Power Electronics is the technology used for the process control and conversion of Electrical Energy by means of power semiconductor devices which operate as switches into form convenient for utilization by machines and other electronic equipments. With the advent of Silicon Controlled Rectifiers (SCRs) in 1950s, the application of Power Electronics spread to various fields of Engineering such as in solid state industrial drives, high frequency converters, inverters, uninterruptible power supplies, Electronic tap changers, lighting control, home appliances and in medical instrumentation.
Gradually since 1970, various Power Electronic devices were developed and were available commercially. The typical classification of the devices based on the controllability characteristics are, Uncontrolled turn on and turn off devices (eg. Diode), Controlled turn on and uncontrolled turn off (eg. SCR) and Controlled turn on and off characteristics (eg. Power Bipolar Junction Transistor (BJT), Metal oxide semiconductor field effect transistor (MOSFET), Gate turn off thyristors (GTOs), Static induction thyristors (SITH), Insulated-gate bipolar transistors (IGBTs), Static Induction thyristors (SITs) and Mos-Controlled thyristors (MCTs))
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The first category of devices such as diodes can be controlled by the power circuits. In the second category, a control signal is required to turn on the device and turning off can be done by the power circuits. The devices belonging to the third category require control signals during turn on as well as during turn off. The evolution of the devices continued with the advanced improvements in current rating, voltage rating and electrical characteristics of the Power Converters. The foremost semiconductor switches which play a vital importance in the Power Electronics converter systems are power diodes, which are the two terminal device consisting of a cathode and an anode. Diodes are broadly classified as general purpose diodes, fast recovery diodes and schottky diodes.
In the case of general purpose diodes, the transfer characteristics of diodes are asymmetric and this device allows the current to flow in one direction i.e., in the forward direction and blocks it in the reverse direction. These diodes start conducting only after a certain value of voltage called cut in voltage or threshold voltage. The reverse recovery time of these diodes are very high and vary between 0.1 µs and 5 µs. Thus the diodes are available with the voltage rating of upto 6000V and the current rating of upto 1100A. Whereas in the case of fast recovery diodes, the reverse recovery times are low and they find wider applications in high frequency switching of power converters.
The third category is the schottky diode which is also called as hot carrier diode. It has very fast switching action and low forward voltage drop. The voltage drop across this diode ranges between 0.15-0.4V. Such lower voltage drop offers high switching speed and higher system efficiency. These diodes are formed by creating a Schottky barrier which is a combination of a metal and a semiconductor. The metals used are molybdenum, platinum, chromium or tungsten and certain silicides. This particular choice usually, establishes the forward voltage of the diode.
These diodes are less stronger than pn junction diodes. In these diodes there is no recovery time since no charge carrier depletion region is present in the junction. The major limitation of these diodes is that the reverse voltage rating is very low typically of the order of 50V and below, where as the reverse leakage current is very high. With the increase in the reverse leakage current, it leads to a thermal instability issue. This in turn places the limitation on higher voltage drops.
The recovery time of schottky diodes are very less in the ranges of nano seconds. They have very low on state voltage. Since the leakage current increases with the voltage rating, it is limited to a range of 100V and 300A. A typical diode conducts when its anode voltage is higher than its cathode voltage. The diode is said to be in blocking mode if the cathode voltage is more than the anode voltage. The forward voltage drop of the power diode is very low in the range of 0.5 V and 1.2 V.
Next to diodes, SCRs occupy a vital significance in many of the applications such as phase controlled converters and rectifier circuits. It is a three terminal device which consists of an anode, a cathode and a gate. When the anode terminal is at higher potential than the cathode terminal, SCR can be turned on by injecting a small current to the gate terminal. They can be turned off by making the anode potential either equal to or less than the cathode potential.
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SCRs are broadly classified into Converter grade and Inverter grade thyristors. Converter grade SCRs are also called as phase controlled thyristors. These thyristors are turned off by natural commutation and their turn off time varies from 50µs to 100µs. Hence they are most widely used in low speed switching applications. The on state voltage of this type of SCRs varies as 1.15V, 2.5V and 1.25 V for 600V, 4000V and for 1200V devices respectively. In modern days these types of thyristors use an amplifying gate which offers very high dynamic characteristics with of 1000 and of 500 . Thus the circuit design is much simplified.
