4.4 Thyristors
4.4.1 Introduction
Thyristors are two- to four-lead semiconductor devices that act exclusively as switches—they are not used to amplify signals, like transistors. A three-lead thyristor uses a small current/voltage applied to one of its leads to control a much larger current flow through its other two leads. A two-lead thyristor, on the other hand, does not use a control lead but instead is designed to switch on when the voltage across its leads reaches a specific level, known as the breakdown voltage. Below this breakdown voltage, the two-lead thyristor remains off.
You may be wondering at this point, Why not simply use a transistor instead of a thyristor for switching applications? Well, you could—often transistors are indeed used as switches—but compared with thyristors, they are trickier to use because they require exacting control currents/voltages to operate properly. If the control current/voltage is not exact, the transistor may lay in between on and off states. And according to common sense, a switch that lies in between states is not a good switch. Thyristors, on the other hand, are not designed to operate in between states. For these devices, it is all or nothing—they are either on or off.
In terms of applications, thyristors are used in speed-control circuits, power-switching circuits, relay-replacement circuits, low-cost timer circuits, oscillator circuits, level-detector circuits, phase-control circuits, inverter circuits, chopper circuits, logic circuits, light-dimming circuits, motor speed-control circuits, etc.
TABLE 4.6 Major Kinds of Thyristors
TYPE |
SYMBOL |
MODE OF OPERATION |
Silicon-controlled rectifier (SCR) |
|
Normally off, but when a small current enters its gate (G), it turns on. Even when the gate current is removed, the SCR remains on. To turn it off, the anode-to-cathode current flow must be removed, or the anode must be set to a more negative voltage than the cathode. Current flows in only one direction, from anode (A) to cathode (C). |
Silicon-controlled switch (SCS) |
|
Similar to an SCR, but it can be made to turn off by applying a positive voltage pulse to a four-lead, called the anode gate. This device also can be made to trigger on when a negative voltage is applied to the anode-gate lead. Current flows in one direction, from anode (A) to cathode (C). |
Triac |
|
Similar to an SCR, but it can switch in both directions, meaning it can switch ac as well as dc currents. A triac remains on only when the gate is receiving current, and it turns off when the gate current is removed. Current flows in both directions, through MT1 and MT2. |
Four-layer diode |
|
It has only two leads. When placed between two points in a circuit, it acts as a voltage-sensitive switch. As long as the voltage difference across its leads is below a specific breakdown voltage, it remains off. However, when the voltage difference exceeds the breakdown point, it turns on. Conducts in one direction, from anode (A) to cathode (C). |
Diac |
|
Similar to the four-layer diode but can conduct in both directions. Designed to switch either ac or dc. |
Table 4.6 provides an overview of the major kinds of thyristors. When you see the phrase turns it on, this means a conductive path is made between the two conducting leads [e.g., anode (A) to cathode (C), MT1 to MT2]. Normally off refers to the condition when no voltage is applied to the gate (the gate is open-circuited). We will present a closer look at these thyristors in the subsections that follow.
4.4.2 Silicon-Controlled Rectifiers
SCRs are three-lead semiconductor devices that act as electrically controlled switches. When a specific positive trigger voltage/current is applied to the SCR's gate lead (G), a conductive channel forms between the anode (A) and the cathode (C) leads. Current flows in only one direction through the SCR, from anode to cathode (like a diode).
FIGURE 4.114
Another unique feature of an SCR, besides its current-controlled switching, has to do with its conduction state after the gate current is removed. After an SCR is triggered into conduction, removing the gate current has no effect. That is, the SCR will remain on even when the gate current/voltage is removed. The only way to turn the device off is to remove the anode-to-cathode current or to reverse the anode and cathode polarities.
In terms of applications, SCRs are used in switching circuits, phase-control circuits, inverting circuits, clipper circuits, and relay-control circuits, to name a few.
How SCRs Work
An SCR is essentially just an npn and a pnp bipolar transistor sandwiched together, as shown in Fig. 4.115. The bipolar transistor equivalent circuit works well in describing how the SCR works.
FIGURE 4.115
THE SCR IS OFF
Using the bipolar equivalent circuit, if the gate is not set to a specific positive voltage needed to turn the npn transistor on, the pnp transistor will not be able to "sink" current from its own base. This means that neither transistor will conduct, and hence current will not flow from anode to cathode.
THE SCR IS ON
If a positive voltage is applied to the gate, the npn transistor's base is properly biased, and it turns on. Once on, the pnp transistor's base can now "sink" current though the npn transistor's collector—which is what a pnp transistor needs in order to turn on. Since both transistors are on, current flows freely between anode and cathode. Notice that the SCR will remain on even after the gate current is removed. This—according to the bipolar equivalent circuit—results from the fact that both transistors are in a state of conduction when the gate current is removed. Because current is already in motion through the pnp transistors base, there is no reason for the transistors to turn off.
Basic SCR Applications
BASIC LATCHING SWITCH
FIGURE 4.116
Here, an SCR is used to construct a simple latching circuit. S1 is a momentary contact, normally open pushbutton switch, while S2 is a momentary contact, normally closed pushbutton switch. When S1 is pushed in and released, a small pulse of current enters the gate of the SCR, thus turning it on. Current will then flow through the load. The load will continue to receive current until the moment S2 is pushed, at which time the SCR turns off. The gate resistor acts to set the SCR's triggering voltage/current. We'll take a closer look at the triggering specifications in a second.
