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Other Electronic Devices

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Contents

• Field-Effect Transistors(FETs)

- JFETs

- MOSFETs

• Insulate Gate Bipolar Transistors(IGBTs)

• H-bridge driver and PWM

• Silicon-Controlled Rectifiers(SCRs)

• TRIACs

• Device Selection

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• “Field-effect” relates to the depletion region formed in the channel of a FET

• Voltage-controlled devices

• Voltage between Gate and Source(VGS) control the current through the device

FETs Main Features

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• Very high input impedance (on the order of 100 MΩ or more)

• A high degree of isolation between input and output.

• Typically produces less noise than a bipolar junction transistor (BJT), and is thus found in noise sensitive electronics such as tuners and low-noise amplifiers for VHF and satellite receivers

• Preferred device in low-voltage switching application

• Faster than BJTs

• Better thermal stability than a BJT

FETs Advantages and disadvantages

• Low gain-bandwidth product compared to a BJT

• Susceptible to overload voltages

• Vulnerable to electrostatic damage

• Low current capability

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MOSFETs Features

• MOSFETs(Metal oxide semiconductor field-effect transistor)

• Gate of MOSFET insulated from the channel by SiO2

• 2 Types: Depletion(D) and Enhancement(E)

1.Depletion Type –the transistor requires the Gate-Source voltage, ( VGS)

to switch the device “OFF”. The depletion mode MOSFET is equivalent to a

“Normally Closed” switch.

2.Enhancement Type –the transistor requires a Gate-Source voltage,( VGS)

to switch the device “ON”. The enhancement mode MOSFET is equivalent

to a “Normally Open” switch.

Note. Enhancement MOSFET is more widely used.

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MOSFET Symbol

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MOSFETs Bias

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Enhancement-mode N-Channel MOSFET Amplifier

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Load connection

A comparison between an NPN transistor and N-channel MOSFET

A comparison between an PNP transistor and P-channel MOSFET

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MOSFETs Summary

The Metal Oxide Semiconductor Field Effect Transistor, or MOSFET for short,

has an extremely high input gate resistance with the current flowing through

the channel between the source and drain being controlled by the gate

voltage. Because of this high input impedance and gain, MOSFETs can be

easily damaged by static electricity if not carefully protected or handled.

MOSFET’s are ideal for use as electronic switches or as common-source

amplifiers as their power consumption is very small. Typical applications for

metal oxide semiconductor field effect transistors are in Microprocessors,

Memories, Calculators and Logic CMOS Gates etc.

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Power Transistor

• Its characteristic is as normal BJT but with higher power rating.

• It is a current controlled switching devices.

• VCE (on-state) is varied depending on current and voltage rating

(a) NPN and PNP

(b) Switching circuit

(c) CharacteristicTO-3 TO-220 TO-92

Power MOSFETs

-Operating as similar as

tradition MOSFET but with

higher power rating.

-High operating frequency

i.e. normally >20kHz

-Switching ON/OFF by

voltage signal; i.e. ±15-20V

IGBTs(Insulated Gate Bipolar Transistor)

• Its operation is similar

to BJT but unlike BJT,

IGBT requires voltage

signal for switching. It

can be considered as

an advantage

combination between

BJT and MOSFET.

• High current capability

• Medium operating

frequency i.e.

normally 5-20 kHz

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Power MOSFETs vs IGBTs

• Power MOSFETs: Higher switching

frequency but low current

• IGBT:Not too high switching frequency but

higher current capability

• On-stage voltage(i.e. conduction or forward)

of IGBT is lower.

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H-Bridge Driver

Reference: http://blog.solutions-cubed.com/wp-content/uploads/2012/06/h-bridge.png

http://www.bristolwatch.com/ele/h_bridge.htm

Truth Table

H Bridge Driver

• L293 is an integrated circuit motor driver that can be used for simultaneous, bi-directional control of

two small motors. The L293 is limited to 600mA to around 1A. L293D has built in free-wheeling

diodes to minimize inductive voltage spikes occurred by switching device and inductive load.

H Bridge Driver

Note. L298 has no free-wheeling diodes.

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H-Bridge Driver

Reference: http://www.bristolwatch.com/ele/h_bridge.htm

Parts list:

Q2, Q4, P-channel MOSFET IRF9630

Q1, Q3, N-channel MOSFET IRF630

Q5, Q6, 2N2222A NPN bipolar transistor.