The inverter grade thyristors are also called as fast switching thyristors. These types of devices are widely used in high speed switching applications and are forced commutated. Their turn off time range from 5 µs to 50 µs. The and of these thyristors are 1000 and 1000 respectively. The fast turn off time and high play a vital role in reducing the size of the commutating components. The on-state voltage is 1.7 V for the capacity of 1800V, 2200A.
The power BJTs, power MOSFETs and power IGBTs gradually occupy an essential position in replacing the SCRs in high power converter applications. BJT is a three layered two terminal device consisting of a base, a collector and an emitter. It is a current controlled device in which a base current can make the transistor to conduct. BJT can withstand only a low rate of change of current since they have no surge current capacity. The forward voltage drop of the conducting BJT varies from 0.3V to 0.8V. Power BJTs are available with lesser voltage and current ratings than thyristors.
Power MOSFET is a three terminal device consisting of drain, source and gate. MOSFETs are widely used in high frequency switched mode power supply and portable applications. These are available in the low power rating in the range of 1000V, 100A at a frequency range of several tons of kilohertz. It is a voltage controlled and unipolar device. The switching speed of MOSFETs is higher than BJTs. These devices require a very small input current and the control signal is a voltage. The switching timing is of the order of 50ns to 100ns.
The development of IGBTs absorbs a crucial significance among the semiconductor devices. These are also three terminal devices comprising of a collector, an emitter and a gate. It is a voltage controlled device which combines the input characteristics of MOSFET and output characteristics of BJT. These devices have high input impedance and low on state conduction losses. The commonly used power devices and their symbols are shown in the table1.1
Table 1.1. Semiconductor devices and its symbols
Silicon Controlled Rectifier
In Industrial applications it is required to control and condition the electric power from one form to another form in order to achieve increased production and high efficiency. This can be easily obtained by power converters. These are broadly classified as Phase controlled converters (AC to DC converters), DC-DC Converters ( Choppers), DC-AC Converters ( Inverters), AC-AC Converters (Cycloconverters) and AC Voltage Controllers ( AC Regulators). AC-DC converters convert fixed AC voltage into a variable DC output voltage. The AC input voltage may be single phase or three phase. These converters use natural commutation and hence called as line commutated or naturally commutated AC to DC converters.
DC-DC converters popularly called as choppers convert fixed dc input voltage into a variable dc output voltage. The choppers can be designed using MOSFETs, IGBTs and Power BJTs. The dc output voltage can be controlled by varying the conducting time of the semiconductor switch. Choppers are fed from a battery or a solar power dc voltage source.
Inverters are the circuits which convert a fixed dc voltage to a variable frequency and of either fixed or variable magnitude. These converters are commonly used in flash light discharge system in a photography camera and in very high industrial systems.
Cycloconverters convert AC input voltage at one frequency to a variable frequency output voltage. The output voltage variation can be effected by varying the triggering signals of the thyristors employed from a control unit. These converters are commonly used for slow speed and high power industrial drives in which the frequency of the output voltage is of the order of 1/3, 1/5 and so as that of the frequency of the input signal.
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AC Voltage Controllers employ thyristorised voltage controller which convert fixed AC voltage into variable AC voltage at same frequency. By varying the firing angle of the thyristors employed, the output voltage can be controlled.
Among the above discussed converters dc-dc converters find wider importance in the field of Power Electronics. The basic dc-dc converter topologies such as Buck, Boost and Buck-Boost converters are the switching circuits which exhibit complex dynamic behavior. They toggle among two different sets of linear and non-linear equations which require a stronger and effective feedback control action. The feedback circuit should be designed in such a way that it offers a robust control over the transient and dynamic behavior of the converter systems under consideration.
In particular, in these basic converter topologies the inductor current ramps up and down during the complementary switching states that exist between the semiconductor switch and the diode. The complementary switching state is that when the switch is turned on, the diode is in off condition and when the switch is turned off the diode will be in on condition and vice versa.