FIGURE 4.117 Here, an SCR is used to rectify a sinusoidal signal that is to be used to power a load. When a sinusoidal waveform is applied to the gate, the SCR turns on when the anode and gate receive the positive going portion of the waveform (provided the triggering voltage is exceeded). Once the SCR is on, the waveform passes through the anode and cathode, powering the load in the process. During the negative going portion of the waveform, the SCR acts like a reverse-biased diode; the SCR turns off. Increasing R1 has the effect of lowering the current/voltage supplied to the SCR's gate. This in turn causes a lag in anode-to-cathode conduction time. As a result, the fraction of the cycle over which the device conducts can be controlled (see graph), which means that the average power dissipated by Rload can be adjusted. The advantage of using an SCR over a simple series variable resistor to control current flow is that essentially no power is lost to resistive heating.
DC MOTOR SPEED CONTROLLER
FIGURE 4.118
An SCR along with a few resistors, a capacitor, and a UJT can be connected together to make a variable-speed control circuit used to run a dc motor. The UJT, the capacitor, and the resistors make up an oscillator that supplies an ac voltage to the SCR's gate. When the voltage at the gate exceeds the SCR's triggering voltage, the SCR turns on, thus allowing current to flow through the motor. Changing the resistance of R1 changes the frequency of the oscillator and hence determines the number of times the SCR's gate is triggered over time, which in turn controls the speed of the motor. (The motor appears to turn continuously, even though it is receiving a series of on/off pulses. The number of on cycles averaged over time determines the speed of the motor.) Using such a circuit over a simple series variable resistor to control the speed of the motor wastes less energy.
Kinds of SCRs
Some SCRs are designed specifically for phase-control applications, while others are designed for high-speed switching applications. Perhaps the most distinguishing feature of SCRs is the amount of current they can handle. Low-current SCRs typically come with maximum current/voltage ratings approximately no bigger than 1 A/100 V. Medium-current SCRs, on the other hand, come with maximum current/voltage ratings typically no bigger than 10 A/100 V. The maximum ratings for high-current SCRs may be several thousand amps at several thousand volts. Low-current SCRs come in plastic or metal can-like packages, while medium and high-current SCRs come with heat sinks built in.
Technical Stuff
FIGURE 4.119
Here are some common terms used by the manufacturers to describe their SCRs:
VT |
On state-voltage. The anode-to-cathode voltage present when the SCR is on. |
IGT |
Gate trigger current. The minimum gate current needed to switch the SCR on. |
VGT |
Gate trigger voltage. The minimum gate voltage required to trigger the gate trigger current. |
IH |
Holding current. The minimum current through the anode-to-cathode terminal required to maintain the SCR's on state. |
PGM |
Peak gate power dissipation. The maximum power that may be dissipated between the gate and the cathode region. |
VDRM |
Repetitive peak off-state voltage. The maximum instantaneous value of the off-state voltage that occurs across an SCR, including all repetitive transient voltages but excluding all nonrepetitive transient voltages. |
IDRM |
Repetitive peak off-state current. The maximum instantaneous value of the off-state current that results from the application of repetitive peak off-state voltage. |
VRRM |
Repetitive peak reverse voltage. The maximum instantaneous value of the reverse voltage that occurs across an SCR, including all repetitive transient voltages but excluding all nonrepetitive transient voltages. |
IRRM |
Repetitive peak reverse current. Maximum instantaneous value of the reverse current that results from the application of repetitive peak reverse voltage. |
Here's a sample section of an SCR specifications table to give you an idea of what to expect (Table 4.7).
TABLE 4.7 Sample Section of an SCR Specifications Table
MNFR # |
VDRM (MIN) (V) |
IDRM (MAX) (mA) |
IRRM (MAX) (mA) |
VT (V) |
IGT (TYP/MAX) (mA) |
VGT (TYP/MAX) (V) |
IH (TYP/MAX) (mA) |
PGM (W) |
2N6401 |
100 |
2.0 |
2.0 |
1.7 |
5.0/30 |
0.7/1.5 |
6.0/40 |
5 |
4.4.3 Silicon-Controlled Switches
A silicon-controller switch (SCS) is a device similar to an SCR, but is designed for situations requiring increased control, triggering sensitivity, and firing predictability. For example, the typical turn-off time for an SCS is from 1 to 10 microseconds as opposed to 5 to 30 microseconds for an SCR. Unlike an SCR, an SCS has lower power, current and voltage ratings, typically with a max anode current from 100 mA to 300 mA and a power dissipation from 100 to 500 mW. Unlike an SCR, a SCS can also switch OFF when a positive voltage/input current is applied to an extra anode gate lead. The SCS can also be triggered into conduction when a negative voltage/output current is applied to that same lead. The figure below shows the schematic symbol for an SCS.
FIGURE 4.120
SCS are used in practically any circuit that needs a switch that turns on and off through two distinct control pulses. They are found in power-switching circuits, logic circuits, lamp drivers, voltage sensors, pulse generators, etc.