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H-Bridge Driver

Reference: http://www.bristolwatch.com/ele/h_bridge.htm

Parts list:

Q2, Q4, P-channel MOSFET IRF9630

Q1, Q3, N-channel MOSFET IRF630

Q5, Q6, 2N2222A NPN bipolar transistor.

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H-Bridge Driver

Reference: http://www.bristolwatch.com/ele/h_bridge.htm

Parts list:

Q2, Q4, P-channel MOSFET IRF9630

Q1, Q3, N-channel MOSFET IRF630

Q5, Q6, 2N2222A NPN bipolar transistor.

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PWM

• PWM(Pulse Width Modulation)Vary the output to the load

T

t

tt

tCycleDuty

on

offon

on

Pulse Width

ton

T

VIN

PWM

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PWM

DC output AC output

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Example

From a given circuit if a PWM signal is a frequency of 10 kHz and 25%

duty cycle draw a voltage waveform across RL and also calculate an average voltage. Assume the switch is ideal.

+ 5 V

PWM

RL

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Deadtime Issue

• Dead time is required to avoid “shoot through” of inverter leg.

• Both switching devices in the same inverter leg cannot ON the same.

• Deadtime depends on turn-on and turn-off times of the devices and also load current.

• Deadtime can be generated either by hardware(RC circuit) or via software.

S1

S4

Gate Signal

Td(dead time)

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SCRs(Silicon-Controlled Rectifiers)

A

GK

G

A

K

Complementary latch: when SCRs stay on, stay on

all by itself and when it stays off, it stays off by itself.

Note. It's called a complementary latch since it's usually made from a

pair of transistors that are said to compliment one another.

• A silicon-controlled rectifier (or semiconductor-controlled rectifier) is a 4-layer solid state device that controls current flow.

• SCRs are used in power switching converter, phase control, dimmer,

chopper, battery chargers, and inverter circuits.

TO-220

STUD

TO-5,TO-92

Capsule

(Disk)

DO-xxx

TO-93

TO-65

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SCR waveform

R load

AC

220 Vrms

50 HzGate

Triggering

Circuit

A

GK

Vload

VAK

IGTGATE CURRENT

(Triggering signal)

Small forward voltage

(on-state voltage)

Firing angle

(delay angle) Conduction angle

i.e. (180-135)=45o

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Output Voltage

Vload

VSCR

)cos1(2

2

)()sin(22

1

RMS

RMSload

V

tdtVV

Output voltage can be calculated by integrating the output

equation(average area in one period)

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Basic Gate Triggering Circuits

• Resistor Circuit: This is the simplest firing circuit shown in Figure below. AC source and voltage are in-phase. As a result, the firing angle can be maximum as 90o. The firing angle can be adjusted via R2. R1 is to prevent over-current when R2 is zero.

Load

R1

R2AC

Source A

GK

Gate triggering

circuit

iG

Fig. 1

VAK

IGT

iG

VAK

IGT

iG

R2 is small

R2 is large

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Exercise

From Figure 1, if AC source =220Vrms

50Hz,R1=10kΩ,IG= 15 mA, 1. what value of R2

to trig SCR at 45 degree? (Neglect 0.7V drop

across Gate-Cathode) 2. Draw the voltage

across SCR and load if the delay angle is 45

degree. iG

AC SOURCE

HW: Calculate the load average voltage when SCR is trigged at 45 degree. Neglect the SCR on-state voltage.

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TRIACs

A2

G

A1

• A TRIAC, or TRIode for Alternating Current is an electronic component approximately equivalent to two silicon-controlled rectifiers (SCRs) joined in inverse parallel.

• Bidirectional switch can conduct current in either direction when it is triggered. It can be triggered by either a positive or a negative voltage being applied to its gate.(very suitable to switch ac circuit).Once triggered, it continues to conduct until the current through it drops below holding current, naturally at the end of a half-cycle of the main supply.

• TRIACs are used in ac system applications such as light dimmers, speed controls of electric fans and electric motors.

A1

A2

G=

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TRIAC Waveforms

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TRIAC Applications

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Applications

Dimmer Circuit

Heater Control Circuit

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Device Selection

• Following are common consideration when selecting

devices.

- Power Rating i.e. maximum current and voltage

- Switching Frequency( How fast the circuit requires

determine control dynamic and switching loss)

- Gate Drive Requirement i.e. simple or bipolar

- Breakdown Voltage (circuit connection)

- Power Loss(when efficiency is critical) i.e. on-stage voltage

or resistance