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Dr. Nasim ZafarElectronics 1
EEE 231 – BS Electrical EngineeringFall Semester – 2012
COMSATS Institute of Information TechnologyVirtual campus
Islamabad
The Diode Circuits-II
Lecture No: 10 Contents: Ideal & Practical Diodes.
Terminal Characteristics of Junction Diodes.
DC Load Line and Quiescent Conditions.
Piecewise Linear Model
Small Signal Analysis of Diodes
Dynamic Resistance, AC Resistance
Capacitance and Switching Response,
3
References:
Microelectronic Circuits: Adel S. Sedra and Kenneth C. Smith.
Electronic Devices and Circuit Theory: Robert Boylestad & Louis Nashelsky ( Prentice Hall ).
Introductory Electronic Devices and Circuits: Robert T. Paynter.
Electronic Devices : Thomas L. Floyd ( Prentice Hall ).
References (Figures):
Chapter 2 Diodes:
Figures are redrawn (with some modifications) from
Introductory Electronic Devices and CircuitsBy
Robert T. Paynter
The Diode Models
1. The Ideal Diode Model
6
The Diode:
p n
Anode Cathode
P-N Junction Diode Schematic Symbol:
The left hand diagram shows the reverse biased junction. No current flows flows.
The other diagram shows forward biased junction. A current flows.
Diode Circuits:
anode
cathode
Forward bias
Reversed bias
- -+ +
8
Forward-Biased Diode Circuit:
IF
R
+V
R
V IF
IF > 0AR
V IF
IF > 0A
IF
R
-V
9
Reverse-Biased Diode Circuit:
R
+V
R
VIT
0AR
VIT
0A
R
-V
10
Effect of VF:
R1
VS5 V
1 k
D1
I
4.3 V 1
1 1
1
1
0.7V5V 0.7V 4.3V
4.3V 4.3mA1kΩ
D
R S D
R
VV V V
VIR
Value Ideal PracticalVF 0 V 0.7 VVR1 5 V 4.3 VI 5 mA 4.3 mA
11
Example-1
R1
VS6 V
10 k
D1
I
V
1 1
1
1
6V 0.7V5.3V
5.3V 530μA10kΩ
R S D
R
V V V
VIR
12
Example-2
R1
VS5 V
1.2 k
D1
I
R2
2.2 k
1
1 2
5V 0.7V1.2kΩ 2.2kΩ1.26mA
S DV VIR R
13
Example-3
R1
VS4 V
5.1 k
D1
I
D2
1 2
1
4V 0.7V 0.7V5.1kΩ
509.8μA
S D DV V VIR
14
Percentage Error:
- '% of error 100
X XX
where X = the measured value
X’ = the calculated value
15
Example-4
1 2
1 2
1 2
10V 3.03mA1.5kΩ 1.8kΩ
10V 0.7V 0.7V1.5kΩ 1.8kΩ
2.61mA2.61mA 3.03mA
% of error = 100 16.1%2.61mA
Sideal
S D Dprac
VI
R RV V VI
R R
R1
VS10 V
1.5 k
D1
I
D2
R21.8 k
Power Dissipation PD(max)
16
17
I0 and PD(max) Relationship:
(max)0
D
F
PI
V
where I0 = the limit on the average forward current PD(max) = the forward power dissipation rating of the diode
VF = the diode forward voltage (0.7V for Si)
18
Forward Power Dissipation PD(max):
VS10 V
D1
I
RL100
Choose a diode with forward power dissipation PD(max) at least 20% greater than actual power dissipation.
1
1
(max)
10V 0.7V 93mA100Ω
0.7V 93mA 65.1mW
1.2 1.2 65.1mW 78.12mW (minimum)
S D
L
D D
D D
V VIR
P V I
P P
19
Example 5.
A diode has a forward power dissipation rating of 500 mW. What is the maximum allowable value of forward current for the device?
(max)0
(max) 0
500mW 714.29mA0.7V
0.8 0.8 714.29mA 571.43mA
D
F
PI
V
I I
20
Complete: Model Diode Curve (Ref 3).
Reverse operating region (also called the reverse breakdown region)
Forward operating region
0.2 0.4 0.6 0.8
2040
6080
100
20406080
1.0
2.0
3.0
IR(A)
IF(mA)
VF(V)VR(V)
IF 0.7 V
RB
Accurate modelComplete model
IR VZ
RZ
I = 0
FB
F
VRI
RZ
R
VRI
21
Another Example:
Determine voltage across diode in Fig. 2.19 (Ref. 3) for the values of IF = 1 mA and IF = 5 mA. Assume that the value for RB = 5 .
IF = 1 mA:
IF = 5 mA:
0.7V 0.7V 1mA 5Ω 0.705VD BV IR
0.7V 0.7V 5mA 5Ω 0.725VD BV IR
Bulk resistance has a significant effect on voltage drop across diode terminals when the forward current is large.
