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.