ET
238B

Four Layer semiconductor Devices
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Another name for “Four Layer semiconductor Devices”
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Some common types of Thyristers
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Diac A two-terminal four-layer semiconductor device (thyristor) that can conduct current in either direction when properly activated.
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Silicon-controlled rectifier (SCR) A type of three-terminal thyristor that conducts current when triggered on by a voltage at the single gate terminal and remains on until the anode current falls below a specified value.
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Silicon-controlled switch (SCS) A type of four-terminal thyristor that has two gate terminals that are used to trigger the device on and off.

Four Layer semiconductor Devices
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Some common types of Thyristors
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Triac A three-terminal thyristor that can conduct current in either direction when properly activated.
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Light-activated silicon-controlled rectifier (LASCR) A four-layer semiconductor device (thyristor) that conducts current in one direction when activated by a sufficient amount of light and continues to conduct until the current falls below a specified value.
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Key values for thyristors
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Forced commutation A method of turning off an SCR.

Four Layer semiconductor Devices
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Key values for thyristors
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Forward-breakover voltage (VBR(F)) The voltage at which a device enters the forward blocking region.
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Holding current (IH) The value of the anode current below which a device switches from the forward-conduction region to the forward-blocking region.
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Switching current (Is) The value of the anode current at the point where the device switches from the forward-blocking region to the forward-conduction region.
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Trigger The activating input of some electronic devices and circuits.

Four Layer semiconductor Devices
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Four-layer pnpn device with three terminals: anode, cathode, and gate
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States of operation
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Off state
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Acts ideally as an almost open circuit between the anode and the cathode.
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Has a very high resistance.
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On state
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Has a small on (forward) resistance.

Four Layer semiconductor Devices
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The SCR operation can best be understood by thinking of its internal pnpn structure as a two transistor arrangement, as shown below.
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The upper pnp layers act as a transistor, Q1
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The lower npn layers act as a transistor, Q2.
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Notice that the two middle layers are "shared."

Four Layer semiconductor Devices
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Turning an SCR On
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Positive pulse (Trigger) applied to the gate when the Anode is more positive than the Cathode activates the SCR
• In the equivalent circuit
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I
B1 through Q
2
B2 turns on Q collector thus turning on Q
1
.
2
, providing a path for

Four Layer semiconductor Devices
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Turning an SCR On
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In the equivalent circuit
• The device stays on (latches) once it is triggered
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Collector of Q
1
. provides additional base current for Q
2 stays in conduction after the trigger pulse is removed
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Turning Off a SCR so that it
• • Anode current interruption
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Forced commutation
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Requires sending enough current through the SCR in the opposite to the forward conduction to reduce forward current below I
H
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Characteristics and Ratings
• Holding current, I
H
This is the value of anode current below which the SCR switches from the forward-conduction region to the forward-blocking region. Value increases with decreasing I maximum for I
G
=0.
G and is

Four Layer semiconductor Devices
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Characteristics and Ratings
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Forward-breakover voltage, VBR(F) This is the voltage at which the
SCR enters the forward-conduction region. The value of VBR(F) is maximum when IG = 0 and is designated VBR(F0). When the gate current is increased, V BR(F) decreases and is designated VBR(F1),
VBR(F2), and so on, for increasing steps in gate current (IG1, IG2, and so on).

Four Layer semiconductor Devices
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Some applications
Power supply overvoltage protection
Power control in a Bridge Rectifier
Note: Trigger circuits not shown

Four Layer semiconductor Devices
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Structure
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See Figures to the right
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Characteristics
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They don’t have gates to turn the devices on
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Turned on by exceeding the
Forward break over voltage for a direction
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They will conduct current in both directions
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Curve looks like two SCRs set for opposite directions without gates
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Turned off in a direction if the current through the device drops below I
H
.

Four Layer semiconductor Devices
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Characteristics
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Operation in an AC circuit

Four Layer semiconductor Devices
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Characteristics
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Like a DIAC with gate or back to back SCRs with a shared gate
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Can conduct current in either direction when triggered on
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Direction depends on the polarity of the voltage across its A1 and A2 terminals.
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Turing it it off
• when the anode current drops below the specified value of the holding current, IH.

Four Layer semiconductor Devices
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Sample circuits
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Because individual SCRs are more flexible to use in advanced control systems, they are more commonly seen in circuits like motor drives, while TRIACs are usually seen in simple, lowpower applications like household dimmer switches.
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A simple lamp dimmer circuit:
Image from:http://www.allaboutcircuits.com/vol_3/chpt_7/6.html