The Diode Models
4. Piecewise-Linear Diode Model
5. Constant-Voltage Diode Model
6. Dynamic Resistance, AC Resistance
Piecewise Linear Diode Model:More accurate than the ideal diode model and does not rely on nonlinear equation or graphical techniques. Diode I-V characteristic approximated
by straight line segments.
We model each section of the diode I-V characteristic with R in series with a fixed voltage source.
𝑉 𝑠𝑠=𝑅𝐼𝐷+𝑉 𝐷 ❑
Constant-Voltage Diode Model:
If VD < VD,on: The diode operates as an open circuit.
If VD VD,on: The diode operates as a constant voltage source with value VD,on.
Example: Diode dc Bias Calculations
This example shows the simplicity provided by a constant-voltage model over an exponential model.
Using an exponential model, iteration is needed to solve for
current. Using a constant-voltage model, only linear equations need to be solved.
S
XTXDXX I
IVRIVRIV ln11
V1for mA 2.0V3for mA 2.2
XX
XX
VIVI
Small-Signal Analysis of Diodes:
Small-signal analysis is performed at a DC bias point by perturbing the voltage by a small amount and observing the resulting linear current perturbation.
If two points on the I-V curve are very close, the curve in-
between these points is well approximated by a straight line:
T
DVV
T
s
VVD
D
D
D
VIe
VI
dVdI
VI
TD
DD
1/1
1
!3!2
132 xxxe x
Small-Signal Analysis of Diodes:
Since there is a linear relationship between the small-signal current and small-signal voltage of a diode, the diode can be viewed as a linear resistor when only small changes in voltage are of interest.
D
Td I
Vr Small-Signal Resistance(or Dynamic Resistance)
28
Small-Signal Analysis of Diodes:
Small-signal analysis is performed around a bias point by perturbing the voltage by a small amount and observing the resulting linear current perturbation.
1DT
D IVVI
29
Small-Signal Analysis of Diodes:
If two points on the IV curve of a diode are close enough, the trajectory connecting the first to the second point is like a line, with the slope being the proportionality factor between change in voltage and change in current.
T
D
T
D
T
s
VDVDD
D
D
D
VI
VI
VI
dVdI
VI
1
1
1
exp
|
30
Small Sinusoidal Analysis:
If a sinusoidal voltage with small amplitude is applied, the resulting current is also a small sinusoid around a value.
tVIV
VVItIItI p
T
TspD cosexpcos)(
0
00 tVVtV p cos)( 0
Resistance Levels:
The operating point of a diode moves from one region to another the resistance of the diode will also change due to the nonlinear shape of the characteristic curve
The type of applied voltage or signal will define the resistance level of interest
Three different types of applied voltage – DC or Static Resistance – AC or Dynamic Resistance – Average AC Resistance
DC or Static Resistance
• The application of a dc voltage to a circuit containing a semiconductor diode will result in an operating point on the characteristic curve that will not change with time
• The resistance of the diode at the operating point can be found simply by finding the corresponding levels of VD and ID
• The lower current through a diode the higher the dc resistance level
AC or Dynamic Resistance
• The varying input will move the instantaneous operating point up and down a region of the characteristics and thus defines a specific change in current and voltage as shown in the Fig.
Temperature Effects:
)(
)1exp(
n
pon
p
nops
s
LneD
LpeD
J
kTeVaJJ
a
ipo
d
ino N
nnNnpstatesteady
22
,:
Js : strong function of temperature
)exp(2
kTE
nJ gis
35
Temperature Effects on Diode Operation:
0.2 0.4 0.6 0.8
IF(mA)
2
4
6
8
10100 C 25 C
1.0
I2
I1
V1V2
VF(V)VR
IR
5
10
15
20
IR = 5 A
IR = 10 A
IR = 20 A
T = 25C
T = 35C
T = 45C
Typical Diodes
37
Diode Maximum Ratings.
Rating Discussion
Peak repetitve reverse voltage, VRRM Maximum allowable reverse voltage.
Nonrepetitive peak reverse voltage, VRSM
Maximum allowable value of a single event reverse voltage. (VRSM > VRRM)
RMS reverse voltage, VR(rms) VR(rms) = 0.707 VRRM
Average rectified forward current, I0 Maximum average diode current.
Nonrepetitive peak surge current, IFSM Maximum allowable value of forward current surge. (30A for 1N400X)
Operating and storage junction temperature, TJ or Tstg
Temperature that diode can withstand.
38
Diode Capacitance:
n p
VR
Insulator
Conductor Conductor
Insulator
Semiconductor Devices
Application of PN Junction:
PN
JUNCTION
PN Junction diode
Junction diode
Rectifiers
Switching diode
Breakdown diode
Varactor diodeTunnel diode
Photo-diode
Light Emitting diode & Laser Diode
BJT (Bipolar Junction Transistor)
Solar cell
Photodetector
HBT (Heterojunction Bipolar Transistor)
FET (Field Effect Transistor)
JFET MOSFET - memory
MESFET - HEMT
40
Summary:
Three diode models.
Diode specifications.
Diode Applications.
Recommended