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EE 348: Lecture Supplement Notes SN2. Semiconductor Diodes: Concepts, Models, & Circuits 22 January 2001. Outline Of Lecture. Rectification Semiconductor Diode Circuit Schematic Symbol Simplified Volt-Ampere Characteristic Model Static Volt-Ampere Relationship

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ee 348 lecture supplement notes sn2

EE 348:Lecture Supplement Notes SN2

Semiconductor Diodes:

Concepts, Models, & Circuits

22 January 2001

outline of lecture
Outline Of Lecture
  • Rectification
  • Semiconductor Diode
    • Circuit Schematic Symbol
    • Simplified Volt-Ampere Characteristic
    • Model
      • Static Volt-Ampere Relationship
      • Time Domain Charge Control Model
  • Diode Circuits
    • Half Wave Rectifier
    • Full Wave Rectifier
    • Simple Limiter

J. Choma, Jr.

power supply system
Power Supply System
  • System
    • Voltage At “1” Has Given RMS Value And Zero Average Value
    • Voltage At “2” Has Non-Zero Average Value; It Is A Time-Varying, Harmonically Rich Half Or Full Wave Rectified Sinusoid
    • Lowpass Filter Attenuates Harmonics At “2” To Produce Constant, Time-Invariant Voltage At “3”
    • Regulator Produces A Very Small Output Resistance Seen Looking Back From “4”
  • Load
    • Effective Load Resistance Is VDC/IDC
    • Voltage Source Nature At “4” Produces Near Constant VDC, Regardless Of Current Value, IDC

J. Choma, Jr.

ac to dc conversion
AC To DC Conversion
  • Sinusoid Input:
  • Output:
  • Open Switch SW Whenever Vs < 0
  • Plot Assumes Vs = 110 VRMS & Rl = 3Rs

J. Choma, Jr.

average output voltage
Average Output Voltage
  • Average Value Calculation
  • Conversion Efficiency Problem

J. Choma, Jr.

semiconductor diode
Semiconductor Diode
  • Schematic Symbol
  • Volt-Ampere

Characteristic

Equation

  • Parametric Definitions
    • Qd(t) Excess Charge Stored In PN Junction
      • Qd(t)  0: Diode Is Forward Biased
      • Qd(t) < 0: Diode Is Reverse Biased Or Back Biased
    • t Storage Time Constant (nSec –to- pSec)
    • vd(t) Diode Voltage (Generally < 800 mV)
    • id(t) Diode Current (Value Depends On Junction Area)
    • Cj(vd) Junction Depletion Capacitance

J. Choma, Jr.

semiconductor diode models
Semiconductor Diode Models
  • Charge Function
  • Forward Bias
  • Reverse Bias

J. Choma, Jr.

diode at dc steady state
Diode At DC Steady State
  • Steady State
    • Input Voltage Is Constant
    • Capacitances Behave As Open Circuits
  • Forward Bias Current (VD 0)
  • Reverse Bias Current (VD < 0)

J. Choma, Jr.

diode dc v i characteristic
Diode DC V–I Characteristic

Is = 10 fA; T = 27 °C; n = 1

J. Choma, Jr.

piecewise linear approximation
Piecewise Linear Approximation
  • Two Segment Approximation
    • ID = 0 For VD V
    • ID – IQ = (VD – VQ)/rD For VD V
      • IQ Expected Quiescent, Or DC, Current Through Diode
      • VQ Corresponding Quiescent, Or DC, Diode Voltage
      • rD Incremental Diode Resistance At (IQ, VQ)
      • V Threshold Or Cut In Voltage Of Diode
  • Operation For Diode Voltage Above Threshold
    • Current
    • Slope
    • Threshold

J. Choma, Jr.

piecewise linear dc diode model
Piecewise Linear DC Diode Model
  • Model Parameters
    • Threshold Voltage, V, Generally Around 700 mV For Silicon
    • For Germanium Diodes, V Is Closer To 200 mV
    • Diode Resistance, rD, Generally Around A Few Ohms
  • Emulates Switch With Resistance And Offset
    • Switch Closed For VD V; Switch Open For VD< V
    • Generally rD Is Negligibly Small
    • For Large Applied Voltages, V Can Often Be Ignored

