ELE 124 - Electronics (1)
Lecture 10 : Special Purpose Diodes
Dr. Said Emam
Outline
• Networks with a dc and ac source
• Special Purpose Diodes
• Zener Diodes
• Light Emitting Diodes (LED)
• Photodiode
• Laser Diode
• Varactor Diodes
• Schottky Barrier diode (SBD)
• PIN diode
• Tunnel Diode
• Photovoltaic Cell
Networks with a dc and ac source
• Using Superposition Theorem.
The response of any network with both an ac and a dc source can be found by
finding the response to each source independently and then combining the
results.
(1) Using Superposition Theorem
DC Source only (AC = S.C.):
• In the absence of the ac source (i.e. replaced by a short-circuit):
AC Source only (DC= S.C.):
• In the absence of the dc source (i.e. replaced by a short-circuit):
The diode will be replaced by the ac resistance given by
π½π»
ππ
=
π°π«
Combining the results of the dc and ac analysis will result in the waveforms of for
ZENER DIODES
• A Zener diode is a silicon diode that is
designed for operation in the breakdown
region.
• The Zener diode is the backbone of voltage
regulators,
• The location of the Zener region can be
controlled by varying the doping levels.
Doping
ππ
ππ = 1.8π π‘π 200π
Zener Effect
• When a diode is heavily doped, the depletion layer becomes very narrow. Because
of this, the electric field across the depletion layer (voltage divided by distance) is
very intense.
• When the field strength reaches approximately 300,000 V/cm, the field is intense
enough to pull electrons out of their valence orbits (breaks the covalent bonds).
• The creation of free electrons in this way is called the Zener effect (also known as
high-field emission).
• This is distinctly different from the avalanche effect, which depends on high-speed
minority carriers dislodging valence electrons.
• When the breakdown voltage is less than 5 V, only the Zener effect occurs.
• When the breakdown voltage is greater than approximately 7 V, the avalanche
effect occurs.
• When the breakdown voltage is between 5 and 7 V, both effects are present
The specification sheet for a 10-V, 500-mW Zener diode
The Zener potential of a Zener diode is very sensitive to
the temperature of operation.
Example1: Find the zener potential if the temperature is increased
to 100°C
Analysis of Zener diode circuits
1) Determine the state of the diode
2) Substitute the appropriate model
3) Determine the unknown quantities
of the circuit.
The Zener diode as a regulator
Vi and R Fixed
Solution:
a. First we compute the Thevenin voltage across
the zener diode as :
• Since π = 8.73π < ππ = 10π , the zener diode
is in the “OFF” state.
• Substituting the open-circuit equivalent results
in:
b. For πΉπ³ = πππ΄
Since π = 12π > ππ = 10π , the zener diode is
in the “ON” state.
The Network in the “ON” state.
The power dissipated is
which is less than the specified
Solution
• To determine π
πΏ(πππ) that will turn the Zener diode ON, we calculate the value of π
πΏ that
will result in a load voltage ππΏ = ππ . That is,
ππΏ
10π
• πΌπΏ(πππ₯) =
=
= 40ππ΄.
π
πΏ(πππ)
250Ω
• πΌπΏ(πππ) ππ πππ‘πππππ ππ‘ πππ₯πππ’π π§ππππ ππ’πππππ‘ πΌππ = 32ππ΄
πΌπΏ πππ = πΌπ
− πΌππ = 40 − 32 = 8mA
• π
πΏ(πππ₯) ππ ππ‘πππππ ππ‘ ππππππ’π ππππ ππ’πππππ‘, Therefore
ππΏ
10π
π
πΏ(πππ₯) =
=
= 1.25πΩ
πΌπΏ(πππ) 8ππ΄
Zener Limiter
Determine the output voltage for each zener limiting circuit in Figure.
HOMEWORK 10
1. At what temperature will the 10-V Zener diode of table. 1 have a nominal voltage of 10.75 V?
Table.
1
2. Determine the temperature coefficient of a 5-V Zener diode (rated 25°C value) if the nominal voltage drops
to 4.8 V at a temperature of 100°C.
3)
4)
i.
Determine and draw the output voltage and current for the zener
limiting circuit shown in Figure.
ii. Draw the transfer characteristics