Power Devices -I

Definition and Application of Power Electronics :-

Power electronics combine power, electronics end control. Power electronics may be
defined as the applications of solid-state electronics for the control and conversion of electric
power. The co-relationship of power electronics with power electronics and control is as shown
in fig. 1.
Power

Control
Analog/Digital

Electronics Power equipment

Devices/ Circuits Static/ Rotating

Electronics
Fig 1: Relationship of Power electronics and control.

The power electronics is based on the switching of power semiconductor devices. With
the development of power semiconductor technology, the power-handling capabilities and the
switching speed of power devices have improved largely. The development of
microprocessors/microcomputer technology has a great impact on the control and synthesizing
the control strategy for the power semiconductors that devices.
Modern power electronics equipment uses,
1. Power semiconductors that can be regarded as the muscle and 2. Microelectronics that as the
power and intelligence of a brain.
The power electronics have already found an important place in modern technology and
are now used in a great variety of high-power applications such as heat, light & motor controls,
Power supplies, and high voltage DC systems. It is difficult to decide the limits for the
applications of power electronics.
Need for Semiconductor Power Devices:
The voltage, current and power ratings of semiconductor power devices are much
higher than the corresponding ratings for low-power or signal devices. Power devices operate
at lower switching speeds whereas signal devices operate at higher switching speeds. The
powers semiconductor devices used extensively in power - electronic circuits. The power
Semiconductor devices are diode, Thyristors, and Transistors. The brief description of power
device is as follows:

1. Power diode:
Diodes play a significant role in power electronics circuits. These are
uncontrolled rectifying devices. It acts as a switch to perform various functions such as
switches in rectifiers, change reversal of capacitor and energy transfer between
components, voltage isolation, energy feedback from the load to power Source.

2. Thyristors:
The thyristors are used extensively in power electronic circuits. They are
operated as bistable switches, operating from of state to conducting state. The member
of thy sister family are SCR, LASCR, RCT, GTO, SITH, and MCT.
3. Transistors:
The power transistors have controlled turn-on and turn off characteristics. The
switching speed of power transistor as much nigher than that of thyristors, and they are
extensively used in dc-dc and dc-ac converters. The power transistors can classifieds
BJT, MOSFETs, SITs and IGBTs.

• Power Diodes
Power semiconductor diodes play a significant role in a power electronics circuits. A
diode acts as a switch to perform various functions, such as switches in rectifiers, energy
transfer between components, voltage isolation, energy feedback from the load to the power
source.
The power diodes are similar to P-N junction signal diode. However, the power
diodes have large power, voltage and current handling capabilities than that of the ordinary
signal diodes. The frequency response (or switching speed) is slow compared to signal diodes.

Construction of Power diode:

The basic vertically oriented structure of a general purpose power diode is as shown in
figure 1(a). The circuit symbol for the power diode is as shown in figure 1(b).
 The vertically oriented structure is preferred in all the power devices as it increases the
surface area for the forward current, therefore reduces the forward resistance and on State
Power dissipation. Figure consists of a heavily doped (n+) substrate on the top of which a
lightly doped (n-) layer of desired thickness is epitaxially grown. A heavily doped (p+) layer
acts as anode. The lightly doped (n-) layer introduced between the heavily doped (p+) and (n+)
layers is the major difference between the power diode and the low power diode. This layer is
known as the “Drift” layer.
The important points from the basic structure are:
1. The anode and cathode layer are heavily doped whereas the drift layer is lightly
doped.
2. The junction is formed between the anode layer (p+) and the drift layer (n-).
3. The width of drift layer (Wd) depends on the desired value of the breakdown
voltage. The breakdown voltage of the power diode increases with increase in
the width Wd.
4. The resistivity of the drift layer due to the low level of doping.
5. The circuit symbol for the power diode is identical to that used for the low
power diode.

• Conductivity Modulation:
(Breakdown voltage and on state losses)
The breakdown voltage of a power diode in the reverse biased region depends on:
1. The doping profile of the junction (step, linearity graded, diffused etc), and the
magnitudes of the doping densities (level).
2. With the structure, doping density on (n-) side much less than that on the (p+) side, the
depletion region gets extended almost entirely in the lightly doped (n-) region.
3. In order to obtain a higher value of breakdown voltage at least one side of the PN
junction must be lightly doped.

