1. Chapter 3 Solid-State Diodes and Diode Circuits
Microelectronic Circuit Design
Richard C. Jaeger Travis N. Blalock
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill
Chap 3 -1

2. Microelectronic Circuit Design
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
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill
Chap 3 -2

3. Microelectronic Circuit Design
Diode Introduction
A diode is formed by interfacing an n-type semiconductor with a p-type semiconductor.
A pn junction is the interface between n and p regions.
Diode symbol
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill
Chap 3 -3

4. Microelectronic Circuit Design
PN Junction Animation
Formation of PN Junction – applet
PN Junction space charge profile and electric field – applet
Biased PN Junction - applet
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill

5. pn Junction Electrostatics
Donor and acceptor concentration on either side of the junction. Concentration gradients give rise to diffusion currents.
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill
Chap 3 -5

6. Microelectronic Circuit Design
Drift Currents
Diffusion currents lead to localized charge density variations near the pn 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 external circuit connections, drift and diffusion currents cancel. There is no actual current, since this would imply power dissipation, rather the electric field cancels the diffusion current ‘tendency.’
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill
Chap 3 -6

7. Space Charge Region Formation at the pn Junction
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill
Chap 3 -7

8. Potential across the Junction
Charge Density
Electric Field
Potential
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill
Chap 3 -8

9. Width of Depletion Region
Combining the previous expressions, we can form an expression for the width of the space-charge region, or depletion region. It is called the depletion region since the excess holes and electrons are depleted from the dopant atoms on either side of the junction.
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill
Chap 3 -9

10. 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:
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill
Chap 3 -10

11. Diode Electric Field (Example)
Problem: Find electric field and size of individual depletion layers on either 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:
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill
Chap 3 -11

12. Internal Diode Currents
Mathematically, for a diode with no external connections, the total current expressions developed in chapter 2 are equal to zero. The equations only dictate that the total currents are zero. However, as mentioned earlier, since there is no power dissipation, we must assume that the field and diffusion current tendencies cancel and the actual currents are zero.
When external bias voltage is applied to the diode, the above equations are no longer equated to zero.
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill
Chap 3 -12

13. Diode Junction Potential for Different Applied Voltages
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill
Chap 3 -13

14. Diode i-v Characteristics
Turn-on voltage marks point of significant current flow. Is is called the reverse saturation current.
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill
Chap 3 -14

15. Microelectronic Circuit Design
Diode Equation
where IS = reverse saturation current (A) vD = voltage applied to diode (V) q = electronic charge (1.60 x C) k = Boltzmann’s constant (1.38 x J/K) T = absolute temperature n = nonideality factor (dimensionless) VT = kT/q = thermal voltage (V) (25 mV at room temp.)
IS is typically between and 10-9 A, and is strongly temperature dependent due to its dependence on ni2. The nonideality factor is typically close to 1, but approaches 2 for devices with high current densities. It is assumed to be 1 in this text.
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill
Chap 3 -15

16. Diode Voltage and Current Calculations (Example)
Problem: Find diode voltage for diode with given specifications
Given data: IS=0.1 fA ID= 300 mA
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
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill
Chap 3 -16

17. Diode Current for Reverse, Zero, and Forward Bias
Reverse bias:
Zero bias:
Forward bias:
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill
Chap 3 -17

18. Microelectronic Circuit Design
Semi-log Plot of Diode Current and Current for Three Different Values of IS
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill
Chap 3 -18

19. 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
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill
Chap 3 -19

20. PTAT Voltage and Electronic Thermometry
The well-defined temperature dependence of the diode voltage is used as the basis for most digital themometers.
PTAT Voltage(Voltage proportional to absolute temperature) VPTAT = VD1-VD2 = VTln(ID1/IS) -VTln(ID2/IS) =VTln(ID1/ID2) = KT/qln(ID1/ID2)
By using two diode, the temperature dependence of Is has been eliminated from the equation.
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill

