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Microelectronic Circuit DesignMcGraw-Hill
Chapter 3Solid-State Diodes and Diode Circuits
Microelectronic Circuit DesignRichard C. JaegerTravis N. Blalock
Chap 3 -1
Microelectronic Circuit DesignMcGraw-Hill
Chapter Goals
• Understand diode structure and basic layout• Develop electrostatics of the pn junction• Explore various diode models including the mathematical model, the
ideal diode model, and the constant voltage drop model• Understand the SPICE representation and model parameters for the
diode• Define regions of operation of the diode (forward, reverse bias, and
reverse breakdown)• Apply the various types of models in circuit analysis• Explore different types of diodes Discuss the dynamic switching
behavior of the pn junction diode• Explore diode applications• Practice simulating diode circuits using SPICE
Chap 3 -2
Microelectronic Circuit DesignMcGraw-Hill
Diode Introduction
• A diode is formed byinterfacing an n-typesemiconductor with a p-typesemiconductor.
• A pn junction is theinterface between n and pregions.
Diode symbol
Chap 3 -3
Microelectronic Circuit DesignMcGraw-Hill
pn Junction Electrostatics
Donor and acceptor concentrationon either side of the junction.Concentration gradients give riseto diffusion currents.
Chap 3 -4
Microelectronic Circuit DesignMcGraw-Hill
Drift Currents
• Diffusion currents lead to localized charge density variations near thepn junction.
• Gauss’ law predicts an electric field due to the charge distribution:
• Assuming constant permittivity,
• Resulting electric field gives rise to a drift current. With no externalcircuit connections, drift and diffusion currents cancel. There is noactual current, since this would imply power dissipation, rather theelectric field cancels the diffusion current ‘tendency.’
Ec
s
E(x)1s
(x)dx
Chap 3 -5
Microelectronic Circuit DesignMcGraw-Hill
Space Charge Region Formationat the pn Junction
Chap 3 -6
Microelectronic Circuit DesignMcGraw-Hill
Potential across the Junction
Charge Density Electric Field Potential
j E(x)dx VT ln NA ND
ni2
, VT kT
q
Chap 3 -7
Microelectronic Circuit DesignMcGraw-Hill
Width of Depletion Region
wd 0 (xn xp) 2s
q1
NA 1
ND
j
Combining the previous expressions, we can form an expressionfor the width of the space-charge region, or depletion region. Itis called the depletion region since the excess holes and electronsare depleted from the dopant atoms on either side of the junction.
Chap 3 -8
Microelectronic Circuit DesignMcGraw-Hill
Width of Depletion Region (Example)
Problem: Find built-in potential and depletion-region width for given diode
Given data:On p-type side: NA=1017/cm3 on n-type side: ND=1020/cm3
Assumptions: Room-temperature operation with VT=0.025 V
Analysis:
m113.01120
jDNANqs
dw
V979.0
/cm10/cm10/cm10V)ln(0.025ln 620
320317
2
i
DATj n
NNV
Chap 3 -9
Microelectronic Circuit DesignMcGraw-Hill
Diode Electric Field (Example)
• Problem: Find electric field and size of individual depletion layers oneither side of pn junction for given diode• Given data:On p-type side: NA=1017/cm3 on n-type side: ND=1020/cm3
from earlier example,
• Assumptions: Room-temperature operation• Analysis:
V979.0j m113.00 dw
m4-1013.1
1
0
110
AN
DNd
wnx
DNAN
pxANDN
nxpxnxdw
kV/cm173m113.0)V979.0(2
0j2
dwMAXE
m113.0
1
0
DN
ANd
wpx
Chap 3 -10
Microelectronic Circuit DesignMcGraw-Hill
Internal Diode Currents
jnT qnnE qDn
nx
= 0
jpT qp pEqDp
px
= 0
Mathematically, for a diode with no external connections, the total currentexpressions developed in chapter 2 are equal to zero. The equations onlydictate that the total currents are zero. However, as mentioned earlier,since there is no power dissipation, we must assume that the field anddiffusion current tendencies cancel and the actual currents are zero.
