1Microelectronics Circuit Analysis and DesignDonald A. NeamenChapter 2Diode CircuitsIn this chapter, we will:• Determine the operation and characteristics of diode rectifier circuits, which is the first stage of the process of converting an ac signal into a dc signal in the electronic power supply.• Apply the characteristics of the Zener diode to a Zenerdiode voltage regulator circuit.• Apply the nonlinear characteristics of diodes to create wave shaping circuits known as clippers and clampers.• Examine the techniques used to analyze circuits that contain more than one diode.Rectifier Circuits• A basic rectifier converts an ac voltage to a pulsating dc voltage.• A filter then eliminates ac components of the waveform to produce a nearly constant dc voltage output.• Rectifier circuits are used in virtually all electronic devices to convert the 120-V 60-Hz ac power line source to the dc voltages required for operation of electronic devices.• In rectifier circuits, the diode state changes with time and agiven piecewise linear model is valid only for a certain time interval.Chap 3 -42Half Wave RectificationIf rfis zero, when diode is on, vo= vs-vγPIV = Vs+VγFigure 2.6Full-wave rectifier: (a) circuit with center-tapped transformer, (b) voltage transfer characteristics, and (c) input and output waveformsFull-Wave RectifiersFull-wave rectifiers cut capacitor discharge time in half and require half the filter capacitance to achieve a given ripple voltage. All specifications are the same as for half-wave rectifiers. Reversing polarity of the diodes gives a full-wave rectifier with negative output voltage.3Figure 2.7A full-wave bridge rectifier: (a) circuit showing the current direction for a positive input cycle, (b) current direction for a negative input cycle, and (c) input and output voltage waveformsFull-Wave Bridge RectificationThe requirement for a center-tapped transformer in the full-wave rectifier is eliminated through use of 2 extra diodes. All other specifications are the same as for a half-wave rectifier except PIV = Vs(max)-vγ.Rectifier Topology Comparison• Filter capacitors are a major factor in determining cost, size and weight in design of rectifiers.• For a given ripple voltage, a full-wave rectifier requires half the filter capacitance as that in a half-wave rectifier. Reduced peak current can reduce heat dissipation in diodes. Benefits of full-wave rectification outweigh increased expenses and circuit complexity (an extra diode and center-tapped transformer).• The bridge rectifier eliminates the center-tapped transformer, and the PIV rating of the diodes is reduced. Cost of extra diodes is negligible.Chap 3 -124Power Supply ApplicationsFilters, Ripple Voltage, and Regulators¾ Most electronic applications require smooth dc currentto operate properly. ¾ Filtering pulsating dc circuits accomplishes this.¾ Adding a capacitor to the output of a half-wave rectifier filters the pulsating dc into smooth dc.Power Supply ApplicationsFilter NetworksOutput Voltage of Full-Wave Rectifier with RC FilterThe ripple on the ‘dc’ output isPMrTffRCVV21 where2==5¾ A capacitive filter added to the output of a full-wave bridge rectifier is shown at the left.¾ One drawback of a half-wave rectifier is the higher level of ripple voltage after filtering. ¾ Full-wave rectification reduces this ripple voltage.Power Supply ApplicationsFull-wave Rectifier with Filter¾ Smoothening the Output Voltage of a RectifierÎ Add a Capacitor across LoadPower Supply CircuitsFilters and Regulators¾ A capacitor-input filter will charge and discharge such that it fills in the “gaps” between each peak. ¾ This reduces variations of voltage. ¾ This voltage variation is called ripple voltage.Power Supply CircuitsFilters and Regulators¾ The advantage of a full-wave rectifier over a half-wave is quite clear. ¾ The capacitor can more effectively reduce the ripple when the time between peaks is shorter.Power Supply CircuitsFilters and RegulatorsVripple6Power Supply ApplicationsHalf-wave Rectifier with FilterFull-wave rectifier with filter designOutput voltage of a full-wave rectifier with an RC filter Design of Filter CapacitorPower Supply CircuitsRCfVVMripple2=RCtMtMoeVeVtv'')(−−==τVoltage across the capacitor:RCTMLeVV'−=Minimum out voltage:T’: discharge time()RCTMLMrippleeVVVV'1−−=−=Ripple voltage:T’ << RC≈RCTVVMripple'T’ ≅ TP≈RCTVVPMripplePTf21=Assume capacitor takes negligible time to chargeOutput voltage of a full-wave rectifier with an RC filter time7Root Mean SquareVV20.707VRMSMM==Mean value of sinusoidal over one period signal is zeroExample 2.3Design a full-wave rectifier to meet particular specification.A full-wave rectifier is to be designed to procedure a peak output voltage of 12 V, deliver 120 mA to the load, and produce an output with a ripple of not more than 5.0 %.An input line voltage of 120 V (rms), 60 Hz is available.P61“Full-wave rectifier design”Variation on Problem 1.62H W # 2Variation con’tFor -0.7V < VI< 0.7V, II= 0The device under test (DUT) acts like an open and can be modeled as such over this voltage range.Variation con’tWhen VI≥ 0.7V, IIchanges linearly with voltageVVkmAVVrf7.0 and 35.227.05=Ω=−=γ8Since the I-V characteristics of the device under test (DUT) are symmetrically about VD= 0, a similar model can be used for VI ≤- 0.7V as for VI≥ 0.7VFor VI≤ -0.7V: Variation con’tVVkmAVVrf7.0 and 35.227.05−=Ω=−=γ9Zener Diodes¾ Zener diodes are available with voltage breakdowns of 1.8 ~ 200 V. This curve illustrates the minimum and maximum ranges of current operation that the Zener can effectively maintain it’s voltage.at least >10%Power Supply ApplicationsZener Voltage Regulator CircuitVariable load conditionsVarying voltage source¾ Output voltage remain constant, even when output load resistance varies over a wide range, and when input voltage varies over a specific range.Limit the current through the Zener diode and drop the “excess” voltage between VPSand VZ.Input resistanceThe Zener diode begins to conduct when VPS= VZ. When VPS≥ VZ:VL= VZIL= VZ/RL,, but VZ≠ constantI1= (VPS–VZ)/RiIZ= I1-ILVoltage Rectifier with nonzero Zener resistanceZener Diode Characteristics10Sizing Series ResistanceLizpsZLZzpsiIRVVI Or IIVVR −−=+−=Case 1:Vz> VzoVps= min, Iz= min, IL= maxCase 2:Pz≤ rated diode dissipationVps= max, Iz= max, Il= minSizing