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1.1
1.2
1.3
1.4
Introduction
Diode characteristics
Reverse Recovery characteristics
Power Diode TypesI)General Purpose
ii)Fast Recovery Diode
iii)Schottky Diode
1.5 Series, Parallel Operation of Diodes
1.6 GTO, Power MOSFET, IGBT- Construction,
Operating principle, specification

A power is a two terminal device[1,2] and p n-junction
is normally formed by alloying ,diffusion ,and epitaxial
growth.

When the anode potential is positive with respect
cathode ,the diode is said to be forward biased and the
diode conducts. A conducting diode has a relatively
small forward voltage drop across it, and the
magnitude of this drop depends on the manufacturing
process and junction temperature .
 When the cathode potential is positive with
respect to the anode, the diode is said to be
reverse biased. Under reverse biased condition,
a small reverse current in the range of micro or
mill ampere , flows and this leakage current
increases slowly in magnitude with the reverse
voltage until the avalanche or zener voltage Is
reached.
 The
current in a forward –biased junction
diode is due to the effect of majority and
minority carriers . Once a diode is in a
forward conduction mode and then its
forward current is reduced to zero ,the
diode continues to conducts due to
minority carriers that remain stored in
the p n – junction and the bulk
semiconductor material.
The minority carriers require a certain time to
recombine with opposite charges and to be
neutralized. This time is called the reverse
recovery tine of the diode . Fig. 1.3 shows two
reverse recovery characteristics of junction
diodes.
The soft - recovery type is more common. The
reverse recovery time is denoted by trr and is
measured from the initial zero crossing of the diode
current to 25 % of maximum reverse Irr
General Purpose
Diode
Fast recovery Diode
Schottky Diode

The general purpose rectifier diodes have relatively high
reverse recovery time, typically 25 microsecond. And are
used in low speed application, where recovery time is
not critical.

These diodes cover current rating from less then 1 A to
several thousands of amperes, with voltage rating from
50V to around 5kV. These diodes are generally
manufactured by diffusion. However, alloyed types of
rectifiers that are used in welding power supply are most
cost effective and rugged, and their rating can go up to
1500 V, 400 A.

The fast recovery diodes have low recovery
time, normally less than 5 microsecond. They
are used in dc-dc and dc-ac converter
circuits, where the speed of recovery is often
of critical importance. These diodes cover
current ratings of voltage from 50 V to
around 3kv, and from less than 1 A to
hundreds of amperes.
 The
charge strong problem of a Pn junction
can be laminated in a schottky diode. It is
accomplished by setting up a barrier with a
contact between a metal and a
semiconductor. A layer of metal is deposited
on a thin epitaxial layer of n type silicon.
The potential barrier simulates the behavior
of a pn junction. The rectifying action
depends on the majority carriers only and as
aresult there are no excess minority carrier
to recombine.
 In
many high voltage applications HVDC
transmission ,one commercially diode can
not meet the required voltage rating and
diodes are connected in series to increase
the reverse blocking capabilities Let us
consider two series connected diodes is as
shown in fig variable ID and VD are the
current and voltage ,respectively in
forward direction.
VD1 and VD2 are the sharing reverse voltages of diode
D1 and D2 ,respectively in practice the V-I characteristics
for the same type of diode differ due to tolerance in
their production process. two v –I characteristics of such
a diode in the forward bias condition both diode conduct
the same amount of current and the forward voltage drop
would be almost equal .
However in the reverse blocking condition, each diode
has to carry the same leakage current and as a result the
blocking voltage may differ significantly.
IS=Is1+IR1=Is2+IR2.but IR1=VD1/R1 and IR2=VD2/R2.if the
resistance are equal then R=!=R2 and two diode voltages
would be significantly different. so final VD1=VD2=VS
In high power application diode are connected
in parallel to increase the current carrying
capability to meet the desired current
requirements .Where current sharing of diode
would be in accord with their respective forward
voltage drop uniform current sharing can be
achieved by providing the equal inductance or by
connecting current sharing resistors and this is
depiciated is shown in fig . It is possible to
minimize this problem by selecting
Resistor help current sharing under steady-state
condition current sharing under dynamic condition can
be accompanishe4d by connecting coupled inductor is
shown in fig . If the current through D1 rises L1 increase
and corresponding voltage of opposite polarity is include
across inductor L2 . The results is a low impendence
path through diode D2 and current is shifted to D2 . The
inductors may generate voltage skips and they may be
expensive and bulky specially at high current.

