CHAPTER 3 DIODES - WordPress.com · 2017. 3. 25. · CHAPTER 3 DIODES Chapter Outline 3.1 The Ideal...
Transcript of CHAPTER 3 DIODES - WordPress.com · 2017. 3. 25. · CHAPTER 3 DIODES Chapter Outline 3.1 The Ideal...
CHAPTER 3 DIODES
Chapter Outline3.1 The Ideal Diode3.2 Terminal Characteristics of Junction Diodes3.3 Modeling the Diode Forward Characteristics3.4 Operation in the Reverse Breakdown Region – Zener Diodes3.5 Rectifier Circuits3.6 Limiting and Clamping Circuits
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3.1 Ideal Diode
Ideal diode characteristicsAn diode is a two-terminal device: Anode: the positive terminal Cathode: the negative terminal
Forward biased turned on shortReverse biased turned off open
Circuit applications
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3.2 Terminal Characteristics of Junction Diodes
I-V characteristics of junction diodesDiode current: IS (saturation current): proportional to diode area n (ideality factor): between 1 and 2. VT (thermal voltage) 25 mV at room temperature
The forward-bias region, determined by v 0.The reverse-bias region, determined by v 0.The breakdown region, determined by v VZK
Forward-bias regionThe simplified forward bias I V relationship:
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The simplified forward-bias I-V relationship: For a given forward voltage: For a given forward current:
Due to the exponential I-V relationship i ≈ 0 for v < 0.5V (cut-in voltage) Fully conduction for 0.6V < v < 0.8V Von = 0.7V
Temperature dependenceIS doubles for every 5C rise in temperature.Volrage decreases 2mV/C for a given current.Current increases with temperature for a given voltage.
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Reverse-bias regionReverse current: Ideally, the reverse current is independent of reverse bias.In reality, reverse current is larger than IS and also increases somewhat with the increase in the reverse bias.Temperature dependence: reverse current doubles for every 10C rise in temperature.
Breakdown regionThe knee of the I-V curve is specified as breakdown voltage VZK for Zener breakdown mechanism.The reverse current increases rapidly with the associated increase in voltage drop being very small.Normally, the reverse current is specified by external circuitry to assure the power dissipation within a safe
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3.3 Modeling the Diode Forward Characteristics
Circuit analysisDetermine the diode current ID and voltage VD for circuit analysisThe equation required for the analysis:
ID = IS exp(VD /nVT) doide I-V characteristicsID = (VDD VD ) /R Kirchhoff loop equation
Need to solve non-linear equationsGraphical analysisPlot the two equations in the same I-V coordinationThe straight line is known as load line.Th i t t i th l ti f I d VThe intersect is the solution for ID and VD
Iterative analysisSet initial value VD = V0
Use ID = (VDD VD ) /R to obtain I1
Use VD = nVT ln (ID /IS) to obtain V2
Repeat until it converges (I3, V4, I5, V6…)Iterations are needed to solve the nonlinear circuit
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The need for rapid analysisRapid analysis using simplified models for initial design.Accurate analysis (iterative analysis or computer program) for final designRapid analysis (I): ideal-diode model The most simplified model and can be used when supply voltage is much higher than the diode voltage Diode on: vD = 0 V and i > 0 Diode off: i = 0 and vD < 0 V Equivalent circuit model as an ideal diode
Rapid analysis (II): constant-voltage-drop model The most widely used model in initial design and analysis phases The most widely used model in initial design and analysis phases Diode on: vD = 0.7 V and i > 0 Diode off: i = 0 and vD < 0.7 V Equivalent circuit model as an ideal diode with a 0.7-V voltage source
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Small-signal approximationThe diode is operated at a dc bias point and a small ac signal is superimposed on the dc quantities:
Under small-signal condition: vd / nVT <<1
ID associates with VD dc operating point Q id associates with vd small signal response
The diode exhibits linear I-V characteristics
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under small-signal conditions (vd 10mV)Diode small-signal resistance and conductance at operating point Q:
The diode small-signal modelChoose proper dc analysis technique or model to obtain the operation point QThe small-signal model is determined once Q is providedThe small-signal model is used for circuit analysis when the diode is operating around Q
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Circuit analysis techniques for total quantities (AC+DC)Eliminate all the time varying signals (ac voltage and current sources) for operation point analysisUse rapid analysis or accurate analysis to obtain dc voltage and current at operating point QDetermine the parameters of small-signal models from QReplace the devices with small-signal models and eliminate all the dc sourcesCircuit analysis under small-signal approximationThe complete response (ac + dc) of the circuit is obtained by superposition of the dc and ac components
Voltage regulator by diode forward dropA voltage regulator is to provide a constant dc voltage regardless changes in load and power-supply voltageThe forward voltage drop remains almost constant at 0 7 V within a wide current rangeThe forward-voltage drop remains almost constant at 0.7 V within a wide current rangeMultiple diodes in series to achieve the required voltage dropBetter regulation can be provided for higher bias current and smaller rd
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3.4 Operation in the Reverse Breakdown Region – Zener Diodes
Symbol and circuit model for the Zener diodeIn breakdown region, a reverse bias (VZ) beyond the knee voltage (VZK) leads to a large reverse current (IZ).The diode in breakdown region is given by VZ = VZ0 + rz Iz
The breakdown diode is modeled by a voltage source VZ0 in series with an incremental resistance rz
Incremental voltage versus current: V = rzI The simplified model is only valid for IZ > IZK (knee current) Equivalent rz increases as IZ decreases
Diode types: Diode: only forward and reverse regions are considered Zener diode: forward reverse and breakdown regions Zener diode: forward, reverse and breakdown regions
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Design of the Zener shunt regulatorOutput voltage of the regulator:
Line regulation:
Load regulation:
Line and load regulation should be minimized
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For rz << R, line regulation can be minimized by choosing small rz
Load regulation can be minimized by choosing small rz and large RThere is an upper limit on the value of R to ensure sufficiently high current IZ (rz increases if IZ is too low)
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3.5 Rectifier Circuits
Block diagram of a dc power supplyDC power supply Generate a dc voltage from ac power sources The ac input is a low-frequency large-signal voltage
Power transformerPower transformer Step the line voltage down to required value and provides electric isolation
Diode rectifier Converts the input sinusoidal to a unipolar output Can be divided to half-wave and full-wave rectifiers
Filter Reduces the magnitude variation for the rectifier output Equivalent to time-average operation of the input waveform
Voltage Regulator Further stabilizes the output to obtain a constant dc voltage Can be implemented by Zener diode circuits
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The half-wave rectifierVoltage transfer curve:
Rectifier diode specifications: Current-handling capability: the largest current the diode is expected to conduct. Peak inverse voltage (PIV): the largest reverse voltage the diode can stand without breakdown.
PIV = Vs (input voltage swing) and the diode breakdown voltage is selected at least 50% higher.
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The full-wave rectifier (center-tapped transformer)Voltage transfer curve:
Transformer secondary winding is center-tapped.Peak inverse voltage (PIV) = 2Vs VD0 .Rectified output waveform for both positive and negative cycles.
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Full-wave rectifier (Bridge rectifier)Voltage transfer curve:
Does not require a center-tapped transformerHigher turn-on voltage (2VD0 ).Peak inverse voltage (PIV) = Vs VD0 .Most popular rectifier circuit configuration.
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