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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. 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

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  1. EE 348:Lecture 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 J. Choma, Jr.

  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 J. Choma, Jr.

  4. AC To DC Conversion • Sinusoid Input: • Output: • Open Switch SW Whenever Vs < 0 • Plot Assumes Vs = 110 VRMS & Rl = 3Rs J. Choma, Jr.

  5. Average Output Voltage • Average Value Calculation • Conversion Efficiency Problem J. Choma, Jr.

  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) < 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.

  7. Semiconductor Diode Models • Charge Function • Forward Bias • Reverse Bias J. Choma, Jr.

  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 < 0) J. Choma, Jr.

  9. Diode DC V–I Characteristic Is = 10 fA; T = 27 °C; n = 1 J. Choma, Jr.

  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 J. Choma, Jr.

  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< V • Generally rD Is Negligibly Small • For Large Applied Voltages, V Can Often Be Ignored J. Choma, Jr.

  12. Half Wave Rectifier • Reverse Bias • Forward Bias J. Choma, Jr.

  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: J. Choma, Jr.

  14. 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.

  15. Ripple, Vr Vomax Vomin DT t = 0 t = Tp Waveforms: Capacitive Filter J. Choma, Jr.

  16. Ripple of Filtered Rectifier • Characteristic Voltage Equations • Ripple Equations • Example • Non-Ideal Large Capacitance J. Choma, Jr.

  17. Vomax Vomin DT 0 Tp Diode Conduction Time Neighborhood Of Time t = 0 Reasonable Result J. Choma, Jr.

  18. 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.

  19. 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.

  20. 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.

  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 J. Choma, Jr.

  22. 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.

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