Thyristors for Students

THYRISTORS

Engr. Mary Charlemaine Abas, MECE

Typical Packaging of THYRISTORS

Intended Learning Outcome • Analyze the characteristics, operations, and circuit implementations of Thyristors. Specifically the following: o o o o o o o

Shockley Diode SCR SCS DIAC TRIAC UJT PUT

o o o

Other solid-state devices used as Triggers Phase Control Other Control applications

What is THYRISTOR? • Constructed from 4 layers semiconductor diode • Essentially an ON-OFF switching device. • Constructed of 4 semiconductor layer (pnpn layer) represented by a pnp and npn transistor. • Used in high-powered switching applications to control very large amount of current. • Widely used as switches, speed controllers, or current controllers.

What is THYRISTOR? • Act as a LATCH. • Uses internal feedback to produce switching actions used for over voltage protection, motor controls, heaters, lighting systems, and other heavy-current loads. • Other names: SHOCKLEY DIODE, 4-LAYER DIODE, PNPN DIODE, SILICON UNILATERAL SWITCH (SUS)

Applications of Thyristor • • • • • • • • •

Light dimmers Power-switching circuit Relays Logic circuits Heaters Welders Battery chargers DC and AC motor speed controls Voltage regulators

SHOCKLEY DIODE • Constructed of 4 pnpn semiconductor layer.

SHOCKLEY DIODE

Basic Construction

Equivalent Circuit

Symbol

SHOCKLEY DIODE • pn junction 1 & 3 – base-emitter junctions – FORWARD BIASED • pn junction 2 – collector-base junction – REVERSED BIASED. • Both transistor in LINEAR region. • At low-bias levels, there is very little anode current, and it is in the OFF state or forward-blocking region.

Basic Operation of SHOCKLEY DIODE • FB: – open switch at Forward Blocking region ( very high forward Resistance) = OFF state where VAK = 0 V up to a value of FORWARD-BREAKOVER VOLTAGE VBR(F).

Basic Operation of SHOCKLEY DIODE • FB: – As IA = IS, VAK = VBR(F) . – VAK drops and diode enters to Forward Conduction region – closed switch or ON state as long as IA > IH.

SHOCKLEY DIODE • HOLDING CURRENT (IH) – Minimum forward current required to maintain conduction.

• SWITCHING CURRENT (IS) – Value of the anode current at the point where the device switches from forward-blocking (OFF) to forward-conduction region (ON). – Always less than the IH.

SHOCKLEY DIODE • Once IA drops below IH until IS, the device is driven back into cutoff. • When VAK reaches forward breakover voltage: The device current IA rapidly increase as the device is driven into saturation.

Breakover-refers to the way a thyristor breaks down and then immediately goes into saturation. (increasing VCC breaks down either Q1 or Q2 and drives both transistors into saturation.

A certain diode is biased in the forward-blocking region with an anode-to-cathode voltage of 20V. Under this bias condition, the anode current is 1µA. Determine the resistance of the diode in the forward-blocking region.

Determine the value of anode current when the device is on. VBR(F) = 10V. Assume the forward voltage drop is 0.9V.

SILICON-CONTROLLED RECTIFIER

SILICON-CONTROLLED RECTIFIER • Sometimes called as Semiconductor Controlled Rectifier. • It is a solid state four layer pnpn device that controls current flow. • Similar to Shockley diode except with three terminals: anode, cathode, and gate. • A most widely known switching device used for high voltage and high current operations.

SILICON-CONTROLLED RECTIFIER • Has two possible operation • OFF state = very high resistance = OPEN CIRCUIT= Forward Blocking Mode • ON state = very small resistance = SHORT CIRCUIT = Forward Conducting Mode • Reverse Blocking Mode

SCR Equivalent Circuit

BASIC CONSTRUCTION

EQUIVALENT CIRCUIT

SCR Symbol

The gate current controls the forward breakover voltage. Once SCR turns on, the gate losses all control.

Operation of SCR • To turn the SCR “ON”: 1. Forward bias the anode-cathode 2. Apply sufficient gate voltage and gate current

How to turn on SCR using Gate Triggering • IG = 0, OFF STATE , OPEN SWITCH, VERY HIGH RESISTANCE • IG = TRIGGERED, ON STATE, CLOSED SWITCH. • Once IG removed, ON STATE , CLOSED SWITCH.

