1 / 22

EE 348: Lecture Supplement Notes SN2

This lecture supplement covers rectification, diode characteristics, circuits, models, DC steady state, and conversion efficiency in semiconductor diodes.

vpeggy
Download Presentation

EE 348: Lecture Supplement Notes SN2

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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.

More Related