Download DESIGN AND FABRICATION OF MOSET BASED

DESIGN OF MOSET BASED QUASI RESONANT CONVERTER FED DC DRIVE
ABSTRACT: In this project of MOSFET based Quasi - Resonant converter Fed DC Drive, high
frequency Quasi Resonant converters have recently gained popularity because they offer many
advantages over phase - controlled converters, DC choppers, and PWM converters. Switching losses
can almost be eliminated in resonant converters, as switching ON and OFF of the device is possible when
the device voltage or current passes through zero value. In the ZCS-QRCs the current produced by
resonant circuit flows through the switch, causing it to turn ON and OFF at zero current. The output
voltage of the converter is controlling by varying switching frequency.The performance of QRCs used in
power supplies in and is found to be superior in reducing armature current ripple and improving
transient response.
Semiconductor–
controlled DC drives usually employ phasecontrolled converters, DC choppers and PWM
converters. Phase-controlled converters have
disadvantage such as low pf on the line side,
less efficiency, poor commutation capability
on the motor side, and rectifier-source
interaction problems [1]. PWM converters and
DC choppers are commonly employed to
improve the power factor and to reduce the
load current ripple. But the frequency of
PWM converters and DC choppers is limited
by the turn-OFF time of semiconductor
device. Also, PWM converters involve the
difficult task of computing specific PWM
instants, and PWM converters as well as DC
choppers have considerable switching losses,
which drastically reduce the efficiency of
power conversion at high frequencies.
1.
INTRODUCTION:
QRCs to control the speed DC drives. In the
ZCS-QRCs, the current produced by
resonant circuit flows through the switch,
causing it to turn ON and OFF at zero
current. The output voltage of the converter
is controlled by varying the switching
frequency. Hence, it is called a frequencymodulated zero- current-switching quasiresonant converter (FM-ZCS-QRC). The
literature on QRCs deals with the
performance of QRCs used in power
supplies but not the performance of QRCs
applied to DC drives. This paper presents
experimental result of a DC motor fed from
series FM-ZCS-QRCs. The FM-ZCS-QRCfed DC drive is found to be superior in
reducing armature current ripple and
improving transient response.
2.RESONANT SWITCH TOPOLOGIES:
A resonant switch is a sub – circuit
consisting of a semiconductor switch and its
associated LC resonant elements for
waveform shaping. The sinusoidal current
wave form generated by the LC resonant
circuit creates a zero current condition for
the switch to turn - OFF without switching
stress and losses. The actual implementation
of the resonant switch can flow bi-
The cause of switching losses in
semiconductor devices is the simultaneous
existence of non-zero device voltage and
current during the switching period of the
device. Switching losses can almost be
eliminated in resonant converters, as
switching ON and OFF of the device is
possible when the device voltage or current
passes through zero value. The paper process
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directionally. There are two types of resonant
switch configurations, L - type and M – type .
In
both cases L1 is connected in series with S1 to
limit the di/dt of the switch current, and C1 is
added as an auxiliary energy storage/transfer
element. L1 and C1 constitute a series
resonant circuit with its oscillation initiated by
the turn - ON of S1.If the ideal switch S1 is
implemented in a unidirectional configuration.
The switch is confined to operate in a half
wave mode. If diode D1 is connected in antiparallel with thyrister . The resonant switch
operates in a full – wave mode and the switch
current can flow bi-directionally. In essence,
the LC resonant circuit is used to shape the
current waveform through switch S1. At turnON, the device voltage (VCE or VDS) can
be driven into saturation before the current
gradually rises in a quasi – sinusoidal fashion.
Because of the resonance between L1 and C1,
current through S1 will oscillate to a negative
value, thus allowing it to be naturally
commutated.
3. BLOCK DIAGRAM DESCRIPTION: This
MOSFET based quasi resonant converter has
three sections [1]. Rectifier [2].Power circuit
[3]. Triggering circuit. A single phase AC 230
Volts is given two step down transformers, the
output of transformers is fed to diode bridge
rectifiers. One diode bridge output voltage fed
to LC tank circuit with power MOSFET and
the output of MOSFET is fed to DC motor.
Second diode bridge output is fed to triggering
circuit. The triggering circuit of the MOSFET
is given by triggering circuit and by varying
the firing angle of the QRC, the output voltage
of converter is varied and smooth variation of
motor is achieved upto high frequency of
8.2Khz.