The on state power loss is the product of on state voltage across the device and
the forward current through it. Whereas the switching loss takes place when the device
is changing its state from off to on or on to off.
The on state power loss in power diodes is high due to the inclusion of the (n-)
drift layer in the structure.
 Conductivity modulation:

However, the on-state resistance of the drift region is much lower than the
estimated value based on the physical size of this region. This is because in the on
state, the resistance of the drift layer reduces substantially due to large amount of excess
carrier injection into the drift region from both sides as shown in figure 1. In this, holes
are injected from the (p+) layer and electrons are injected from the (n+) layer into the
lightly doped drift layer (n-). Due to this excess charge injection, the conductivity of the
drift layer increases. This phenomenon is known as “Conductivity Modulation” and it
reduces the on state power dissipation to a great extent.

• I-V characteristics of power diode:

The I-V characteristic of a power diode is as shown in figure 1. The forward

current appears to rise exponentially with increase in the forward bias voltage. The
conclusions that can be drawn from characteristics are as follows:
 (a) The on state voltage across a power diode is higher than 1 volt which is higher than the
on state voltage across a low power diode.
(b) With the power diode reverse biased, a very small leakage current flows through it. The
leakage current is independent of applied reverse bias Voltage.
(c) When the reverse voltage reaches the reverse breakdown voltage VBD, avalanche
breakdown takes place.
(d) As the voltage across and current through the device is large simultaneously, when the
device breakdown, the power dissipation is dangerously high which can destroy the
device. This must therefore be avoided,
(e) The on state resistance offered by the diode (Ron) can be found from the
characteristics.

• Reverse recovery effect of power diodes:

The current in a forward biased junction diode is due to the net effect of majority and
minority carriers. Once a diode is in a forward conduction mode and then its forward current is
reduced to zero (due to the natural behaviour of diode circuit or by applying a reverse
voltage), the diode continues to conduct due to minority Carriers which remain stored in the
PN junction and the bulk semiconductor material. The minority Carriers require certain time to
recombined with opposite changes and to be neutralized. This time is called the reverse
recovery time of the diode. Figure 1 shows the reverse recovery characteristics of junction
diodes.

The soft recovery type is more common. The reverse recovery time is denoted as trr and
is measured from the initial zero crossing of the diode current to 25% of maximum (or
peak) reverse currentIRR. The trr consists of two components ta and tb , ta is due to charge
storage in the depletion region of the junction(and represents the time between the zero
crossing and the peak reverse current IRR), tb is due to charge storage in the bulk semiconductor
material. The ratio tb/ta is known as the softness factor SF. For practical purposes, the total
recovery time trr and the peak value of the reverse current IRR.
 trr =ta+ tb (1)
The peak reverse current can be expressed in reverse di/dt as
IRR= ta di/dt (2)
Reverse recovery time trr, may be defined as the time interval between the instant the
current passes through zero during the change over from forward conduction toreverse
blocking condition and the moment the reverse current has decayed to 25% of its peak reverse
value IRR. The trr is dependent on the junction temperature, rate of fall of forward current and
the forward current prior to commutation.
Reverse recovery Charge QRR, is the amount charge carriers that flow across the diode
in the reverse direction due to changeover from forward conduction to reverse blocking
condition.
The storage Charge,which is the area enclosed by the path recovery current is
approximately
 From equation (6) and (7) the reverse recovery time trr and the peak reverse recovery
current IRR depend on the storage charge QRR and the reverse di/dt. The storage charge is
dependent on the forward diode current IF.

• Types of Power Diode :

Ideally a diode should have no reverse recovery time. However, the manufacturing cost of
such a diode will increase. Depending on the recovery characteristics and manufacturing
techniques the power diodes can be classified into three categories:
1. Standard or general-purpose Diodes
2. Fast recovery diode
3. Schottky diode

1. General-purpose Diodes:
The general purpose rectifier diodes have a relatively high reverse recovery
time, typically 25 μs and are used in low speed applications. These diodes cover current
rating from less than 1A to several thousands of amperes, with voltage ratings from
50V to around 5kV. These diodes are generally manufactured by diffusion. However,
alloyed types of rectifiers that are used in welding power supplies are most cost
effective and their ratings can go up to 300A and 1000 V.