21. Microelectronic Circuit Design
PTAT Thermometer
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill

22. Microelectronic Circuit Design
Reverse Bias
External reverse bias adds to the built-in potential of the pn junction. The shaded regions below illustrate the increase in the characteristics of the space charge region due to an externally applied reverse bias, vD.
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill
Chap 3 -22

23. Microelectronic Circuit Design
Reverse Bias (cont.)
External reverse bias also increases the width of the depletion region since the larger electric field must be supported by additional charge.
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill
Chap 3 -23

24. Reverse Bias Saturation Current
We earlier assumed that reverse saturation current was constant. Since it results from thermal generation of electron-hole pairs in the depletion region, it is dependent on the volume of the space charge region. It can be shown that the reverse saturation gradually increases with increased reverse bias.
IS is approximately constant at IS0 under forward bias.
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill
Chap 3 -24

25. Microelectronic Circuit Design
Reverse Breakdown
Increased reverse bias eventually results in the diode entering the breakdown region, resulting in a sharp increase in the diode current. The voltage at which this occurs is the breakdown voltage, VZ.
2 V < VZ < 2000 V
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill
Chap 3 -25

26. Reverse Breakdown Mechanisms
Avalanche Breakdown Si diodes with VZ greater than about 5.6 volts breakdown according to an avalanche mechanism. As the electric field increases, accelerated carriers begin to collide with fixed atoms. As the reverse bias increases, the energy of the accelerated carriers increases, eventually leading to ionization of the impacted ions. The new carriers also accelerate and ionize other atoms. This process feeds on itself and leads to avalanche breakdown.
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill
Chap 3 -26

27. Reverse Breakdown Mechanisms (cont.)
Zener Breakdown Zener breakdown occurs in heavily doped diodes. The heavy doping results in a very narrow depletion region at the diode junction. Reverse bias leads to carriers with sufficient energy to tunnel directly between conduction and valence bands moving across the junction. Once the tunneling threshold is reached, additional reverse bias leads to a rapidly increasing reverse current.
Breakdown Voltage Temperature Coefficient Temperature coefficient is a quick way to distinguish breakdown mechanisms. Avalanche breakdown voltage increases with temperature, zener breakdown decreases with temperature. For silicon diodes, zero temperature coefficient is achieved at approximately 5.6 V.
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill
Chap 3 -27

28. Breakdown Region Diode Model
In breakdown, the diode is modeled with a voltage source, VZ, and a series resistance, RZ. RZ models the slope of the i-v characteristic.
Diodes designed to operate in reverse breakdown are called Zener diodes and use the indicated symbol.
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill
Chap 3 -28

29. PN Junction Capacitance
The capacitance is important under dynamic signal conditions because it prevents the voltage across the diode from changing instantaneously.
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill

30. Reverse Bias Capacitance
Changes in voltage lead to changes in depletion width and charge. This leads to a capacitance that we can calculate from the charge voltage dependence.
Cj0 is the zero bias junction capacitance per unit area.
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill
Chap 3 -30

31. Reverse Bias Capacitance (cont.)
Diodes can be designed with hyper-abrupt doping profiles that optimize the reverse-biased diode as a voltage controlled capacitor.
Circuit symbol for the variable capacitance diode (varactor)
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill
Chap 3 -31

32. Forward Bias Capacitance
In forward bias operation, additional charge is stored in neutral region near edges of space charge region.
tT is called diode transit time and depends on size and type of diode.
Additional diffusion capacitance, associated with forward region of operation is proportional to current and becomes quite large at high currents.
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill
Chap 3 -32

33. Schottky Barrier Diode
One semiconductor region of the pn junction diode is replaced by a non-ohmic rectifying metal contact.A Schottky contact is easily added to n-type silicon,metal region becomes anode. n+ region is added to ensure that cathode contact is ohmic.
Schottky diode turns on at lower voltage than pn junction diode, has significantly reduced internal charge storage under forward bias.
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill
Chap 3 -33

