Thyristor Shawn Standfast. About Thyristors Thyristors can take many forms but they all have certain aspects in common Act as Solid-State Switches Become.

ThyristorShawn Standfast

About Thyristors

• Thyristors can take many forms but they all have certain aspects in common• Act as Solid-State Switches• Become low impedance current paths when triggered

• Remain on until input current drops below minimum threshold “holding” level• Once triggered, most devices remain “on” even if trigger signal is removed

SCR Structure• Most devices typically consist of four

alternating layers of P-type and N-type materials• Exact structure depends on the type of device• Simplest form is the Semiconductor Controlled

Rectifier (SCR)

• Current flows into one leg of the device (shown here as base terminal for the NPN device) and is amplified by the gain of the transistors

Thyristor Theory and Design Considerations, On-Semiconductor ©2005

About Thyristors• SCR space charge regions

prior to reverse break-down• Junctions 1 and 3 are

forward biased while junction 2 is reverse biased

• As Avalanche breakdown begins of the J2 junction, the n1 region becomes more negative which increases V1 & V3

• Device is capable of blocking voltages up to the reverse breakdown voltage of the J2 P-N junction• Typical voltage ratings

can vary between 50V to over 1KV Semiconductor Physics and Devices, Neamen, (c) 2003

About Thyristors• SCR device in the low-

impedance, “ON” state• Common Base current

gains α1 and α2 have increased to unity gain and regeneration occurs

• Both transistors are driven into saturation

• Device “Latches” into the on state

Semiconductor Physics and Devices, Neamen, (c) 2003

I-V curve

Thyristor Theory and Design Considerations, On-Semiconductor ©2005

Increasing the gate current results in lower reverse voltage required to enter breakdown

Typically device turn-off requires removal of external current source.

Thyristor Limitations

• dI/dT limits – Because these device switch rather slowly, turn-on results in large instantaneous power dissipation unevenly distributed in the device which can lead to failure

• dV/dT limits – because of the internal capacitance of the J2 junction, fast voltage changes can lead to large collector currents, initiating device turn-on

• Noise sensitivity• Thermal stability

http://en.wikipedia.org/wiki/File:Triac_structure.svg

TRIAC’s

• TRIAC – Triode for Alternating Current• Two SCR’s in Anti-Parallel• Creates a four-Quadrant Switch• Used to switch Bi-Polar signals

http://en.wikipedia.org/wiki/File:TRIAC_Equivalent_Circuit.png

About Triacs• In quadrants 1 and 2, MT2 is positive, and

current flows from MT2 to MT1 through P, N, P and N layers. The N region attached to MT2 does not participate significantly.

• In quadrants 3 and 4, MT2 is negative, and current flows from MT1 to MT2, also through P, N, P and N layers. The N region attached to MT2 is active, but the N region attached to MT1 only participates in the initial triggering, not the bulk current flow.

In most applications, the gate current comes from MT2, so quadrants 1 and 3 are the only operating modes.

Typical Applications of SCR’s and Traics• Over-Voltage protection• “Crow-bar” protection devices• Synchronous rectification• Power Control

Other types of Thyristor Devices

• BCT — Bidirectional Control Thyristor — A bidirectional switching device containing two thyristor structures with separate gate contacts• BOD — Breakover Diode — A

gateless thyristor triggered by avalanche current• DIAC — Bidirectional trigger device• GTO — Gate Turn-Off thyristor• ETO — Emitter Turn-Off Thyristor[9]

LASCR — Light-activated SCR, or LTT — light-triggered thyristorMOS-controlled thyristor (MCT) — MOSFET Controlled Thyristor — It contains two additional FET structures for on/off control.BRT — Base Resistance Controlled Thyristor