ene 210 211 class03 - kmuttwebstaff.kmutt.ac.th/~werapon.chi/m2_3/1_2015/ene_210_211_clas… ·...

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Lecture 3 ‐ Diodes & Applications : Outline

‐ Introduction

‐ Diode biasingDiode biasing

‐ Diode model

‐ Testing a diode

‐ Diode application: Rectifiers

‐ Diode application: Voltage multipliers

‐ Diode application: Optoelectronics

1KMUTT Radburi: Electronics Module

Diodes: Symbol

‐ From Greek words “Di”+ “Ode” (odos

means path or way)p y)

‐ A device that conducts current in only one

direction.

‐Diode has 2 terminals : Anode and Cathode


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Diodes: Early Diodes

‐ Early diodes were made from wires and crystal (galena‐ PbS)

‐ Diode is firstly used in radio a called “Crystal Set”

‐ This early diode property is not consistent, due to contact between the wire and crystal

‐ Users have to readjust the contact


Diodes: Revolution

‐ Then we have vacuum tube diodes and metal‐oxide diodes

‐ Today, most diodes are made from semiconductors


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Diodes : Biasing

‐ Bias refers to the use of a dc voltage to establish certain operating conditions for an electronic device

‐ There are two bias conditions

‐ Forward Bias and Reverse Bias


Diodes : V‐I Curve of Ideal Diode

ResistorsChange direction of applied bias voltage,

change current flow direction.

DiodeChange direction of bias, current flows or

stops flowing, depending on bias.

ON

V

I

6R1 > R2 > R3

OFF

KMUTT Radburi: Electronics Module

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Diodes : Model of Ideal Diode


Diodes : Practical Model


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Diodes : Complete Model


Diodes : Shockley Diode Equation

where

I is the diode current,

IS is the reverse bias saturation current (or scale current),

h l h d d

10

VD is the voltage across the diode,

VT is the thermal voltage(≈ 26 mV @ 300 K), and

n is the ideality factor (n =1 for Ge and n = 2 for Si)


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Diodes : Complete Model (cont.)


Diodes : Diode testing (Pass)


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Diodes : Diode Testing (Fail)

Short Open

13

circuitcircuit


Diodes : Bias Summary


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Diodes Applications: Rectifiers

All semiconductor PN junction devices need DC bias voltage to

function. Hence, all have rectifiers to convert AC to DC voltage

‐Half‐Wave Rectifiers‐Half‐Wave Rectifiers

‐ Full‐Wave Rectifiers

‐ Filter and Ripple Voltage

‐ Voltage Regulators


Diodes Application: Rectifiers

≈ 311 V

R tifi t lt i t d lt

311 V

‐ Rectifiers convert ac voltage into dc voltage

‐ Found in all dc power supplies operate from ac voltage source, i.e. your cell phone charger, rectifier unit in all computers


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Diode Application : DC Power Supply

‐ DC power supply composes of rectifier, filter and regulator


Diode Application: Half‐wave rectifier

Mode IFirst half of cycley

Mode IISecond half of cycle

(0,T/2)

(T/2 T)

‐ Simplest form of rectifiers

(T/2,T)


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Diode Application: Ideal Diode

‐ Forward Bias, diode is “ON” . Reverse Bias , diode is “OFF”

‐ Diode is a switch.


Diode Application: Average Voltage


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Diode Application: Effect of Forward Voltage

‐ The effect of forward voltage drop VF can be neglected if Vm

is much higher than VF


Diode Application: Peak Invers Voltage ‐PIV

‐ The peak inverse voltage (PIV) or peak reverse voltage (PRV)

rating of the diode is of primary importance in the design of rectification systems.(Normal avalance diodes have PIV range from

50 V to 1000 V )


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Diode Application : HW Rectifier with Transformer

•Transformer coupling provides two advantages

• First, it allows the source voltage to be stepped up or stepped down as needed

• Second, the ac source is electrically isolated from the rectifier,

thus preventing a shock hazard in the secondary circuit.


Diode Application: Full Wave Rectifiers

A full‐wave rectifier allows unidirectional (one‐way) current

through the load during the entire 360˚ of the input cycle,

whereas a half‐wave rectifier allow current through the load

only during one‐half of the cycle


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Diode Application: Center‐Tapped FW

‐ Requires center‐tapped transformer


Diode Application : Center‐Tapped FW

Mode IMode I

Mode II


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Diode Application: PIV Rating


Diode Application : Bridge Rectifier

Di d i di l iti t t th‐ Diodes in diagonal positions operate together

‐ D1 & D2 “ON” during positive half cycle‐ D3 & D4 “ON” during negative half cycle


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Diode Application: Bridge Rectifier

Mode I

Mode II


Diode Application: PIV of Bridge Rectifier


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Diode Application: Filters

‐ Filter is required to reduce voltage fluctuation


Diode Application: Capacitor‐Input Filter


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Diode Application: Effect of Time Constant τ

Medium τ

L

Medium τ

33

Large τ

We recall that the time constant τ is equal to R x C


Diode Application : HW vs. FW


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Diode Application: Ripple Approximation

• Small ripple approximation

• Vp(rect) is unfiltered peak rectified voltage

• Where f = fL for HW rectifiers &f = 2fL for FW rectifiers


Diode Application: Surge Current

• Worst cast occurs at first half cycle whereas the charge in capacitor is empty


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Diode Application: Surge Current Limiter

NTC

•Thermistors (temperature dependent resistance) with NTC are used to limit the surge current.


Diode Application: Voltage Regulator

• A voltage regulator is connected to the output of a g g p

filtered rectifier and maintains a constant output voltage

(or current) despite changes in the input, the load current,

or the temperature


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Diode Application : Basic Regulated Power Supply

• 3‐terminal regulator integrated circuit (IC) is

designed to produce fixed O/P voltage

• It requires only small output capacitor, usually 0.1 μF

to 1.0 μF


Diode Application: Percent Regulation

•These are figure of merit, used to specify the performance of voltage regulator

• VNL is no‐load voltage, VFL is full‐load Voltage


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Another Example of Rectifier: Alternator

Polyphase rectifier used in car’s alternator

• 6 power diodes mounted to the case that act as heat sink


Alternator (cont.)

• AC voltage from 3 phase voltage• AC voltage from 3‐phase voltage

generator is converted to DC voltage with small ripple voltage


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Voltage Doublers

• Half‐Wave Voltage Doubler• Half‐Wave Voltage Doubler


Voltage Doublers (cont.)

• Full‐Wave Voltage Doubler


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Voltage Tripler and Quadrupler

Tripler

Quadrupler


Voltage Multiplier Applications

‐ Cockcroft‐Walton voltage

multiplier provides 5 8 kVdc tomultiplier provides 5‐8 kVdc to generate ozone (O3)


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Voltage Multiplier Applications (cont.)

• Used ( to produce high voltage pulses) in bug killers


Optoelectronics: LED Application

• Text Display and RGB Display for Big Screen TV (VF of

LEDs are range from 1.6 to 3 V)


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LED Applications (cont.)

‐Traffic light‐ Solid State Lighting


IR Emitting Diode Applications

• Infrared Emitting Diode & Photo Diode are used in remote

control units for TV sets.


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Diode Applications: Special Purpose Diodes

There are diodes specially designed for using in reverse bias.

‐ Zener diodes

Semiconductor devices

‐ Varacter didoes (or Varicap diodes)

‐ Photo diodes

‐ Hall effect sensors

‐ Thermisters

‐ Thermoelectric Modules


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