It is inevitable to design a feedback circuit in order to control the duty cycle of the switching converters to maintain a fixed value of output voltage irrespective of the input voltage and load variations. The simplest feedback circuits which are widely used for these converter topologies compare the output voltage with the reference voltage and thus the control signal is generated using Pulse width modulation scheme. This control signal in turn adjusts the duty cycle so that the converter tracks the reference voltage at the output and thus the error gets minimized. Alternately both the output voltage and the inductor current can be used for the feedback circuit. Most commonly used feedback circuits for the dc-dc converters are voltage mode control, and current mode control. The voltage mode control uses only the output voltage in the feedback where as the current mode control uses both the output voltage and the inductor current. Both these modes are discussed in detail now.
Voltage mode control
The voltage mode control scheme is explained by using a simple Buck converter as shown in the Figure 1.1. Here the output voltage is compared with the reference voltage and the error voltage is given to the controller or compensator. The compensator processes the error and produces a control voltage signal, VC. This control signal is fed to the comparator and compared against the ramp signal. This in turn generates a pulse width modulated signal to drive the semiconductor switch.
The pulse width modulation is implemented in such a way that, if the VRAMP is lesser than or equal to control signal, the switch is turned on and if the ramp signal voltage is higher than the control signal, the switch will be turned off. Thus the duty cycle which is defined as the ratio of, is adjusted to track the reference value.
Figure 1.1. Voltage mode control of dc-dc converter
This results in tight output voltage regulation and improved dynamic characteristics of the converter systems under consideration. The pulse width modulation scheme is illustrated in Figure 1.2.
Figure 1.2. Pulse width modulation scheme
Current mode control
In this scheme, an additional inner loop is provided in order to speed up the response of the system. This approach is mainly used in Boost and Buck-Boost converters which suffer from non-minimum phase response. The current mode control scheme is shown in the Figure 1.3 using boost converter and the corresponding pulse width modulation scheme is shown in the Figure 1.4 respectively.
Figure 1.3. Current mode control of dc-dc converter
Figure 1.4. Driving pulses using current mode control
This scheme comprises of two loops, an outer loop and an inner loop. The outer loop is much slower in which the output voltage is compared against the reference and the resulting signal in turn acts as a current reference for the inner loop. The inner loop is much faster in which the reference value produced by the outer voltage loop is compared against the actual value of the inductor current. It is assumed that the switches are turned on and off by a clock pulse generated by the RS flip flop circuits. As the clock pulse turns on the switch, the inductor current rises up. When this current reaches the reference current value given by the outer loop, the comparator output goes high and hence the switch is turned off. Thus the inductor current ramps down and this process gets repeated in a cyclic manner thereby controlling the duty cycle ratio of the converter system. The purpose of the outer voltage loop is to set a reference value for the inductor according to the error voltage signal. The important feature of this method is that the response of the system will be much faster and the performance of the system will be better. Inspite of this, during the compensator design, it should be kept in mind that this method suffers from inherent high frequency instability due to sub harmonics and chaos.
The DC-DC converters are extensively used in computer peripherals, communication systems, medical electronics, adapters of consumer electronic devices, space stations, ships and aircraft to provide the required level of dc voltages. The demand for the development of light weight, compact and highly efficient switched mode power supplies have been increased in recent years which lead to the stabilization of the converter systems. The stability of the converters is achieved by designing appropriate feedback loops. The conventional design approaches lead to the performance degradation of the closed loop system due to component deprivation and voltage changes, resulting in the design of robust controller to achieve good dynamic performance (Middlebrook 1988).
There are several DC-DC converters among which the Buck and Boost converters play a vital role in portable consumer electronics, since these converters are simpler in construction and highly efficient. When these converters are used in high power applications, they cannot withstand voltage or current stress. To overcome this drawback the converters of the same type are connected either in series or in parallel. Such parallel connected converters are called interleaved converters. The interleaved converters have the advantages of reducing the input current ripple, inductor size, current rating of the semiconductors, I2R losses and inductor AC losses. Further these interleaved converters are highly reliable with good current sharing among the converter modules and easier for system maintenance.
