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CHAPTER 1
INTRODUCTION
1.1
GENERAL
The DC - DC step-up converters are widely used in computer
hardware and industrial applications such as computer periphery power
supplies, car auxiliary power supplies, aerospace, servo-motor drives and
medical equipment. In recent years, the DC-DC conversion technique has
developed greatly. High gain voltage converters are necessary for the above
mentioned applications. The Pulse Width Modulation (PWM) technique used
to get the variable output voltage from these converters results in high
switching losses, high switching stresses, reduced reliability and increased
Electromagnetic Interference (EMI) when the converters are operated at high
frequencies. The main objective of this thesis is to overcome these drawbacks
and to design new converters to achieve high efficiency, high power density
and voltage gain. To increase the power density and to reduce the size and
weight of the converter with high voltage gain, Voltage Boost (VB)
converters operating at higher switching frequencies are used.
However, elementary DC-DC converter topologies employing hard
switching are unable to achieve high efficiency, because of the following
reasons.
In order to achieve high power density, the switching
frequency is normally increased. At increased frequency, the
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converter can employ smaller sized components. However, as
the switching frequency increases, the switching losses
associated with the turning on and off of switches will
become excessive. These losses remarkably reduce the
converter efficiency.
Generally MOSFETs are used when high switching frequencies are
required whereas IGBTs are used at high power level requirements. However,
MOSFET is a resistive device when it is turned on, the conduction losses are
higher when compared to IGBT. But MOSFET is the fast switching device
and is able to operate above 1 MHz.
To overcome the above mentioned difficulties, Quasi Resonant
Converters (QRCs) are used. The efficiency of these converters can be
improved by having Zero Voltage Switching (ZVS) or Zero Current
Switching (ZCS) in the converter operation.
In QRCs the voltage across the device (or) current through the
device is forced to pass through zero crossing by using an LC resonant circuit,
so that ZVS (or) ZCS can be achieved in the converter circuit. There are
many ways to achieve the ZVS (or) ZCS in the converter circuit at high
frequency by rearranging the components of the converter circuit. The so
called soft switching has many advantages over the hard switching such as
reduced voltage and current stresses across the switching devices, reduced
EMI as dv/dt and di/dt problems can be minimized.
The soft switching techniques can greatly reduce and even
completely eliminate the switching losses in a converter and increase
efficiency. Therefore, to improve the efficiency and boost the output voltage
of the converters, it is proposed to have ZVS/ZCS resonant soft switching
techniques in the existing voltage boost DC-DC converter. In this thesis, the
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analysis and design of ZVS QR voltage boost DC-DC converters are
presented. Simulation and experimentation of these converters are also carried
out to validate the analysis. In the experimentation, a simple method has been
adopted to generate the gating pulse to achieve the ZVS condition at all loads.
1.2
REVIEW OF LITERATURE
The converters operating at high frequencies are subjected to high
voltage stresses and switching power losses across the devices. The switching
devices in the converters with a PWM control can be gated to have a desired
shape of the output voltage (or) current. But the devices are switched on and
off at the load current with a high di/dt value. The turn on and off losses are
significant portion of the total power loss in the converter circuit. The EMI is
also produced due to high di/dt and dv/dt in the converter waveforms (Fang
Lin and Luo Hong Ye: 2004). The voltage boost Luo converters with constant
and variable output voltages find many applications such as telemetry,
computer and communication systems. To increase the power density and to
reduce the size and weight of the converter, voltage boost converters with
high voltage conversion ratio are used (Luo and Ye 2006).
A Continuous Conduction Mode (CCM) boost converter can be
selected as a possible candidate for a step-up converter with a continuous
input current. Since a continuous input current can simplify the input filter
stage, it is one of the major advantages of this solution. However, its
switching losses and the reverse-recovery problem of the output diode
degrade the system performance in terms of efficiency and EMI noise
(Zhao et al 2001).
A new boost converter with a ripple free input current was
suggested. In the new boost converter, an extra LC circuit with coupled
inductors was utilized. The ripple component of the input current was reduced
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by carefully designing the parameters of the coupled inductor. However, the
efficiency could not be improved due to the switching losses of the power
switch (Cheng et al 1998).
Super Lift (SL) technique using geometric progression voltage
conversion ratio is implemented in positive and negative output PWM Luo
converters (Fang Lin Luo 2003, Luo and Ye 2003). The analysis of SL PWM
Luo converters with capacitor drop was reported by Fang Lin Luo (2008).
It is important to design converter circuit capable of operating at
high frequencies owing to the ever-increasing demand of high power density
switched mode converters. At high frequencies, hard switched converters are
unsuitable because of high switching losses, switching stresses, reduced
reliability, EMI and acoustic noise (Kasskian et al 1991, Issa Bataresh 2004,
Ned Mohan and Undeland 2004). VB methods were used in buck, boost,
buck-boost and Cuk converters with PWM signal where the output voltage
increases in arithmetic progression. These converters also experience the
above mentioned problems (Ye and Luo 2005, Luo 1999a, Luo 1999b, Miao
Zhu and Fang Lin Luo 2007, Zhu and Luo 2008). A novel resonant switch
operating under the principle of ZVS has been proposed (Liu and Lee 1986).