Fig Block Diagram of QRC
3.1 TRIGGERING CIRCUIT: In
this triggering circuit we used IC 555
timer and is used as Astable
multivibarator and it generates
rectangular pulses. For
Astable
multivibrator
Charging time of
capator
Tc = 0.693 (R1+R2)C, Discharging
time of capacitor Td = 0.693 R2C,
Duty cycly = Ton/Ton +Toff
=Tc/Tc+Td =R1+R2/R1+2R2. The
Duty cycle can be adjusted to any
desired percentage by proper selection
of the resistors R1 and R2.
Triggering Circuit
3.2 RECTIFIER: Rectifier is nothing
but which converts AC to DC. In this
project we are using full wave bridge
rectifier and it consists 4 diodes.
During +ve half cycle of secondary
voltage, diodes D2 and D3 are forward
biased, and during –ve half cycle D1
and D4 are forward biased, and wave
forms and circuit diagram are shown in
below fig..
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voltage VGSP which is required to drive
the transistor into linear region. The
turn-OFF delay time td (OFF) is time
required for the input resistance to
discharge from the overdrive gate
voltage V1 to the pinch off region. VGS
must decrease significantly before VDS
begins to rise.
4.3 Gate Drive
The turn ON time of a
MOSFET depends on the charging
time of the input or gate capacitance.
The turn - OFF time can be reduced by
connecting an RC circuit as shown in
fig to charge the gate capacitance
faster. When the gate voltage is turned
ON. The initial charging current of the
capacitance is
IG = VG / RS
Steady state value of the gate
voltage VGS = (RG VG) / RS + R1 +RG).
In order to achieve switching
speeds of the order of 100 ns or less,
the gate drive circuit should have a low
output impedance and the ability to
sink and source relatively large
currents.
A totempole arrangement that
is capable of sourcing and sinking a
large current. The PNP and NPN
transistors acts as emitter followers
and offers a low output impedance.
MOSFET BASED QRC FED DC DRIVE
4. MOSFET
It is unlike a JFEET; however,
the gate is insulated from the channel.
Because of this, Gate current is small.
Whether gate is positive or negative
(IGFET).
There are two types:
1. Depletion type MOSFET
2. Enhancement Mode MOSFET
4.1Switching Characteristics
With out any gate signal an
enhancement type MOSFET may be
considered as two diodes connected back
to back or as an NPN transistor.
The gate structure has parasitic
capacitance to the source Cgs and to the
drain ‘Cgd”. The NPN transistor has a
reverse bias junction from drain to the
source and offers a capacitance Cds.
The base to emitter region of
NPN transistor is shorted at chip by
metalling source terminal and the
resistance from base emitter due to bulk
resistance of n and p region Rbe is small.
Hence a MOSFET may be
considered as having internal diode and
the equivalent circuit.
4.2 Switching Models of MOSFETs
The turn ON delay td (ON) is the
time that is required to charge input
capacitance to threshold voltage level.
The rise time tr is the gate charging time
from the threshold level to the full gate
4.4 Steady state characteristics of
power MOSFET
The MOSFETs are voltage
controlled devices and have very high
input impedance, the gate draws a very
small current, in the order of nano
amperes.
The current gain, which is the
ratio of drain current ID to the input
gate current IG, it is typically on the
order of (10-9).
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The trans - conductance, which is
the ratio of drain current to gate voltage,
defines the transfer characteristics and is
a very important parameter.
There are three regions of operations,
they are
1. Cut off region, where VGS ≤ VT
2. Pinch off (or) saturation region where
VDS ≥ VGS
- VT
1. Linear region, where VDS ≤ VGS - VT
The pinch off occurs at VDS = VGS VT
a. In the linear region, the drain
current ID varies in proportion to
the drain source voltage VDS.
b. Due to high drain current and
low drain voltage, the power
MOSFETs are operated in the
linear region for switching
actions.
c. In the saturation region, the drain
current remains almost constant
for any increase in the value of
VDS and the transistors are used
in the region for voltage for
voltage amplification. It should
be noted that saturation has the
opposite meaning to that for
BJTs.
d. The steady state model, which is
the same for both Depletion and
Enhancement type MOSFET.