The main challenge in the field of Power Electronics is emphasized more on the control aspects of the DC-DC converters. The control approach requires effective modelling and a thorough analysis of the converters. The control based on conventional methods results in difficulty of system alteration and higher functions, thereby leading to low reliability and higher sensitivity to noise (Chander et al 2011). To overcome the above mentioned drawbacks, robust controllers have to be implemented.
In recent years digital controller for power switching converters are being popular due to several advantages such as advanced control strategies, low sensitivity to variations, robustness to ageing and environmental changes, noise immunity and ease of programming. In digital approach, the highest resolution Digital Pulse Width Modulation (DPWM) signals and digital control algorithm have found significance in the research, in order to obtain high accuracy in required output voltage and maximum utilization of the controller (Morroni et al 2009).
The control of DC-DC converters are mainly focused on obtaining stiff output voltage regulation. The typical control strategy which is widely implemented through pulse width modulation can be categorized into voltage mode control and current mode control. The current mode control is advantageous over voltage mode control in which the system responds quickly to the disturbances (Sreekumar and Agarwal 2008). But this technique suffers from an inherent instability and sub harmonic oscillations at constant frequency operation and hence a dynamic compensation has to be designed. The major constraint in the design of control based on frequency domain is the presence of a zero in the right hand side of the plane in many of the averaged models. The average value of the inductor current is inversely proportional to the location of this zero and therefore any increase in the value of the inductor current may reallocate this zero to the lower frequency region of the right hand side of the plane. This creates a considerable phase lag which restrains the existing bandwidth for a constant operation of the converter and makes the design to be carried out in time domain (Shuibao Guo 2009).
In low power DC-DC converters, the overload protection, increased efficiency and improved dynamic response are obtained by current sensing or measurement methods. The measurement methods are generally voltage drop and observer based methods. The major drawback in voltage drop method is that it decreases the efficiency of the system under consideration and requires an amplifier with wide bandwidth, which is very difficult to implement. The control circuits for the DC-DC converters are more complex and when the converters are operated in continuous conduction mode, the current imbalances due to intrinsic device parameter variation occur which is quite critical. To overcome the above mentioned control problems an observer based controller for the Buck, Boost, Interleaved Buck and Interleaved Boost converters are designed and presented, which is based on deriving a control law defined as, where k is the state feedback matrix and x(t) is the state vector. An Observer Controller (which estimates the unmeasurable variables) with state feedback matrix (control law) has been designed for Buck and Boost Converters using pole placement technique and Separation principle, both in analog and digital structure.
Observer is designed based on time domain in which the converter specifications such as rise time, settling time, maximum peak overshoot and steady state error are met. The converters are modeled using state space averaging technique. The Separation Principle makes the design procedure much simpler in which the state feedback gain matrix is designed by pole placement and then the full order state observer by the same technique, finally which can be combined together to provide a better dynamic compensation for both the Buck and Boost converters. The main advantage of this principle is that the design of control law and the observer can be carried out independently and when both are used together the roots remain unchanged.
The proposed Observer Controller is designed in twofold. First, the appropriate Observer poles are chosen and the controller is designed by combining state feedback matrix and Observer poles by using Separation Principle. The main advantage of this principle is that the design of state feedback matrix and the observer can be carried out independently and when both are used concurrently the roots remain unchanged. Next the state feedback matrix obtained for the interleaved converters are optimized by deriving Riccati matrix and using the same observer poles chosen as in the first step, a Linear quadratic optimal regulator (LQR) is designed for Interleaved Buck and Interleaved Boost converters.
The simulation and experimental setup were carried out to validate the proposed controller design. Thus the Observer Controller designed for the Buck, Boost, Interleaved Buck and Interleaved Boost converters gives an excellent output voltage regulation, improved dynamic response, robust, rejects the disturbances, highly efficient with much lesser settling time in the range of milli second and good current sharing among the converter modules.
