Lecture 3 Diodes & Applications :Outline

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Dorcas Welch
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Lecture 3 Diodes & Applications :Outline Introduction Diode

Rectifiers Diode application: Voltage multipliers Diode

words Di + Ode (odos means path or way) A device that

1 Lecture 3 Diodes & Applications :Outline Introduction Diode biasing Diode model Testing a diode Diode application: Rectifiers Diode application: Voltage multipliers Diode application: Optoelectronics 1 Diodes: Symbol From Greek words Di + Ode (odos means path or way) A device that conducts current in only one direction. Diode has 2 terminals : Anode and Cathode 2 1

Diodes: Early Diodes Early diodes were made

firstly used in radio a called Crystal Set This

contact between the wire and crystal Users have

Then we have vacuum tube diodes and metal oxide

2 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 3 Diodes: Revolution Then we have vacuum tube diodes and metal oxide diodes Today, most diodes are made from semiconductors 4 2

3 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 5 Diodes : V I Curve of Ideal Diode Resistors Change direction of applied bias voltage, change current flow direction. Diode Change direction of bias, current flows or stops flowing, depending on bias. I ON V OFF R 1 > R 2 > R 3 6 3

4 Diodes : Model of Ideal Diode 7 Diodes : Practical Model 8 4

5 Diodes : Complete Model 9 Diodes : Shockley Diode Equation where I I S V D V T n is the diode current, is the reverse bias saturation current (or scale current), is the voltage across the diode, d is the thermal voltage( K), and is the ideality factor (n =1 for Ge and n = 2 for Si) 10 5

6 Diodes : Complete Model (cont.) 11 Diodes : Diode testing (Pass) 12 6

7 Diodes : Diode Testing (Fail) Open circuit Short circuit 13 Diodes : Bias Summary 14 7

8 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 Full Wave Rectifiers Filter and Ripple Voltage Voltage Regulators 15 Diodes Application: Rectifiers 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 16 8

9 Diode Application : DC Power Supply DC power supply composes of rectifier, filter and regulator 17 Diode Application: Half wave rectifier Mode I First half of cycle (0,T/2) Mode II Second half of cycle (T/2,T) T) Simplest form of rectifiers 18 9

10 Diode Application: Ideal Diode Forward Bias, diode is ON. Reverse Bias, diode is OFF Diode is a switch. 19 Diode Application: Average Voltage 20 10

11 Diode Application: Effect of Forward Voltage The effect of forward voltage drop V F can be neglected if V m is much higher than V F 21 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 ) 22 11

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

12 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. 23 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 24 12

13 Diode Application: Center Tapped FW Requires center tapped transformer 25 Diode Application : Center Tapped FW Mode I Mode II 26 13

14 Diode Application: PIV Rating 27 Diode Application : Bridge Rectifier Diodes in diagonal positions operate togetherth D 1 & D 2 ON during positive half cycle D 3 & D 4 ON during negative half cycle 28 14

15 Diode Application: Bridge Rectifier Mode I Mode II 29 Diode Application: PIV of Bridge Rectifier 30 15

16 Diode Application: Filters Filter is required to reduce voltage fluctuation 31 Diode Application: Capacitor Input Filter 32 16

17 Diode Application: Effect of Time Constant τ Medium τ Large τ We recall that the time constant τ is equal to R x C 33 Diode Application : HW vs. FW 34 17

18 Diode Application: Ripple Approximation Small ripple approximation V p(rect) is unfiltered peak rectified voltage Where f = f L for HW rectifiers & f = 2f L for FW rectifiers 35 Diode Application: Surge Current Worst cast occurs at first half cycle whereas the charge in capacitor is empty 36 18

19 Diode Application: Surge Current Limiter NTC Thermistors (temperature dependent resistance) with NTC are used to limit the surge current. 37 Diode Application: Voltage Regulator A voltage regulator is connected to the output of a filtered rectifier and maintains a constant output voltage (or current) despite changes in the input, the load current, or the temperature 38 19

20 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 39 Diode Application: Percent Regulation These are figure of merit, used to specify the performance of voltage regulator V NL is no load voltage, V FL is full load Voltage 40 20

21 Another Example of Rectifier: Alternator Polyphase rectifier used in car s alternator 6 power diodes mounted to the case that act as heat sink 41 Alternator (cont.) AC voltage from 3 phase voltage generator is converted to DC voltage with small ripple voltage 42 21

22 Voltage Doublers Half Wave Voltage Doubler 43 Voltage Doublers (cont.) Full Wave Voltage Doubler 44 22

23 Voltage Tripler and Quadrupler Tripler Quadrupler 45 Voltage Multiplier Applications Cockcroft Walton voltage multiplier provides 5 8 kvdc to generate ozone (O 3 ) 46 23

24 Voltage Multiplier Applications (cont.) Used ( to produce high voltage pulses) in bug killers 47 Optoelectronics: LED Application Text Display and RGB Display for Big Screen TV (V F of LEDs are range from 1.6 to 3 V) 48 24

25 LED Applications (cont.) Traffic light Solid State Lighting 49 IR Emitting Diode Applications Infrared Emitting Diode & Photo Diode are used in remote control units for TV sets

26 Diode Applications: Special Purpose Diodes There are diodes specially designed for using in reverse bias. Zener diodes Varacter didoes (or Varicap diodes) Photo diodes Semiconductor devices Hall effect sensors Thermisters Thermoelectric Modules 51 26

Chapter 2. Diodes & Applications

Chapter 2. Diodes & Applications Chapter 2 Diodes & Applications The Diode A diode is made from a small piece of semiconductor material, usually silicon, in which half is doped as a p region and half is doped as an n region with a pn