5. PRINCIPLE
REALISATION:
OPERATION
minimizes the switching losses.If the
gate pulse is given the MOSFET, the
LC circuit in the output of the
converters starts oscillations. The
capacitor voltage tends to charge the
peak voltage of the input DC supply. If
the voltage level of the capacitor and
input equals the resultant potential
difference between the MOSFET
becomes zero. So the voltage stress
also
minimum.
Free
running
multivibrator NE 555 is used as a
PWM generator 555 IC generates
gating signal at a frequency of 2
Khz.The ON - time of the
multivibrator can be varied from 0 –
100% by adjusting the pot in the main
board. So we can run the DC motor at
different speeds with minimum
switching loses. Quasi resonant
converters are widely used for DC
motors for noiseless run and for the
linear speed controlled operations.
AND
The switching loses in ordinary
converter are high due to the high
switching frequency of the power
electronics components. Due to high
voltage and current stress on thyristor
the life time of the static switch leads to
limited value. In order to minimize the
switching losses in thyristor switches we
are going for resonant converters. In
resonant conveters the resonant circuit in
the output side of the converters
O/p. Characteristics of a Power MOSFET
EXPERIMENTAL
AND RESULTS :
INVESTIGATION
In this chapter, we
can see how it is operated practically.
We investigate each block and each
circuit operation experimentally about
QRC. Practically this fixed frequency
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MOSFET based QRC having three
sections, they are 1. Rectifier circuit
2. Power circuit 3. Triggering circuit.
The 555 timer is basically a monolithic
timer circuit and it is one of the most
versatile linear integrated circuit devices
in practical use. It can operate in both
monostable and Astable
mode. As
monostable multivibrator it finds many
applications such as frequency divider,
pulse width modulator, linear ramp
generator etc. As Astable multivibrator,
it has numerous applications like voltage
control oscillator, FSK generator, Square
wave generator etc.This chapter
discusses mainly the basic features of
the timer, the block diagram, the
operation of the device as monostable
(or one shot ) multivibrator and as
Astable multivibrator ( or free running )
multivibrator, and some practical
applications. The system described
above has been constructed and
thoroughly tested in the laboratory. The
switching frequency was varied from
1khz to 2 khz. It is noted that the
maximum switching frequency with
MOSFETs QRC is 2 khz for a resonant
frequency.
(owing to elimination of discontinuous
conduction region). The QRC – fed
DC drives have the fallowing
disadvantages. The power devices
have to be rated to a voltage twice the
DC supply voltage. The half-wave
series and parallel QRC-fed DC drives
require non-linear controllers, as the
average output voltage of these
converters is not linear function of the
switching frequency. The resonant
capacitor has to be rated for full-load
motor current. The switch-ON period
of the MOSFET is load dependent.
The armature current ripple is found to
be greater in full wave mode of
operation.
REFERENCES
1]G.K. Dubey, Power semiconductor
controlled drives (Engle – Wood
Cliffs, NJ: Prentice Hall, 1989).
2].R.S.Gaonkar(1989), Microprocessor
Architecture,
Programming
and
Applications ‘,
Wiley Eastern
Limited.
3]. J.G. Kassakian, M.E. Schiect and
G.C.Verghese (1991),‘Principles of
power Electronics’, Addison Wesley
Electrical Engineering Series
4].F.C.Y.Lee, K.H. Liu and R.
Oruganti (1987), Quasi – resonant
Converters
–Topologies
and
Characteristics’, IEEE Trans. On
power Electronics, Vol. No. 1, pp. 6274.
5].F.C.Y
Lee,
R.B.Ridley,
W.A.Tabisz,
V. Vorperian (1991), ‘Multi – Loop
Control for Quasi – Resonant
Converters’, IEEE Trans. On Power
Electronics, Vol. 6, No. 1, pp. 28 –38.
6]N. Mohan, T.M. Undeland and
William .Robbins (1989), ‘Power
Electronics: Converters, Applications
and Design’, John Wiley and Sons.
CONCLUSIONS:
Design and fabrication of QRCs
were constructed and successfully
employed to control the speed of a DC
motor. The drive system was found to
operate satisfactorily in closed loop. The
QRC fed DC drives are found to have
several desirable features, the most
important among which are improved
efficiency, high power density, reduction
Ia armature current ripple, high
reliability, fewer switching losses and
switching stresses, less EMI, snubber
circuits not required, an additional filter
not required (as armature inductance
itself acts as L – filter), and improved
transient response and speed regulation
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