When external bias voltage is applied to the diode, the above equations areno longer equated to zero.
Chap 3 -11
Microelectronic Circuit DesignMcGraw-Hill
Diode Junction Potential for DifferentApplied Voltages
Chap 3 -12
Microelectronic Circuit DesignMcGraw-Hill
Diode i-v Characteristics
Turn-on voltage marks point of significant current flow. Is is called thereverse saturation current.
Chap 3 -13
Microelectronic Circuit DesignMcGraw-Hill
where IS = reverse saturation current (A)vD = voltage applied to diode (V)q = electronic charge (1.60 x 10-19 C)k = Boltzmann’s constant (1.38 x 10-23 J/K)T = absolute temperaturen = nonideality factor (dimensionless)VT = kT/q = thermal voltage (V) (25 mV at room temp.)
IS is typically between 10-18 and 10-9 A, and is strongly temperature dependent dueto its dependence on ni
2. The nonideality factor is typically close to 1, butapproaches 2 for devices with high current densities. It is assumed to be 1 in thistext.
Diode Equation
iD I S exp qv D
nkT
1
I S exp vD
nVT
1
Chap 3 -14
Microelectronic Circuit DesignMcGraw-Hill
Diode Voltage and Current Calculations(Example)
Problem: Find diode voltage for diode with given specifications
Given data: IS=0.1 fA ID= 300 A
Assumptions: Room-temperature dc operation with VT=0.025 V
Analysis:
With IS=0.1 fA
With IS=10 fA
With ID= 1 mA, IS=0.1 fA
V718.0)A16-10A4-103V)ln(10025.0(11ln
SIDI
TnVDV
V603.0DV
V748.0DV
Chap 3 -15
Microelectronic Circuit DesignMcGraw-Hill
Diode Current for Reverse, Zero, andForward Bias
• Reverse bias:
• Zero bias:
• Forward bias:
iD I S exp vD
nVT
1
IS 01 I S
iD IS exp vD
nVT
1
I S 11 0
iD I S exp vD
nVT
1
I S exp vD
nVT
Chap 3 -16
Microelectronic Circuit DesignMcGraw-Hill
Semi-log Plot of Diode Current andCurrent for Three Different Values of IS
I S[ A]10IS [B ]100IS [C ]
Chap 3 -17
Microelectronic Circuit DesignMcGraw-Hill
Diode Temperature Coefficient
Diode voltage under forward bias:
Taking the derivative with respect to temperature yields
Assuming iD >> IS, IS ni2, and VGO is the silicon bandgap energy at 0K.
For a typical silicon diode
vD VT ln iD
IS1
kTq
ln iD
IS1
kTq
ln iDIS
dvD
dT
kq
lniD
IS
kTq
1I S
dIS
dT
vD
TVT
1I S
dI S
dT
vD VGO 3VT
TV/K
dvD
dT
0.651.120.075 V300K
=-1.82 mV/K - 1.8 mV/C
Chap 3 -18
Microelectronic Circuit DesignMcGraw-Hill
Reverse Bias
External reverse bias adds to the built-in potential of the pnjunction. The shaded regions below illustrate the increase in thecharacteristics of the space charge region due to an externallyapplied reverse bias, vD.
Chap 3 -19
Microelectronic Circuit DesignMcGraw-Hill
Reverse Bias (cont.)
External reverse bias also increases the width of the depletionregion since the larger electric field must be supported byadditional charge.
wd (xn x p)2s
q1
NA
1ND
j vR
wherewd0 (xn xp) 2s
q1
NA 1
ND
j
wd wd 0 1vR
j
Chap 3 -20
Microelectronic Circuit DesignMcGraw-Hill
Reverse Bias Saturation Current
We earlier assumed that reverse saturation current was constant.Since it results from thermal generation of electron-hole pairs inthe depletion region, it is dependent on the volume of the spacecharge region. It can be shown that the reverse saturation graduallyincreases with increased reverse bias.