Lueng et al (1991) proposed a control problem based on Linear quadratic optimal regulator for PWM type switching dc-dc converters. The authors suggested that the state feedback theory provides an important solution for obtaining the dynamic performance of the dc-dc converters.
Pinheiro et al (1999) have proposed a control of Interleaved Boost Converter using Lyapnov control technique for power factor correction application. The state feedback gain is obtained using Linear quadratic optimal regulator method. Additionally a feed forward voltage loop is employed to improve the system response which is quite complex and more computations are required.
Sun et al (2001) have designed the reduced order and full order averaged models for the DCM PWM Boost converter. The authors have taken the duty cycle ratio as a constraint and suggested it as a frame work for comparing different models. The authors also have suggested that this method accurately predicts that, the responses of the small signals are upto the maximum frequency that can be handled by the linear time invariant frequency domain model. Comparative evaluation of all the relevant models developed earlier was made which overcomes the existing problems in modelling.
Zhang et al (2001) have designed a Boost converter under critical current mode for PFC rectifier. They highlighted the fact that when the converter is operated in the critical mode, it requires a larger differential mode EMI filter which is quite undesirable. Next, the interleaved boost converter under critical mode of operation with variable frequency control is also carried out and they highlighted the fact that the boost converter suffers from non minimal phase response due to phase lags and subharmonic oscillations. In this study the dynamic performances of both the Boost and interleaved Boost converters are not verified under variable voltage and under disturbances.
Tse et al (2002) have analyzed most of the non linear phenomena existing in power electronic systems. The chaotic and bifurcation behavior of the Buck, Boost and Buck-Boost converters are discussed. The authors focused the bifurcation behavior and dynamic behavior of the power electronic converters. This work paves the way for the essential analytical approaches that can be performed to utilize the power electronic converters in several applications.
Gonzalez et al (2005) have designed an Observer controller for the basic Buck, Boost and Buck Boost converter topologies by using passivity based non linear design. The output thus obtained shows overshoots and undershoots which are undesirable.
Padimiti et al (2005) have evaluated the various digital control techniques including the predictive control and a dead beat control techniques. They have suggested that the digital control and implementation of the dc-dc converter topologies are highly advantageous and easy to realize. The authors highlighted that the sensorless current mode control which estimates the inductor current without measuring is one of the best choice to replace the current sensors especially in industries. They emphasized that these control techniques provide better results when used with dc-dc converters and they are the best choice for the automotive industries in order to improve the dynamic performances of the electric vehicles.
Feng et al (2006) have proposed a new switching cycle compensation algorithm to optimize the transient performance of the dc-dc converter under input voltage changes. The converter is feedback to steady state, driven by an optimized value of two switching cycle series. This was done by using the principle of capacitor charge balance. The system shows improved dynamic performance. The authors implemented it using FPGA and used sensing resistors in series with the switch to measure the inductor current. This type of current measurement is quite disadvantageous since the efficiency of the system gets reduced due to higher power loss.
Peretz et al (2006) have designed a digital PID controller for the PWM dc-dc converters based on the time domain responses. The averaged models were derived and the discrete controller shows an improved dynamic performances. Hence the authors suggested that the time domain approach bypasses some of the errors due to the s to z transformation and hence it is highly compatible.
Zou et al (2006) have proposed a method to control the chaos in the Buck converter using Pole placement technique. Sate feedback matrix has been derived and the stability of the converter is achieved. But the state estimation has not been done in order to verify the robustness of the control method.
Huang et al (2007) have analyzed the slow scale and fast scale bifurcations of the parallel connected buck converter using master-slave current sharing method. The PI controller is implemented and the boundaries between these bifurcations are identified. The results offered a reference for the practical implementation where the occurrences of slow-scale bifurcation are least bothered.
Hyung-Su Bae et al (2007) have proposed a digital state feedback current control using pole placement technique for the synchronous Buck-Boost converter. This paper mainly focuses the current sharing among the converter modules during the transition from one mode to another mode. It does not show the performance parameters of the converters in terms of output voltage, settling time, rise time and maximum peak overshoot.