How an SCS Works
Figure 4.121a shows a basic four-layer, three-junction P-N-P-N silicon model of an SCS with four electrodes, namely the cathode (C), cathode gate (G1), anode gate (G2), and anode (A). An equivalent circuit of the SCS can be modeled by the back-to-back bipolar transistor network shown in Fig. 4.121c. Using the two-transistor equivalent circuit, when a negative pulse is applied to the anode gate (G2), transistor Q1 switches ON. Q1 supplies base current to transistor Q2, and both transistors switch ON. Likewise, a positive pulse at the cathode gate G1 can switch the device on. Because the SCS uses only small currents, it can be switched off by an appropriate polarity pulse at one of the gates. At the cathode gate, a negative pulse is required to switch the device off, while at the anode gate a positive pulse is needed.
FIGURE 4.121
Specifications
When buying an SCS, make sure to select a device that has the proper breakdown voltage, current, and power-dissipation ratings. A typical specification table will provide the following ratings: BVCB, BVEB, BVCE, IE, IC, IH (holding current), and PD (power dissipation). Here we have assumed the alternate lead name designations.
4.4.4 Triacs
Triacs are devices similar to SCRs—they act as electrically controlled switches—but unlike SCRs, they are designed to pass current in both directions, therefore making them suitable for ac applications. Triacs come with three leads, a gate lead and two conducting leads called MT1 and MT2. When no current/voltage is applied to the gate, the triac remains off. However, if a specific trigger voltage is applied to the gate, the device turns on. To turn the triac off, the gate current/voltage is removed.
FIGURE 4.122
Triacs are used in ac motor control circuits, light-dimming circuits, phase-control circuits, and other ac power-switching circuits. They are often used as substitutes for mechanical relays.
How a Triac Works
Figure 4.123 shows a simple n-type/p-type silicon model of a triac. This device resembles two SCRs placed in reverse parallel with each other. The equivalent circuit describes how the triac works.
FIGURE 4.123
TRIAC IS OFF
Using the SCR equivalent circuit, when no current/voltage is applied to the gate lead, neither of the SCRs' gates receives a triggering voltage; hence current cannot flow in either direction through MT1 and MT2.
TRIAC IS ON
When a specific positive triggering current/voltage is applied to the gate, both SCRs receive sufficient voltage to trigger on. Once both SCRs are on, current can flow in either direction through MT1 to MT2 or from MT2 to MT1. If the gate voltage is removed, both SCRs will turn off when the ac waveform applied across MT1 and MT2 crosses zero volts.
Basic Applications
SIMPLE SWITCH
FIGURE 4.124
Here is a simple circuit showing how a triac acts to permit or prevent current from reaching a load. When the mechanical switch is open, no current enters the triac's gate; the triac remains off, and no current passes through the load. When the switch is closed, a small current slips through RG, triggering the triac into conduction (provided the gate current and voltage exceed the triggering requirements of the triac). The alternating current can now flow through the triac and power the load. If the switch is open again, the triac turns off, and current is prevented from flowing through the load.
DUAL RECTIFIER
FIGURE 4.125 A triac along with a variable resistor and a capacitor can be used to construct an adjustable full-wave rectifier. The resistance R of the variable resistor sets the time at which the triac will trigger on. Increasing R causes the triac to trigger at a later time and therefore results in a larger amount of clipping (see graph). The size of C also determines the amount of clipping that will take place. (The capacitor acts to store charge until the voltage across its terminals reaches the triac's triggering voltage. At that time, the capacitor will dump its charge.) The reason why the capacitor can introduce additional clipping results from the fact that the capacitor may cause the voltage at the gate to lag the MT2-to-MT1 voltage (e.g., even if the gate receives sufficient triggering voltage, the MT2-to-MT1 voltage may be crossing zero volts). Overall, more clipping results in less power supplied to the load. Using this circuit over a simple series variable resistor connected to a load saves power. A simple series variable resistor gobbles up energy. This circuit, however, supplies energy-efficient pulses of current.
AC LIGHT DIMMER
FIGURE 4.126
This circuit is used in many household dimmer switches. The diac—described in the next section—acts to ensure accurate triac triggering. (The diac acts as a switch that passes current when the voltage across its leads reaches a set breakdown value. Once the breakdown voltage is reached, the diac releases a pulse of current.) In this circuit, at one moment the diac is off. However, when enough current passes through the resistors and charges up the capacitor to a voltage that exceeds the diac's triggering voltage, the diac suddenly passes all the capacitor's charge into the triac's gate. This in turn causes the triac to turn on and thus turns the lamp on. After the capacitor is discharged to a voltage below the breakdown voltage of the diac, the diac turns off, the triac turns off, and the lamp turns off. Then the cycle repeats itself, over and over again. Now, it appears that the lamp is on (or dimmed to some degree) because the on/off cycles are occurring very quickly. The lamp's brightness is controlled by R2.
AC MOTOR CONTROLLER
FIGURE 4.127
This circuit has the same basic structure as the light dimmer circuit, with the exception of the transient suppressor section (R2C2). The speed of the motor is adjusted by varying R1.