2. Fast Recovery Diodes:

The fast recovery diodes have low recovery time, normally less than 5μs. They
are used in DC-DC and DC-AC converter circuits, where the spirit of recovery is often
of critical importance. These diodes cover current ratings from less than 1A to hundreds
of amperes, with voltage ratings from 50V to around 3kV.

3. Schottky Diodes:
The charge storage problem of a PN junction can be eliminated or minimized in
a Schottky diode. It is accomplished by setting up a barrier potential 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 behaviour of a PN junction.
(The rectifying action depends on the majority carriers only and as a result there are no
excess minority carriers to recombine) the recovery effect is due solely to the self
capacitance of the semiconductor junction.
The recovered charge of a Schottky diode is much less than that of an equivalent
PN junction diode. (Since it is due only two the junction capacitance, it is largely
independent of the reverse di/dt). A Schottky diode has a relatively low forward voltage
drop.
The leakage current of a Schottky diode is higher than that of a PN junction
diode. The Schottky diodes are ideal for high current and low voltage DC power
supplies.
Spice Diode Model :

The SPICE model of a diode is shown in figure 1(a). The diode current ID that
depends on its voltage is represented by a current source. RS is the series resistance and
it is due to resistance of the semiconductor. RS is also known as bulk resistance, is
dependent on the amount of doping. The small signal and static models that are
generated by SPICE are shown in Figure 1(b) ,(c) and (d) Respectively. Where CD is a
nonlinear function of the diode voltage VD and is equal to CD=dqd/dVd, Where qd is the
 depletion layer Charge. SPICE generates the small signal parameters from the operating
point.
The SPICE model statement of a diode has the general form

MODEL DNAME D(P1=V1 P2=V2 P3=V3........ PN=VN)

DNAME is the model name and it can begin with any character; but its word
size is normally limited to 8. D is the type symbol for diodes. P1, P2,... and V1, V2,....
are the model parameters and their values respectively.

• Series-connected Diodes:
In many high voltage applications (e.g. HVDC transmission lines), one
commercially available diode cannot meet the required voltage rating, then diodes are
connected in series to increase the reverse blocking capabilities.

Let us consider two series connected diodes as shown in Figure 1(a). In practice,
the V-I characteristics for the same type of diodes differ due to tolerances in their
production process, as shown in figure 1(b). In the forward biased condition, both
diodes conduct the same amount of current and the forward voltage drop of each diode
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 voltages will differ
significantly.
A simple solution to this problem, is to force equal voltage sharing by connecting a
resistor across each diode, as shown in figure 2(a).
 Due to equal voltage sharing the leakage current of each diode would be
different and this is shown in figure 2(b). Since the total leakage current must be shared
by a diode and its resistor.

Is= IS1 +IR1 =IS2 + IR2 (1)

VD1 + VD2 = VS
• Parallel Connected Diodes:
In high power application, diodes are connected in parallel to increase the current
carrying capability to meet the desired current requirements. The current sharing of diodes
would be in accord with their respective forward voltage drops. Uniform current sharing can be
achieved by providing equal inductances(e.g. in the leads) or by connecting current sharing
resistors(Which may not be practical due to power losses), as shown in Figure 1. It is possible
to minimize this problem by selecting diodes with equal forward voltage drops or diodes of the
same type. Since the diodes are connected in parallel, the reverse blocking voltages of each
diode would be the same.
 The resistors of figure 1(a) will help Current sharing under steady state conditions.
Current sharing under dynamic conditions can be accomplished by connecting coupled
inductors as shown in figure 1(b). If the current through diode D1 rises, the L di/dt Across L1
increases and a corresponding voltage of opposite polarity is induced across inductor L2. The
result is a low impedance path through diode D2 and the current is shifted to D2. The inductors
would generate voltage spikes and they may be expensive and bulky, especially at high
currents.