Abu et al (2008) proposed the sensorless voltage mode in multiphase buck converter and achieved optimum current sharing among the converter modules. The proposed method is implemented using digital controller and shows good performance under dynamic loading variations. The voltage loop control when compared with current mode control is less advantageous. With the voltage mode control, a type three compensator is required to stabilize. Also with this mode of control crossover frequency of the converter should be higher than the resonant frequency otherwise the filter will ring. It is quite complex to design a compensator with voltage mode control since good dynamic performance cannot be obtained under both CCM and DCM.
Bo-Cheng et al (2008) have dealt with a feedback control using sampled inductor current for the Boost converter. The stability criterion has been achieved successfully and it is shown that the chaotic behavior of the boost converter can be controlled. But the performance parameters of the converters are not taken into account and the load estimation was not done to ensure the robustness of the control law.
Chen et al (2008) have made a thorough analysis to identify the stable operating region of the Buck and Boost converters. The authors focused their study on fast scale and slow scale bifurcations which occur both in current mode and voltage mode control. They have suggested that the effects can be eliminated by increasing the feedback gain. They have also extended their investigation by inspecting the Eigen values. This in particular is very much useful to design the input voltage parameter and load resistance for the converters which operate under stable operating regime.
Geyer et al (2008) have modeled the DC-DC converter as hybrid system for the whole operating regime by deriving a piecewise affine model. Dynamic programming is used for formulating a constrained optimal control problem. The authors validated that this method can be easily implemented by creating a look up table instead of going in for an online optimization. But the authors say that the derived controller is quite complex and higher in cost.
Huber et al (2008) have focused the master slave converter synchronization under open loop conditions for interleaved boost converters operating under CCM and DCM. The slave converter is synchronized with the master converter at turn on instant of the master converter there by achieving stable operating conditions. The analysis illustrates the effects of mismatched inductances, phase shift error, switching frequency limit and valley switching on the input current ripple and input current distortion. Since the feedback loop is absent, the output response may be inaccurate and unreliable. Further, the response of the system to the external disturbances is not discussed.
Jonathan et al (2008) have dealt with the stabilization of Input series and output parallel connected converters. This method uses a single outer loop to generate a reference. It is quite disadvantageous when compared with two loop system and especially when it is employed for Boost converters lots of disturbances will be generated.
Lukic et al (2008) have proposed a self tuning current estimator for the synchronous buck converter with digital implementation using CMOS technologies. The estimator works well and automatically adjusts the coefficients using test current sink and a tunable IIR filter for the variations in the inductor parameters. The operation of the self tuned estimator is proved to be faster and accurate. Here the input voltage is sampled at a frequency much lesser than the switching frequency of the converter. This is quite undesirable since if the sampling frequency is lesser than the highest frequency of the system, then aliasing effect is produced and most of the information will be lost.
Mariethoz et al (2008) introduced a new predictive control method for interleaved dc-dc converters. The major control is emphasized on the good transient performance and to achieve protection of the semiconductor devices. The load estimation is achieved based on the capacitor voltage measurement method and is implemented. This in turn achieves inherent limitation on the leg current during transients. Instead of all the performance benefits it is found that observer can be seen as an algorithm which processes the data thereby reconstructing the variable state system from the input mathematical model and output measurement for the practical systems. From this literature it is evident that it is less expensive and more reliable. It presents advantages from the mathematical model itself. It can easily be implemented through digital computers.
Sreekumar and Agarwal (2008) have presented a new hybrid control algorithm for the boost converter to obtain output voltage regulation under continuous and discontinuous mode of operations. The authors prove that the system is stable under operating conditions and can easily be implemented since the computations are minimum. But the main constraint in this algorithm is that it can only be applied for lower switching frequency and when it is applied for the higher frequency systems, limitations are to be imposed on various circuit conditions such as power loss, driver circuit limitations and device speed.
