EE 348: Lecture Supplement Notes SN2

1
EE 348Lecture Supplement Notes SN2

• Semiconductor Diodes
• Concepts, Models, Circuits
• 22 January 2001

2
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

3
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

4
AC To DC Conversion

• Sinusoid Input
• Output
• Open Switch SW Whenever Vs lt 0
• Plot Assumes Vs 110 VRMS Rl 3Rs

5
Average Output Voltage

• Average Value Calculation
• Conversion Efficiency Problem

6
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) lt 0 Diode Is Reverse Biased Or Back
  Biased
• t ? Storage Time Constant (nSec to- pSec)
• vd(t) ? Diode Voltage (Generally lt 800 mV)
• id(t) ? Diode Current (Value Depends On Junction
  Area)
• Cj(vd) ? Junction Depletion Capacitance

7
Semiconductor Diode Models

• Charge Function
• Forward Bias
• Reverse Bias

8
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 lt 0)

9
Diode DC VI Characteristic
Is 10 fA T 27 C n 1
10
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

11
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 lt
  V?
• Generally rD Is Negligibly Small
• For Large Applied Voltages, V? Can Often Be
  Ignored

12
Half Wave Rectifier

• Reverse Bias
• Forward Bias

13
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

14
FilteringContd

• 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

15
Waveforms Capacitive Filter
16
Ripple of Filtered Rectifier

• Characteristic Voltage Equations
• Ripple Equations
• Example
• Non-Ideal Large Capacitance

17
Diode Conduction Time

• Neighborhood Of Time t 0

Reasonable Result
18
Maximum Diode Current

• Diode Current
• Occurs At Diode Cut
• In Point, t DT Load Voltage Nearly Constant
  At Vomax

19
Transformer Input

• Ideal Transformer
• N Is Turns Ratio Generally, N gtgt1
• 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

20
Full Wave Rectifier

• CenterTapped Transformer
• Operation
• When Vs1 Is Positive, Vs2 Vs3 gt 0 ? ID2 0
  Il ID1
• When Vs1 Is Negative, Vs2 Vs3 lt 0 ? ID1 0
  Il ID2
• Result Is Full Wave Sinusoid For Unfiltered Case

21
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

22
Bridge Full Wave Rectifier

• Operation
• When Vs gt 0, Current Flows From Vs Through
  D1-Rl-D1A-Back To Vs
• When Vs lt 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