• Power Transistor:
The power bipolar transistor is supposed to block a high voltage in the off state and
have a high current carrying capacity in the on state. Power transistors have controlled turn on
and turn off characteristics. In order to have these characteristics the power bipolar transistor
(BJT), must have a substantially different structure than the small low power BJT.

Power BJT structure:

A power transistor has a vertically oriented four layer structure of P type and N type
as shown in figure 1. The power transistor has three terminals collector, base and
emitter. The circuit symbol for the BJT is as shown in figure 1(b). (The npn transistor
are more widely used than the pnp transistors as power switches).

1. The vertical structure increases the cross sectional area through which the
device current flows. This will reduce the on state resistance of the device and
hence the on state power dissipation in the transistor. (The vertical structure also
minimises the thermal resistance of the transistor, thus keeping the problem of
power dissipation under control.)
 2. Doping levels- Figure 1(a) shows the doping levels in each of the layers. The
thickness of the different layers will have a significant effect on the
characteristics of the device. As shown in figure 1(a), the emitter layer is heavily
doped. The base is moderately doped. The (n-) region is known as the collector
drift region and it is lightly doped. (The (n+) region that terminates the drift
region has doping level similar to that of emitter). The (n+) region serves as
collector terminal.
3. If the structure of power transistor is compared with that of transistor, it is found
that the (n-) drift layer is included in the power transistor. Due to the low doping
level the (n-) drift layer will increase the voltage blocking capacity of the
transistor. The width of this layer will decide the breakdown voltage of power
transistor. The disadvantage of including this layer is that the on state device
resistance increases, increasing the on state power loss.
4. The current gain β of a transistor depends on the base thickness. As the base
thickness reduces the gain increases but the breakdown voltage of the transistor
will decrease. Thus the base thickness is a compromise between these two
factors. In power transistors high breakdown voltage is more important than
high current gain. Therefore the base thickness is much larger in power
transistors. This reduces the current gain β of power transistors 5 to 10.

Operation of Power Transistor

The difference between the power transistor and that of logic level (low power)
transistor is that in the power transistor an additional region called collector drift
region is included. However the basic mechanism operation for the power transistor
is same as that in the logic level transistors, even the current gain mechanism is
similar. The collector drift region does not play any role when the power BJT is in
the active region. The drift region will affect the breakdown voltage, on state losses
and switching time of power BJT. A very important factor in the operation of a
power transistor is the mechanism to obtain a high value of current gain β. There are
three prime requirements for large values of β in a BJT.
(1) Heavy doping of emitter region
(2) Long minority carrier lifetime in the base and
(3) Short base Thickness
These factors will conflict with the other characteristics desired for the power transistor
and hence a compromise will be required between the large gain and other parameters such as
fast switching times, breakdown voltage etc. The effect of this compromise is that the thickness
of base region in power transistor is larger than that in the logic level transistors and the β of
power transistor is typically 5 - 20 i.e. 10.
The β of the power transistors as well as the logic level transistors is not constant, in
fact it is dependent on the value of collector current as shown in figure 2. The two important
reasons for this are(1) conductivity modulation in the base region and (2) emitter current
crowding .
 At large value of collector current the β is approximately inversely proportional to the
IC. The effect of emitter current crowding on the β values can be reduced by using a structure in
which the emitter is separated into many rectangular areas. The value of VCE(sat) increases
sharply after the collector current exceeds the value of IC(max). This fact is utilized to design the
protection circuit against overcurrent condition.

• Power MOSFET
A Power MOSFETs (Metal oxide semiconductor field effect transistors) with improved
on the state current carrying capacity and off state blocking voltage capability are now
available and are replacing the power BJTs in the many applications, especially where high
switching speeds are important.
MOSFETs are two types (1) Depletion MOSFET and (2) Enhancement MOSFET