J. Choma, Jr.

half wave rectifier
Half Wave Rectifier
  • Reverse Bias
  • Forward Bias

J. Choma, Jr.

filtered half wave rectifier
Filtered Half Wave Rectifier
  • Load Resistance, Rl, Is Ratio Of Desired DC Output Voltage –To– Desired DC Output Current
  • Diode Conducts (Vs Vo + V)
    • Capacitor Charges With Time Constant, [Rl||(rD + Rs)]Cl
    • For Small Time Constant, Output Voltage Follows Input
    • Maximum Output Voltage

To Which Capacitor Charges:

J. Choma, Jr.

filtering cont d
Filtering–Cont’d
  • Diode Non-Conductive
    • Capacitor Voltage Does Not Change Instantaneously
    • When Capacitor Charges To Its Maximum Voltage And The Input Sinusoid Diminishes from Its Maximum Value, The Diode Open Circuits And The Capacitor Discharges Through The Load Resistance, Rl
    • Diode Begins To

Conduct Again

When The

Unfiltered Output

Rises To Meet The

Decaying Capacitor Voltage

At Time Tp. At This Point, The Output Voltage Is Vomin

    • See Plots On Next Slide

J. Choma, Jr.

waveforms capacitive filter

Ripple, Vr

Vomax

Vomin

DT

t = 0

t = Tp

Waveforms: Capacitive Filter

J. Choma, Jr.

ripple of filtered rectifier
Ripple of Filtered Rectifier
  • Characteristic Voltage Equations
  • Ripple Equations
  • Example
  • Non-Ideal Large Capacitance

J. Choma, Jr.

diode conduction time

Vomax

Vomin

DT

0

Tp

Diode Conduction Time

Neighborhood Of Time t = 0

Reasonable

Result

J. Choma, Jr.

maximum diode current

Vomax

Vomin

DT

0

Tp

Maximum Diode Current

Diode Current

Occurs At Diode Cut

In Point, t = –DT; Load Voltage Nearly Constant At Vomax

J. Choma, Jr.

transformer input
Transformer Input
  • Ideal Transformer
    • N Is Turns Ratio; Generally, N >>1
    • Voltage On Primary Winding Is Stepped Down By Factor Of N
    • Current In Primary Winding Is Stepped Down By Factor Of N
  • Impedance Transformation
    • Set Vs = 0 To Find Effective Source Resistance Seen By Diode
    • Marked Resistance Reduction

J. Choma, Jr.

full wave rectifier
Full Wave Rectifier
  • Center–Tapped Transformer
  • Operation
    • When Vs1 Is Positive, Vs2 = Vs3 > 0ID2 = 0 & Il = ID1
    • When Vs1 Is Negative, Vs2 = Vs3 < 0ID1 = 0 & Il = ID2
    • Result Is Full Wave Sinusoid For Unfiltered Case

J. Choma, Jr.

full wave performance
Full Wave Performance
  • Half Wave Analysis Can Be Replicated With Minor Modifications
  • Unfiltered Average Is Twice As Large As Half Wave Case Because Current Is Now Continually Supplied To Load
  • Ripple Is Factor Of Two Smaller Because Capacitor Now Decays For Only ½ Period
    • For Same Ripple, Filter Capacitor Can Be ½ As Large In Full Wave Rectifier As In Half Wave Unit
    • Maximum Diode Current, Expressed In Terms Of Ripple, Is The Same As for Half Wave Case

J. Choma, Jr.

bridge full wave rectifier
Bridge Full Wave Rectifier
  • Operation
    • When Vs > 0, Current Flows From Vs Through D1-Rl-D1A-Back To Vs
    • When Vs < 0, Current Flows From Vs Through D2-Rl-D2A-Back To Vs
    • Full Wave Unfiltered Output Results
  • Comments
    • Two Threshold Voltages In Each Current Path
    • Does Not Require Center Tap Transformer

J. Choma, Jr.