Kinds of Triacs
Triacs come in low-current and medium-current forms. Low-current triacs typically come with maximum current/voltage ratings no bigger than 1 A/(several hundred volts). Medium-current triacs typically come with maximum current/voltage rating of up to 40 A/(few thousand volts). Triacs cannot switch as much current as high-current SCRs.
FIGURE 4.128
Technical Stuff
Here are some common terms used by the manufacturers to describe their triacs:
ITRMS,max |
RMS on-state current. The maximum allowable MT1-to-MT2 current |
IGT,max |
DC gate trigger current. The minimum dc gate current needed to switch the triac on |
VGT,max |
DC gate trigger voltage. The minimum dc gate voltage required to trigger the gate trigger current |
IH |
DC holding current. The minimum MT1-to-MT2 dc current needed to keep the triac in its on state |
PGM |
Peak gate power dissipation. The maximum gate-to-MT1 power dissipation |
Isurge |
Surge current. Maximum allowable surge current |
Here's a sample section of a triac specifications table to give you an idea of what to expect (Table 4.8).
TABLE 4.8 Sample Section of a Triac Specifications Table
MNFR # |
IT,RMS MAX. (A) |
IGT MAX. (mA) |
VGT MAX. (V) |
VFON (V) |
IH (mA) |
ISURGE (A) |
NTE5600 |
4.0 |
30 |
2.5 |
2.0 |
30 |
30 |
4.4.5 Four-Layer Diodes and Diacs
Four-layer diodes and diacs are two-lead thyristors that switch current without the need of a gate signal. Instead, these devices turn on when the voltage across their leads reaches a particular breakdown voltage (or breakover voltage). A four-layer diode resembles an SCR without a gate lead, and it is designed to switch only dc. A diac resembles a pnp transistor without a base lead, and it is designed to switch only ac.
FIGURE 4.129
Four-layer diodes and diacs are used most frequently to help SCRs and triacs trigger properly. For example, by using a diac to trigger a triac's gate, as shown in Fig. 4.105a, you can avoid unreliable triac triggering caused by device instability resulting from temperature variations, etc. When the voltage across the diac reaches the breakdown voltage, the diac will suddenly release a "convincing" pulse of current into the triac's gate.
FIGURE 4.130
The circuit in Fig. 4.130 right is used to measure diac characteristics. The 100-kΩ variable resistor is adjusted until the diac fires once for every half-cycle.
Specifications
Here's a typical portion of a specifications table for a diac (Table 4.9).
TABLE 4.9 Sample Section of a Diac Specifications Table
MNFR # |
VBO (V) |
IBO MAX (μA) |
IPULSE (A) |
VSWITCH (V) |
PD (mW) |
NTE6411 |
40 |
100 |
2 |
6 |
250 |
Here, VBO is the breakover voltage, IBO is the breakover current, Ipulse is the maximum peak pulse current, Vswitch is the maximum switching voltage, and PD is the maximum power dissipation.
4.5 Transient Voltage Suppressors
There are numerous devices that can be used to stomp out unwanted transients. Earlier on, we saw how a decoupling capacitor could absorb supply line fluctuations, and we also saw how diodes could clip transient spikes caused by inductive switching action. These devices work fine for such low-power applications, but there are times when transients get so large and energetic that a more robust device is required. Here we'll take a look at various transient suppressor devices, such as TVSs, varistors, multilayer varistors, Surgectors, and PolySwitches. But before we do that, here's a little lecture on transients.
4.5.1 Lecture on Transients
Transients are momentary surges or spikes in voltage or current that can wreak havoc within circuits. The peak voltage of a transient can be as small as a few millivolts or as large as several thousand volts, with a duration lasting from a few nanoseconds to more than 100 ms, depending on their origin. In some cases, the transients are repetitive, recurring in a cyclic manner, as in the case of an inductive ringing transient caused by faulty wiring of a motor.
Transients are generated both internally within a circuit and externally—where they enter the circuit via power input lines, signal input/output lines, data lines, and other wires entering and leaving the circuit's chassis. Internal transients, the predominant of the two, can result from inductive load switching, transistor/logic IC switching, arcing effect, and faulty wiring, to name a few.
In the case of inductive loads, such as motors, relay coils, solenoid coils, and transformers, the sudden switching off of these devices will cause the inductive component within the device to suddenly dump its stored energy into the supply line, creating a voltage spike—recall the inductor equation V = LdI/dt. In many cases, these induced voltages can exceed 1000 V, lasting anywhere from 50 ns to more than 100 ms. Any transistor or logic driver ICs as well as circuits that use the same supply line will suffer, either by getting zapped with the transient spike or suffering from erratic behavior due to propagation of the transient along the power line. (Power lines, or rails, are not perfect conductors and don't have zero output impedance.)
Switching of TTL and CMOS circuits can also result in transient current spikes of a much smaller threat, yet enough to cause erratic behavior. For example, when the output transistors of a TTL gate switch on, a sudden surge in current is drawn from the supply line. This surge is often quick enough that the supply rail or PCB trace will dip in voltage (due to the fact that a conductor has built-in impedance). All circuits connected to the rail will feel this voltage dip, and the resulting consequences lead to oscillation or some sort of instability that can cause distortion or garble digital logic levels.