Structure of power MOSFET

The power MOSFET has the vertically oriented four layer structure as shown in figure
1(a) and the circuit symbol is shown in figure 1(b). The vertically oriented structure reduces the
on state resistance and therefore on state loss. The n+ p n- n+ Structure shown in Figure 1, is
termed as enhancement mode n-channel MOSFET. A structured with the opposite doping
profile can also be fabricated and is termed as p-channel MOSFET. The doping in the two
n(+)regions of figure 1(a) labelled “source and drain” is approximately the same in the both
layers and it is quite large. The p-type middle layer is termed as ‘body’. The n- layer is the
‘drift region’ and it is lightly doped as compared to the drain and source layers. This drift
region determines the breakdown voltage of the power MOSFET.
[The circuit symbol for n- channel and p-channel MOSFETs are shown in figure1(b).
Three terminals are Drain, Source and gate. The body Terminal is shorted with the source
terminal internally.]
The direction of arrow on the body lead indicates the direction of current flow. (If the
body source pn junction were forward biased by removing the short link between the body and
source.) Therefore n-channel MOSFET has a p-type body region and the arrow points into the
MOSFET symbol.
The MOSFET has two different modes of operation. If VGS>0 it works in the
enhancement mode where the conductivity increases with increase in the gate to source
voltage. If VGS<0 the device work in the depletion mode where the conductivity decreases with
increase in the gate to source voltage (VGS).

The simplified structure of enhancement MOSFET is shown in figure 2 the structure is

very much same as a transistor. The p-type layer is known as the body layer that gives the MO
normally off nature.
As seen from figure2, the gate terminal is not connected directly to the semiconductor
(p-layer), instead there exist an oxide layer (SiO2) between the metal and semiconductor. The
oxide layer acts as a layer of dielectric between the metal and the semiconductor to form a
MOS (metal oxide semiconductor) capacitance at the input of the MOSFET. This MOS
capacitance does not exist in the low power JFET. The input capacitance of MOSFET is large
(greater than 1000 pf). The SiO2 oxide layer isolates the gate terminal from the body layer and
gives the device insulating properties.]
Operation of Power MOSFET
With gate to source voltage VGS=0 the MOSFET is equivalent to two back to back
diodes connected as shown in figure 2(a). The diodes are formed between n+ and P layers. The
basic structure of MOSFETis very much similar to the BJT. The only difference is the presence
of a MOS capacitor that isolates the gate from the body region.

When the gate to source voltage is applied, the MOSFETs turns on. The operation takes
place as explained below.
The basic structure of the N type MOSFET as shown in figure 2(b). Due to the presence
of P-layer (base layer) it is consider that the conduction cannot takes place through MOSFET
from drain to source. But practically it is possible due to a phenomenon called “Inversion
layered creation.”

Formation of depletion region:

The MOSFET is forward biased by connecting a positive voltage to its drain terminal
with respect to source terminal and the gate is made positive with respect to the body layer as
shown in figure 2(b). the P-layer consist of a large number of holes and few electrons. (Even
though the number of electrons is far less as compared to the number of holes, still the number
of electrons present in the P- layer is sufficiently large.)
Due to the positive voltage applied between gate and body, these electrons are attracted
towards the gate and greater below the SiO2 layer and produce depletion layer by combining
with the holes.

Creation of inversion layer:

If the gatevoltage is increased further, the number of electrons below the SiO2layer will
be greater than the number of holes. Thus the conductivity of portion of the layer close two
SiO2 layer will change from positive to negative. That means on n-type of sub layer is formed
below the SiO2layer as shown in figure 3. This process is known as creation of the inversion
layer and the process of generation of an inversion layer due to the externally applied gate
voltage is known as the field effect .
In this way now the n-type channel gets created in the p-type body layer and conduction
can take place through this layer. The MOSFET now acts as a variable resistor where the
resistance of the channel is dependent on the magnitude of gate to base (body) voltage. With
increase in the gate to body voltage, the resistance will decrease. (However this resistance
cannot decrease below a certain minimum value; Even with the increase in the gate. To body
voltage. There is a limitation on the maximum value of voltage applied between gate and body.
If it is exceeded the SiO2 dielectric will breakdown).
I-V characteristics of MOSFET:

