CHAPTER 1 INTRODUCTION

Transcription

1 1 CHAPTER 1 INTRODUCTION 1.1 INTRODUCTION Pulse Skipping Modulation (PSM) is a control technique and a DC-DC converter under PSM control has significant merits such as constant frequency operation, absence of the need for compensation circuit design, higher light load efficiency, better Electro Magnetic Interference (EMI) performance, inherent stability as it is a variant of hysteretic control with absence of chaos under Discontinuous Conduction Mode (DCM) and good response to changes in input and load with capability to regulate over a wider range of input voltage. The PSM converter operates with higher ripple and large device peak current making its use questionable as a whole time mode. The ripple performance and means to reduce it along with the use of Pulse Skipping Modulation (PSM) as a whole time mode is studied in this thesis. Since a PSM converter under DCM works free of chaos, a possibility of operating a converter in DCM with circuit parameters corresponding to continuous conduction mode is also studied. Ripple minimization with inductor current control is proposed and studied. For power supply applications the method proposes a PSM converter cascaded with linear regulator and SIMPLIS is used for studying. In hybrid mode converters a method by which the avoidance of chaotic operation during Pulse Width Modulation (PWM) mode

2 2 through mode hopping is suggested due to chaos free operation of PSM converters under DCM. 1.2 BACKGROUND DC supply is required in most applications involving electronic circuitry. A power converter circuit is used to convert the available AC mains voltage or DC voltage from battery or other sources to higher or lower level voltages desired. The power converter accordingly may be one of either a buck or a boost converter or their variant. The power converter located typically as shown in Figure 1.1 is controlled with a low power analog or digital control circuit to achieve the desired output under different conditions. The working voltage of electronic apparatus becomes lower and lower with the development of science and technology. Therefore Buck converters that step down voltages to lower level is increasingly finding applications with integrated circuits being designed to operate at lower voltages with supply and battery voltages remaining same as discussed by Liu Shulin et al (2005), Xue Yali et al (2003), Liu Jian et al (2005). E lec tric E nergy S o u rce P o w er E lec tro nic C irc u it E lec tric a l L o a d C o ntrolle r Figure 1.1 Power converter in an electric power system

3 Types of DC-DC Buck Converters Linear and switching converters DC-to-DC buck converters are of linear or switching type. Linear ones depend on dissipation for stepping down and their efficiency is poor. The linear regulator like Low Drop Out (LDO) is used often but the transfer efficiency is lower when drop voltage between supply and output is large. Switching regulators operate at higher efficiency under these conditions. A comparison of linear and switching regulators is shown in Table 1.1. Linear voltage regulators have merits such as low noise EMI, low ripple at output voltage, good load and line regulation, wide bandwidth and quick response to changes in load and line quantities, but the efficiency is poor and can only operate as buck regulators. Due to the efficiency being lower and loss higher, the heat produced is high. This in turn increases the weight and volume. Table 1.1 Comparison between Linear and switching regulators Linear Switching Function Step down Both step up and down Efficiency Low to medium. High if supply load voltage diff. is small High except at very light loads. Waste heat High Low Complexity Low Medium to high Size Medium to large for moderate to high loads Low to medium Cost Low Medium to high Ripple Low Medium to high

4 4 Switching converters employ electronic switches that operate either in cut off or saturation region. Since the voltage across or current through the switch is almost zero always, the losses are minimum and since the operating point of switch does not dwell in active region for long, the efficiency is much higher. Heat dissipation is less and this in turn makes thermal management easier, reduces the weight and volume. Hence Switching converters are preferred over linear converters due to their high efficiency making them suitable for power critical applications. With reduced losses, the thermal management is improved with no heat sink requirement mostly as pointed out by Liu Shulin et al (2005) Isolated and non isolated switching buck converters Converters can be classified into various types, and in accordance with the provision of input output isolation there are two types known as isolated and non-isolated types. In isolated type converters, including a transformer, which works at low frequency or high frequency, provides transformer isolation. Conversion involving rectification almost always includes a transformer at power frequency that serves the isolation purpose. These converters are usually heavy and these are used as battery eliminators at low power levels and also for DC power sources in UPS and motor control applications. They always produce power quality problems at AC supply end. In applications such as emergency lamps and adaptors used for printers, set top boxes, MODEMs etc., a high frequency transformer is usual with double or triple stage conversion. There are forward and flyback type converters, which are almost single stage converters and employ an isolation transformer that also serves the purpose of storing energy for short time duration. This transformer is made of ferrite core material that is light and occupies less space and does not produce much heat. These converters are

