage is needed before suppression begins. This is called a weak knee. The device will also only operate for only a limited number of transient pulses. Some of the varistor’s advantages are voltage-current characteristics comparable to zener diodes, good bipolar suppression capabilities, and high power dissipation ability. It Figure F-7: Primary Method for Assuring Automatic Voltage Protection of Motor Starter Thyristors andDiodes also has a fast response time, which is in the nanosecond range. Figure F-7 illustrates one method for assuming automatic voltage protection of motor starters, thyristors, and diodes. F.3.6 Suppressor Lead Inductive Effects Inductive effects can be, and often are, a source of abnormally high peakclamping voltages which can nullify the capability of a transient voltage suppressor. These high clamping voltages can result in failure of vulnerable electronic parts. Therefore, transient voltage suppressors can be rendered ineffective due to inductive effects. To minimize the inductive effect, a zero inductance suppressor element can be used. MOV Vswitch<VRMS
- 13. F-13 A s F.3.6.1 Inductive Effects in Component Leads The voltage developed across an inductor under a voltage step is expressed as: V L di L dt Where: VL = peak voltage in volts L = inductance in henries di/dt = time rate of change of current The inductance caused by the lead wires between the suppressor and the circuitry is usually overlooked when designing suppressors into a system. Although the inductance of such lead wires is quite low (about 1 H/m), for fast rise time transients even a few centimeters of lead wire can drastically reduce supressor effectiveness. Inductance causes a voltage overshoot across the leads. For example, if a 20 kV voltage transient occurs, and the total wire length were 10 cm, then the voltage at point A(reference Figure F-8), would be: V 10X102 106 H 4X109 A Where: di/dt = 4 X 109 A/s Thus: VA = 400 V Even if this voltage overshoot may not be permanently destructive, it could still cause circuit malfunction. The effect of lead length on peak clamping voltage is shownin Figure F- for an ICT-5 type Avalanche (TranZorb) suppressor. This device was designed to protect low voltage logic circuits. It was pulsed at 100, 200, 300, 400 and 500 A with a 1.2 x 50 second waveform. The voltage drop was measured across the 0.030cm diameter straight wire leads at distances of 0, 1.0 and 2.0 cm from the body of the + Input _ A Suppressor Element Package + Output to Device Under Protection _ Figure F-8: Transient Suppressors with Lead WireInductance. m
- 14. F-14 5 4 3 2 1 0 5 10 15 20 25 Peak Clamping Voltage Above Breakdown Voltage (V) Figure F-9: Effects of Lead Length on ClampingVoltage. package. Figure F-9 shows that clamping voltage increases with both pulse current and lead. The increase in clamping voltage with lead length is due to inductive effects. For fast rise time transients (e.g., ESD which has rise times on the order of 10 ns) lead lengths present a real problem. Therefore, if the protector lead lengths could be reduced to virtually zero, more effective suppression would result. F.4 Applications The intent of adding suppression elements is to improve reliability of the equipment, but the opposite effect can occur if the suppressor is not properly chosen. For example, adding a suppressor having marginal capability can be worse than no suppressor at all. Selection of the proper transient suppressor is, for most applications, a five step process: Determine the necessary steady-state voltage ratings. Establish the transient energy absorbed by the suppressor. Calculate the peak transient current through the suppressor. Determine any power dissipation requirements. Select a device to provide therequired voltage limiting characteristics. Quite often adequate transient protection maynot be attainable with only one type of suppressor. For example, high energy transient sources may require the use of spark gaps followed by semiconductor protective devices. A spark gap can divert thehigh current surge, and a varistor, Zener diode or Avalanche diode can then rapidly clamp theresidual voltage. Figure F-10: Power Line Transient Suppressor. 100 A 200 A 300 A 400 A 500 A + _ TotalLeadLength(cm)