How to turn on SCR without using Gate Triggering

• Increasing VAK > VBR(F), ON STATE, CLOSED SWITCH.

How to turn on SCR without using Gate Triggering

• VBR(F) decreases, IG increases above 0V. • The gate current controls the value of VBR(F).

How to turn “forced OFF” SCR • ANODE CURRENT INTERRUPTION – Using series or parallel (shunt) arrangement – Series reduced the anode current to zero – Parallel (shunt) routes part of the total current away from SCR, reducing anode current below holding current.

• FORCED COMMUTATION – Forcing current in the direction opposite to the forward conduction, reducing anode current below holding current.

How to turn “forced OFF” SCR • FORCED COMMUTATION – Commutation circuitry – a switching device connected in parallel with the SCR. – A control signal activates the switching circuitry and provides a low impedance bypass for the anode to cathode current. This momentary loss of current through the SCR will turn it off. – The switching circuitry can also apply a reverse bias voltage across the SCR, which also will turn the SCR off.

ANODE CURRENT INTERRUPTION

FORCED COMMUTATION

SCR CHARACTERISTICS AND RATINGS • FORWARD-BREAKOVER VOLTAGE – At maximum when gate current is zero. – When gate current increases, forward-breakover voltage decreases.

• HOLDING CURRENT – Value of anode current from forward-conduction region to forward-blocking region. – Increases as gate current decreases. – Maximum at gate current zero.

SCR CHARACTERISTICS AND RATINGS • GATE TRIGGER CURRENT – Value necessary to switch the SCR from the forward-blocking region to forward-conduction region.

• FORWARD AND REVERSE BLOCKING REGIONS – Regions corresponding to the open circuit condition for the controlled rectifier which block the flow of charge (current) from anode to cathode.

• REVERSE BREAKDOWN VOLTAGE – Maximum reverse bias voltage for the SCR. It is equivalent to the Zener or avalanche region of the fundamental two-layer semiconductor diode.

SCR APPLICATION

SILICON-CONTROLLED SWITCH

SILICON-CONTROLLED SWITCH • Similar to SCR • Has two gate terminals, cathode and anode gate • Four-terminal thyristor used to trigger the device ON and OFF. • FASTER turn-off time than SCR. • Used in counters, registers, and timing circuits.

How to turn on SCS • A POSITIVE pulse applied on the CATHODE gate or a NEGATIVE pulse applied on the ANODE gate.

How to turn off SCS • A POSITIVE pulse applied on the ANODE gate or a NEGATIVE pulse applied on the CATHODE gate.

How to turn off SCS • Reducing the anode current below the holding current by using BJT as a switch to interrupt anode current.

DIAC

DIAC Internal Constructions • Two-terminal four-layer thyristor which conduct in EITHER direction when properly activated. • RIGHT SIDE: PNPN • LEFT SIDE: NPNP • Require breakover voltage to initiate conduction with either polarity is across the 2 terminals.

DIAC Internal Constructions • Neither terminal is referred to as CATHODE. • Contains 2 anodes, anode 1 (electrode 1) and anode 2 (electrode 2) • When anode 1 is positive, the applicable layers are p1 n2 p2 and n3. • When anode 2 is positive, the applicable layers are p2n2p1 and n1.

DIAC Equivalent Circuit and Basic Operation From A1 to A2: • Q1 & Q2 forward-biased • Q3 & Q4 reversed-biased • Operate on the upper right portion of the characteristic curve.

DIAC Characteristic Curve

DIAC Equivalent Circuit and Basic Operation From A2 to A1: • Q3 & Q4 forward-biased • Q1 & Q2 reversed-biased • Operate on the lower right portion of the characteristic curve.

DIAC Characteristic Curve

DIAC Equivalent Circuit

DIAC Applications Trigger circuit for the Triac Proximity Sensor circuit

TRIAC

TRIAC and its Basic Construction • A bi-directional thyristor used to control the power in AC circuits. • A Diac with a gate control or two SCRs in parallel and in opposite directions with a common gate terminal. • Has two leads designated MT1 and MT2 or A1 and A2.