I S I S 0 1v R
j
IS is approximately constant at IS0 under forward bias.
Chap 3 -21
Microelectronic Circuit DesignMcGraw-Hill
Reverse Breakdown
Increased reverse biaseventually results in the diodeentering the breakdownregion, resulting in a sharpincrease in the diode current.The voltage at which thisoccurs is the breakdownvoltage, VZ.
2 V < VZ < 2000 V
Chap 3 -22
Microelectronic Circuit DesignMcGraw-Hill
Reverse Breakdown Mechanisms
• Avalanche BreakdownSi diodes with VZ greater than about 5.6 volts breakdown according toan avalanche mechanism. As the electric field increases, acceleratedcarriers begin to collide with fixed atoms. As the reverse biasincreases, the energy of the accelerated carriers increases, eventuallyleading to ionization of the impacted ions. The new carriers alsoaccelerate and ionize other atoms. This process feeds on itself andleads to avalanche breakdown.
Chap 3 -23
Microelectronic Circuit DesignMcGraw-Hill
Reverse Breakdown Mechanisms (cont.)
• Zener BreakdownZener breakdown occurs in heavily doped diodes. The heavy dopingresults in a very narrow depletion region at the diode junction.Reverse bias leads to carriers with sufficient energy to tunnel directlybetween conduction and valence bands moving across the junction.Once the tunneling threshold is reached, additional reverse bias leadsto a rapidly increasing reverse current.
• Breakdown Voltage Temperature CoefficientTemperature coefficient is a quick way to distinguish breakdownmechanisms. Avalanche breakdown voltage increases withtemperature, zener breakdown decreases with temperature.
For silicon diodes, zero temperature coefficient is achieved atapproximately 5.6 V.
Chap 3 -24
Microelectronic Circuit DesignMcGraw-Hill
Breakdown Region Diode Model
In breakdown, the diode ismodeled with a voltage source,VZ, and a series resistance, RZ.RZ models the slope of the i-vcharacteristic.
Diodes designed to operate inreverse breakdown are calledZener diodes and use theindicated symbol.
Chap 3 -25
Microelectronic Circuit DesignMcGraw-Hill
Reverse Bias Capacitance
Qn qN D xn A qN A N D
N A N D
wd A Coulombs
C j dQn
dvR
C j 0 A
1vR
j
F/cm2 where C j 0 s
wd 0
Changes in voltage lead to changes in depletion width and charge.This leads to a capacitance that we can calculate from the chargevoltage dependence.
Cj0 is the zero bias junction capacitance per unit area.
Chap 3 -26
Microelectronic Circuit DesignMcGraw-Hill
Reverse Bias Capacitance (cont.)
Diodes can be designed with hyper-abrupt doping profiles thatoptimize the reverse-biased diode as a voltage controlled capacitor.
Circuit symbol for the variablecapacitance diode (varactor)
Chap 3 -27
Microelectronic Circuit DesignMcGraw-Hill
Forward Bias Capacitance
CoulombsTDD iQ
F
T
TD
T
TSD
D
Dj V
iV
IidvdQ
C
In forward bias operation, additional charge is stored in neutralregion near edges of space charge region.
T is called diode transit time and depends on size and type ofdiode.
Additional diffusion capacitance, associated with forward regionof operation is proportional to current and becomes quite large athigh currents.
Chap 3 -28
Microelectronic Circuit DesignMcGraw-Hill
Schottky Barrier Diode
One semiconductor region of the pnjunction diode is replaced by a non-ohmic rectifying metal contact.ASchottky contact is easily added ton-type silicon,metal region becomesanode. n+ region is added to ensurethat cathode contact is ohmic.
Schottky diode turns on at lowervoltage than pn junction diode, hassignificantly reduced internalcharge storage under forward bias.
