Power electronics integrates the use of three areas of specialization in

learnedbawledElectronics - Devices

Nov 24, 2013 (3 years and 10 months ago)

85 views


1

Power electronics integrates the use of three areas of specialization in
engineeri
ng, namely, Electronics, Power
,

and Control. It deals with
the use of electronic for the conversion and control of electronic for
electronic power in various industrial, com
mercial, residential and
aerospace applications. The evolution in power electronic is the
synthesis of multiple technological disciplines. Today, a specialist in
this area is supposed to have expertise in power semi
-
conductor
devices, converter circuits,
electrical machines, analog/ digital
electr
onic, control theory, computer
-
aided design, micro
computers,
and the newly emerging VISI electronic.





1.

M H Rashid, “Power Electronic : Circuits, Devices and
Applications,” Prentice
-
Hall, 1993.

2.

N Mohan, T

M Undeland, and W P Robins, “Power Electronics:
Converters, Applications, and Design,” John Wiley and Sons,
1995.


2

Applications of power electronics Systems



DC
-
AC regulated power supplies



Electro
-
chemical processes



Heating and Lighting Control



Electron
ic Welding



Power Line Var. and Harmonic Compensation



High voltage DC system



Variable speed constant frequency system



Solid State Circuit Breaker



Induction Heating


Types of Converter systems (power conditioning system)



Rectifier (Unc
ontrolled

& Controlled
)



AC voltage controller



DC
-
DC converters



Inverters (Uncontrolled

& Controlled
)



Cyclo
converters



3

Generalized power converter


Input filter
-

filters off any harmonic noise generated in the pow
er
converter, such that the feedback switching harmonic jammed
into the input line are minimized.

Power circuits
-

interconnects all the power switching devices together to form a
bulk power module.

Output filter
-

smoothes out any switchi
ng harmonic in the output so as to
obtain the desired output waveform.


4

Switching Control
-

provides gate signals to all the power devices according to the
switching strategy used and the external control signals.


Electrical Power control

Achieved by :

(1)

a switch



(2)

an adjustable stepless impedance

When the power flow is regulated by the controlled variation of series
resistance, there occurs a waste of heat and loss of system efficiency.
For this reason, the control of high power c
ircuits is invariably
approached via the
use of semi
-
conductor power devices as switches
.


Limitations of practical semi
-
conductor switches



Finite on
-
resistance



Finite leakage current in “ off ” state



Finite turn
-
on and turn
-
off times.



Switching losses.



5

Classifications of semi
-
conductor switches



Device conducts automatically when forward bias is applied, e.g.
DIODE.



Device begins to conduct in the forward direction upon command
of a control signal and continue to conduct until the next current
zero cross
ing , e.g.
THYRISTOR
.



Forward conduction can be initiated and interrupted by control
signals applied to the control terminal , e.g.
TRANSISTOR
,
GTO
,
MOSFET
,
IGBT
.


DIODES

The basic concepts will not be discussed since they should be learnt
in previous yea
rs. However, several criteria will be discussed.

A.

Reverse voltage breakdown

Increasing applied reverse bias eventually to a junction reverse
voltage breakdown and the diode current is controlled by the external
circuit. Breakdown may be due to one of t
he following three
phenomena :


6


1. Punch
-
through voltage

The reverse voltage extends the depletion layer to at least one of
the ohmic contacts and the device presents a short
-
circuit to that
voltage in excess of the punch
-
through voltage in excess of
punc
h
-
through voltage. Punch
-
through occurs with devices
which employ a low
-
concentration region, as is usual with high
voltage devices.


2. Avalanche Breakdown

Avalanche breakdown is the most common mode of breakdown
and occurs when the electric field in th
e depletion layer at
junction exceeds a certain level which is dependent on the
doping level of the lighter doped region. Minority carriers
associated with the leakage current are accelerated to kinetic
energies high enough for them to ionize silicon atoms

on
collision , thereby creating a new hole
-

electron pair . These are
accelerated in opposite directions, because of the high electric
field strength , colliding and ionising again and again
-

hence
lead to carrier multiplication or avalanche.


7

3.

