EE100Su08 Lecture #15 (July 30th 2008)Slide 2Diode Ideal (Perfect Rectifier) ModelI-V Characteristicspn-Junction Reverse BreakdownSlide 6Ideal Diode Model of PN DiodeDiode Large-Signal Model (0.7 V Drop)Large-Signal Diode ModelHow to Analyze Circuits with DiodesSlide 11Rectifier CircuitFull Wave Rectifier & AC-DC converterSlide 14Slide 15Another Example CircuitSlide 17Peak Detector CircuitSlide 19Load Line Analysis MethodSlide 21Zener DiodeSlide 23Slide 1EE100 Summer 2008 Bharathwaj MuthuswamyEE100Su08 Lecture #15 (July 30th 2008)•Outline–MultiSim:•Step 1: Download program from (257.5 MB): http://ftp.ni.com/support/softlib/Circuit_Design_Suite/10.0/10.0.1/NI_CDS_10_0_1_Stu.exe•Step 2: Use license key given out in class–Project labs START NEXT WEEK (see updated schedule online tonight). For this week, make sure you finish Strain Gauge.–QUESTIONS?–Diodes: Wrap up•Reading–Chapter 2 from your reader (Diode Circuits)Slide 2EE100 Summer 2008 Bharathwaj MuthuswamyDiode Physical Behavior and EquationNtypePtypeSchematic DeviceVIISymbolVQualitative I-V characteristics:IVV positive, easy conductionV negative, no conductionQuantitative I-V characteristics:)1e(IIkTqV0In which kT/q is 0.026V and IO is a constant depending on diode area. Typical values: 10-12 to 10-16 A. Interestingly, the graph of this equation looks just like the figure to the left.A non-ideality factor n times kT/q is often included.Slide 3EE100 Summer 2008 Bharathwaj MuthuswamyDiode Ideal (Perfect Rectifier) ModelThe equationis graphed below for 1)kTqVexp(II0A10I150The characteristic is described as a “rectifier” – that is, a device that permits current to pass in only one direction. (The hydraulic analog is a “check value”.) Hence the symbol: VISimple “Perfect Rectifier” ModelIf we can ignore the small forward-bias voltage drop of a diode, a simple effective model is the “perfect rectifier,” whose I-V characteristic is given below:VIReverse bias0Vany ,0I Forward bias0Iany ,0V A perfect rectifier0246810-5 0 5 10Current in mAForward Voltage in VSlide 4EE100 Summer 2008 Bharathwaj MuthuswamyI-V CharacteristicsIn forward bias (+ on p-side) we have almost unlimited flow (very low resistance). Qualitatively, the I-V characteristics must look like:VFIcurrent increases rapidly with VVFIThe current is close to zero for any negative biasIn reverse bias (+ on n-side) almost no current can flow. Qualitatively, the I-V characteristics must look like:Slide 5EE100 Summer 2008 Bharathwaj Muthuswamypn-Junction Reverse Breakdown•As the reverse bias voltage increases, the peak electric field in the depletion region increases. When the electric field exceeds a critical value (Ecrit 2x105 V/cm), the reverse current shows a dramatic increase:ID (A)VD (V)reverse (leakage) currentforward currentbreakdown voltage VBDSlide 6EE100 Summer 2008 Bharathwaj MuthuswamyThe pn Junction I vs. V EquationIn EECS 105, 130, and other courses you will learn why the I vs. V relationship for PN junctions is of the form)1e(IIkTqV0where I0 is a constant proportional to junction area and depending on doping in P and N regions, k is Boltzman constant, and T is absolute temperature. a typical value for I0 is ,106.1hargec electronicq19,Kat300 0.026VqKT A10101512 We note that in forward bias, I increases exponentially and is in the A-mA range for voltages typically in the range of 0.6-0.8V. In reverse bias, the current is essentially zero.I-V characteristic of PN junctionsSlide 7EE100 Summer 2008 Bharathwaj Muthuswamyreverse biasforward bias•An ideal diode passes current only in one direction. •An ideal diode has the following properties:• when ID > 0, VD = 0• when VD < 0, ID = 0Ideal Diode Model of PN DiodeID (A)VD (V)ID+VD –+VD –IDCircuit symbol I-V characteristicDiode behaves like a switch: • closed in forward bias mode • open in reverse bias modeSwitch modelSlide 8EE100 Summer 2008 Bharathwaj MuthuswamyDiode Large-Signal Model (0.7 V Drop)Improved “Large-Signal Diode” Model:If we choose not to ignore the small forward-bias voltage drop of a diode, it is a very good approximation to regard the voltage drop in forward bias as a constant, about 0.7V. the “Large signal model” results.Reverse bias0Vany ,0I Forward bias0Iany ,0.7V0100200300400-5 -3 -1 1forward bias (V)C u rre n t (m ic ro a m p )VIThe Large-Signal Diode Model- 0.7+VI0.7Slide 9EE100 Summer 2008 Bharathwaj MuthuswamyLarge-Signal Diode Modelreverse biasforward biasID (A)VD (V)ID+VD –+VD–IDCircuit symbol I-V characteristicSwitch modelVDon+VDonRULE 1: When ID > 0, VD = VDonRULE 2: When VD < VDon, ID = 0Diode behaves like a voltage source in series with a switch: • closed in forward bias mode • open in reverse bias mode For a Si pn diode, VDon 0.7 VSlide 10EE100 Summer 2008 Bharathwaj MuthuswamyA diode has only two states:• forward biased: ID > 0, VD = 0 V •reverse biased: ID = 0, VD < 0 V (or 0.7 V)Procedure:1. Guess the state(s) of the diode(s)2. Check to see if KCL and KVL are obeyed.3. If KCL and KVL are not obeyed, refine your guess4. Repeat steps 1-3 until KCL and KVL are obeyed.Example: vs(t)If vs(t) > 0 V, diode is forward biased (else KVL is disobeyed – try it)If vs(t) < 0 V, diode is reverse biased (else KVL is disobeyed – try it)How to Analyze Circuits with Diodes++vR(t)–Slide 11EE100 Summer 2008 Bharathwaj MuthuswamySlide 12EE100 Summer 2008 Bharathwaj MuthuswamyRectifier CircuitVS(t)t+VR(t)+VS(t)VR(t)“rectified” version of input waveformtAssume the ideal (perfect rectifier) model.Slide 13EE100 Summer 2008 Bharathwaj MuthuswamyFull Wave Rectifier & AC-DC converterSlide 14EE100 Summer 2008 Bharathwaj MuthuswamyFull Wave Rectifier & AC-DC converterSlide 15EE100 Summer 2008 Bharathwaj MuthuswamyFull Wave Rectifier & AC-DC converterSlide 16EE100 Summer 2008 Bharathwaj MuthuswamyAnother Example CircuitSlide 17EE100 Summer 2008 Bharathwaj MuthuswamyAnother Example CircuitSlide 18EE100 Summer 2008 Bharathwaj MuthuswamyPeak Detector Circuit+VC(t)Vi(t)+CVi(t)VitVC(t)VC+Assume the ideal (perfect rectifier) model.Key Point: The capacitor charges due to one way current behavior of the diode.Slide 19EE100 Summer 2008 Bharathwaj MuthuswamyPeak Detector Circuit+VC(t)Vi(t)+C+Assume the ideal (perfect rectifier) model.Key Point: The capacitor
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