Chap 3 -29
Microelectronic Circuit DesignMcGraw-Hill
Diode Spice Model
Rs is inevitable series resistance of areal device structure. Currentcontrolled current source representsideal exponential behavior of diode.Capacitor specification includesdepletion-layer capacitance forreverse-bias region as well asdiffusion capacitance associated withjunction under forward bias.
Typical default values: Saturationcurrent= 10 fA, Rs = 0, Transittime= 0 second
Chap 3 -30
Microelectronic Circuit DesignMcGraw-Hill
Diode Layout
Chap 3 -31
Microelectronic Circuit DesignMcGraw-Hill
Diode Circuit Analysis: Basics
V and R may represent Theveninequivalent of a more complex 2-terminal network.Objective ofdiode circuit analysis is to findquiescent operating point fordiode, consisting of dc current andvoltage that define diode’s i-vcharacteristic.
Loop equation for given circuit is:
This is also called the load line forthe diode. Solution to this equationcan be found by:• Graphical analysis using load-linemethod.• Analysis with diode’s mathematicalmodel.• Simplified analysis with idealdiode model.• Simplified analysis using constantvoltage drop model.
DD VRIV
Chap 3 -32
Microelectronic Circuit DesignMcGraw-Hill
Load-Line Analysis (Example)
Problem: Find Q-pointGiven data: V=10 V, R=10k.Analysis:
To define the load line we use,VD= 0VD= 5 V, ID =0.5 mA
These points and the resultingload line are plotted.Q-point isgiven by intersection of load lineand diode characteristic:
Q-point = (0.95 mA, 0.6 V)
1010 4DD VI
mA1)k10/V10( DI
Chap 3 -33
Microelectronic Circuit DesignMcGraw-Hill
Analysis using Mathematical Model forDiode
DD
DT
DSD
VV
VnVV
II
140exp101010
140exp101exp
134
13
Problem: Find Q-point for given diodecharacteristic.Given data: IS =10-13 A, n=1, VT=0.0025 VAnalysis:
is load line, given by a transcendentalequation. A numerical answer can befound by using Newton’s iterativemethod.
•Make initial guess VD0 .
•Evaluate f and its derivative f’ forthis value of VD.•Calculate new guess for VD using
•Repeat steps 2 and 3 till convergence.
Using a spreadsheet we get :Q-point = ( 0.9426 mA, 0.5742 V)
Since, usually we don’t have accuratesaturation current and significanttolerances for sources and passivecomponents, we need answers preciseup to only 2or 3 significant digits. DD VVf 140exp101010 134
)(' 0
001
D
DDD Vf
VfVV
Chap 3 -34
Microelectronic Circuit DesignMcGraw-Hill
Analysis using Ideal Model for Diode
If diode is forward-biased, voltage across diodeis zero. If diode is reverse-biased, currentthrough diode is zero.vD =0 for iD >0 and vD =0 for vD < 0Thus diode is assumed to be either on or off.Analysis is conducted in following steps:• Select diode model.• Identify anode and cathode of diode and labelvD and iD.• Guess diode’s region of operation from circuit.• Analyze circuit using diode model appropriatefor assumed operation region.• Check results to check consistency withassumptions.
Chap 3 -35
Microelectronic Circuit DesignMcGraw-Hill
Analysis using Ideal Model for Diode:Example
Since source is forcing positive currentthrough diode assume diode is on.
Q-point is(1 mA, 0V)0
m1k10
V)010(
D
D
I
AI
our assumption is right.
Since source is forcing currentbackward through diode assume diodeis off. Hence ID =0 . Loop equation is:
Q-point is (0, -10 V)V10
01010 4
D
DD
V
IV
our assumption is right.
Chap 3 -36
Microelectronic Circuit DesignMcGraw-Hill
Analysis using Constant Voltage DropModel for Diode
Analysis:
Since 10V source is forcing positive currentthrough diode assume diode is on.
vD = Von for iD >0 andvD = 0 for vD < Von.
mA94.0k10
V)6.010(k10
V)10(
on
D
VI
Chap 3 -37
Microelectronic Circuit DesignMcGraw-Hill
Two-Diode Circuit Analysis
Analysis: Ideal diode model is chosen. Since15V source is forcing positive current throughD1 and D2 and -10V source is forcing positivecurrent through D2, assume both diodes are on.