5 5 compact, efficient but expensive at present compared to those employing power frequency transformers. Converters that are of low wattage and voltage ratings deployed in applications that do not necessarily require isolation may not include a transformer. A non-isolated buck converter is shown in Figure 1.2. Figure 1.2 Buck converter It includes a switching cell that chops the input voltage into a rectangular wave and an LC filter that filters the wave to get a DC voltage. The magnitude of the output voltage is proportional to the duty ratio of the chopper (Mohan et al 2003). Switching frequency is the chopping frequency and the LC filter is designed to filter this out. But in a practical converter a low or sometimes considerable amount of ripple always would be present in the output. The ripple frequency usually equals the switching frequency.

6 Feedback Control of Converter Output voltage of a converter depends on input voltage, load and other conditions and hence may not be constant throughout the operation as load, input voltage, temperature or other parameters may vary. But most applications require the output quantity be regulated and held constant at a desired value for which regulators are used. The main function of voltage regulators is the regulation of the dc output voltage against changes in the load current, the input voltage, and other parameters. They should also provide dc isolation, ripple voltage reduction, and good transient response to abrupt changes in the load current and the input voltage. Hence feedback control that suitably controls the converter to regulate the output constant is designed such that it also improves the performance of the converter with better responses to changes or transients in load current or input voltage. The controller usually measures the actual voltage and compares with the desired voltage to generate a signal that undergoes further processing and switching signals are generated based on the decision made by the controller. The control methods generally adopted in switch mode regulators include Hysteretic, PWM, PFM, Zero Voltage Switching (ZVS) resonant technique and PSM recently. PWM control is well developed and popular but the efficiency is poor at light loads. PFM requires operation at different frequencies, making optimum component selection for filter, difficult. This problem is solved in Constant Frequency Zero Voltage Switching Quasi Resonant Converter (CF-ZVS-QRC) (Dananjayan and Chellamuthu 1996) but ZVS enhances conduction losses and hence is not suitable for applications involving energy-limited sources (Erickson and Maksimovic 2000). Pulse Skipping Modulation reduces the switching loss and improves efficiency at

7 7 light loads during standby mode operation of portable devices (Ankititrakul and Hu 2008). Hysteretic and PSM controllers are closely related to on-off controller. On-off controller is the simplest controller and it has some important advantages. It is economical, simple to design and it does not require any parameter tuning. If oscillations will hamper the operation of the system and if controller parameter tuning is to be avoided, on-off controller is a good solution. In addition, if actuators work in only two modes (on and off), then it is almost always the only controller that can be used with such actuators. That is a reason why on-off controllers are often used in home appliances (refrigerators, washers etc.) and in process industry when control quality requirements are not high (temperature control in buildings etc.) Additional advantage of on-off controllers is that they in general do not require any maintenance (Zoran Vukic 2002 ) Comparison between PWM and PSM PWM control is well developed and popular but the efficiency is poor at light loads. PSM control is new but efficiency at light loads is high. Pulse Skipping Modulation reduces the switching loss and improves efficiency at light loads, during standby mode operation of portable devices. PWM control requires parameter tuning and not easy to design, whereas PSM converter is economical, simple to design and it does not require any parameter tuning. PWM converter involves varying pulse width whereas PSM has a constant frequency constant duty cycle clock. Hence operation at high frequencies is easier. PSM converter has better transient response and regulates over a wide load and supply voltage range. PWM converter requires filtering due to poor EMI performance where as PSM converter has a widely spread out spectrum with better EMI performance. PWM converter operates