Arcing is another transient generator that comes from a number of sources, such as faulty contacts in breakers, switches, and connectors, where arcs jump between the gaps. When electrons jump the gap, the voltage suddenly rises, usually resulting in an oscillatory ringing transient. Faulty connections and poor grounding can also result in transients. For example, motors with faulty windings or insulation can generate a continuous stream of transients exceeding a few hundred volts. Poor electrical wiring practices can also aggravate load-switching transients.
Transients can also attack circuits from external sources through power input lines, signal input and output lines, data lines, and any other wire coming into or going out of a chassis containing the electronics. One cause of external transients is a result of induced voltages onto lines (power, telephone, distributed computer systems, etc.) due to lightning strikes near the lines or the switching of loads, capacitor banks, and so on, at the power utility. External transients may also enter the power line to a circuit due to inductive switching that occurs within a home, such as turning on a hair dryer, microwave, or washing machine. Usually the transient is consumed by other parallel loads, so the effects aren't as pronounced. For valuable electrical equipment, such as computers, monitors, printers, fax machines, phones, and modems, it is a good idea to use a transient power surge/battery backup protector, with a phone line, too, which will handle the surges and dips in the power and signal lines.
Electrostatic discharge (ESD) is another common form of external transient that can do damage to sensitive equipment and ICs. It usually enters a system through the touch of a fingertip or handheld metal tool. Static electricity that is humidity-dependent can generate low-current transients up to 40,000 V. Systems that are interconnected with long wires, such as telephones and distributed computer systems are efficient collectors of radiated lightning energy. Close-proximity strikes can induce voltages of 300 V or more on signal lines.
Transients are to be avoided; they can cause electronics to operate erratically, perhaps locking up or producing garbled results. They can zap sensitive integrated circuits, causing them to fail immediately or sometime down the road. Today's microchips are denser than older chips and a transient voltage can literally melt, weld, pit, and burn them, causing temporary or permanent malfunctions to occur. They might also be the cause for decreased efficiency—say, a motor running at higher temperatures due to transients, which interrupt normal timing of the motor and result in microjogging. This produces motor vibration, noise, and excessive heat.
4.5.2 Devices Used to Suppress Transients
There are several devices that can be used when designing circuits to limit the harmful effects of transients. Table 4.10 provides an overview of the most popular devices.
TABLE 4.10
DEVICE TYPE |
SYMBOL |
APPLICATIONS |
ADVANTAGES |
DISADVANTAGES |
Bypass capacitor
Logic: 0.01–0.22 µF Power: 0.1 µF and up |
|
Used for low-power applications, such as RC snubbers and decoupling of digital logic rails to provide clean power |
Low cost, available, simple to apply, fast action, bipolar |
Uneven suppression, may fail unpredictably, high capacity |
Zener diodes |
|
Diversion/clamping in low-energy circuits running at high frequencies (e.g., high-speed data lines) |
Low cost, fast, calibrated clamping voltage, easy to use, standard ratings, bidirectional |
Low energy handling, tend to fail open (which can hurt circuit); actually used more for regulation than transients |
Transient voltage suppressor diodes (TVS) |
|
Diversion/clamping in low-voltage, low-energy systems, modest frequency |
Fast, calibrated low clamping voltage, available, easy to use; fails short-circuited |
High capacitance limits frequency, low energy, more expensive than zeners or MOVs |
Metal oxide varistors (MOVs) |
|
Diversion/clamping in most low to moderate frequency circuits at all voltage and current levels |
Low cost, fast, available, calibrated clamping voltage, easy to use, standard ratings, and bidirectional; handles more total power than TVS; fails short-circuited |
Moderate to high capacitance limits high frequency performance |
Multilayer varistor (MLTV) |
|
Diversion/clamping in low voltage (3–70 V) systems with modest frequencies |
Fast, compact, high energy, low calibrated voltage bidirectional, surface mount |
More expensive than zeners or MOVs, high capacitance limits frequency |
Surgectors |
|
Diversion (crowbar) for moderate to high energy and frequency circuits and data lines |
High speed/moderate energy, sharp clamp voltage, moderate cost |
Cost more than other methods, exhibit follow-on current |
Avalanche diode |
|
Low-voltage, high-speed logic protection |
Very fast (sub-nanosecond response), low shunt capacitance (50 pF) |
Low surge capability |
Gas discharge and spark gap TVSs |
Diversion (crowbar) for very high-energy/voltage applications |
Very high-energy capability—upward of 20,000 A in some cases; leakage current is almost nonexistent (within the pA range) |
Cost more than other methods, slow response time |
|
PolySwitches |
|
Overcurrent protection for speakers, motors, power supplies, battery packs, etc. |
Low cost, easy to use, overcurrent protection |
Requires a cooling-down period to reset |
Transient Voltage Suppressor Diodes (TVSs)
Transient voltage suppressor diodes (TVSs) are popular semiconductor devices used to instantly clamp transient voltages and currents (electrostatic discharge, inductive switching kickback, induced lightning surges, etc.) to safe levels before they can do damage to a circuit. Earlier, in the section on diodes, you saw how standard diodes and zener diodes could be used for transient suppression. Though standard diodes and zener diodes can be used for transient protection, they are actually designed for rectification and voltage regulation and are not as reliable or robust as TVSs.
