lecture week6

1© Unitec New Zealand

Power Electronics

By : Anthony

Outline of this week presentation

• Review DC biasing analysis on FET• The family of thyristors• Tutorial questions• Assignment 1 due today

Learning Outcomes

• At the end of the lesson, students should be able to :• Explain the characteristic and operation of SCR• Explain the characteristic and operation of Diac• Explain the characteristic and operation of Triac

Power devices

The family of thyristor

Controllers

Triggers

Thyristors

4 layer unidirectional

5 layer bi-directional

4 layer unidirectional

Bi-directional

GTO(0.5A- 2000A, 100-200V)SCR(0.5A- 2000A, 100-1800V)SCS(1A, 100V)LASCR (1A, 200V)

Triac (0.5A- 200A, 100-1000V)

SUS(0.2A, 6-10V)PUT(1A, 50V)Shockley diode

SBS{5 layer}(0.2A, 6-10V)DIAC{4 layer}(2A, 28-36V)

The history of thyristor

Mercury arc rectifier Vacuum- tube rectifier

Thyratron

Invention of Thyristor

Application of fast-switching fully-controlled semiconductor

devices

Power diode Thyristor

GTO GTR

Power MOSFET Thyristor

(microprocessor)

IGBT Power MOSFET

Thyristor (DSP)

Pre-history

1st phase

2nd phase

3rd phase

late 1980s

mid 1970s

The Thyristor or SCR

• The thyristor is a four layer P-N-P-N device with a third terminal.

• A SCR is designed to handle large currents.• It may be latched by break over voltage/current or • by exceeding the critical rate of voltage rise between

anode and cathode.

Characteristic of SCR

• Is inherently a slow switching device compared to BJT or MOSFET.

• Used as a latching switch that can be turned on by the control terminal but cannot be turned off by the gate.

Different types of Thyristors

• Silicon Controlled Rectifier (SCR).• TRIAC.• DIAC.• Gate Turn-Off Thyristor (GTO).

V-I Characteristics

• + ve at anode• - ve at cathode• No conduction

• + ve at gate• - ve at cathode• Lower PN junction • Forward biased• acts like a closed switch

• By applying a small voltage between gate and cathode, the lower transistor will be forced on by the resulting base current, which will cause the upper transistor to conduct, which then supplies the lower transistor's base with current so that it no longer needs to be activated by a gate voltage.

Effects of gate current

Turn-on Characteristics

on d rt t t

• + ve at gate• + ve at anode• - ve at cathode• both PN junction forward biased• act like closed switches

• After conduction,• + ve remove at gate• + ve at anode• - ve at cathode• both PN junction forward biased• act like closed switches

• The necessary gate current to initiate latch-up will be much lower than the current through the SCR from cathode to anode

• SCR does achieve a measure of amplification, or more appropriately behaves as if it were a switch

• Using the gate to activate the SCR’s conduction is called triggering,

• and it is by far the most common way that SCRs are latched in actual practice.

• In practice, SCRs are usually chosen so that their break over voltage is far beyond the greatest voltage expected to be experienced from the power source,

• so that it can be turned on only by an intentional voltage pulse applied to the gate.

• Dropout is accomplished by reducing current until one or both internal transistors fall into cutoff mode

PB 1 to trigger on the SCRPB 2 to turn off the SCR

• Conduction

• Thermal Turn-on.• Light.• High Voltage.• Gate Current.• dv/dt.

Methods of Thyristor Turn-on

Anode currentbegins todecrease

Commutation didt

Recovery Recombination

t1 t2 t3 t4 t5

tr r tgr

tq=device off tim etc=circuit off tim e

Turn-off Characteristics

Applications of SCRs

• SCR’s are often used in power electronics applications for the control of AC Voltage.

• They are also used frequently in motor controllers. • Usually an AC voltage controller circuit for an SCR will

comprise of a switching method that will switch the SCR on partway through the cycle of an AC waveform, only delivering part of the voltage.

Application of S C R

• Circuit above only shows the gate connections for two out of the four SCRs. Pulse transformers and triggering sources for SCR1 and SCR3, as well as the details of the pulse sources themselves, have been omitted for the sake of simplicity.

• Controlled bridge rectifiers are not limited to single-phase designs.

• In most industrial control systems, AC power is available in three-phase form for maximum efficiency, and solid-state control circuits are built to take advantage of that. A three-phase controlled rectifier circuit built with SCRs, without pulse transformers or triggering circuitry shown, would look like this-

• A two terminal device• Conduct in either direction when a certain breakover

voltage is reached•

• AC repeatedly reverses direction, DIACs will not stay latched longer than one-half cycle.

• If a DIAC becomes latched, it will continue to conduct current only as long as there is voltage available to push enough current in that direction.

• When the AC polarity reverses, the DIAC will drop out due to insufficient current, necessitating another breakover before it conducts again.

Diac Operating Characteristics

Unstable region

• With the DIAC, that breakover voltage limit was a fixed quantity.

• With the SCR, we have control over exactly when the device becomes latched by triggering the gate at any point in time along the waveform.

• By connecting a suitable control circuit to the gate of an SCR, we can "chop" the sine wave at any point to allow for time-proportioned power control to a load.

Testing Diacs

• Diacs are thyristors without any gate terminal. They depend on the leakage current to switch them on once the voltage across the device exceeds their specified ratings. With an ohmmeter, they can be tested only for shorts. Resistance should be infinite in both directions.

• SCRs are unidirectional (one-way) current devices, making them useful for controlling DC only.

• If two SCRs are joined in back-to-back parallel, we have a new device known as the TRIAC

• Individual SCRs are more flexible to use in advanced control systems, they are more commonly seen in circuits like motor drives,

• TRIACs are usually seen in simple, low-power applications like household dimmer switches.

• A simple lamp dimmer circuit

light dimmer circuit

• TRIACs are notorious for not firing symmetrically.

• One way to make the TRIAC's current waveform more symmetrical is to use a device external to the TRIAC to time the triggering pulse.

• A DIAC placed in series with the gate does a fair job of this:

• DIAC breakover voltages tend to be much more symmetrical (the same in one polarity as the other) than TRIAC triggering voltage thresholds.

• The DIAC prevents any gate current until the triggering voltage has reached a certain, repeatable level in either direction, the firing point of the TRIAC from one half-cycle to the next tends to be more consistent,

• The waveform more symmetrical above and below its centerline.

• Practically all the characteristics and ratings of SCRs apply equally to TRIACs,

• Except that TRIACs of course are bidirectional (can handle current in both directions).

• Not much more needs to be said about this device except for an important caveat concerning its terminal designations.

• Main terminals 1 and 2 on a TRIAC are not interchangeable.

• To successfully trigger a TRIAC, gate current must come from the main terminal 2 (MT2) side of the circuit!

• Identification of the MT1 and MT2 terminals must be done via the TRIAC's part number with reference to a data sheet or book.

Triac Testing

• As a triac is two opposing SCRs in parallel, it is possible to test each triac individually using the SCR testers.

Simplified AC Power Control Circuit using a Triac

The Opto Triac

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