8 8 with low ripple whereas a PSM converter suffers from higher ripple content and also there is a possibility of frequency components entering into audio frequency range resulting in noise and weakening of board components. A dc-dc buck converter, controlled by naturally sampled, constantfrequency pulse width modulation, in continuous conduction mode, has been reported to exhibit nonlinear phenomena, such as bifurcation and chaos, depending on the values of the parameters of the circuit. It has been verified and reported that, PSM converters, especially in DCM, is chaos free even though bifurcation is present. PWM can be employed as whole time mode whereas PSM requires improvement to reduce ripple and make it suitable for whole time operation Nonlinear Phenomena in Buck Converters Research in nonlinear systems recently led to findings that simple deterministic systems may behave in an apparently random fashion. This behaviour, known as chaos, is due to nonlinearity as discussed in power electronics circuits that behave nonlinear exhibiting a variety of complex behaviour. Chaos is an aperiodic, bounded, and random like behaviour exhibited by a deterministic system and is mostly not desired. Bifurcation is another phenomenon characterized by a sudden change of qualitative behaviour with expansion of operating regions potential enough to cause damage to elements. Bifurcation may be observed when a parameter is varied and may lead to chaos. Occurrence of bifurcations and chaos in power electronics was first reported by Hamill and Jefferies (1988), Deane and Hamill (1990) and are often encountered in systems of this type and considerable attention has been focused on the analysis of DC-DC converters (Di Bernadi et al 1998,

9 9 Fossas and Oliver 1996, Gautham Poddar et al 1995, Di Bernado and Francesca Vasca 2000, Maity et al 2005). A dc-dc buck converter controlled by naturally sampled, constantfrequency pulse width modulation in continuous conduction mode has been reported to exhibit nonlinear phenomena, depending on the values of the parameters of the circuit (Fossas and Olivar 1996). The papers by Deane, Hamill and Wood discuss bifurcation and chaos that occur in PWM buck dc-to-dc switching converters under continuous conduction mode (Deane and Hamill 1990, Hamill et al 1992, Wood 1989 ). DC-DC converters are always designed for period-1 operation in which waveforms repeat at the driving clock's rate. Under all possible disturbances converters are expected to work in this regime stably. Under certain conditions, the circuit may lose stability and follow period - n operation in which the periods of waveforms are n times that of the clock. Due to parameter variations operation in one regime may fail and converter abruptly may start operating in another regime. This phenomenon is known as bifurcation and often leads to chaos. Such behaviours are undesirable and are always eliminated by adjusting circuit components and parameters. In order either to avoid or make use of for better design it is necessary to understand the behaviour of the systems being designed under all possible practical conditions. Knowing when and how a bifurcation occurs is important and it requires appropriate modeling and analysis (Tse 2003) Using computer simulation a summary chart of the different types of its behaviour when some parameters are varied is obtained. This is known as the bifurcation diagram and is a plot, where a state variable is plotted against the chosen bifurcation parameter. This bifurcation diagram clearly