Since voltage at node D is zero due to shortcircuit of ideal diode D1,
Q-points are (-0.5 mA, 0 V) and (2.0 mA, 0 V)
But, ID1 <0 is not allowed by diode, so try again.
mA50.1k10
V)015(1
I mA00.2k5
V)10(02
DI
211 DIDII mA50.025.11 DI
Chap 3 -38
Microelectronic Circuit DesignMcGraw-Hill
Two-Diode Circuit Analysis (contd.)
Since current in D1 is zero, ID2 = I1,
Q-points are D1 : (0 mA, -1.67 V):off
D2 : (1.67 mA, 0 V) :on
Analysis: Since current inD2 but that in D1 is invalid,the second guess is D1 offand D2 on.
V67.17.16151000,10151
mA67.115,000
V251
0)10(2000,51000,1015
IDV
IDII
Chap 3 -39
Microelectronic Circuit DesignMcGraw-Hill
Analysis of Diodes in ReverseBreakdown Operation
Choose 2 points (0V, -4 mA) and (-5 V, -3mA) to draw the load line.It intersects with i-vcharacteristic at Q-point (-2.9 mA, -5.2 V).
Using piecewise linear model:
DIDV 500020 Using load-line analysis:
0 DIZI
mA94.25100
V)520(
05510020
ZI
ZI
Since IZ >0 (ID <0), solution is consistentwith Zener breakdown assumption.
Chap 3 -40
Microelectronic Circuit DesignMcGraw-Hill
Voltage Regulator using Zener Diode
Zener diode keeps voltageacross load resistor constant.For Zener breakdownoperation, IZ >0.
mA3k5
V)520(
RZ
VS
VSI
mA1k5V5
L
RZ
VLI
mA2 LISIZI
011
LRRZVRS
VZI
For proper regulation, Zener current must bepositive. If Zener current <0, Zener diode nolonger controls voltage across load resistorand voltage regulator is said to have“dropped out of regulation”.
min1
R
ZVSVR
LR
Chap 3 -41
Microelectronic Circuit DesignMcGraw-Hill
Voltage Regulator using Zener Diode:Example (Including Zener Resistance)
Problem: Find output voltage andZener diode current for Zener dioderegulator.Given data: VS=20 V, R=5 k, RZ=0.1 kVZ=5 VAnalysis: Output voltage is afunction of current through Zenerdiode.
0mA9.1100V5V19.5
100V5
V19.5
05000100V5
5000V20
LVZI
LV
LVLVZV
Chap 3 -42
Microelectronic Circuit DesignMcGraw-Hill
Line and Load Regulation
Line regulation characterizes how sensitive output voltage is to inputvoltage changes.
SdV
LdV
Line Regulation mV/V
For fixed load current, Line regulation =
Load regulation characterizes how sensitive output voltage is to changesin load current withdrawn from regulator.
Load Regulation Ohms
For changes in load current, Load regulation =
Load regulation is Thevenin equivalent resistance looking back intoregulator from load terminals.
LdI
LdV
ZRRZ
R
)( RZR
Chap 3 -43
Microelectronic Circuit DesignMcGraw-Hill
Rectifier Circuits
• Basic rectifier converts an ac voltage to a pulsating dc voltage.
• A filter then eliminates ac components of the waveform toproduce a nearly constant dc voltage output.
• Rectifier circuits are used in virtually all electronic devices toconvert the 120 V-60 Hz ac power line source to the dc voltagesrequired for operation of the electronic device.
• In rectifier circuits, the diode state changes with time and agiven piecewise linear model is valid only for a certain timeinterval.
Chap 3 -44
Microelectronic Circuit DesignMcGraw-Hill
Half-Wave Rectifier Circuit withResistive Load
For positive half-cycle of input, source forces positive current throughdiode, diode is on, vo = vs.