Zen
er breakdown

Zener breakdown occurs with heavily doped junction regions. It
occurs when the depletion layer is too narrow for avalanche yet
electric field grows very large and electrons tunnel directly from
the valence band to the conduction band. These mo
des of
reverse voltage breakdown are not necessarily destructive
provided that the current is uniformly distributed . If the current
density in a particular area is too high, a local hot spot may
occur, leading thermal destruction .


B.

Turn
-
off characteri
stic

Once a diode is conducting, the junction cannot instantaneously revert
to the blocking mode because during forward conduction there is an
excess of minority carriers in each diode region (i.e. holes in n
-
r
egion
and electrons in p
-
region
) and these mu
st be removed at turn
-
off.


8



The recovery charge has two components, one due to internal excess
charge recombination and the other d
ue to the reverse diode current
.
These charges must first be swept out before the diode can regain its
reverse blocking capability
. The time for reverse current to flow is
called the reverse recovery time,
t
rr

=
t
4

+
t
5
. The time
t
5

is defined by
projecting
I
rr

though 0.25
I
rr
.
V
rr

is the reverse over
-
shoot voltage
produced by the high
di
rr
/
dt

effect and

the external circuit inductance
L
. Afterwards,
di
rr
/
dt

reduces to zero , the circuit supports zero volts
and the diode blocks the reverse voltage .



9

Schottky Barrier Diode

Apart from the conventional type p
-
n function diode. There exists a
low
-
voltage

, high
-
speed type , called the
Schottky barrier diode

(or
Schottky Diode). It has very low forward voltage drop but with a
very high reverse leakage current relative to application imposing a
reverse bias of less than 400V. Another key feature is the hig
h
switching speed.


POWER TRANSISTORS



Bipolar Junction transistor (
BJT
)



Metal
-
Oxide Semi
-
conductor Field Effect transistor (
MOSFET
)


A. BJT

Types :
PNP

and
NPN


In switching application, the
common emitter configuration

is
generally used.




10

Operating Regio
ns



Cut
-
Off†



Active (linear)



Saturation†




The operating modes that the transistors will be used in power
electronics.


Switching Characteristics





11

A
. Turn
-
on time

It consists of a delay time (
t
d

) and a rise time (
t
r

). The delay
time is
mainly due to the charging of the BE junction capacitance and can be
significantly reduced by increasing the applied rate and magnitude of
the forward base current (
I
bf

). The rise time (
t
r

) depends on the time
constant determined by capacitances

of the transistors.


B. Turn
-
off time

It is the sum of storage time (
t
s

) and the fall time (
t
f
). The storage
time is the time required to remove the saturating charge from the
base. The fall time (
t
f
) depends on the time constant determined by
th
e capacitance of the reverse
-
biased BE junction.


Power Darlington Transistor

Since a power bipolar transistors consume considerable input control
power, an intermediate power driver must be cascaded with transistor
before a unit may be considered for op
eration from a signal
-

power
level source.


12


T
he complementary connection is particularly useful as it uses as low
level PNP driver to convert a high power NPN bipolar into a high
power PNP unit. It is found that NPN units can be const
ructed on an
integrated basis, sometimes with three devices in cascade, with
protecting diodes and thermal stabilizing resistors also included.


POWER MOSFET

Types :

Depletion type (normally on )

Enhancement type (n
ormally off
)


13



O
perating Region



cut
-
off region†



constant current region

(saturation)




constant resistance region

(triode)






The operating modes that the transistors will be used in power
electronics.

Since MOSFETs do not have inherent minority
-
carrier delay as in
bipolar devices. The
turn
-
on and turn
-
off times depend on the ability

14

of the gate drive circuit to charge and discharge a tiny input capacitor
C
iss

= C
gs
+C
gd

. Typical s
witching times are 150 to 200ns
.


Synchronous Rectifier

For the n
-
type MOSFETs, a negative drain voltage is

applied to the
device with thei
r gates connected to the source and

th
e p
-
n junction
becomes forward
-
biased. When the drain voltage exceeds a knee
voltage,
V
N

of approxi
mately 0.7V at room temperature
, the p
-
n
junction begins to conduct. If a positive gat
e voltage is applied to the
device which create a channel, an alternate path for current flow
between drain and source is created. Consequently, the power
MOSFET will exhibit a very low voltage drop, significantly lower
than a p
-
n junction diode.