10 10 shows the behavioural change of the converter within the parameter range of interest Applications of Buck Converters Applications of dc-to-dc buck converters include drives for electric vehicles, in the D.C link for variable frequency inverters and in switched mode power supplies. In drives applications, winding inductances and mechanical inertia of motors act as filters resulting in high-quality armature currents. The average output voltage of step-down choppers is a linear function of the switch duty ratio. The converter can also work in two or four quadrants. Two-quadrant choppers are found in autonomous power supply systems containing battery packs and such renewable energy sources such as photovoltaic arrays, fuel cells, or wind turbines. Four-quadrant choppers are found in drives in which regenerative breaking of dc motor is required (Rashid 2007). In many low power, low voltage, battery powered dc-dc converter applications, the high efficient conversion at light load condition, or stand by mode, is required to gain power saving and battery life extension. Dual mode control scheme, PWM and pulse skipping mode are generally employed in buck converter to obtain highly efficient conversion over wider load range extended from light load to heavy loads (Arbetter and Maksimovic 1997, Peterchev and Sanders 2006, Huang et al 2006). 1.3 REVIEW OF RELEVANT LITERATURE To obtain high efficiency in switching power converters, recently switching control ICs such as MIC2178, Si9118 and LM2647 etc, started adding pulse skipping mode to PWM mode when they are on light loads or working in sleeping mode. But except some application examples, the

11 11 theories or mathematical models of pulse skipping mode are hardly found. Luo Ping et al (2005) considered PSM mode as the whole time control mode of a switching converter and modeled and analyzed the characteristics and behaviors of PSM converter. It is shown that as load becomes lighter, more cycles are skipped. In their improved PSM mode besides the PSM regulating rule, the duty ratio of the control pulse becomes a little smaller, when the load turns light, which result in smaller output ripple. It was reported that the PSM mode has high efficiency, especially with light loads, quick response speed and good EMI characteristic. The work suggested that PSM can be an independent regulating mode for a DC/DC converter during its whole operating time, not just an assist mode if ripple performance is improved. Ankititrakul and Hu (2008) discussed on the light load efficiency of the converter employing pulse-skipping mode. The improvement in efficiency is determined by the periods of skipping and active operations. The impact of the output inductance and capacitance on active interval and conversion efficiency was explained. The operation of synchronous buck converter with pulse-skipping mode can be divided into two portions: 1) Skipping interval, and 2) Active interval. Current pulses are applied to the filter during active interval and are skipped during skipping interval. By controlling the length of active and skipping periods, the switching loss is reported to be reduced and this could benefit dc-dc conversion applications in the low power, low voltage, and battery-powered equipments. A dual mode converter was suggested in which the mode under light load can be skipping mode as the losses due to switching dominates under light load as shown in Figure 1.3.

12 12 Figure 1.3 Rapid decrease of both conduction and switching losses in skipping mode A method of realizing PWM/PSM dual-mode controller, for high efficiency DC-DC buck converters was proposed by Yidie (2010). A minimum duty cycle control module was added to the PWM control loop, and with a simple logic control, the system could work in PSM mode when the load is light. The PWM control, at higher loads, operates with higher efficiency as conduction loss dominates. At lighter loads the switching loss dominates and hence PSM mode operates with better efficiency. Thus the efficiency of the system is improved over wide load range.

13 13 Figure 1.4 The schematic of PWM/PSM dual-mode controller The One-shot module is designed to provide a minimum duty cycle, which depends on V IN and V OUT. Once the duty cycle of the PWM comparator output is less than the minimum duty cycle, refer to the logic as seen in Figure 1.4, the PWM wave is on a non-functional, and Q will depend on the output of the One- shot module. At this time the load is light, and the system is limited by the minimum duty cycle, which is larger than the real needed duty cycle, so V OUT will increase. As a result, V fb is bigger than V ref, therefore the output of EA, V comp will get down, and PWM Comparator put logic 1 out, until V fb is less than V ref. During the time, several cycles are skipped, and it works in PSM mode. An integrated DC-DC (Buck) converter was proposed by Ahmed Emira et al (2010). The Buck converter has two modes of operation. The continuous mode is used for heavy loads, and the PSM mode is used for light loads. To optimise the Buck converter efficiency in PSM mode, an ON-time control loop is utilized. The loss due to residual current in the inductor being discharged through the drain-bulk diode during the skipping time, is reduced by controlling the ON time on which the residual energy in the inductor is proportional to. Researches were done subsequently and an Energy Balance (EB) model, based on energy conservation, was proposed by Ping Luo et al (2010)

