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
𝑽𝑻
𝒓𝒅 =
𝑰𝑫
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
𝑉𝑍
𝑉𝑍 = 1.8𝑉 𝑡𝑜 200𝑉
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 𝑉 = 8.73𝑉 < 𝑉𝑍 = 10𝑉 , the zener diode
is in the “OFF” state.
• Substituting the open-circuit equivalent results
in:
b. For 𝑹𝑳 = 𝟑𝒌𝜴
Since 𝑉 = 12𝑉 > 𝑉𝑍 = 10𝑉 , 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 𝑅𝐿(𝑚𝑖𝑛) that will turn the Zener diode ON, we calculate the value of 𝑅𝐿 that
will result in a load voltage 𝑉𝐿 = 𝑉𝑍 . That is,
𝑉𝐿
10𝑉
• 𝐼𝐿(𝑚𝑎𝑥) =
=
= 40𝑚𝐴.
𝑅𝐿(𝑚𝑖𝑛)
250Ω
• 𝐼𝐿(𝑚𝑖𝑛) 𝑖𝑠 𝑜𝑏𝑡𝑎𝑖𝑛𝑒𝑑 𝑎𝑡 𝑚𝑎𝑥𝑖𝑚𝑢𝑚 𝑧𝑒𝑛𝑒𝑟 𝑐𝑢𝑟𝑟𝑒𝑛𝑡 𝐼𝑍𝑀 = 32𝑚𝐴
𝐼𝐿 𝑚𝑖𝑛 = 𝐼𝑅 − 𝐼𝑍𝑀 = 40 − 32 = 8mA
• 𝑅𝐿(𝑚𝑎𝑥) 𝑖𝑠 𝑏𝑡𝑎𝑖𝑛𝑒𝑑 𝑎𝑡 𝑚𝑖𝑛𝑖𝑚𝑢𝑚 𝑙𝑜𝑎𝑑 𝑐𝑢𝑟𝑟𝑒𝑛𝑡, Therefore
𝑉𝐿
10𝑉
𝑅𝐿(𝑚𝑎𝑥) =
=
= 1.25𝑘Ω
𝐼𝐿(𝑚𝑖𝑛) 8𝑚𝐴
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