15

THYRISTORS

Thyristor is a family name
of
bi
polar devices
,

which c
omprise 4 semi
-
conductor layers
, including

1.

Silicon
-
Controlled Rectifier (SCR)

2.

Triac
(Bi
-
directional thyristor)

3.

Reverse
-
conducting thyristor (RCT)

4.

Gate
-
turn
-
off thyri
stor (GTO)


1. SCR


When the anode voltage V
AK

is
positive
, the junctions J
1

and J
3
are forward
-
biased, but junction J
2

is reverse
-
biased and the leakage
current flows from A to K. The SCR is then said to be in forward
blocking or off
-
state conditio
n and the leakage current is known as
off
-
state current I
D
.


16

If V
AK

is increased to sufficiently large value i.e.> V
BO

( forward
breakdown voltage ) and avalanche breakdown occurs in the reverse
-

biased junction J
2
. Since the other junction J
1

and J
3

are
already
forward
-
biased, there will be free movement of carrier across all three
junctions resulting in large forward anode current. The device will
then be in a conducting state or on state. The on
-
state voltage drop
across the 4 layers is small.



In the on
-
state, I
AK

is limited by the external impedance or
resistance. However, to maintain the required amount of carrier flow
across the junction , the anode current I
AK

must be more than a value
known as
Latching Current

( I
L

), otherwise , the dev
ice will revert to
the blocking condition as the V
AK

is reduced. Once a SCR is
conducting, it behaves like a conducting diode and there is no control
over the device. The device will continue to conduct because there is
no depletion layer on the junction

J
2

due to free movements of carrier.
If the forward anode current is reduced below a level known as the
Holding Current

(
I
H
), depletion region would develop around
junction J
2

due to the reduced no. of carriers and the SCR would be in
the blocking state
.


17



An increase of reverse voltage can cause SCR failure by punch
-
through of the reverse
-
biased junction J
1
. In high voltage SCR this is
prevented by using a thick N
2
layer.


Thyristor turn
-
on

1.

Gate triggering

2.

Forward br
eakdown

3.

Irradiation methods (light
-
activated SCR)

4.

dv
/
dt

triggering

5.

Temperature elevation





18

Thyristor turn
-
off

1.

Insertion of series impedance

2.

Reversal of the anode voltage


The main objective is to make the anode current is lower than the
ho
lding current
. Typical turn
-
off times lie in the range of 10
-
300

s.


Ratings of Thyristors

1.

di/dt Limitation

In many

applications
, a thyristor is subjected to a very steep rise
of curr
ent at and after switching ‘on’
.

The ‘vertical’ regenerative
trigge
ring action occur
s

exceedingly quickly but the ‘horizontal’
migration of triggering charge in the base region take place very
slowly.

The result is that only the region near the gate contact actually
triggers. Therefore, unless limited by external means, t
he full
-
load
current passes through a small fraction of cathode surface near the
gate. The thermal capacity of silicon is very small and it is a poor
thermal conductor with the result that a hot spot is created. The result

19

is damaged to the crystalline s
tructure of die or cracks in it due to
mechanical stress as silicon is brittle. The performance can be
improved by using centre gate, dual gate, and gat
e
-
cathode amplifying
dual gate.


2.

dv/dt Limitation

Even if a small amplitude fast transient of forwa
rd voltage is
applied to the device, it may trigger as displacement current will flow
through the depletion layer of J
2
.


Anode voltage ratings


1.

Crest working reverse voltage


( V
RWM

)

2.

Repetitive peak reverse voltage

( V
RRM

)


20

3.

Non
-
re
petitive peak reverse voltage

( V
RSM

)

4.

Crest working off
-
state voltage

( V
DSM
)

5.

Repetitive perk off
-
state voltage

(V
DRM

)

6.

Non
-
repetitive peak off
-
state voltage

(V
DSM

)


Current Ratings

1.

Continuous on
-
state current

2.

Mean
-
on state current

3.

Repetitive peak on
-
state current

4.

Non
-
repetitive surge on
-
state current

5.

Surge current capability for fusing

6.

Repetitive peak reverse current

7.

Rate of rise of forward current


Gate ratings

1.