34. Microelectronic Circuit Design
Schottky Diode
Key uniqueness: fast switching from ON to OFF and back.
Widely used:
in analog circuits: in track and hold circuits in A/D converters,pin drivers of IC test equipment
in communications and radar applications: as detectors and mixers,also as varactors
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill

35. Fermi Level & Fermi Function
The Fermi function f(E) gives the probability that a given available electron energy state will be occupied at a given temperature.
Fermi level is the top of the collection of electron energy levels at absolute zero temperature. At higher temperatures, f(E) = 0.5 at Fermi level.
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill

36. Density of Energy States
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill

37. Population of Conduction Band for a Semiconductor
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill

38. Energy Band View of Diode Operation
For a p-n junction at equilibrium, the fermi levels match on the two sides of the junctions. Electrons and holes reach an equilibrium at the junction and form a depletion region.
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill

39. Microelectronic Circuit Design
Reverse Bias
To reverse-bias the p-n junction, the p side is made more negative, making it "uphill" for electrons moving across the junction. The conduction direction for electrons in the diagram is right to left, and the upward direction represents increasing electron energy.
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill

40. Microelectronic Circuit Design
Forward Bias
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill

41. Schottky Diode Energy Band Diagram
Electrons flow into the metal until equilibrium is reached between the diffusion of electrons from the semiconductor into the metal and the drift of electrons caused by the field created by the ionized impurity atoms. This equilibrium is characterized by a constant Fermi energy throughout the structure.
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill

42. Microelectronic Circuit Design
Ohmic Contact
a contact between a metal and a semiconductor is typically a Schottky barrier contact. 
However, if the semiconductor is very highly doped, the Schottky barrier depletion region becomes very thin.  At very high doping levels, a thin depletion layer becomes quite transparent for electron tunneling. 
a practical way to make a good ohmic contact is to make a very highly doped semiconductor region between the contact metal and the semiconductor.
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill

43. Microelectronic Circuit Design
Diode Spice Model
Rs is inevitable series resistance of a real device structure. Current controlled current source represents ideal exponential behavior of diode. Capacitor specification includes depletion-layer capacitance for reverse-bias region as well as diffusion capacitance associated with junction under forward bias.
Typical default values: Saturation current= 10 fA, Rs = 0W, Transit time= 0 second
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill
Chap 3 -43

44. Microelectronic Circuit Design
Diode Layout
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill
Chap 3 -44

45. Diode Circuit Analysis: Basics
Loop equation for given circuit is:
This is also called the load line for the diode. Solution to this equation can be found by:
Graphical analysis using load-line method.
Analysis with diode’s mathematical model.
Simplified analysis with ideal diode model.
Simplified analysis using constant voltage drop model.
V and R may represent Thevenin equivalent of a more complex 2-terminal network.Objective of diode circuit analysis is to find quiescent operating point for diode, consisting of dc current and voltage that define diode’s i-v characteristic.
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill
Chap 3 -45

46. Load-Line Analysis (Example)
Problem: Find Q-point
Given data: V=10 V, R=10kW.
Analysis:
To define the load line we use,
VD= 0
VD= 5 V, ID =0.5 mA
These points and the resulting load line are plotted.Q-point is given by intersection of load line and diode characteristic:
Q-point = (0.95 mA, 0.6 V)
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill
Chap 3 -46

47. Analysis using Mathematical Model for Diode
Problem: Find Q-point for given diode characteristic.
Given data: IS =10-13 A, n=1, VT = V
Analysis:
is load line, given by a transcendental equation. A numerical answer can be found by using Newton’s iterative method.
Make initial guess VD0 .
Evaluate f and its derivative f’ for this value of VD.
Calculate new guess for VD using
Repeat steps 2 and 3 till convergence.
Using a spreadsheet we get :
Q-point = ( mA, V)
Since, usually we don’t have accurate saturation current and significant tolerances for sources and passive components, we need answers precise up to only 2or 3 significant digits.
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill
Chap 3 -47