- 15. F-15 F.4.1 Avalanche Suppressors Avalanche operated devices have proven to be effective EMPsuppressors. However, each situation must be evaluated on its own particular set of boundary conditions (i.e, circuit operating voltage and frequency, circuit destruct threshold, maximum peak current anticipated, etc). Integrated circuits can be protected by placing an avalanche suppressor across the power line. The suppressor, having a low "on" resistance will reduce unwanted transients while maintaining the circuit Figure F-11: Operational Amplifier Transient Supression. voltage level. In case of abnormal transients beyond the maximum current orpower ratings of the suppressor, the suppressor will usually fail "short." This trips the system’s circuit breaker or fuse, butprotects the equipment circuitry. Typical applications of avalanche transient suppressors are asfollows: a. Transient suppressors on power lines prevent IC failures caused by surges, power supply +5V MOS LSI Memory -10V reversals, or transients (see Figure F-10). ICT 5 ICT 8 ICT 10 b. The suppressor protects internal MOSFET devices from transients introduced on the power supply line (see Figure F- 22). Figure F-22: Transient Suppression forMOS Devices. c. A suppressor on the output of an operational amplifier will prevent a voltage transients from being transmitted into the output stage (see Figure F-1). It will also reduce effective capacitance at the output. d. Input stages of operational amplifiers are vulnerable to low energy, highvoltage static discharges and EMPs. Limited protection is provided by a clamping diode or a diode input network within the IC substrate (Figure F-14). The diode, however, must have a breakdown voltage greater than the supply voltage (Vcc)). Such diodesare limited in current capacity. e. An avalanche diode across the output terminals of Schottky rectifiers provides protection from load voltage transients. It simultaneously provideseffective protection from secondary leakage inductance voltage spikes (see FigureF-13). + _
- 16. F-16 Or Figure F-43: Input Circuit Protection for Operational Amplifiers. F.4.2 Metal Oxide Varistors (MOVs) MOVs are a cost effective means of dealing with high, medium, and lowlevel transients. Most MOVs are electrically symmetrical, resembling back-to-back Zener diodes. They are useful in protecting both AC and DC circuits. However, they are not effective in clamping highly repetitive spikes. This is because the MOV’s characteristics Figure F-34: Avalanche Diode Suppression for Schottky Rectifiers. degrade each time it suppresses a spike. As a consequence, pulse life deratingcurves identify the operating life of a MOV as a function of peak current, pulse width, andmean number of pulses. A MOV should not be used in applications were repetitive pulses will occur. For example, assume a high power MOV is capable of suppressing 1,000,000 pulses, 20ms wide at 50A peak. Operating at 10 kHz, the MOV can function for only 100s. By that time the characteristics have deteriorated enough so that rated specifications are no longer guaranteed. MOVs are recommended for use in circuits where medium to high surge currents are expected on a random basis, and where maximum insulation resistance andminimum discharge voltage are not important. MOVs provide good protection together with small size, low cost, and large energy handling capacity. They are available withclamping voltages between 22 and 1,800 V, peak surge capacities from 250 to 20,000 A,and response times in the order of 50 ns. MOV ratings of energy, power dissipation AC + _ + _
- 17. F-17 capability, voltage, and peak current, are temperature dependent. Therefore, it is important to use derating curves when selectingMOVs. MOVs have some drawbacks for high frequency applications. They can exhibit high shunt capacitance in the range of 10 pf to about .02 f. Most varistors are also slightly inductive. Table F-4 is a general guide for comparing various types of transient suppressors with ratings on their performance characteristics. The higher the rating number (e.g., 4), the better the device performance characteristics. Table F-2: Comparison of Transient Supressor Properties. Characteristics Silicon Avalanche Suppressors Zener TVS MOVs Voltage Capability 3 1 3 Peak Current Capability (non- repetitive) 2 1 2 Leakage Current 4 3 2 Energy Absorption Capability (Repetitive) 4 2 2 Response Time 4 2 2 Operating Temperature 3 2 2 Life 3 2 1 Size 3 4 4 Cost 1 3 3 Note: Ratings are based on a scale of 1 to 4. The higher thenumber, the better the rating.
- 18. F-18