EE‐321 N

Lecture‐7Gate Turn OFF (GTO) Thyristor

Introduction

• The conventional SCR is the ruling device forthe power electronics industry due to veryhigh ratings

• However, it is a semi‐controlled devicebecause it can’t be turned OFF by the gateterminal

• GTO achieves this objective of being turnedOFF by –ve gate current and thus is a fullycontrolled switch

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Contd...

• Two configurations available: asymmetric andsymmetric

• Available ratings up to : 6 kV, 6 kA (Mitsubishi)

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Popular

Device Symbol

Structure

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Contd...

• Turn OFF capability by supplying –ve IG can beguaranteed if the access resistance from thegate to all parts of the device cathode is verylow

• This is done by using an “interdigitated”structure in which the gate‐cathode area isdistributed over the whole pellet andinterleaving their respective regions

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Interdigitized Structure(Front View)

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Interdigitized Structure(Top View)

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Cathode

Gate

Appearance

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Characteristics

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Real Ideal

Turn OFF Mechanism 

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Remember ?

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Analysis

• Above circuit was used to explain the turn ONmechanism of SCR

• Now we use this to understand how turn‐OFFis achieved

• Both transistors are in saturation• A –ve gate current can bring T2 out ofsaturation and eliminate this “regenerative”effect thereby turning OFF the device

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Contd...

• The anode current is given by:

• Suppose, we neglect ICBO’s then IA = 0 means that 

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)(1 21

212

ααα

+−++

= CBOCBOGA

IIII

( )22

1 2 1 21 ( ) 1 ( )GG

A

IIIαα

α α α α−−

= =− + − +

Contd...

• Turn‐OFF gain βOFF for the turn‐OFF process ofa GTO is defined as:

• βOFF lies in the range 4‐5 (low) but this calls fora very high –IG. E. g. if IA = 2000 A then –IG =500 A (around)

• A high value of βOFF can be obtained bymaking α2 high and α1 low

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( )2

OFF1 2 1

A

G

II

αβα α

= =+ −

Comparison with Conventional SCR

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Advantages DisadvantagesElimination of commutatingcomponents in forced commutationresulting in reduction in cost, weightand volume.

ON state voltage drop and associatedlosses are higher in GTO thanthyristor

Reduction in acoustic and electromagnetic noise due to theelimination of commutation chokes

Triggering gate current required forGTOs is more than those of thyristor

Faster turn OFF permitting highswitching frequency

Latching and holding current is morein GTO than those of thyristor

Improved converters efficiency Gate drive circuit loss is more thanthose of thyristor, and,

It has more di/dt rating at turn ON Its reverse voltage block capability isless than its forward blockingcapability. (But this is no disadvantagefor inverters/choppers)

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HOME ASSIGNMENT

Q1. Discuss the switching characteristics of SCR.Emphasize on the following terms: Delaytime, rise time, spread time, turn‐OFF time,circuit turn OFF time.

Q2. Discuss briefly other devices of the thyristorfamily. Highlight their circuit symbol,characteristics, ideal characteristics, mainapplications

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Reminder

• Internet connection is now available• Please complete the Today’s Task assigned in lecture‐2

• Will be checked randomly

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End of Unit‐1

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