48. Analysis using Ideal Model for Diode
If diode is forward-biased, voltage across diode is zero. If diode is reverse-biased, current through diode is zero.
vD =0 for iD >0 and iD =0 for vD < 0
Thus 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 label vD and iD.
Guess diode’s region of operation from circuit.
Analyze circuit using diode model appropriate for assumed operation region.
Check results to check consistency with assumptions.
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill
Chap 3 -48

49. Analysis using Ideal Model for Diode: Example
Since source is forcing positive current through diode assume diode is on.
Q-point is(1 mA, 0V)
Since source is forcing current backward through diode assume diode is off. Hence ID =0 . Loop equation is:
Q-point is (0, -10 V)
our assumption is right.
our assumption is right.
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill
Chap 3 -49

50. Analysis using Constant Voltage Drop Model for Diode
Since 10V source is forcing positive current through diode assume diode is on.
vD = Von for iD >0 and iD = 0 for vD < Von.
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill
Chap 3 -50

51. Two-Diode Circuit Analysis
Analysis: Ideal diode model is chosen. Since 15V source is forcing positive current through D1 and D2 and -10V source is forcing positive current through D2, assume both diodes are on.
Since voltage at node D is zero due to short circuit 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.
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill
Chap 3 -51

52. Two-Diode Circuit Analysis (contd.)
Since current in D1 is zero, ID2 = I1,
Q-points are D1 : (0 mA, V):off
D2 : (1.67 mA, 0 V) :on
Analysis: Since current in D2 but that in D1 is invalid, the second guess is D1 off and D2 on.
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill
Chap 3 -52

53. Microelectronic Circuit Design
Three Diode Circuit
A more complicated circuit. CVD model will be used in the analysis to improve accuracy.
First make an initial guess on the state of the diodes then verify with KCL
Repeat until correct diode states are found
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill

54. Three diode Circuit, all diodes on
-0.6V
-0.6V 0V
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill

55. Microelectronic Circuit Design
Iteration 1 Analysis
VC = V, VB = = 0 V,
VA = 0 – 0.6 = V
I1 = (10 – 0) V/10 k = 1 mA
I2 = [-0.6 – (-20)]/10K = 1.94 mA
I3 = [-0.6 – (-10)]/10K = 0.94 mA (3.36)
Using Kirchhoff’s current law
I2 = ID1, I1 = ID1 + ID2, I3 = ID2 + ID3 (3.37)
Combining Eqs. (3.39) and (3.40) yields the three diode currents:
ID1=1.94 mA > 0, ID2= mA < 0, ID3 = 1.86 mA > 0 (3.38)
Check : ID2 < 0 contradicted with our initial assumption.
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill

56. Three diode circuits, D1 and D3 on only
-0.6V
10-R1*I1
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill

57. Microelectronic Circuit Design
Iteration 2 Analysis
ID1 = 29.4 / 20K = 1.47 mA > 0,
ID3 = I3 = -0.6 – (-10) /10K = 0.94 mA > 0
The voltage across diode D2 is given by
VD2 = 10 – 10K * I1 – (-0.6) = 10 – = V < 0
Check : ID1>0, ID3>0, and VD2<0 are consistent with our assumptions.
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill

58. Analysis of Diodes in Reverse Breakdown Operation
Choose 2 points (0V, -4 mA) and (-5 V, -3 mA) to draw the load line.It intersects with i-v characteristic at Q-point (-2.9 mA, -5.2 V).
Using piecewise linear model:
Using load-line analysis:
Since IZ >0 (ID <0), solution is consistent with Zener breakdown assumption.
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill
Chap 3 -58

59. Voltage Regulator using Zener Diode
For proper regulation, Zener current must be positive. If Zener current <0, Zener diode no longer controls voltage across load resistor and voltage regulator is said to have “dropped out of regulation”.
Zener diode keeps voltage across load resistor constant. For Zener breakdown operation, IZ >0.
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill
Chap 3 -59