Peak forward gate voltage

2.

Peak reverse gate voltage

3.

Peak forward rate current

4.

Peak reverse rate current


21

5.

Peak gate power

6.

Mean gate power


Power Losses of Switching Devices

1.

Switching Losses

2.

Off
-
state leakage power loss

3.

Conduction Power Loss

4.

Device

input Device power Loss


1.

Switching Losses


A
n approximation of straight line switching is assumed. For
resistive load,

)
W
(
6
)
(
S
ON
ON
OFF
ON
sw
f
t
I
V
P



22

)
W
(
6
)
(
S
OFF
ON
OFF
OFF
sw
f
t
I
V
P


For an inductive load ,

)
W
(
2
)
(
S
ON
ON
OFF
ON
sw
f
t
I
V
P


)
W
(
6
)
(
S
OFF
ON
OFF
OFF
sw
f
t
I
V
P



2.

O
ff
-
state leakage power loss


)
1
(



off
off
off
V
I
P



: Duty cycle of the switch


3.

Conduction Power Loss




on
on
on
V
I
P


4.

Drive Input Device Power Loss


Usually very small compared to other losses. For bipolar
transistors,


B
BE
g
I
V
P
.


23

Thus,

Total Power Loss =
P
sw(on)

+ P
sw(off)
+ P
off
+ P
on
+ P
g


Thermal considerations and Heat sinks

Power Losses in semi
-
conductor devices are dissipated in the
form of heat. This heat must be transferred in the device to maintain
the

operating junction temper
ature within the specific range
. It should
be noted that the reliability and life expectancy of any power semi
-

conductor are directly related to the maximum device junction
temperature experienced.


P
d


-

average power los
s in the device (W)

R
jc

-

thermal resistance from junction to case (
o
C/W)

R
cs


-

thermal resistance from case to sink (
o
C/W)


24

R
sa


-

thermal resistance from sink to ambient (
o
C/W)

R
ca


-

thermal resistance from case top ambient (
o
C/W)



Total thermal resistance ,
sa
cs
ca
sa
cs
ca
jc
ja
R
R
R
R
R
R
R
R





)
(

In general ,
sa
cs
ca
R
R
R


,

sa
cs
jc
ja
R
R
R
R




R
jc

and
R
cs

are normally specified by the power device
manufacturers. If the device power loss
P
d

is known, the required
thermal resista
nce of the heat sink can be calculated for a specified
ambient temperature
T
a
. The next step is to choose a heat sink and its
size which would meet the thermal resistance requirement.



Methods of reducing switching tim
es

1.

Speed
-
up capacitor

2.

Speed
-
up diode

3.

Overdrive the gate signal and protect by collector clamping
diode.




25

Driving of switching devices

A.

Power Transistors



provide adequate base current to saturate the output power
transistor.



provide
electrical isolation between the power circuit and the
switching command signals.



being high switching speed, low power consumption and simple
circuitry.


Remark :

When operated in the saturation region, the conduction loss of a
power transistor is minimi
zed. However, the base drive current
should not be too high to heavily saturate the transistor since the turn
-
off is lengthened due to large quantity of base charge.


B.

Power MOSFET



Do not suffer secondary breakdown



Drive by a current source followed by
a voltage source.


26



Protect the

gate from damaging overvoltage
, zener diode protection
is necessary.



Suitable for low power, high frequency applications.


C.

Thyristors

Three basic types :



DC firing signals



Pulse signals



AC phase signals


Commutation is a
chieved by a capacitor



Self commutation by parallel resonant turn
-
off



Controlled resonant turn
-
off



Parallel capacitance turn
-
off


Protection


1.

Snubber circuits

a.

to shift the device switching loss to the snubber circuit


27

b.

to avoid second breakdown

c.

to control

dv
/
dt




2.

Series connection



Steady
-
state voltage sharing is forced by using a resistor



Transient response is controlled by a low non
-
inductive
resistor and capacitor in series are placed in parallel.




28

3.

Parallel connec
tion

When the load current is greater than the thyristor current rating,
operation is parallel. Current division may vary due to different
thyristors unless matched thyristors are used. To attempt to get equal
current sharing, external balancing reactors
can be used .