TVSs come in both unipolar (unidirectional) and bipolar (bidirectional) types. The unipolar TVS breaks down in one direction (current flows in the opposite direction of the arrow—like a zener diode) when its specified breakdown voltage VBR is exceeded. The bipolar TVS, unlike the unipolar TVS, can handle transients in either direction, when the applied voltage across it exceeds its breakdown voltage. See Fig. 4.131.
In terms of design, the TVS should be invisible until a transient occurs. Electrical parameters such as breakdown voltage, standby current (leakage current), and capacitance should have no effect on normal circuit performance. The TVS's VBR is typically 10 percent above VRWM, which approximates the circuit operating voltage to limit standby current and to allow for variations in VBR caused by the temperature coefficient of the TVS. (In catalogs, they give you both—VBR/VRWM: 12.4 V/11.1 V, 15.2 V/13.6 V, 190 V/171 V, etc.) VRWM should be equal to, or slightly greater than, the normal operating voltage of the protected circuit. When a transient occurs, the TVS clamps instantly to limit the spike voltage to a safe voltage level, while diverting current from the protected circuit. VC should be, of course, less than the maximum voltage the protected circuit can handle. Note that in ac circuits, you should use the peak voltage (Vpeak) values, not the RMS values for selecting VRWM and VBR (Vpeak = 1.4 VRMS ≤ VRWM). Also, make sure to choose a TVS that can handle the maximum expected transient pulse current. Figure 4.132 shows various TVS applications.
FIGURE 4.131
FIGURE 4.132
Fig. 1–3: Very high transient voltages are generated when an inductive load is disconnected, such as motors, relay coils, and solenoids. Here, TVSs provide protection to the driving circuitry, as well as limiting damage to relay and solenoid metal contacts.
Fig. 4–7: Typical power sources employing TVS for transient protection. The TVS is chosen for the breakdown voltage that is equal to or greater than the dc output voltage. In most applications, a fuse in the line is desirable.
Fig. 8–9: Input states are vulnerable to low-current, high-voltage static discharges or crosstalk transmitted on the signal wires. Usually an op amp or other IC will have an internal clamp diode, but this provides limited protection for high currents and voltage. Here, an external TVS diode is used to provide additional protection. The second circuit has a TVS on the output of an op amp to prevent a voltage transient due to a short circuit or an inductive load from being transmitted into the output stage.
Fig. 10: Transients generated on the line can vary from a few microseconds' to several milliseconds' duration and up to 10,000 V. This threat has given rise to high-noise-immunity ICs. However, the input diodes to these devices, again, have limited internal diode protection, and IC damage is still possible, resulting in either an open circuit or slow degradation of the circuit's performance with time. Here, a TVS located on the signal line can absorb this excess energy and prevent damage.
Fig. 11: A selection of transient suppressor arrangements for RF coupling.
Metal Oxide Varistors (MOVs) and Multilayer Varistors
A metal oxide varistor (MOV) is a bidirectional semiconductor transient suppressor that acts like a voltage-sensitive variable resistor. Internally, it consists of a complex ceramic crystal structure with various multidirectional metal oxide p-n junction boundaries between crystal grains, all sandwiched between two electrodes. Each individual p-n junction is highly resistive, up until a voltage across the grain boundary in excess of around 3.6 V, where it then is bias on—has a very small resistance. The voltage at which the MOV itself switches is dependent on the average number of grains between its electrode leads. During the manufacturing process, this value can be varied to create any desired breakdown threshold. Due to the random orientation of the boundaries within the MOV, there is no directional, so the MOV acts as a bipolar device—it can be used for ac or dc applications.
In terms of applications, an MOV is usually connected across the mains input of the equipment or the circuit it's protecting, with a series filter inductor and/or fuse thrown in to protect the MOV itself. In the presence of a transient, the MOV's resistance switches from high resistance (several megaohms) to very low resistance (a few ohms), transforming itself into a high-current shunt for the transient current. MOVs are made with various clamping voltages, peak current ratings, and maximum energy ratings—reflecting the fact that an MOV can absorb a very large amount of power for a very brief time or smaller amounts over a longer time. For example, an MOV rated at 60 J can absorb 60 W for 1 s, or 600 W for 0.1 s, or 6 kW for 10 ms, or 60 kW for 1 ms, and so on.
FIGURE 4.133
In many regards, MOVs resemble back-to-back zener diodes. However, unlike diodes, MOVs can handle higher-energy transients than zener diodes, since there is no single p-n junction, but rather numerous p-n junctions throughout its structure. The highly conductive ZnO grains act as heat sinks, ensuring a rapid and even distribution of thermal energy throughout the device and minimizing temperature rise. (Note that MOVs can dissipate only a relatively small amount of average power and are unsuited to applications that demand continuous power dissipation.) They are about as fast as zeners and clamp surge voltage to safe levels. Leakage is very low, which means little power is stolen from the circuit. Unlike the zener and other devices, the varistor fails shorted. A zener will also fail in an open-circuit condition that leaves equipment unprotected during a subsequent surge. This helps protect the circuit against subsequent surges; a shorted varistor across an ac or other line might fracture if the energy is high. MOVs should be fused or located where they won't effect other components should this happen.