The MOSFET is a three terminal device like power BJT, in which the gate terminal
controls the drain current of the device. The source terminal is common between the input and
output of the MOSFET. The output characteristics i.e. drain current iD as a function of drain to
source voltage VDS with gate to source voltage VGS as a parameter.
The active, cut off and ohmic regions of the characteristics are shown in figure. In the
power electronic applications the MOSFET is used as switch, the device must be operated in
the cut off and ohmic region when turned off and on respectively. The operation in the active
region should be avoided to reduce the power dissipation in the on state.
The MOSFET is in cut off state when the gate source voltage is less than the threshold
voltage VGS(th). The device must withstand to the applied voltage and to avoid the breakdown,
the drain to source breakdown voltage should be greater than the applied voltage. When
breakdown occurs it is due to the avalanche breakdown of the drain body junction. When
longer gate to source voltage is applied the device is driven into the ohmic region where the
drain to source voltage VDS(on) is small (In this regions of operation the power dissipation can
be kept reasonably low, by minimising the on state voltage ).
In the active region the drain current iD is almost independent of the drain to source
voltage VDS. It is only dependent the gate to source voltage VGS as shown . The gate voltage
VGS greater than the threshold voltage VGS(th). The power dissipation in the MOSFET is high in
the active region.
The important conclusions from the I-V Characteristics of MOSFET are as follows:
(i) The MOSFETs are voltage controlled devices in the output current can be controlled by
varying the gate-to-source voltage.
(ii) with increase in VGS the drain current will increase
(iii) The gate- to-source voltage (VGS) should be large enough to drive the MOSFET into
ohmic region. (Practically the minimum VGS required is about 12V. If VGS is less than 12V, the
MOSFET will operate in the active region which is not desired).
(iv) When the forward voltage applied to the MOSFET exceeds the beakdown voltage BVDSS,
the avalanche breakdown takes place. (Operation above the breakdown voltage must be
avoided).
(v) The second breakdown does not exist in MOSFETS.
* Advantages of power MOSFET over power BJTs are as follows:
1 Fast switching
2. Absence of second breakdown
3. Wide safe operating area
4. Extremely high gain.
*Applications of Power MOSFETs:
1. High frequency switching power supplies.
2. Chopper of inverter systems for dc and acmotor speed control.
3. High frequency generators for induction heating.
4. Ultrasonic generators, audio amplifiers.

Exercise:
01) Select the correct Alternative:
a) The process of conductivity modulation results in ________________.
i) Increase in the switching frequency
ii) Increase in the on state loss.
iii) Increase in the breakdown voltage.
iv) Reduction in on state loss.
b) Operation of MOSFET is basically dependent on __________________
i) the principle of conductivity modulation
ii) the principle: of current multiplication
iii) principle of creation of inversion layer
iv) none of the above
c) The process of conductivity modulation results in the reduction in__________________
i) current ii) on state loss iii) voltage iv) voltage and current
d) A TRIAC is equivalent to__________________
i) two diodes in antiparallel
ii) two thyristor in parallel
iii) two thyristor in antiparallel
iv) two diodes in parallel
e)________modulation is a process where holes and electrons both are injected into
the drift layer.
i) Conductivity ii) Resistivity iii) Amplitude iv) Phase
f) The MOSFET is a ___________controlled device
i) Current ii) Voltage iii)Power iv) None of these
g) Which of the following device has the terminals collector , emitter, gate?
i) BJT
ii) MOSFET
iii) SCR
iv) IGBT
h) A power diode uses the vertically oriented structure as________________
i) It reduces on state voltage drop
ii) It increases the switching frequency
iii) It gives the device properties of majority carrier device
iv) It decreases the size of the diode

I) Secondary breakdown occurs in________________

i) MOSFET
ii) MOSFET & BJT
iii) BJT
iv) none of these

02) Attempt any Two (Eight marks each):

a) Explain the basic structure and working of power MOSFET.
b) Explain the basic structure and working of power diode.
c) Explain the basic structure of power MOSFET and give its principle of operation
d) Explain the basic structure and I-V characteristics of Power diode with necessary diagrams

03) Attempt any Four (Four marks each);

a) Explain with neat diagram the construction of a p-n junction power diode.
b) Explain the operation of Power MOSFET.
c) Explain the operation of parallel connected diodes.
d) Explain the basic structure of power diode.
e) What is meant by conductivity modulation and what is its effect on the on state power loss
of the device?