TRIAC • Has a gate lead which is used to control its conduction, which can be turned on by a pulse of gate current and does not require the breakover voltage to initiate conduction.

TRIAC Characteristic Curve

TRIAC Characteristic Curve Current in direction depending on the polarity across the terminal.

It turn OFF when the current drop sufficient low level. Breakover potential decrease as the gate current increase.

TRIAC Basic Operaton

Terminal A1 is biased positive with respect to A2.

TRIAC

Terminal A2 is biased positive with respect to A1.

UNIJUNCTION TRANSISTOR

UJT • A three-terminal semiconductor device that has only one pn junction. • A breakover type switching device whose characteristics make it useful in timers, oscillators, waveform generators, and gate control circuits for SCRs and TRIACs. • Two base lead B1 and B2 and an emitter E lead. • Interbase resistance, RBB of a UJT is the resistance of its n-type silicon bar.

UJT 𝑅𝐵1 𝑅𝐵1 𝑅𝐵2

• The ratio is called the INTRINSIC STANDOFF + RATIO, designated as η (eta). • Used with SCRs and Triacs to control their conduction angle.

Basic Construction of UJT

Equivalent Circuit of UJT

UJT • Vpn –barrier potential of the pn junction • VP = ηVBB + Vpn where VP is the peak-point voltage • After turn-on, the UJT operates in a negative

• • • •

resistance region up to a certain value of IE. At peak-point, VE = VP and IE = IP. Then, VE decreases as IE continues to increase, thus producing the negative resistance characteristic. At valley point, VE = VV and IE = IV. Beyond the valley point , the device is in saturation, and VE increases very little with an increasing IE.

UJT

UJT • All UJT circuits, the burst of current from E to B1 is short-lived, and the UJT quickly reverts back to the OFF condition.

Programmable UJT (PUT or PUJT)

PUT or PUJT • A four-layer pnpn device with a gate connected directly to the sandwiched n type layer. • Unlike in UJT, RBB, η, and VP can be controlled through RB1 and RB2 (external to the device). • type of three-terminal thyristor that is triggered into conduction when the voltage at the anode exceeds the voltage at the gate.

PUT or PUJT • The gate is connected to the n region adjacent to the anode. • This pn junction controls the on and off states of the device. • The gate is always biased positive with respect to the cathode. • When the anode voltage exceeds the gate voltage by approximately 0.7 V, the pn junction is forward biased and the PUT turns on. • The PUT stays on until the anode voltage falls back below this level, then the PUT turns off.

PUT or PUJT

PUT or PUJT circuit

PUT or PUJT characteristic curve

Problem Solving

SCR Calculate the value of anode current when the Shockley diode is on, VBR(F) = 9 V. The forward voltage drop of the 4layer diode is assumed to be 0.9 V.

Answer: 21.1mA

SCR • Determine the trigger voltage and trigger current when SW1 is CLOSED. • Assume: VAK = 0.2V VGK = 0.7V IH= 5mA • Will the SCR turn on if VA is reduced to 12V?

SCR: Operation of Phase-control circuit

• At 180 degrees conduction.

SCR: Operation of Phase-control circuit

• Conduction angle – number of degrees of an AC cycle during which the SCR is turn on. • Firing Delay angle – number of degrees that elapse before the SCR is turned on. • Total cycle time is 360 degrees. • Conduction Angle + Firing Delay Angle = 180 degrees

SCR: Operation of Phase-control circuit

• At 90 degrees conduction.

SCR: Operation of Phase-control circuit

• At 135 degrees conduction.

UJT • The datasheet of a certain UJT gives η = 0.6. Determine the peak-point emitter voltage VP if VBB = 20V.

• How can the peak-point emitter voltage of a UJT be increased? – By increasing the VBB.

UJT • Determine a value of R1 that will ensure proper turn-on and turnoff of the UJT. The characteristic of UJT are as follows: η=0.5 VV = 1 V IV = 10 mA IP = 20 μA VP = 14 V

COURSEWORK 1.B