Xu et al (2008) have presented a closed loop control for Interleaved Boost converter operating in critical mode of conduction. It includes a master and a slave converter which operate ideally under critical mode and there is a much reduced input ripple current which finds wider applications especially in power factor corrections. Inspite of the improved results it has several disadvantages such as, when the semiconductor switch is turned on, the hard reverse recovery takes place resulting in the generation of high frequency EMI and harmonics and more over additional circuitry should be involved to soften the reverse current transient of the diode and the switching semiconductor device turn on.
Al-Saffar et al (2009) have designed a new non isolated single stage switch high power factor correction converter. Both Buck and Buck Boost Converters are selected due to their intense step down capabilities. This paper proposes a way to overcome the inrush current problem and acts as a protection against over load current. The major drawback in going for Buck-Boost converter is that it is very difficult to operate in the continuous conduction mode.
Carlos et al (2009) achieved the stability and the performance of the PWM converters using LQR method. Here the Linear matrix inequalities are dealt in which more than one plant can be taken into account. The authors suggested that the controller exhibits a more predictable response thereby assuring an upper band of a performance index.
Chen et al (2009) introduced a four stage interleaved boost convert cells. The authors suggest that the interleaved converters can reduce the input current and output voltage ripple thereby resulting in improved efficiency.
Grote et al (2009) have achieved the low cost implementation of interleaved boost converter for PFC application by combining together a digital and analog control parts. The analog circuitry is retained by the conventional analog structure. This results in high current control bandwidth and improved efficiency.
Jonathan et al (2009) discussed a PFC control without the current sensor in which the current sensor is replaced by a Kalman like filter acting as an Observer. It has two extended kalman filters which derive the information from the input and bus voltage measurements. In this work kalman filters are used for voltage sensing. Though it gave near unity power factor, the observer can better be used when it is employed for current sensing, which is more advantageous.
Jong et al (2009) have obtained average current information by sensing the diode current in the literature. A resistance is in series with the converter circuit is employed and it obviously results in reduced efficiency due to power loss. The power losses are more pronounced under low voltage conditions which further reduce the efficiency.
Morroni et al (2009) proposed a closed loop operation of SMPS converter based on adaptive tuning method. In this method, the system cross over frequency and phase margin of the converter under consideration is closely monitored. The adaptive tuning of the compensator parameters are done with MIMO closed loop system. The compensator thus designed includes the design of PID compensator and an adaptive tuning compensator. Though the method yields a satisfactory performance, the computations are quite complex and time consuming.
Chander et al (2010) have proposed the FPGA implementation of PID Controller for DC-DC converter. The modelling and simulation of a synchronous buck converter with discrete PID controller is carried out. The performance parameters are much improved and better results were obtained. But when FPGA is used for prototyping, it may have significant changes when migrated to higher performance design and package solution. The other disadvantages include limited size options and limited performances.
Mariethoz et al (2010) have designed five different control methodologies for Buck and Boost converters by taking into consideration the response times and capability to reject the disturbances. The systems settle down faster and reject the disturbances thereby improving the dynamic performances of the systems. Authors have suggested that the performance of the converter and the robustness of the controller mainly depend upon the controller tuning. State feedback control has been derived and load estimation was done which can be taken as a point of reference result for the design based on the observer controller.
Mayo et al (2011) have proposed the full order and reduced order non linear dynamic model of the multilevel boost converter. Here the inductor current is controlled by using input-output feedback linearization. The state feedback gain matrix is derived using pole placement technique and output voltage regulation is obtained and implemented using RTAI Lab as a Linux based real time platform and NI PCI-6024 E data acquisition board. But the robustness of the control law has not been presented and the dynamic performance of the converter has not been checked by varying the inductor, L and the capacitor, C parameters of the converter.
Based on the above literature survey it can be very well understood that the state feedback control method proffers an efficient stability for the dc-dc converters under all operating conditions. But in most of the conventional current control methods, peak current mode control is being used which is highly detrimental leading to the higher noise sensitivity. In most of the industrial applications, small value of a precision resistor is being used which in turn leads to the false firing of the power transistors employed in the dc-dc converters. The important criteria to be considered for most of the industrial applications are the reduction in the sensor number. Hence a cost effective sensors are required. A feasible solution is obtained by using a well known Observer Controller.