Download Diode Circuits - 2. - COMSATS Institute of Information Technology

COMSATS Institute of Information Technology
Virtual campus
Islamabad
Dr. Nasim Zafar
Electronics 1
EEE 231 – BS Electrical Engineering
Fall Semester – 2012
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,
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 ).
3
References (Figures):
Chapter 2 Diodes:
Figures are redrawn (with some modifications) from
Introductory Electronic Devices and Circuits
By
Robert T. Paynter
The Diode Models
1. The Ideal Diode Model
The Diode:
P-N Junction Diode Schematic Symbol:
Anode
Cathode
p
n
6
Diode Circuits:
anode
Reversed bias
+
-
+
-
Forward bias
cathode
The left hand diagram shows the reverse biased junction.
No current flows flows.
The other diagram shows forward biased junction.
A current flows.
Forward-Biased Diode Circuit:
R
R
IF > 0A
IF > 0A
IF
V
IF
V
+V
-V
R
R
IF
IF
8
Reverse-Biased Diode Circuit:
R
R
V
0A
0A
IT
IT
V
+V
R
-V
R
9
Effect of VF:
VD1  0.7V
4.3 V
I
VS
5V
VR1  VS  VD1  5V  0.7V  4.3V
R1
1 k
VR1 4.3V
I

 4.3mA
R1 1kΩ
D1
Value
VF
VR1
I
Ideal
0V
5V
Practical
0.7 V
4.3 V
5 mA
4.3 mA
10
Example-1
V
I
VS
6V
R1
10 k
VR1  VS  VD1  6V  0.7V
 5.3V
D1
VR1 5.3V
I

 530μA
R1 10kΩ
11
Example-2
I
R1
1.2 k
D1
VS
5V
R2
2.2 k
VS  VD1
I
R1  R2
5V  0.7V

1.2kΩ  2.2kΩ
 1.26mA
12
Example-3
R1
5.1 k
I
D1
VS
4V
D2
VS  VD1  VD 2
I
R1
4V  0.7V  0.7V

5.1kΩ
 509.8μA
13
Percentage Error:
% of error 
X -X'
X
100
where X = the measured value
X’ = the calculated value
14
Example-4
I
R1
1.5 k
I ideal 
VS
10V

 3.03mA
R1  R2 1.5kΩ  1.8kΩ
I prac 
VS  VD1  VD 2 10V  0.7V  0.7V

R1  R2
1.5kΩ  1.8kΩ
D1
R2
1.8 k
VS
10 V
 2.61mA
D2
% of error =
2.61mA  3.03mA
2.61mA
100  16.1%
15
Power Dissipation PD(max)
16
I0 and PD(max) Relationship:
I0 
PD (max)
VF
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)
17
Forward Power Dissipation PD(max):
I
VS
10 V
D1
RL
100 
Choose a diode with forward power
dissipation PD(max) at least 20% greater
than actual power dissipation.
VS  VD1 10V  0.7V
I

 93mA
RL
100Ω
PD  VD1 I   0.7V  93mA   65.1mW
PD (max)  1.2 PD  1.2  65.1mW   78.12 mW (minimum)
18
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?
I0 
PD (max)
VF
500mW

 714.29mA
0.7V
I (max)  0.8I 0   0.8 714.29mA   571.43mA
19
Complete: Model Diode Curve (Ref 3).
IF(mA)
100
IR
VZ
80
80
Reverse operating
region (also called
the reverse
breakdown
region)
60
Forward
operating
region
60
RZ
VR
RZ 
I R
VR(V)
Complete model
Accurate model
40
20
40
20
0.2
0.4
0.6
VF(V)
0.8
1.0
I=0
IF
0.7 V
2.0
RB
3.0
IR(A)
RB 
VF
I F
20
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:
VD  0.7V  IRB  0.7V  1mA  5Ω  0.705V
IF = 5 mA:
VD  0.7V  IRB  0.7V   5mA  5Ω  0.725V
Bulk resistance has a significant effect on voltage drop
across diode terminals when the forward current is large.
21
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.
𝑉𝑠𝑠 =𝑅𝐼𝐷 + 𝑉𝐷
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
IX
VX  I X R1  VD  I X R1  VT ln
IS
I X  2.2mA for VX  3V
I X  0.2mA for VX  1V
 This example shows the simplicity provided by a constantvoltage 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.
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 inbetween these points is well approximated by a straight line:
I D
dI
 D
VD dVD

VD VD1
I s VD1 / VT I D1
e

VT
VT
x 2 x3
e  1 x 

 
2! 3!
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.
Small-Signal Resistance
(or Dynamic Resistance)
VT
rd 
ID
Small-Signal Analysis of Diodes:
I D 
V
I D1
VT
Small-signal analysis is performed around a bias point by
perturbing the voltage by a small amount and observing
the resulting linear current perturbation.
28
Small-Signal Analysis of Diodes:
I D dI D

|VD VD1
VD dVD
Is
I D1
 exp
VT
VT
I D1

VT
 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.
29
Small Sinusoidal Analysis:
 If a sinusoidal voltage with small amplitude is applied, the
resulting current is also a small sinusoid around a value.
V (t )  V0  Vp cos t
I D (t )  I 0  I p cos t  I s exp
V0 VT
 V p cos t
VT I 0
30
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:
eVa
J  J s exp(
 1)
kT
eD p pno eDn n po
Js  (

)
Lp
Ln
ni2
ni2
steady state: pno 
, n po 
Nd
Na
Js : strong function of temperature
J s  n  exp( 
2
i
Eg
kT
)
Temperature Effects on Diode Operation:
IF(mA)
100 C
10
25 C
V2
8
V1
I2
6
4
2
I1
VR
T = 25C
T = 35C
0.2
IR = 5 A
IR = 10 A
0.4
0.6
0.8
1.0
VF(V)
5
10
15
T = 45C
IR = 20 A
20
IR
35
Typical Diodes
Diode Maximum Ratings.
Rating
Discussion
Peak repetitve reverse voltage, VRRM
Maximum allowable reverse voltage.
Nonrepetitive peak reverse voltage, Maximum allowable value of a single
event reverse voltage. (VRSM > VRRM)
VRSM
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
temperature, TJ or Tstg
junction Temperature that diode can withstand.
37
Diode Capacitance:
Insulator
Conductor
Conductor
n
p
Insulator
VR
38
Application of PN Junction:
BJT (Bipolar Junction Transistor)
P
N
J
U
N
C
T
I
O
N
HBT (Heterojunction Bipolar Transistor)
Rectifiers
Switching diode
Junction diode
Tunnel diode
PN Junction diode
Breakdown diode
Varactor diode
Solar cell
Photo-diode
Photodetector
Light Emitting diode & Laser Diode
JFET
FET (Field Effect Transistor)
MOSFET - memory
MESFET - HEMT
Semiconductor Devices
Summary:
 Three diode models.
 Diode specifications.
 Diode Applications.
40