14 14 and the model of a boost converter was proposed. The relationship between output voltage, modulation factor, M and load R were depicted. It was verified that a PSM converter may have bifurcation but under DCM the converter do not exhibit chaos whereas it happens for PWM converter. In this work the closed loop control strategy is based on the capacitance energy being above or below a critical value at the beginning of a cycle. The cycle will be a charging cycle if the energy is below and skipped if the energy is above the critical value. The model for a boost converter under DCM was studied and observed that there were period two bifurcation at M=0.5 and period 2,3,4 bifurcations with (n+m)= 2,3,4 respectively as shown in Figure 1.5 and there was no chaos observed as expected. At steady state there is always stabilization with nj=1 or mj=1 whenever a PSM converter operates in DCM as was observed in simulation. V0(V) V ref M Figure 1.5 The output voltage V O with modulation factor M

15 15 As is seen in the diagram shown in Figure1.5 in zone A there is period 2 bifurcation where M=0.5 and on the region left to A the skipping cycles are less than the charging cycle and it is just the opposite on to the right of A. In region B we find period-3 bifurcation with one skipping cycle out of three total number of cycles and in region C. Pulse skipping modulation mode is shown to be permitting higher conversion efficiency compared to that by PWM mode under the same operation conditions especially under light loads as reported by Ankititrakul and Hu (2008). PSM converter has better EM1 characteristic and the response speed is quicker. The ripple of the output voltage for a PSM converter is a little large and the actual frequency of power device enters the audible noise range due to skipped cycles as reported by Ping Luo et al (2006). 1.4 AIMS AND OBJECTIVES The main aim is to find means to reduce the ripple, which would improve the PSM converter, since ripple is the main drawback. Due to this demerit the PSM mode is preferred only at light loads, mostly during sleep/wait cycles, in devices using a buck converter and all the converter circuits use mostly combined PWM/PSM modes. During normal loads the control switches to PWM mode. Main objectives of the thesis are To study a PSM Buck Converter for the suitability of PSM as a whole time mode under continuous and discontinuous conduction. To improve the performance of the PSM converter leading to reduced ripple and device current transient.

16 16 To study applications of PSM converter when PSM is a whole time mode. To design a buck converter power circuit and study the operation of the converter with PWM and PSM controller separately to study if the two control techniques can be used interchangeably. 1.5 ORGANISATION OF THE THESIS Chapter 2 includes a DC/DC buck converter studied with PWM and PSM controller under continuous conduction mode for the same power circuit and operating frequency. The converters are studied under variation of input voltage and load current. Chapter also includes state space average modeling of converter and simulation study for exhibition of nonlinear phenomena with input voltage as parameter. The study includes bifurcation diagrams of the PWM and PSM converters and the strange attractor of the chaotic converter just after the chaos sets in, in the case of PWM converter. Chapter 3 deals with PSM converter under discontinuous conduction mode. The converter is modelled with and without Equivalent Series Resistances (ESR) and studied for input and load variation. Conditions for discontinuous operation is discussed and a converter under forced discontinuous conduction mode that would be useful when it is to be adopted through mode hopping from PWM operation under continuous conduction mode to improve stability. The chapter includes study of nonlinear phenomena to verify the non-existence of chaos with variation of input voltage as parameter. Chapter 4 discusses the factors affecting ripple in buck converter and includes a method to minimize the ripple in a PSM DC-DC buck converter considering change in input voltage and load over nominal range of

17 17 operation. A power supply application that cascades a PSM regulator with a linear regulator to add the merits of better ripple performance of linear regulator under near unity conversion ratios is proposed. Experimental investigations on the same are performed and the result included. Results show an improved ripple performance. Avoiding chaos through mode hopping is studied in a hybrid PWM/PSM converter. Chapter 5 gives a summary of the findings and other minor demerits, which may be considered while adopting PSM control for converters. The chapter also discusses other applications and possible modifications and improvements in methods of control for future work.