In comparison to TVS diodes, MOVs can handle more total power/energy, while leaving a smaller footprint. TVS diodes, however, exhibit much better clamp ratios (better-quality protection) and faster response times (1 to 5 ns compared to about 5 to 200 ns for MOVs). The speed limitation of MOVs, however, is a result of parasitic inductance in the package and leads, and can be minimized with short lead design. MOVs also exhibit an inherent wear-out mechanism within their structure. As the device absorbs transient energy, the electrical characteristics (e.g., leakage, breakdown voltage) tend to drift. On the other hand, TVS diodes have no inherent wear-out mechanism. MOVs have an effective capacitance range from 75 pF for small MOVs to as high as 20,000 pF for large ones. This, combined with the lead inductance, makes practical MOVs slower than TVS diodes, but still fast—in the range of 5 to 200 ns, depending on the device. However, transients that they are designed to remove are usually much longer, so they are usually perfect for the job.
MOVs are found in power supplies of computers and other sensitive equipment, and in mains filters and stabilizers to prevent damage from mains-borne transients due to switching or lightning. They are used in telecommunication and data systems (power supply units, switching equipment, etc.), industrial equipment (control, proximity switches, transformers, motors, traffic lighting), consumer electronics (televisions and video sets, washing machines, etc.), and automotive products (all motor and electronic systems).
One variation of the MOV found in surface-mount form is the multilayer varistor, or MLTV. By having surface-mount contacts, lead self-inductance and series resistance are minimized, allowing for much quicker response time—less than 1 ns. A decrease in series resistance also translates into a massive increase in peak current capability per component unit volume. Even though this is the case, the energy ratings of MLTVs are rather conservative when compared to those of other varistors. One of MLTVs' strong points is their ability to survive many thousands of strikes, at full rated peak current, without degradation. MLTVs have a characteristic similar to capacitors, having an effective dielectric constant of around 800—much lower than conventional capacitors. Because of this feature, MLTVs are also used in filter circuits. MLTVs come with an operating voltage from 3.5 V to around 68 V, and they are used extensively for transient voltage protection for ICs and transistors, as well as for many ESD and I/O protection schemes.
The following are specifications for MOV and MLTVs:
- Maximum continuous dc voltage (VM(DC)): The maximum continuous dc voltage that may be applied up to the maximum operating temperature of the device. The rated dc operating voltage (working voltage) is also used as the reference point for leakage current. This voltage is always less than the breakdown voltage of the device.
- Maximum continuous ac voltage (VM(AC)): The maximum continuous sinusoidal RMS voltage that may be applied at any temperature up to the maximum operating temperature of the device. It's related to the previous dc rating by VM(DC) = 1.4 × VM(AC). This means that if a nonsinusoidal waveform is applied, the recurrent peak voltage should be limited to 1.4 × VM(AC).
- Transient energy rating (WTM): Energy is given in joules (watt-seconds). This represents the maximum allowable energy for a single 10/1000-µs impulse current waveform with continuous voltage applied.
- Peak current rating (IPK): The maximum current rating for a given maximum clamping voltage VC.
- Varistor voltage (VB(DC) or VNOM): The voltage at which the device changes from the off state to the on state and enters its conduction mode of operation. The voltage is usually characterized at the 1-mA point and has a specified minimum and maximum voltage listed.
- Clamping voltage (VC): The clamping voltage across an MOV at a peak current IPK.
- Leakage at rated dc voltage (IL): The leakage current when the device is in nonconducting mode, when a specified voltage is applied.
- Capacitance (Cp): The capacitance of the device, typically specified at a frequency of 1 MHz at a bias of 1 Vpp. This capacitance is usually 100 pF or lower for smaller devices, and up to a few thousand for larger ones.
In terms of design, a varistor must operate under both a continuous operating (standby) mode and the predicted transient (normal) mode. Determine the necessary steady-state voltage rating (working voltage), and then establish the transient energy absorbed by the varistor. Calculate the peak transient current through the varistor and determine the power dissipation requirements. Select a model to provide the required voltage-clamping characteristics.
FIGURE 4.134
Surgector
There are other transient voltage suppression devices out there, such as the Surgector, gas discharge, and spark gap TVSs. The Surgector utilizes silicon thyristor technology to provide bidirectional "crowbar" clamping action for transients of either polarity. This is accomplished with a five-layer p-n junction structure. Surgectors remain in a low-leakage, reverse-bias state, presenting effectively no load to the circuit as long as the applied voltage is at or below its VDRM rating. A transient voltage exceeding this value will cause the device to avalanche (breakdown), beginning the clamping action across the line to which it is connected. As the leading edge of the transient voltage attempts to rise higher, the Surgector current will increase through the circuit's source impedance until the VBO, or breakover voltage mode, is reached. Thyristor action is then rapidly triggered, and the Surgector switches to its "on," or latched state. This very low impedance state crowbars the line with effectively the characteristics of a forward p-n junction, thereby short-circuiting the transient voltage.
