Diode application presentationnnnnnnnnnn

CHAPTER 17
DIODES AND APPLICATIONS

Diode Approximations
 The ideal diode model

การป้ อนไฟให้ Diode
 Forward bias:
 ให้ศักย์ไฟฟ
้ าด้าน Anode ( P ) สูงกว่า
Cathode ( N )
 VF > VB (VB (Si ) = 0.7 V; VB (Ge
) = 0.3 V )

Diode Approximations
 The complex diode model

DIODE CHARACTERISTICS
 Diode Characteristic
Curve
 Reverse bias
 As the voltage (VR)
increases to the
left, the current
remains near zero
until the
breakdown voltage
(VBR) is reached
 When breakdown
occurs, there is a
large reverse
current that can
destroy the diode

การป้ อนไฟให้ Diode
 Reverse bias:
 ให้ศักย์ไฟฟ
้ าด้าน Anode ( P ) ต่ากว่า
Kathode ( N )
 ป้ อนไฟให้กระแสไหลในทิศย้อนลูกศร ของ
สัญลักษณ์ไดโอด
 Diode จะทาตัวเหมือน switch เปิ ด ทาให้

Breakdown Voltage
(VBR)
 Reverse bias สูงมาก จนกระทั่ง
 Depletion region ขยายจนเต็มพื้นที่ N และ P
 Avalanche Effect
 Free e- ฝั่ง P มีพลังงานสูงมากจนกระทั่ง ชน valence e-
ใน P กลายเป็ น Free e-
 ทาให้จานวน Free e- เกิดเป็ นจานวนเท่าทวีคูณ ( rapid
multiplication )
 ทาให้เกิด Free e- ในฝั่ง P จานวนมาก ไหลผ่าน
รอยต่อได้
 กลายเป็ นกระแสไหลในทิศย้อนกลับ

Diode Testing
 Working diode
 Forward bias:
 Voltmeter: 0.7 v
 Reverse bias:
 Voltmeter: 2.5 – 3.5 v
(internal meter voltage)
 Open diode
 Forward & Reverse bias
 Voltmeter: 2.5-3.5 v
 Ohm meter: High resistance
 Shorted diode
 Forward & Reverse bias
 Voltmenter: 0 v
 Ohm meter: Low resistance

การใช้งาน diode
 กาหนดทิศการไหลของกระแสในวงจร
 ตัวอย่างวงจรที่ใช้งาน diode
 วงจร rectifier ใน DC Power Supply
 กระแสสลับ (AC) 2 ขั้ว (+/-) ให้เป็ น กระแสตรง (
DC) (ขั้วเดียว)
 วงจร Diode Limiter (จากัดความต่างศักย์
ของสัญญาณ AC)
 วงจร Diode Clamper (ยกระดับความต่าง
ศักย์ของสัญญาณ AC)

Basic DC power supply
 The dc power supply converts the standard
220 V,50 Hz into a constant dc voltage
 They consist of three parts : Rectifier, Filter,
and Regulator
 The dc voltage produced by a power supply is
used to power all types of electronic circuits,
such as television receivers, stereo systems,
VCRs, CD player

The Half-Wave Rectifier
Figure 17-2 Average Value of
the Half-wave
output voltage
VAVG=Vp (out) / 

Effect of Diode Barrier Potential on
Half-Wave Rectifier Output Voltage
 During the positive half-cycle, the input
voltage must overcome the barrier
potential before the diode becomes
forward-biased
Vp (out) =Vp (in) - 0.7 V
Figure 17-5

Peak Inverse Voltage (PIV)
 The maximum value of reverse voltage,
sometimes designated as PIV, occurs at the
peak of each negative alternation of the input
cycle when the diode is reverse-biased
Figure 17-7

 The full-wave rectifier is the most commonly used
type in dc power supplies
 allowing unidirectional current to the load during
the entire input cycle differ from the half-wave
rectifier that allows only during one-half of the
cycle
FULL-WAVE RECTIFIERS
Figure 17-9
Average Value of the Full-wave rectified output voltage
VAVG= 2Vp (out) / 

Center-Tapped Full-Wave Rectifier
 Using two diode connected to the secondary of
a center-tapped transformer
 At the positive half-cycle
 Forward-biases the upper diode D1
 Reverse-biases the upper diode D2
 At the negative half-cycle
 Reverse-biases the upper diode D1
 Forward-biases the upper diode D2

Center-Tapped Full-Wave Rectifier

Effect of the Turn Ratio on Full-Wave
Output Rectifier
 If the turn ratio of the transformer is
1:1, the output of the rectifier is equal
to ½ of the input voltage Vp
 Owing to the voltage input is
approximately equal to the output
voltage, we must use the step-up
transformer

