Power control devices

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Power control devices

• Silicon Controlled Rectifier

• Triac

• Diac

• GTO

• BJT

• MOSFET

Can be turned off while anode current is still flowingCan be turned off while anode current is still flowing

Once they are triggered into conduction, they can only be stopped by turning off current external to the device.

Once they are triggered into conduction, they can only be stopped by turning off current external to the device.

ThyristorsThyristors

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• SCRs can switch high currents from about 1A – 2000A

• SCRs turn ON when the +ve potential is applied to the gate terminal provided the anode is +ve with respect to the cathode.

• An SCR can only turn off if the anode current is reduced below its holding value with the gate open.

Power devices - SCRs

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NE185 Power Control Systems

Silicon controlled rectifier (thyristor)

Small Currentto “trigger” SCR

Large CurrentLarge Current

Anode (A)

Cathode (K)

Gate (G)

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Note: The process of turning an SCR ON is called TRIGGERINGThe process of turning an SCR OFF is called COMMUTATION


Large Current

Anode (A)

Cathode (K)

Gate (G)



Anode (A)

Cathode (K)

Gate (G)

Anode (A)

Cathode (K)

Gate (G)

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Large Current

Anode (A)

Cathode (K)

Gate (G)



Anode (A)

Cathode (K)

Gate (G)

Anode (A)

Cathode (K)

Gate (G)

Note: The ONLY way to turn them off is to stop the current flowing.They must then be re-triggered on.

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Silicon controlled rectifiers

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Silicon controlled rectifier

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Gate

Anode

Cathode

N

P

N

P

N

P

N

P

P

N

Cathode

Anode

Gate

4-layer device

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Anode

Cathode

Gate

Q1

Q2

Load

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SCR control: phase control

Silicon Controlled Rectifier (Thyristor)

Current required for triggering SCRCurrent required for triggering SCR

Load CurrentLoad Current

A N

Load

A NA N

Load

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Load CurrentLoad Current

Lower resistance on potentiometer. Capacitor

charges quicker

A N

Load

A NA N

Load

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Load Current

Higher resistance on potentiometer. Capacitor

charges slower

A N

Load

A NA N

Load

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SCR controlSilicon Controlled Rectifier (Thyristor)

Control

Control

A N

Load

Control

Control

A N

Load

SCR’s only conduct in one direction. Above is one way top overcome this.

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T1

T2

GATE

TRIAC

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T2T1 GT2T1 G

SC151D

T2

T1GST4

500k

10k

120V

0.27µF

TRIAC Light Dimmer

Diac

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Phase control

+V

-V

180° 270° 360°90°

SCR1 conducts

SCR2 conducts

SCR1 conducts

SCR2 conducts

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Phase control

+V

-V

180° 270° 360°90°

SCR1 conducts

SCR2 conducts

SCR1 conducts

SCR2 conducts

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Phase control

+V

-V

180° 270° 360°90°

SCR1 conducts

SCR2 conducts

SCR1 conducts

SCR2 conducts

The main disadvantage of Phase Control (indeed SCR control) is RFI.

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-VA

IHOLDINGVBR(R)

IHOLDING VBR(F)

FORWARD

REVERSE

- IA

+IA

+VA

28-36V typical28-36V typical

Diac characteristic curves

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Gate turn-off thyristers (GTO’S)

• A gate turn-off switch, also known as a gate-controlled switch (GCS) or gate turn-off thyristor (GTO), is similar to an SCR but can be turned off by a negative signal on the gate terminal.GTOs generally handle much lower currents than SCRs

Anode

Cathode

Gate

Anode

Cathode

Gate

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Transistor• BJT Transistors • A transistor is a device which acts like a controlled valve. The

current flow permitted can be controlled. • The bipolar junction transistor (BJT) is a three-terminal

electronic valve - the output (collector) terminal current-voltage characteristics are controlled by the current injected into the input port (base). The BJT is a semiconductor device constructed from two pn junctions. There are two types of BJT: pnp and npn.

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Transistor

• A transistor may be thought of as an electronic tap able to control a large flow of electrons with only small variations of the 'handle'. The 'handle' in the case of a transistor is called the "base". The in and out 'pipes' are called the "emitter" and the "collector".Voltage changes at the base of the transistor result in changes to the flow of electricity through the transistor.

• A transistor can be thought of as a 'tap'.• Symbol for a Transistor

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Transistor operation

Emitter

Collector

Base

1. Bi-Junction Transistor (BJT)

Collector

Emitter

Base

NPN

PNP

P

PN

N

NP

Transistors are three layer devices

P

PN

P

PN

N

NP

Transistors are three layer devices

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Emitter

Collector

Base

1. Bi-Junction Transistor (BJT)

Collector

Emitter

Base

NPN

PNP

Small currentSmall current

Large currentLarge current

Large currentLarge current

Small currentSmall current

BJT’s are current operated devices

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2. MOSFET Transistor

Drain

Gate

Source Source

Drain

Gate

Source

Drain

Gate

Source

Drain

Gate

N-ChannelEnhancement

Mode only

P-ChannelEnhancement

Mode only


/Depletion Mode


/Depletion Mode

Drain

Gate

Source

Drain

Gate

Source

GateGate

Source Source

Drain

Gate

Source

Drain

Gate

Source

Drain

Gate

Source

Drain

Gate

Source

Drain

Gate

Source

Drain

Gate


Mode only


Mode only


/Depletion Mode


/Depletion Mode

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2. MOSFET Transistor

Drain

Gate

Source

Negative

Positive

Negative

Positive

A positive voltage (around +5V) on the

gate turns the transistor fully on

In other words, they are VOLTAGE DRIVEN DEVICES.

Remember?… BJT’s are CURRENT DRIVEN DEVICES.

RDS off = MΩ

RDS on = <0.1Ω

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• Advantages of transistors over thyristors:– Lower voltage drop across transistors when

conducting (0.1V as compared to 2V) This causes less heat to be developed.

– Faster switching times – (less time to turn on = less heat developed while

turning on)– Can easily be turned off.

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Education

Power control devices