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 cur- rent 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 cur- rent/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 cir- cuits, level-detector circuits, phase-control circuits, inverter circuits, chopper circuits, logic circuits, light-dimming circuits, motor speed-control circuits, etc.

TABLE 4.3 Major Kinds of Thyristors

TYPE SYMBOL MODE OF OPERATION

Silicon-controlled Normally off, but when a small current enters its gate (G), it turns on. rectifier (SCR) 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 flowsin only one direction, from anode (A) to cathode (C).

Silicon-controlled Similar to an SCR, but it can be made to turn off by applying a positive switch (SCS) 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 a 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 gateis 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 itsleads 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 4.3 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). I 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).

G

A C

FIGURE 4.89

anode gate

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 trig- gered 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 cath- odes polarities.

S

In terms of applications, SCRs are used in switching circuits, phase-control cir- cuits, inverting circuits, clipper circuits, and relay-control circuits, to name a few.

FIGURE 4.90

How SCRs Work

An SCR is essentially just an npn and a pnp bipolar transistor sandwiched together, as shown in Fig. 4.90. The bipolar transistor equivalent circuit works well in describing how the SCR works.

N

Anode

P

anode

"anode"

N

Gate N P

N

Cathode

gate

cathode

equivalent to "gate"

"cathode"

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 tran- sistor’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

V+ Here, an SCR is used to construct a

simplelatching circuit. S1 is a momentary contact, nor-mally open pushbutton switch, while S2 is a momentary contact, normally closed pushbut-

2

S 1

normally open

RG

normallyclosed

load

ton switch. When S1 is pushed in and released,a small pulse of current enters the gate of theSCR, 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 resis- tor acts to set the SCR’s triggering voltage/cur- rent. We’ll take a closer look at the triggering specifications in a second.

FIGURE 4.91

ADJUSTABLE RECTIFIER

R1

Vsource

Vsource

FIGURE 4.92

Rload

Vload

Vload

Vtrig

set by R1

Here, an SCR is used to rectify a sinusoidal signal that is to be used to power a load. When a sinu-soidal waveform is applied to the gate, the SCR turns on when the anode and gate receive the pos- itive going portion of the waveform (provided the triggering voltage is exceeded). Once the SCRis 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

+10V

R1

100

+3 to 6V

dc motor

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 volt-

FIGURE 4.93

4.7 F

100K

2N4819

100

age to the SCR’s gate. When the voltage at thegate 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 fea- ture 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/100V. 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.

FIGURE 4.94

Low current Medium current High current

Technical Stuff

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 volt- age 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 cur- rent that results from the application of repetitive peak off-state voltage.

VRMM 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.

IRMM 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 4.4).

TABLE 4.4 Sample Section of an SCR Specifications Table

VDRM IDR IRR IG VG IH(MIN) (MAX) (MAX) V

T

(TYP/MAX) (TYP/MAX) (TYP/MAX) PG

MMNFR # (V)

(mA) (mA) (V) (mA) (V)

(mA) (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-controlled switch (SCS) is a device similar to an SCR, but it is designed to turn off when a positive voltage/input current pulse is applied to an additional anode gate lead. The device also can be triggered into conduction by applying a negative voltage/output current pulse to the same lead. Other than this, the SCS behaves just like an SCR (see last section for the details). Figure 4.95 shows the symbol for an SCS. Note that the lead names may not appear as cathode, gate, and anode gate. Instead, they may be referred to as emitter (cathode), base (gate), and collector (anode gate).

gate(base)

anode cathode(emitter)

FIGURE 4.95

anode gate(collector)

SCSs are used in counters, lamp drivers, power-switching circuits, and logic cir- cuits, as well as in essentially any circuit that requires a switch that can be turned on and off by two separate control pulses.

How an SCS Works

Figure 4.96 shows a basic n-type/p-type silicon model of an SCS, along with its bipo- lar equivalent circuit. As you can see, the equivalent circuit looks a lot that the SCR equivalent circuit, with the exception of the anode gate connection. When a positive pulse of current is applied to the gate, the npn transistor turns on. This allows currentto exit the pnp transistor ’s base, hence turning the pnp transistor on. Now that both transistors are on, current can flow from anode to cathode—the SCS is turned on. The SCS will remain on until you remove the anode-to-cathode current, reverse the anode and cathode polarities, or apply a negative voltage to the anode gate. The negative anode gate voltage removes the transistor ’s self-sustaining biasing current.

Anode

"anode"

anodeP

N

Gate P

Anode gate

gate

anode gate

equivalent to "gate""anode gate"

N cathode

"cathode"

FIGURE 4.96

Cathode

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 pro- vide the following ratings: BVCB, BVEB, BVCE, IE, IC, IH (holding current), and PD (power dissipation). Here I 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.

gate

FIGURE 4.97

MT1 MT2

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 4.98 shows a simple n-type/p-type silicon model of a triac. This device resem- bles two SCRs placed in reverse parallel with each other. The equivalent circuit describes how the triac works.

MT2

MT2

P

"MT2"

N equivalent to

Gate P

gate

MT1 "gate" "MT1"

N

FIGURE 4.98

MT1

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 suf- ficient 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 theac waveform applied across MT1 and MT2 crosses zero volts.

Basic Applications

FIGURE 4.99

SIMPLE SWITCH

ac input(e.g., 120 V)

load

RG

Here is a simple circuit showing how a triac acts to permit or prevent current from reachinga 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 pre- vented from flowing through the load.

DUAL RECTIFIER

Vsource

R

Rload

C

Vload

Vsource

Vload

FIGURE 4.100

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 clip- ping (see graph).The size of C also determines the amount of clipping that will take place. (The capac- itor 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 maybe 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

120 V100 W

R1

1K

R2

120 V 500Kac

0.1 F50V

FIGURE 4.101

diac

triac

This circuit is used in many household dimmerswitches. The diac—described in the next sec- tion—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 release 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 volt- age, 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

motor

FIGURE

4.102

120 Vac

C1

0.1 F100V

R1

100K2W

diac

triac R2

100 1/2W

C2

0.22 F200V

This circuit has thesame basic structure as the light dimmer circuit, with the excep- tion 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 ratingof up to 40 A/(few thousand volts). Triacs cannot switch as much current as high- current SCRs.

FIGURE 4.103

Low current High current

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 trig- ger 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 4.5).

TABLE 4.5 Sample Section of a Triac Specifications Table

IT,RMS IG VGMAX. MAX. MAX. VFON IH ISURGE

MNFR # (A) (mA) (V)

(V)

(mA) (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.

four-layer diode diac

FIGURE 4.104

anode cathode

Four-layer diodes and diacs are used most frequently to help SCRs and triacs trig- ger 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 break- down voltage, the diac will suddenly release a “convincing” pulse of current into the triac’s gate.

FULL-WAVE PHASECONTROL CIRCUIT

CIRCUIT USED TO MEASURE DIACCHARACTERISTICS

120 VAC

load(<1500W)

200K

3.3K

diac

triac

120 Vrms

47K100K

0.1 F

diac under test

IR

L

FIGURE 4.105

60 Hz

0.1 F100V

60 Hz

IC

L 220

The circuit in Fig. 4.105 right is used to measure diac characteristics. The 100-k

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 4.6).

TABLE 4.6 Sample Section of a Diac Specifications Table

IBVBO MAX IPULSE VSWITCH PD

MNFR # (V) (A) (A) (V)

(mW)

NTE6411 40 100

2 6 250

Here, VBO is the breakover voltage, IBO is the breakover current, Ipulse is the maximumpeak pulse current, Vswitch is the maximum switching voltage, and PD is the maximum power dissipation.