lecture 24 basic diode concepts - university of … 24 basic diode concepts electrical engineering:...

ELECTRICAL ENGINEERING: PRINCIPLES AND APPLICATIONS, Fourth Edition, by Allan R. Hambley, ©2008 Pearson Education, Inc.

Lecture 24Basic Diode Concepts


Diodes

1. Understand diode operation and select diodes for various applications.

2. Analyze nonlinear circuits using the graphical load-line technique.


3. Analyze and design simple voltage-regulator circuits.

4. Solve circuits using the ideal-diode model and piecewise-linear models.

5. Understand various rectifier and wave shaping circuits.


Fluid Analogy


Diode-Fluid Analogy

Seat

Flapper

Pivot

Schematic diagram of a flapper valve


Thermionic Diode


Cat’s Whisker


Galena (Sulphide Ore of Lead)


Cat’s Whisker


Semiconductor Diode


P-Type and N-Type Materials

http://www.ineer.org/Events/ICEE1999/Proceedings/papers/334/334.htm


P-N Junction



Carrier Recombination



Depletion Layer



I-V Characteristic

“On”

“Off”

Reverse Breakdown

Non-linear I-V Characteristic


Shockley Equation

⎥⎦

⎤⎢⎣

⎡−⎟⎟

⎠

⎞⎜⎜⎝

⎛= 1exp

T

DsD nV

vIiq

kTVT =

n = Emission coefficient (1<n<2)k = 1.38 × 10–23 J/K is Boltzmann’s constant and q = 1.60 × 10–19 C is the magnitude of the electrical charge of an electron. At a temperature of 300 K, we have

mV 26≅TV


Shockley Equation

breakdown reverse reaching until-for TDsD VvIi <<−≈

⎥⎦

⎤⎢⎣

⎡−⎟⎟

⎠

⎞⎜⎜⎝

⎛= 1exp

T

DsD nV

vIi mVq

kTVT 26≅=

TVfor exp >>⎟⎟⎠

⎞⎜⎜⎝

⎛≈ D

T

DsD v

nVvIi


Zener Diodes

Diodes that are intended to operate in the breakdown region are called Zener diodes.


Zener Diodes


Load-Line Analysis of Diode Circuits

DDSS vRiV +=

Assume VSS and R are known. Find iD and vD


DDSS vRiV +=


DDSS vRiV +=



mAViv

vivi

kRVV

DD

DD

DD

ss

21000

20

201000212

=Ω

=→=

=→=+=

Ω==



mAViv

offscalevi

mAViv

vikRVV

DD

DD

DD

DD

ss

8.0000,10

82

)(100

1000,10

100

000,10101010

==→=

=→=

=Ω

=→=

+=Ω=

=


Ideal Diode Model

The ideal diode acts as a shortcircuit for forward currentsand as an open circuit withreverse voltage applied.

iD > 0 vD < 0 → diode is in the “on” state

vD < 0 ID = 0 → diode is in the “off” state


Assumed States for Analysis of Ideal-Diode Circuits

1. Assume a state for each diode, either on (i.e., a short circuit) or off (i.e., an open circuit). For n diodes there are 2n possible combinations of diode states.

2. Analyze the circuit to determine the current through the diodes assumed to be on and the voltage across the diodes assumed to be off.


3. Check to see if the result is consistent with the assumed state for each diode. Current must flow in the forward direction for diodes assumed to be on. Furthermore, the voltage across the diodes assumed to be off must be positive at the cathode (i.e., reverse bias).

4. If the results are consistent with the assumed states, the analysis is finished. Otherwise, return to step 1 and choose a different combination of diode states.



Assume D1 off and D2 on:vD1= 7V

D1 would be on!



Assume D1 off and D2 off:

vD1= 10V vD2= 3VBoth diodes should be on!



Assume D1 on and D2 on:



i1 i2

i3

mAmAmAiiiiii

mAKVi

mAK

VVi

25.175.15.0

5.063

75.14

310

132321

3

1

−=−=−=→=+

=Ω

=

=Ω−

=

Not consistent with direction of current through D2!



Assume D1 on and D2 off:

iD1=1mA

6VvD1= -3V

This works!



D1 on

iD

D1 off−+

D1 would be on!

This works!



D2 on D2 off

+

−iD

Current is going the wrong way

through the diode!

VD consistent with D2 off



D3 on

D4 on

D3 on

D4 off

D3 off

D4 off

D3 off

D4 on



D3 on

D4 on

6.67 mA

-1.67 mA

V

mAimAiV

VVV

mAk

VkV

67.667.1310103

10202

51

)10(2

21 =−=→−

=

−==++

=Ω−−

+Ω



D3 on

D4 off10V

20V

+ -

- +



D3 off

D4 off

Not consistent with 5 mA

current source!



D3 off

D4 onV

50105−=

=−+−v

v

5V

5mA

+ -

+ -

lecture 24 basic diode concepts - university of … 24 basic diode concepts electrical engineering:...

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