60. Microelectronic Circuit Design
Voltage Regulator using Zener Diode: Example (Including Zener Resistance)
Problem: Find output voltage and Zener diode current for Zener diode regulator.
Given data: VS=20 V, R=5 kW, RZ= 0.1 kW, VZ=5 V
Analysis: Output voltage is a function of current through Zener diode.
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill
Chap 3 -60

61. Photo Diodes and Photodetectors
If depletion region of pn junction diode is illuminated with light with sufficiently high frequency, photons can provide enough energy to cause electrons to jump the semiconductor bandgap to generate electron-hole pairs:
h =Planck’s constant= 6.626e-34 J.s
υ = frequency of optical illumination
l = wavelength of optical illumination
c =velocity of light=3e+8m/s
Photon-generated current can be used in photo detector circuits to generate output voltage
Diode is reverse-biased to enhance depletion-region width and electric field.
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill
Chap 3 -61

62. Microelectronic Circuit Design
CMOS Image Sensor
(LR) : Sony full frame sensor
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill

63. Diode i-v curve with illumination
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill

64. Optical Fiber Application
In optical fiber communication systems, the amplitude of the incident light is modulated by rapidly changing digital data, and iPH is a time-varying signal. The time-varying signal voltage at vo is fed to additional electronic circuits to demodulate the signal and recover the original data that were transmitted down the optical fiber.
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill

65. Solar Cells and Light-Emitting Diodes
In solar cell applications, optical illumination is constant, dc current IPH is generated. Aim is to extract power from cell, i-v characteristics are plotted in terms of cell current and cell voltage. For solar cell to supply power to external circuit, ICVC product must be positive and cell should be operated near point of maximum output power Pmax.
Light-Emitting Diodes use recombination process in forward-biased pn junction diode. When a hole and electron recombine, energy equal to bandgap of semiconductor is released as a photon.
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill
Chap 3 -65

66. Microelectronic Circuit Design
Solar Cell Videos
Next generation cheap solar cell – link
NASA Helios – link
2008 Solar Challenge - link
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill

67. Solar Power for the Home
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill

68. Microelectronic Circuit Design
LED
Organic LED introduction – link
Sony OLED TV XEL1 – link
Cool LED light ad - link
UR : LED light bulb : 7.5W for 60W light bulb, $120, 10years
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill

69. Microelectronic Circuit Design
Rectifier Circuits
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 the electronic device.
In rectifier circuits, the diode state changes with time and a given piecewise linear model is valid only for a certain time interval.
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill
Chap 3 -69

70. Microelectronic Circuit Design
Delta Electronics
台達電子集團 (The Delta Group) holds the world's number one position in switching power supplies.
Power Product:
Switching Power Supply
DC / DC converter
Uninterrupted Power Supply (UPS)
AC / DC adapter
Telecom Power
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill

71. Delta Electronics

72. Half-Wave Rectifier Circuit with Resistive Load
For positive half-cycle of input, source forces positive current through diode, diode is on, vo = vs.
During negative half cycle, negative current can’t exist in diode, diode is off, current in resistor is zero and vo =0 .
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill
Chap 3 -72

73. Half-Wave Rectifier Circuit with Resistive Load (contd.)
Using CVD model, during on state of diode vo =(VP sinwt)- Von. Output voltage is zero when diode is off.
Often a step-up or step-down transformer is used to convert 120 V-60 Hz voltage available from power line to desired ac voltage level as shown.
Time-varying components in circuit output are removed using filter capacitor.
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill
Chap 3 -73

74. Microelectronic Circuit Design
Peak Detector Circuit
As input voltage rises, diode is on and capacitor (initially discharged) charges up to input voltage minus the diode voltage drop.
At peak of input, diode current tries to reverse, diode cuts off, capacitor has no discharge path and retains constant voltage providing constant output voltage
Vdc = VP - Von.
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill
Chap 3 -74