During negative half cycle, negative current can’t exist in diode, diode isoff, current in resistor is zero and vo =0 .
Chap 3 -45
Microelectronic Circuit DesignMcGraw-Hill
Half-Wave Rectifier Circuit withResistive Load (contd.)
Using CVD model, during on state of diode vo=(VP sint)- Von. Output voltage is zero whendiode is off.
Often a step-up or step-down transformer is usedto convert 120 V-60 Hz voltage available frompower line to desired ac voltage level as shown.
Time-varying components in circuit output areremoved using filter capacitor.
Chap 3 -46
Microelectronic Circuit DesignMcGraw-Hill
Peak Detector Circuit
As input voltage rises, diode is on andcapacitor (initially discharged) chargesup to input voltage minus the diodevoltage drop.
At peak of input, diode current tries toreverse, diode cuts off, capacitor has nodischarge path and retains constantvoltage providing constant output voltage
Vdc = VP - Von.
Chap 3 -47
Microelectronic Circuit DesignMcGraw-Hill
Half-Wave Rectifier Circuit with RCLoad
As input voltage rises during first quartercycle, diode is on and capacitor (initiallydischarged) charges up to peak value of inputvoltage.
At peak of input, diode current tries to reverse,diode cuts off, capacitor dischargesexponentially through R. Discharge continuestill input voltage exceeds output voltage whichoccurs near peak of next cycle. Process thenrepeats once every cycle.
This circuit can be used to generate negativeoutput voltage if the top plate of capacitor isgrounded instead of bottom plate. In this case,Vdc = -(VP - Von)
Chap 3 -48
Microelectronic Circuit DesignMcGraw-Hill
Half-Wave Rectifier Circuit with RCLoad (contd.)
Output voltage is not constant as in ideal peak detector, but has ripplevoltage Vr.
Diode conducts for a short time T called conduction interval duringeach cycle and its angular equivalent is called conduction angle θc.
PVrVTc
PVrV
PV
onVP
V
RCTT
CT
RonV
PV
TT
RCT
onVPVrV
2
21)(21
)(1)(
Chap 3 -49
Microelectronic Circuit DesignMcGraw-Hill
Half-Wave Rectifier Analysis: Example
Problem: Find dc output voltage, output current, ripple voltage, conductioninterval, conduction angle.Given data: secondary voltage Vrms 12.6 (60 Hz), R= 15 , C= 25,000 F,Von = 1 VAnalysis:
Using discharge interval T=1/60 s,
A12.11516.8V
16.8VV)126.12(
RonVPV
dcI
onVPVdcV
ms769.012029.0
2
fccT
V747.0)(
CT
RonV
PV
rV
06.16rad290.02 PVrVTc
Chap 3 -50
Microelectronic Circuit DesignMcGraw-Hill
Peak Diode Current
In rectifiers, nonzero current exists indiode for only a very small fraction ofperiod T, yet an almost constant dccurrent flows out of filter capacitor toload.
Total charge lost from capacitor in eachcycle is replenished by diode duringshort conduction interval causing highpeak diode currents. If repetitive currentpulse is modeled as triangle of height IPand width T,
using values from previous example.
A6.482 TT
dcIPI
Chap 3 -51
Microelectronic Circuit DesignMcGraw-Hill
Surge Current
Besides peak diode currents, when power supply is turned on, there is aneven larger current through diode called surge current.
During first quarter cycle, current through diode is approximately
Peak values of this initial surge current occurs at t = 0+:
using values from previous example.
Actual values of surge current won’t be as large as predicted because ofneglected series resistance associated with rectifier diode as well astransformer.
tPCVtP
VdtdCtcitdi cossin)()(
A168 PCVSCI
Chap 3 -52
Microelectronic Circuit DesignMcGraw-Hill
Peak Inverse Voltage Rating
Peak inverse voltage (PIV) rating of therectifier diode gives the breakdownvoltage.