A design oriented analysis of boost ZVS resonant DC-DC converter has been
given by Kazimierczuk (1998). The analysis of ZVS quasi resonant buck and
flyback converters were given by Tabisz and Lee (1989).
To overcome the above mentioned difficulties, QRCs are used.
Extensive work on the control of QRCs using Frequency Modulation (FM)
technique was reported by Rama Reddy et al (1995). However, these QRCs
have limited switching frequency variation and limited load range of
operation. Hence, optimization of the circuit components will be difficult.
Therefore, it is preferable to have a QRC operating at constant frequency.
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Various hard and soft switching topologies at low voltage and power levels in
DC-DC Converters were reported by Carriero, 1999. The efficiency of the
DC-DC converters can be improved by having ZVS or ZCS in the converter
operation (Ned Mohan and Undeland , 2004). Boosting the output voltage and
increasing the efficiency of the DC-DC converter was reported by Hyun-Lark
Do (2011), Amini et al (2009) ,Mantaro Nakamura et al (2004) and Jun-Ho
Kim(2009) using coupled inductor. It was also reported in Lin et al, 2009
that, the output voltage is boosted with increased voltage conversion ratio
using active clamp method. It was reported by Bassett (1995) that ZVS with
power factor correction and boosting the output voltage in the converters .It
was also reported by Barbosa et al ,1997 that there exists some difficulty in
achieving ZVS at light load and the circuit becomes too complex in boosting
the output voltage of DC-DC converter. It was report by Gang Yao et al ,
2007 and Miwa et al, 1992, that ZVS interleaved boost DC-DC converter has
got extra branch added across the main switch to achieve the ZVS for
boosting the output voltage and efficiency.
The soft switching have many advantages over the hard switching
such as decreased the voltage and current stresses across the switching
devices, dv/dt , di/dt and EMI problems.
Soft switching techniques can be classified into two groups:
1)
Zero Voltage Switching (ZVS)
2)
Zero Current Switching (ZCS)
The above techniques can greatly reduce and even completely
eliminate the switching losses in a converter there by increasing the efficiency
of the system. The soft switching technique involves many methods
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implementing resonance characteristics. Most popularly ZVS-QR converter
technique is used in order to determine the two zero crossing points for switch
on and off state (Fang Lin and Luo Hong Ye 2004.). To improve the
efficiency and boost the output voltage of the converters, it is proposed to
have ZVS/ZCS resonant switching techniques to the existing DC-DC Luo
converter.
The positive boost DC-DC converters perform the voltage
conversion from positive to positive voltages using Voltage Lift (VL)
technique. They work in the first quadrant with large voltage amplification.
These converters are driven by a PWM signal with repeating frequency (f)
and conduction duty cycle (k). These converters can transfer large amount of
power with high power density from input to output. However, its power
losses are usually high during the transfer of large amount of power since the
power losses across the switching devices are high. The soft switching
technique is an effective way to reduce the converter power losses and
improve the efficiency. Usually, the repeating frequency is not very high and
a converter works in the resonance state, the higher order harmonics are very
low, so that the EMI is low.
The regulation of output voltage for these converters against the
load and supply variations is one of the most important criteria for designing
high density power supplies. In order to improve the speed of response and to
achieve voltage regulation, it is necessary to have a closed loop control
system. The modeling and control techniques of various converters have been
described by Simon Ang (1995), Rashid (2001) and Varienina (2002). The
small signal transfer function of the converter obtained from state space
averaging technique is used to develop the feedback controllers
(Kazimierczuk and Robert Cravens 1994, Erickson and Maksimovic 2004),
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which provides good accuracy up to 1/10 th of the switching frequency. The
two general approaches to design feedback controllers for DC-DC converters
are Voltage Mode Control (VMC) and Current Mode Control (CMC) (Ned
Mohan and Undeland 2004). The PID controller design for the voltage control
of converters based on average modeling has been reported (Uma and
Chellamuthu 2000a, Uma et al 2000b, Uma 2001, Ogata 2002). An output
voltage regulation of a high efficiency, high step up DC-DC power converter
is reported by Malo and Grino (2007). Also, regulation is achieved by using
analog PI controller and switching pulses are generated using analog pulse
generating circuits. Moreover, a driver circuit is also used to drive the
switches. In addition, separate power supplies are needed to activate the
circuits. This in turn increases the size and weight of the converter. These
drawbacks can be overcome by using a dedicated analog controller IC
UC3861 for voltage regulation of QRCs under VMC (Texas Instruments
1998). The above controller derives power from converter circuit itself and
generates switching pulses at zero crossing instances of resonant capacitor
voltage. This increases the power packing density and efficiency of the
converter. The application of analog resonant controller UC3861 to Constant
Frequency (CF) ZVS QRC is reported (Arulselvi 2006, Ramareddy and
Chellamuthu 1995, Ramareddy et al 1996).