Peak Inverse Voltage (full-wave
rectifier)
(sec)
p
(sec)
p
(sec)
p
)
out
(
p
(sec)
p
(sec)
p
(sec)
p
2
D
V
2
2
V
2
V
V
V
2
V
2
V
V














)
out
(
p
(sec)
p V
2
V
PIV 


Bridge Output Voltage
)
out
(
p
(sec)
p V
V
PIV 


POWER SUPPLY FILTER AND
REGULATORS
 After passed the rectifier, the output of the
power supply is filtered for reduce the
ripple, on the other hand, for make an
output smoothly
 Capacitor-Input filter
 Ripple Voltage
 The voltage which change due to charging and
discharge of the capacitor is called “ripple voltage”

Ripple Voltages for half-wave and
full-wave

Ripple Factor (r)
 Ripple factor is the ratio of the Vr to VDC,
expressed as :
%
100
V
V
r
DC
r


NOTE: the frequency in the full-wave rectifier is
twice of the half-wave rectifier

Surge Current in the Capacitor-Input
Filter

IC Regulators
 An integrated circuit regulator is a device that
is connected to the output of a filtered
rectifier and maintains a constant output
voltage
 The capacitor-input filter reduces the input
ripple to the regulator to an acceptable level
and it is combined in IC regulator.
 The most IC regulators have three terminal
 Input terminal
 Output terminal
 Reference terminal

Percent Regulation
 Line regulation
 Specifies how much change occurs in the output
voltage for a given change in the input voltage
 Load regulation
 Specifies how much change occurs in the output
voltage over a certain range of load current value
∆VOUT
∆VIN
%
100

Line regulation =
VNL- VFL
VFL
%
100

Load regulation =

Diode Limiter
 Diode Limiters
 Diode limiters (clipper) cut off above or below specified levels
in
L
S
L
out V
R
R
R
V 










Diode Limiting and Clamping
Circuits
 Diode Limiters
 Adjustment of the limiting level

Diode Clampers
 Diode clamper known as a dc restorer
 Add a dc level to an ac signal

ชนิดของ Diode
 Regular diode
 Zener diode
 Varactor diode
 Light-Emitting diode (LED)
 Photo diode

ZENER DIODES
 The zener diode is a
silicon pn junction device
and operate in the
reverse breakdown region
symbol

Zener Breakdown (Vz)
 Two types of reverse breakdown in
a diode
 Avalanche
 also occurs in the rectifier diode
(regular diode)
 Zener
 Occurs in a zener diode at low reverse
voltages
NOTE : Zeners with breakdown voltage of 1.8 to
200 V are commercially available

Breakdown Characteristic
ZM
zener
ZK I
I
I 

Regulator: ตัวคงค่าควา

Zener Equivalent Circuit
Regulator: ตัวคงค่าความต่างศักย์

Zener diode impedance
 The ratio of ∆Vz to ∆Iz is the zener
diode impedance
 Normally, ZZ is specified at IZT
 ZZ is approximately constant over the
full range of reverse-current values
∆VZ
∆IZ
Zz =

Zener Voltage Regulation
 Zener diodes can be
used for voltage
regulation in
noncritical low-current
applications
ZM
zener
ZK I
I
I 


 As the input voltage varies, the zener
diode hole the constant voltage across
the output terminals
Zener Voltage Regulation

Zener Regulation with a Varying Load
 The zener diode maintains a constant voltage
across RL as long as the zener current is
greater than IZK and less than IZM, this
process is called load regulation

VARACTOR DIODES
 A varactor is basically a reverse-biased pn
junction that utilizes the inherent capacitance
of the depletion region
 The depletion region acts as a capacitor
dielectric
d
A
C



VARACTOR DIODE APPLICATIONS
LC
fr

2
1


LEDs and PHOTODIODES
 There are two types of optoelectronic
devices
 The Light Emitting Diode (LED)
 The photodiode (light detector)

The Light Emitting Diode (LED)
 When the device is forward-biased,
electrons across the pn junction from the n-
type material and recombine with the holes
in the p-type material
 When recombination takes place, the
recombining electrons release energy in the
form of heat and light

 The semiconductive materials used in LEDs are
gallium arsenide(GaAs), galium arsenide
phosphide (GaAsP), and Gallium phosphide (GaP)
 Silicon and Germanium are not used because they
are very poor at producing light
 GaAs LEDs emit infrared (IR) radiation
 GaAsP produces either red or yellow visible light
 GaP emits red or green visible light

symbol
electroluminescence

Applications

The Photodiode
 The photodiode is a pn junction device
that operates in reverse bias

Diode application presentationnnnnnnnnnn

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