Power electronics basic and introductions

POWER ELECTRONICS
Subject Code: PCT-803
Dr. Ramachandra A C
Professor & Head,
Department of Electronics and Communication Engineering,
Nitte Meenakshi Institute of Technology, Yelahanka, Bengaluru-560064,
Karnataka, India.
Lecture-03

26/03/2025 AIRFORCE TECHNICAL COLLEGE, BENGALURU 2
TEXTBOOKS

SYLLABUS

TRAINING OBJECTIVE
Students to understand the
• Power Electronic Components switching Characteristics.
• Its Circuit Protection .
• Power Devices Cooling Effect

LEARNING OBJECTIVE
After the class, the cadets would have learned about the
• Basics of Power Electronics Concepts
• Working operation of power electronics devices and
working of high power rating power devices.
• Application of power devices.

COVERAGE
Module 803-1.1 : Overview of Power Electronics
• Introduction to Power Diode
• Construction and working
• V-I Characteristics.
• Concept Reverse Recovery
• Application of Power Diode

PREVIOUS LESSON TIE UP
Communication Basics

QUOTE
“Continuous learning is the minimum requirement for
success in any field”
— Brian Tracy

Introduction to Power Diodes
• A power diode is built differently than a conventional diode to allow
for this increased current rating. To improve its power handling
capability, the power diode’s structure is modified slightly from a
simple p-n junction diode.

Construction of Power Diode

Construction of Power Diode (Contd…)
• An n+ strongly doped substrate is taken into consideration, over
which an n- layer is developed epitaxial.
• Diffusion of the p+ layer also occurs over the n- area. While the n+
substrate serves as the cathode connection, this p+ area serves as
the anode connection.
• Thus, the p+ and n+ areas serve as the anode and cathode,
respectively, while the n- region serves as the drift region.
• The depletion zone is absorbed inside the drift region in the
reverse-biased state.
• The breakdown voltage of the diode when reverse biased exhibits
a direct correlation with the thickness of the n-region.
• Therefore, the breakdown voltage will be higher the broader the n-
region.

26/03/2025 AIRFORCE TECHNICAL COLLEGE, BENGALURU
Working of Power Diode
12
In forward biased condition: The positive terminal of the battery is
connected to the anode and the negative terminal forms a
connection with the cathode.
Working Principle of Power Diode

Working of Power Diode (Contd…)
13
• Majority carriers (holes) from the p+
region start injecting into the
n- drift region.
• When the injection rate is low then the holes of the p+
region will
recombine with electrons of the n–
region.
• As the injection rate is increased then holes will penetrate and
recombine with electrons of the n+
region. This is called double
injection.
• Due to this flow of carriers and recombination within the drift
region, the diode starts conducting heavily once the threshold is
surpassed.

14
• In reverse biased condition: The negative terminal of the battery
is connected to the anode and the positive terminal forms a
connection with the cathode.
• The junction becomes reverse biased and like a normal diode,
power diode also stops conduction in this condition.

15
• The depletion region is extended up to the drift region that will cause
difficulty for the minority carriers to penetrate the junction and recombine.
• But it is to be noted here that a sudden change in polarity of applied
potential will not immediately stop the flow of current.
• Moreover, the minority charges stored in the junction will lead to the flow of
small leakage current (of order 100 mA) through the diode in opposite
direction. This reverse current shows dependency on variation in junction
temperature.
• Once the applied potential becomes equal to the breakdown voltage, then
impact ionization will take place.

V-I Characteristics of Power Diode
• Initially with no supply voltage
forward current is 0 but as the
supply input increases, and reaches
the threshold value (of about 0.7 V),
a small amount of forward current
flows through the device.
• Once the threshold value is
surpassed, a considerable increase
in diode current (at 1V) is noticed as
it starts conduction.
• Here linear rise in forward current is
noticed when voltage increases
beyond the threshold.

Reverse Recovery Characteristics of
Power Diode

Power Diode (Contd…)
• Even after removal of forward applied voltage, the diode conducts
due to stored charge in the depletion region and the
semiconductor layer.
• The time being for which this leakage current flows is called
reverse recovery time, trr. The blocking ability of the diode is
regained till the time the leakage current becomes 0.
• The trr is the time between the moment forward current vanishes
and the moment the reverse recovery current remains only 25% of
its peak value IRM.
• From the figure, it is clear that
trr = ta + tb

Power Diode (Contd…)
• During ta the charge within the depletion region, is vanished. While
tb is the duration from the peak of reverse current IRM to 0.25% IRM.
• During tb, the charge from the layers of semiconductors is removed.
• The ratio of tb and ta is termed as softness factor given by S.
• It is generally unity, hence such diode with S equal to 1 is called soft
recovery diode.
• While if S>1 hence it is called fast recovery or snappy recovery
diode.

Types of Power Diode
The classification of power diodes is done on the basis of reverse
recovery characteristics possessed by them.

Types of Power Diode
1. General Purpose Diodes: These possess trr quite high i.e., of about 25
microseconds. Low-frequency applications such as rectification, converters operated
nearly up to 1KHz uses this diode. Its current rating lies between 1 A to several
thousand amperes and its voltage rating lies between 50 V to 5 KV.
2. Fast recovery Diodes: These exhibit trr quite low i.e., of about 5 microseconds.
Mainly used in electrical power conversion systems. Its current rating lies between 1 A
to several thousand amperes and its voltage rating lies between 50 V to 3 KV.
3. Schottkey Diodes: In these diodes instead of p-n junction, metal-semiconductor
junction is formed where generally aluminium is preferred for metal and silicon for
semiconductor. Its current rating lies between 1 A to 300 A while the reverse voltage
rating is about 100 V.

Applications of Power Electronics
(Contd…)
Due to the switching characteristics of power diodes, these
are mainly used as:
 freewheeling diodes, in ac to dc and dc to ac
conversion systems, battery charging, etc.
 Along with these, power diodes are also used in
electroplating, UPS, choppers, SMPS, and
induction heating as well.

Questions by the class???

ASSIGNMENT QUESTIONS
1) With a diagram explain the construction and working of Power
Diode, also elaborate on Reverse Recovery Time

THANK YOU

Power electronics basic and introductions

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