FIGURE 4.135
PolySwitch
A PolySwitch (also known as polyfuse, multiswitch, and generically resettable fuse) is a special positive temperature coefficient resistor that is constructed from a conductive polymer mix. It resembles a varistor and PTC thermistor in one. At normal temperatures, the conductive particles within the polymer form densely packed low-resistance chains, allowing current to flow easily. However, if the current flow through the PolySwitch increases to a point where its temperature rises above a critical level, the crystalline structure of the polymer suddenly changes into an expanded amorphous state. At this point, the device's resistance dramatically increases, causing a sudden drop in current flow. The point at which this occurs is referred to as the trip current. If the voltage level is maintained after tripping, enough holding current will generally flow, keeping the device in a tripped state. The PolySwitch will reset itself only if the voltage is reduced and the device is allowed to cool, at which point the polymer particles rapidly return to their densely packed state, and the resistance drops.
PolySwitches can be used in numerous applications wherever you need a low-cost, self-resetting solid-state circuit breaker. They are used to limit over-current in speakers, power supplies, battery packs, motors, etc. For example, Fig. 4.136 shows how a PolySwitch used to protect a speaker from excessive current sourced by an amplifier. The PolySwitch is rated with a trip current that is slightly higher than that rated for the power level the speaker can handle. For example, an 8-Ω, 5-W speaker has a maximum current rating determined by the generalized power law.
FIGURE 4.136
Avalanche Diodes
Avalanche diodes are designed to break down and conduct at a specified reverse-bias voltage. This behavior is similar to that of a zener diode, but its operation is caused by a different mechanism, called the avalanche effect (a reverse electric field applied across a p-n junction causes a wave of ionization, reminiscent of an avalanche, leading to a large current). However, unlike zener diodes that are rather restricted in maximum breakdown voltage, avalanche diodes are available with breakdown voltage of over 4000 V. Avalanche diodes are used in circuits to guard against damaging high-voltage transients. They are connected to a circuit so that they're reverse-biased (the cathode is set positive with respect to the anode). In this configuration, the avalanche diode is nonconducting and doesn't interfere with the circuit. However, if the voltage rises beyond a safe design limit, the diode goes into avalanche breakdown, eliminating the harmful voltage by shunting current to ground. Avalanche diodes are specified with a clamping voltage VBR and a maximum-size transient that it can absorb, specified either in terms of joules of energy or as I2t. The avalanche breakdown event is not destructive, provided the diode isn't overheated. One side effect that occurs in avalanche diodes is RF noise generation.
4.6 Integrated Circuits
An integrated circuit (IC) is a miniaturized circuit that contains a number of resistors, capacitors, diodes, and transistors stuffed together on a single chip of silicon no bigger than your fingernail. The number of resistors, capacitors, diodes, and transistors within an IC may vary from just a few to millions.
The trick to cramming everything into such a small package is to make all the components out of tiny n-type and p-type silicon structures that are embedded into the silicon chip during the production phase. To connect the little transistors, resistors, capacitors, and diodes together, aluminum plating is applied along the surface of the chip. Figure 4.137 shows a magnified cross-sectional view of an IC showing how the various components are embedded and linked together.
FIGURE 4.137 The structure of an IC
ICs come in analog, digital, or analog/digital form:
- Analog (or linear) ICs produce, amplify, or respond to varying voltages. Some common analog ICs include voltage regulators, operational amplifiers, comparators, timers, and oscillators.
- Digital (or logic) ICs respond to or produce signals having only high and low voltage states. Common digital ICs include logic gates (such as AND, OR, or NOR), microcontrollers, memories, binary counters, shift registers, multiplexers, encoders, and decoders.
- Analog/digital ICs share properties common with both analog and digital ICs. Analog/digital ICs may take a number of different forms. For example, the IC may be designed primarily as an analog timer but may contain a digital counter. Alternatively, the IC may be designed to read in digital information and then use this information to produce a linear output that can be used to drive, say, a stepper motor or LED display.
ICs are so pervasive that you are likely to use them in any project that you will undertake. You will find them used in many of the chapters that follow.
4.6.1 IC Packages
ICs come in many and various packages (see Fig. 4.138). The determining factors for the package type are the number of pins and the power dissipation. For example, a high-power voltage regulator IC may have three pins and look just like a high-power transistor.
However, the majority of ICs have many more pins and are arranged in a dual in-line (DIL) package (see Fig. 4.138) of 8, 14, 16, 20, 24, or 40 pins. There are also surface-mount versions of the DIL packages, as well as packages arranged as a square with pins on all sides. Some of the surface-mount packages have extremely small spacing between pins—sometimes as small as 0.5 mm, which is two pins every millimeter and not really intended for hand soldering.
FIGURE 4.138 IC packages
Some of the most common packages are listed in Table 4.11.
You will often find that the same ICs are available in multiple packages, making it possible to prototype in something easy to solder like DIL or SO, and then switch to a smaller package for the final product.
TABLE 4.11
PACKAGE |
LONG NAME |
PITCH (mm) |
NOTES |
DIL |
Dual in-line |
2.54 |
|
SO/SOIC/SOP |
Small outline IC package |
1.27 |
|
MSOP/SSOP |
Mini/shrink small outline package |
0.65 |
|
SOT |
Small outline transistor |
0.65 |
|
TQFP |
Thin quad flat pack |
0.8 |
Pins on four sides |
TQFN |
Thin quad flat no leads |
0.4-0.65 |
No pins or pads underneath body |