 The
basic structure of GTO is shown in fig. (a). As
seen from the Construction of GTO.
 Fig.(a)
it is basically a four layer structure similar
to a conventional SCR
 Fig.
(b) shows the circuit symbol for the GTO.As
seen from the Fig.(b) GTO is a three terminal
device. Gate is control terminal.
 Note
the two arrows marked on the gate terminal.
They indicate that the gate current for GTO can be
either positive or negative. (Whereas in SCR the
gate current is only positive.)
 The
turn on mechanism of GTO is exactly
same as that of a conventional SCR.
 The
dependence of GTO’s breakover voltage
on the magnitude of gate current is also
same as that of SCR .
 In
short the working principle of GTO at the
time of turn on and in the on state is same as
that of SCR.
 However
the operation at the time of turn off is
entirely different.
 To
turn off the conducting GTO ,we have to
apply a negative gate current pulse at the gate
terminal .
 The
basic operation of GTO is same as that of the
conventional SCR.
 But
the major difference between them is that the
conducting GTO can be turned off by applying a
negative gate current to it .Thus a positive gate
current turns it on and negative gate current turns
it off.
 Refers
Fig.(C)
to the equivalent circuit shown in
 Both
the transistors Q1 and Q2 are in
saturation when the GTO is in it’s on state .
 However
if the base current of Q2 could briefly
be made less than the value needed for
maintaining it in saturation ,then Q2 will come
out of saturation and will be in the active
state, this will reduce the regeneration and
GTO will begin to turn off.
In order to reduce the base current of Q2,a
negative gate current IG(-) must flow in the
direction as shown in Fig.(c).
It can be proved that the negative gate
current required for turning off a conducting
GTO is given by,
 IG(-) >
Where β OFF =
The parameter β OFF is known as the turn off
gain.
 Equation (c) clearly indicates that to turn
off the GTO , the negative current I(G)(-) must
be greater than the anode current divided by
the turn off gain.
 The
long form of MOSFET is metal oxide
semiconductor field effect transistor .
 Power
MOSFETs with improved current
carrying capacity and off state blocking
voltage capacity are now available and are
replacing the power transistor in many
applications.
 Power MOSFETS are capable of switching at very high
frequency up to about 100 KHz.
 A power BJT is a current controlled device. The
collector current is dependent on the base current hence
the current is highly dependent on temperature. This is the
serious disadvantage of power BJT.
 To overcome this disadvantage .we can use a voltage
controlled device such as a power MOSFET. The other
advantage of MOSFET is that it require only a small input
current.

With gate to source voltage VGS=0 the MOSFET is
equivalent to two back to back diodes connected
as shown in Fig (a). The diodes are formed
between n and p layers as shown.

The basic structure of MOSFET is very much
similar to the BJT. The only difference is the
presence of MOS capacitor that isolates the gate
from the body region.
 When the gate to source voltage is applied, the
MOSFET turns on. The operation takes place.
BASIC STRUCTURE OF IGBT The
vertically oriented structure of IGBT is as
shown in fig.
 Like
al other device IGBT also uses the
vertically oriented structure in order to
maximize the area available for the current
flow.
 This
will reduces the resistances offered
to the current flow and hence the on
state power loss taking place in the
device.
 This
device also uses the n-type drain
drift layer which also improves its
breakdown voltage capacity .this is same
as that in case of power MOSFETS.
 This
layer improves the operation of IGBT
in two important aspects. B
 1)The
I-V characteristics of IGBT is as shown
in fig .in the forward direction the yare
similar to those of logic level bipolar
transistor he only difference here is that the
controlling parameter is the gate to sources
voltage and the parameter seeing0
controlled is the drain current
 2)the
IGBT is the voltage controlled devices
with an insulated gate.
 3)the
IGBT possesses all the advantage of
MOSFETS due to the insulated gate it also
has all the advantages of the BJT due to
bipolar conduction
 4)as
seen from the drain current increase
with the increase in the voltage between
gate and
 breakdown
voltage this is the voltage at
which the avalanche breakdown take place
.at this point the voltage across the devices
and current through it both are high