When diode is off, reverse-bias acrossdiode is Vdc - vs. When vs reachesnegative peak,
PIV value corresponds to minimumvalue of Zener breakdown voltage forrectifier diode.
PVPVonVPVsvdcV 2)(minPIV
Chap 3 -53
Microelectronic Circuit DesignMcGraw-Hill
Diode Power Dissipation
dcIonV
TTPI
onVdtT
tD
ionVT
dtT
tD
itD
vTDP
20
)(1
0)()(1
Average power dissipation in diode is given by
The simplification is done by assuming the triangular approximation ofdiode current and that voltage across diode is constant at Vdc.
Average power dissipation in the diode series resistance is given by
This power dissipation can be reduced by minimizing peak currentthrough use of minimum required size of filter capacitor or using full-wave rectifiers.
SRdcITT
TT
SRPIdtT
SRt
Di
TDP 2342
31
0)(21
Chap 3 -54
Microelectronic Circuit DesignMcGraw-Hill
Full-Wave Rectifiers
Full-wave rectifiers cut capacitor dischargetime in half and require half the filtercapacitance to achieve given ripple voltage.All specifications are same as for half-waverectifiers.Reversing polarity of diodes gives a full-wave rectifier with negative output voltage.
Chap 3 -55
Microelectronic Circuit DesignMcGraw-Hill
Full-Wave Bridge Rectification
Requirement for a center-tappedtransformer in the full-waverectifier is eliminated through useof 2 extra diodes.All otherspecifications are same as for ahalf-wave rectifier exceptPIV=VP.
Chap 3 -56
Microelectronic Circuit DesignMcGraw-Hill
Rectifier Topology Comparison
• Filter capacitor is a major factor in determining cost, size and weight indesign of rectifiers.
• For given ripple voltage, full-wave rectifier requires half the filtercapacitance as that in half-wave rectifier. Reduced peak current canreduce heat dissipation in diodes. Benefits of full-wave rectificationoutweigh increased expenses and circuit complexity (a extra diode andcenter-tapped transformer).
• Bridge rectifier eliminates center-tapped transformer, PIV rating ofdiodes is reduced. Cost of extra diodes is negligible.
Chap 3 -57
Microelectronic Circuit DesignMcGraw-Hill
Rectifier Design Analysis
Problem: Design rectifier with given specifications.Given data: Vdc =15 V, Vr < 0.15 V, Idc = 2 AAnalysis: Use full-wave bridge rectifier that needs smaller value of filtercapacitance, smaller diode PIV rating and no center-tapped transformer.
A7.940.352mss1/60
A22T2
ms352.017V
V)15.0(2120
121T
F111.0V15.0
1s120
1A22/
rms12VV2
2152
2
2
TdcIPI
PV
rVrV
Tdc
IC
onVdc
VP
VV
A711)17)(111.0(120surgeI PCV
V17PIV PV
Chap 3 -58
Microelectronic Circuit DesignMcGraw-Hill
Three-terminal IC Voltage Regulators
• Uses feedback with high-gain amplifier to reduce ripple voltage atoutput.Bypass capacitors provide low-impedance path for high-frequencysignals to ensure proper operation of regulator.
• Provides excellent line and load regulation, maintaining constant voltageeven if output current changes by many orders of magnitude.
• Main design constraint is VREG which must not fall below a minimumspecified “dropout” voltage (a few volts).
Chap 3 -59
Microelectronic Circuit DesignMcGraw-Hill
DC-to-DC converters: Boost Converter
When switch is closed, diode is off:
onTLSV
LidtonT
LS
VLionTLi )0(
0)0()(
When switch is open, diode is on:
offTLoVSV
onTLSV
LiTLi
)0()(
)0()( LiTLi
1sV
oV
where = Ton/T is duty cycleSince 0<<1, Vo > VS.
Chap 3 -60
Microelectronic Circuit DesignMcGraw-Hill
DC-to-DC converters: Boost Converter(contd.)