The application of analog
resonant controller IC UC3861 to capacitor charging power supply
(Lippincott and Nelms 1991) and FM ZVS forward converter (Bill Andreycak
1999) have been discussed. The above drawbacks can be overcome by using
digital controller. The digital controller is programmed for open loop and
closed loop operation of the converter by generating proper triggering pulses
at constant frequency.
Hence, the literature review points out that there exists a need for
the design and implementation of ZVS boost DC-DC converters for several
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applications. In this work, an attempt is made to design and implement ZVS
QR voltage boost DC-DC converters, to increase the output voltage and the
power packing density with reduced switching losses. It is also proposed to
verify the performance of the converter at all load conditions and various duty
cycles.
1.3
OBJECTIVE OF THE THESIS
To analyze, design and implement ZVS elementary, self and
re-lift DC-DC converters.
To analyze, design and control ZVS super boost and double
boost DC-DC converters.
It is proposed to use dSpic microcontroller for pulse
generation and for voltage regulation of the proposed soft
switched converters.
1.4
ORAGANIZATION OF THE THESIS
A chapter-wise summary of the thesis is presented in the sections
that follow:
1.4.1
Introduction
Chapter 1 presents the importance of soft switching techniques over
the hard switching in the DC-DC converters. Various topologies of DC-DC
converters and their control techniques are discussed. Closed loop control
techniques available for DC-DC converters are reviewed. The literature
review of various power DC-DC converters and the objective of the thesis are
also presented.
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1.4.2
Design and Implementation of High Gain ZVS Elementary
Boost DC-DC Converter
Chapter 2 presents the design and implementation of ZVS in the
voltage boost DC-DC converter. A design procedure has been developed and
the characteristics of the converter are studied. The converter is simulated
using MATLAB software. The simulation results agree closely with the
theoretical predictions. The prototype of this converter operating at a
switching frequency of 40 kHz is fabricated and tested. The required pulse is
generated using the microcontroller. The experimental waveforms of resonant
capacitor voltage, inductor current and output voltage are furnished.
1.4.3
Design and Implementation of High Gain ZVS Self Boost
DC-DC Converter
Chapter 3 presents the design, analysis and implementation of ZVS
in the self boost DC-DC converter. The entire system has been modeled and
simulated using MATLAB software. Prototype of this converter operating at
a switching frequency of 40 kHz is constructed and tested. The test results are
compared with the simulated results.
1.4.4
Design and Implementation of High Gain ZVS Re-lift DC-DC
Converter
Chapter 4 deals with the implementation of ZVS in the re-lift DC-
DC converter. The converter circuit is simulated using the MATLAB
software and its characteristics are studied. The entire system is fabricated
and tested at a switching frequency of 40 kHz. The pulse required for the
converter is generated using the digital controller. The experimental
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waveforms obtained agree closely with the simulated waveforms of the
converter.
1.4.5
Design and Implementation of High Gain ZVS Super Boost
DC-DC Converter
Chapter 5 deals with open loop and closed loop operation of super
boost DC-DC converter implementing ZVS. The converter has been designed
and analyzed for ZVS operation for changes in the supply voltage. The
converter operating at a switching frequency of 50 kHz is modeled and
simulated using MATLAB software. The hardware implementation is carried
out for a constant load and change in the supply voltage in open loop and
closed loop. The experimental waveforms are obtained and compared with
the simulated waveforms. It is found that the results obtained experimentally
agree closely with the simulated waveforms.
1.4.6
Design and Implementation of High Gain Double Boost DC-DC
Converter
Chapter 6 gives the operation of the double boost DC-DC converter
both in open loop and closed loop. The digital controller used generates the
gating pulse for the operation of the converter and performs closed loop
operation of the converter by changing the duty cycle of the pulse. The
converter is simulated for a switching frequency of 11 kHz and respective
waveforms are obtained. The hardware implementation of the converter is
carried out and the required waveforms are obtained. The experimental and
simulated waveforms are compared and it is found that they closely agree.
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1.4.7
Conclusion
Chapter 7 presents a complete review of the work carried out and
reported in the thesis. It also presents a detailed report on the contributions
made in the thesis in increasing the operating performance and efficiency of
the converter by employing soft switching. This chapter also briefly presents
the future work that can be carried out in the various boost converters. The
results obtained reveal that the high gain ZVS - QR voltage boost
converters will be an alternative to conventional PWM converters for
telecommunication, aerospace, computer and industrial applications.