75. A 175,000 uF, 15-V filter Capacitor
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill

76. Half-Wave Rectifier Circuit with RC Load
As input voltage rises during first quarter cycle, diode is on and capacitor (initially discharged) charges up to peak value of input voltage.
At peak of input, diode current tries to reverse, diode cuts off, capacitor discharges exponentially through R. Discharge continues till input voltage exceeds output voltage which occurs near peak of next cycle. Process then repeats once every cycle.
This circuit can be used to generate negative output voltage if the top plate of capacitor is grounded instead of bottom plate. In this case, Vdc = -(VP - Von)
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill
Chap 3 -76

77. Ripple Voltage and Conduction Interval
During the discharge period, the voltage across the capacitor is described by
vo(t’) = (Vp- Von) exp(-t’/RC)
for t’ = (t – T/4)  0 (3.53)
The ripple voltage Vr is given by
Vr = (Vp – Von) – (Vp – Von) exp(-(T - T)/RC) (3.54)
Using exp(-x) ~= (1-x) for small x to simplify.
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill

78. Half-Wave Rectifier Circuit with RC Load (contd.)
Output voltage is not constant as in ideal peak detector, but has ripple voltage Vr.
Diode conducts for a short time DT called conduction interval during each cycle and its angular equivalent is called conduction angle θc.
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill
Chap 3 -78

79. Half-Wave Rectifier Analysis: Example
Problem: Find dc output voltage, output current, ripple voltage, conduction interval, conduction angle.
Given data: secondary voltage Vrms 12.6 (60 Hz), R= 15 W, C= 25,000 mF, Von = 1 V
Analysis:
Using discharge interval T=1/60 s,
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill
Chap 3 -79

80. Microelectronic Circuit Design
Peak Diode Current
In rectifiers, nonzero current exists in diode for only a very small fraction of period T, yet an almost constant dc current flows out of filter capacitor to load.
Total charge lost from capacitor in each cycle is replenished by diode during short conduction interval causing high peak diode currents. If repetitive current pulse is modeled as triangle of height IP and width DT,
using values from previous example.
T*Idc
Ip* DT/2
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill
Chap 3 -80

81. Microelectronic Circuit Design
Surge Current
Besides peak diode currents, when power supply is turned on, there is an even 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 of neglected series resistance associated with rectifier diode as well as transformer.
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill
Chap 3 -81

82. Peak Inverse Voltage Rating
Peak inverse voltage (PIV) rating of the rectifier diode gives the breakdown voltage.
When diode is off, reverse-bias across diode is Vdc - vs. When vs reaches negative peak,
PIV value corresponds to minimum value of Zener breakdown voltage for rectifier diode.
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill
Chap 3 -82

83. Diode Power Dissipation
Average power dissipation in diode is given by
The simplification is done by assuming the triangular approximation of diode current and that voltage across diode is constant at Vdc. PD=1*1.1=1.1W
Average power dissipation in the diode series resistance is given by
This power dissipation can be reduced by minimizing peak current through use of minimum required size of filter capacitor or using full-wave rectifiers. Rs=0.2ohm yields PD=7.3W
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill
Chap 3 -83

84. Microelectronic Circuit Design
Full-Wave Rectifiers
Full-wave rectifiers cut capacitor discharge time in half and require half the filter capacitance to achieve given ripple voltage. All specifications are same as for half-wave rectifiers.
Reversing polarity of diodes gives a full-wave rectifier with negative output voltage.
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill
Chap 3 -84

85. Full Wave vs. Half Wave Rectifier
Same formulas for dc output voltage, ripple voltage, and ΔT, except the discharge interval is T/2 rather than T.
The ripple voltage Vr is halved, the conduction interval ΔT and peak current are reduced. The PIV rating are the same.
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill

86. Full Wave Rectifier with Negative Output Voltage
By reversing the polarity of the diodes, a full-wave rectifier circuit with a negative output voltage is realized
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill

87. Full-Wave Bridge Rectification
Requirement for a center-tapped transformer in the full-wave rectifier is eliminated through use of 2 extra diodes.All other specifications are same as for a half-wave rectifier except PIV=VP.
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill
Chap 3 -87

88. Rectifier Topology Comparison
Filter capacitor is a major factor in determining cost, size and weight in design of rectifiers.
For given ripple voltage, full-wave rectifier requires half the filter capacitance as that in half-wave rectifier. Reduced peak current can reduce heat dissipation in diodes. Benefits of full-wave rectification outweigh increased expenses and circuit complexity (a extra diode and center-tapped transformer).
Bridge rectifier eliminates center-tapped transformer, PIV rating of diodes is reduced. Cost of extra diodes is negligible.
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill
Chap 3 -88

89. Rectifier Comparison Table
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill

90. Rectifier Design Analysis
Problem: Design rectifier with given specifications.
Given data: Vdc =15 V, Vr < 0.15 V, Idc = 2 A
Analysis: Use full-wave bridge rectifier that needs smaller value of filter capacitance, smaller diode PIV rating and no center-tapped transformer.
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill
Chap 3 -90

91. Three-terminal IC Voltage Regulators
Uses feedback with high-gain amplifier to reduce ripple voltage at output.Bypass capacitors provide low-impedance path for high-frequency signals to ensure proper operation of regulator.
Provides excellent line and load regulation, maintaining constant voltage even if output current changes by many orders of magnitude.
Main design constraint is VREG which must not fall below a minimum specified “dropout” voltage (a few volts).
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill
Chap 3 -91

92. Microelectronic Circuit Design
DC-to-DC Converters
Multiple power supply voltages are required in most electronic systems.
Transformer + rectifier method : more expensive; size / weight of transformers a issue.
High efficiency dc-dc converters that can operate in much higher frequencies, reduces the size and weight of inductances in the circuit.
Boost converter : output voltage greater than input voltage
Buck converter : output voltage less than input voltage
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill

93. DC-to-DC converters: Boost Converter
When switch is closed, diode is off:
When switch is open, diode is on:
where d = Ton/T is duty cycle
Since 0<d<1, Vo > VS.
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill
Chap 3 -93

94. 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 power delivered to load resistor. or
Ripple voltage is given as
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill
Chap 3 -94

95. DC-to-DC converters: Buck Converter
When switch is closed, diode is off:
When switch is open, diode is on:
where d is switch duty cycle
Since 0<d<1, Vo < VS.
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill
Chap 3 -95

96. 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 power delivered to load resistor.
Ripple voltage is given as
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill
Chap 3 -96

97. Clamping or DC-Restoring Circuit
After the initial transient lasting less than one cycle in both circuits, output waveform is an undistorted replica of input.
Both waveforms are clamped to zero. Their dc levels are said to be restored by the clamping circuit.
Clamping level can also be shifted away from zero by adding a voltage source in series with diode.
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill
Chap 3 -97

98. Clipping or Limiting Circuits
Clipping circuits have dc path between input and output, whereas clamping circuits use capacitive coupling between input and output.
The voltage transfer characteristic shows that gain is unity for vI < VC, and gain is zero for vI > VC.
A second clipping level can also be set as shown or diodes can be used to control circuit gain by switching resistors in and out of circuits.
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill
Chap 3 -98

99. Dynamic Switching Behavior of Diode
Non-linear depletion-layer capacitance of diode prevents voltage from changing instantaneously and determines turn-on and recovery times. Both forward and reverse current overshoot final value when diode switches on and off as shown. Storage time is given by:
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill
Chap 3 -99

100. Microelectronic Circuit Design
Homework 3
Written Homework
3.2
3.24
3.56
3.69 for figure (b)
3.72 for figure (a)
3.91
SPICE Homework for rectifier
Jaeger/Blalock
7/1/03
Microelectronic Circuit Design
McGraw-Hill