Ripple current is given by or
In ideal converter, power delivered to input of converter is same as powerdelivered to load resistor.
or
Ripple voltage is given as
onTLS
VrI
RC
ToV
TonT
RC
ToV
RConToV
onTCoIrV
frI
SV
TonT
rI
TS
VonT
rIS
VL
oIoVSISV
1
oI
offT
ToI
SV
oVoISI
offTL
SVoV
rI
Chap 3 -61
Microelectronic Circuit DesignMcGraw-Hill
DC-to-DC converters: Buck Converter
When switch is closed, diode is off:
onTLoVSV
LidtonT
LoV
SV
LionTLi
)0(0
)0()(
When switch is open, diode is on:
offTLoV
onTLoVSV
LiTLi
)0()(
)0()( LiTLi
sVTonT
sVoV
where is switch duty cycleSince 0<<1, Vo < VS.
Chap 3 -62
Microelectronic Circuit DesignMcGraw-Hill
DC-to-DC converters: Buck Converter(contd.)
Ripple current is given by or
In ideal converter, power delivered to input of converter is same as powerdelivered to load resistor.
Ripple voltage is given as
offTLoV
rI
CQoff
TonT
onTdtriCrV
)2/(
2/
1
frI
oV
Toff
T
rI
ToVoffT
rIoV
L
oITonT
oI
SV
oVoISI
onTL
oVS
VrI
82221 TrIoff
TonTrI
Q
)1(8
2
8
LT
rVoV
rV
TrIC
Chap 3 -63
Microelectronic Circuit DesignMcGraw-Hill
Clamping or DC-Restoring Circuit
After the initial transient lastingless than one cycle in bothcircuits, output waveform is anundistorted replica of input.
Both waveforms are clamped tozero. Their dc levels are said tobe restored by the clampingcircuit.
Clamping level can also beshifted away from zero byadding a voltage source in serieswith diode.
Chap 3 -64
Microelectronic Circuit DesignMcGraw-Hill
Clipping or Limiting Circuits
Clipping circuits have dc path between input andoutput, whereas clamping circuits use capacitivecoupling between input and output.The voltage transfer characteristic shows thatgain is unity for vI < VC, and gain is zero for vI >VC.A second clipping level can also be set as shownor diodes can be used to control circuit gain byswitching resistors in and out of circuits.
Chap 3 -65
Microelectronic Circuit DesignMcGraw-Hill
Dynamic Switching Behavior of Diode
Non-linear depletion-layer capacitanceof diode prevents voltage from changinginstantaneously and determines turn-onand recovery times. Both forward andreverse current overshoot final valuewhen diode switches on and off asshown. Storage time is given by:
R
IF
ITS 1ln
Chap 3 -66
Microelectronic Circuit DesignMcGraw-Hill
Photo Diodes and Photodetectors
If depletion region of pn junction diode is illuminated withlight with sufficiently high frequency, photons can provideenough energy to cause electrons to jump the semiconductorbandgap to generate electron-hole pairs:
GEhchPE
h =Planck’s constant= 6.626e-34 J.sυ= frequency of optical illumination= wavelength of optical illuminationc =velocity of light=3e+8m/sPhoton-generated current can be used in photodetectorcircuits to generate output voltage
Diode is reverse-biased to enhance depletion-region widthand electric field.
Riov PH
Chap 3 -67
Microelectronic Circuit DesignMcGraw-Hill
Solar Cells and Light-Emitting Diodes
In solar cell applications, opticalillumination is constant, dc current IPH isgenerated. Aim is to extract power fromcell, i-v characteristics are plotted in termsof cell current and cell voltage. For solarcell to supply power to external circuit,ICVC product must be positive and cellshould be operated near point of maximumoutput power Pmax.
Light-Emitting Diodes use recombinationprocess in forward-biased pn junctiondiode. When a hole and electronrecombine, energy equal to bandgap ofsemiconductor is released as a photon.
Chap 3 -68
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