Transcript Recall-Lecture 4 - International Islamic University Malaysia

Recall-Lecture 5

DC Analysis
 Representation of diode into three models


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Ideal case – model 1 with V = 0
Piecewise linear model 2 with V has a given value
Piecewise linear model 3 with V and forward resistance, rf
• Diode AC equivalent model
– During analysis, must perform DC analysis first to
calculate ID in order to obtain rd
DC ANALYSIS
DIODE = MODEL 1 ,2
OR 3
CALCULATE DC
CURRENT, ID
AC ANALYSIS
CALCULATE
rd
DIODE = RESISTOR,
rd
CALCULATE AC
CURRENT, id
Other Types of Diodes
Photodiode
The term ‘photo’ means light. Hence, a photodiode converts optical energy
into electrical energy. The photon energy breaks covalent bond inside the
crystal and generate electron and hole pairs
Solar Cell
Solar cell converts visible light into
electrical energy. The working
principle is the same as photodiode
but it is more towards PROVIDING the
power supply for external uses
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ECE 1231
Light Emitting Diode
An LED is opposite of photodiode this time, it converts electrical energy into light
energy – Normally GaAs is used as the material for LED. During diffusion of
carriers – some of them recombines and the recombination emits light waves.
Schottky Barrier Diode
A Schottky Barrier diode is a metal
semiconductor junction diode. The
metal side is the anode while the ntype is the cathode. But the turn on
voltage for Schottky is normally
smaller than normal pn junction
diode
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ECE 1231
Breakdown Voltage
The
breakdown voltage is a function of the doping concentrations
in the n- and p-region of the pn junction.
Large doping concentrations result in smaller break-down voltage.
Reverse biased voltage – ET 
The electric field may become large enough for the covalent bond
to break, causing electron-hole pairs to be created.
So, electrons from p-type are swept to n-region by the electric
field and holes from the n-type are swept to the p-region
The movement will create reverse biased current known as the
Zener Effect.
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Zener Effect and Zener Diode
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The applied reverse biased voltage cannot increase without limit since at
some point breakdown occurs causing current to increase rapidly.
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The voltage at that point is known as the breakdown voltage, VZ

Diodes are fabricated with a specifically design breakdown voltage and are
designed to operate in the breakdown region are called Zener diodes. Circuit
symbol of the Zener diode:
NOTE: When a Zener diode is reversebiased, it acts at the breakdown region,
when it is forward biased, it acts like a
normal PN junction diode

Such a diode can be used as a constant-voltage reference in a circuit.
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The large current that may exist at breakdown can cause heating effects and
catastrophic failure of the diode due to the large power dissipated in the device.

Diodes can be operated in the breakdown region by limiting the current to a
value within the capacities of the device.
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ECE 1231
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Avalanche Effect
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While these carriers crossing the space-charge
region, they also gain enough kinetic energy.
Hence, during collision with other atoms, covalent
bond is broken and more electron-holes pairs are
created, and they contribute to the collision process
as well. Refer to figure below
Electron with
high kinetic
energy
e
© Electronics
e
atom
h
e
atom
h
eh
atom
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Zener Diode
10 k
Calculate the value of the current ID if VZ = 10V
ANSWER: ID = 0.2 mA
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ECE 1231
The Zener diode with VZ = 4.7V is used in the circuit shown below and the input
signal is a 20 V peak-to-peak square wave. Use V = 0 V
Sketch the output signal, vo.
+
+
Vin
R
Vo
-
-
HINT: When a Zener diode is reverse-biased, it acts at the breakdown
region, when it is forward biased, it acts like a normal PN junction diode
© Electronics
ECE 1231
Chapter 3
Diode Circuits
Voltage Regulator
Voltage Regulator - Zener Diode
A voltage regulator supplies constant voltage to a load.
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The breakdown voltage of a Zener
diode is nearly constant over a wide
range of reverse-bias currents.

This make the Zener diode useful in
a voltage regulator, or a constantvoltage reference circuit.
3. The remainder
of VPS drops
across Ri
1. The zener diode holds
the voltage constant
regardless of the current
2. The load
resistor
sees a
constant
voltage
regardless
of the
current
Example
A Zener diode is connected in a voltage regulator circuit. It is given that VPS = 20V, the Zener
voltage, VZ = 10V, Ri = 222  and PZ(max) = 400 mW.
a. Determine the values of IL, IZ and II if RL = 380 .
b. Determine the value of RL that will establish PZ(max) = 400 mW in the diode.
For proper function the circuit must satisfied the following conditions.
1. The power dissipation in the Zener diode is less than the rated value
2. When the power supply is a minimum, VPS(min), there must be minimum current in
the zener diode IZ(min), hence the load current is a maximum, IL(max),
3. When the power supply is a maximum, VPS(max), the current in the diode is a
maximum, IZ(max), hence the load current is a minimum, IL(min)
AND
Or, we can write
For general thumb of rule for design this circuit is, so from the last Equation
Maximum power dispassion in the Zener diode is
EXAMPLE 1 (Example 3.3 from textbook)
Consider voltage regulator is used to power the cell
phone at 2.5 V from the lithium ion battery, which
voltage may vary between 3 and 3.6 V. The current in
the phone will vary 0 (off) to 100 mA(when talking).
Calculate the value of Ri and the Zener diode power
dissipation
simple Zener diode voltage
regulator circuit
Solution:
The stabilized voltage VL = 2.5 V, so the Zener diode voltage must be VZ = 2.5 V. The
maximum Zener diode current is
The maximum power dispassion in the Zener diode is
The value of the current limiting resistance is
• Example 2
Range of VPS : 10V– 14V
RL = 20 – 100 
VZ = 5.6V
Find value of Ri and calculate the maximum power rating of the diode