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BUCK-BOOST CONVERTER Introduction: In a large number of industrial applications, it is required to convert a dc voltage to a different dc voltage level, often with a regulated output. To perform this task, a dc-dc converter is needed. A dc-dc converter directly converts a dc voltage of one level to another. It can be used to step-down (buck), or step-up (boost) a dc voltage source. Higher switching frequency would reduce the size of the filter used.1.2 Basic Principle of Buck-Boost converter:The buck-boost is a popular non-isolated inverting power stage topology, sometimes called a step-up/down power stage. Power supply designers choose the buck-boost power stage because the required output is inverted from the input voltage, and the output voltage can be either higher or lower than the input voltage. The input current for a buck-boost power stage is discontinuous, or pulsating, because of the power switch current that pulses from zero to IL every switching cycle. The output current for a buck-boost power stage is also discontinuous or pulsating because the output diode only conducts during a portion of the switching cycle.Equivalent circuit1.2.1 Analysis for the Switch Closed:When the switch is closed, the voltage across the inductor isThe rate of change of inductor current is a constant, indicating a linearly increasing inductor current. The preceding equation can be expressed as1.2.2 Analysis for the Switch Open:When the switch is open, the current in the inductor cannot change instantaneously, resulting in a forward-biased diode and current into the resistor and capacitor. In this condition, the voltage across the inductor isAgain, the rate of change of inductor current is constant, and the change in current is1.2.3 Inductor DesignPower absorbed by the load must be the same as that supplied by the source.Average source current is related to average inductor current bySubstituting for Vo derived above and solving for IL, we findFor continuous current, the inductor current must remain positive. To determine the boundary between continuous and discontinuous current, Imin is set to zero resulting in1.2.4 Output Voltage Ripple:The output voltage ripple for the buck-boost converter is computed from the capacitor current waveform.The converter consists of dc input voltage source VS, controlled switch S, inductor L, diode D, filter capacitor C, and load resistance R. With the switch on, the inductor current increases while the diode is maintained off. When the switch is turned off, the diode provides a path for the inductor current. Note the polarity of the diode that results in its current being drawn from the output. The condition of a zero volt-second product for the inductor in steady state yieldsFig 1.3 Circuit diagram of buck boost converterHence, the dc voltage transfer function of the buck-boost converter isThe output voltage VO is negative with respect to the ground. Its magnitude can be either greater or smaller (equal at D = 0.5) than the input voltage as the name of the converter implies. The value of the inductor that determines the boundary between the CCM and DCM isDESIGN PROBLEM:Vs = 24 VD = 0.4R = 5 OhmL = 20 uHC = 80 uFf = 100 kHzLimitation = 0 V to 36 V(by simulation)Output Voltage: Vo = -16Volt.Inductor Current: IL = VsD(R*1-D^2 = 5.33 ARipple current: IL=VsDTL =4.8 A ILmax= IL+?IL2 = 7.33 AILmin=IL-?IL2 = 2.93 AOutput voltage ripple:Vo=DRCF = 0.01=1%Inductor Design:Type : Power InductorInductance : 20uHMaximum DC current : 7.8AmpsCore Material : Powdered Iron CoreMaximum DC resistance : 26mOhmOPEN LOOP SIMULATION:Fig 1.4 Open Loop Simulation of buck boost converterFig 1.5 Simulation resultsCLOSED LOOP SIMULATION:Fig 1.6 Output VoltagePI CONTROLLER:(Tuning by trial and error method)KpKiPeak overshootSettling time0.0002215-340.0320.000510-340.0520.00051-340.0930.00095-340.0680.0028-340.0550.00510-340.0530.0095-340.0920.0018-340.0600.015-340.089Thus if Kp, Ki values are increased or decreased Overshoot remains the same.When Ki value is increased and Kp value decreased further Settling time decreases.The above table is achieved by trial and error method.To further decrease the peak overshoot Derivative controller can be added.M-File Coding for Open loop Buck Boost Converter:function my_ode()global A D Cf Lf Vsf = 100000;R = 5;Vo = 16;Io = Vo/R;Vs = 24;D = Vo/(Vo+ Vs);L = 20E-6;C = 80E-6;Lf = L*f;Cf = C*f;A = [0 -1/Lf; 1/Cf -1/(R*Cf)];x0 = [D*Io; Vo];tf = 50;tic[t,X] = ode23(@bukboost,[0 tf],x0);toc;IL = X(:,1);VC = X(:,2);subplot(2,1,1),plot(t,IL),gridaxis([0 tf 0 20])title('Inductor Current')subplot(2,1,2),plot(t,VC),gridaxis([0 tf 0 120])title('Output Voltage')xlabel('cycles') endfunction dx = bukboost(t, x)global A D Cf Lf VsiL = x(1);vC = x(2);B = [(vC+Vs)/Lf; -iL/Cf];u = 0.5*(1-sign(t-fix(t)-D));dx=A*x+B*u;endFig 1.7 Inductor Current and Capacitor VoltageTic?starts a stopwatch timer to measure performance. The function records the internal time at execution of the?tic?command. Display the elapsed time with the?toc?function.APPLICATIONS:Battery-powered systems: In battery powered systems , where the input voltage can vary widely, starting at full charge and gradually decreasing as the battery charge is used up. At full charge, where the battery voltage may be higher than actually needed by the circuit being powered, a buck regulator would be ideal to keep the supply voltage steady. However as the charge diminishes the input voltage falls below the level required by the circuit, and either the battery must be discarded or re-charged; at this point the ideal alternative would be the boost regulator. Hence buck boost converter will be the preferred choiceIn Solar PV for MPPT:Buck/Boost converters make it possible to efficiently convert a DC voltage to either a lower or higher voltage. Buck/Boost converters are especially useful for PV maximum power tracking purposes, where the objective is to draw maximum possible power from solar panels at all times, regardless of the load. ................
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