Calculating the Operating Parameters of the DC-DC Boost Converter for Automobile Electronics - Coursework Example

Table of Contents 2 Introduction 3 Objective 3 System 4 Voltage Mode Control 6 Current Mode Control 6 Component Selection 6 MOSFET selection 7 Boost converter inductor selection 7 Diode Selection 8 Capacitor Selection 8 Circuit Simulation 9 Detailed Practical Design 11 Circuit Diagram 11 System Evaluation 11 Overall Performance Evaluation 13 References 15 Abstract The current project involves developing and calculating the operating parameters of the DC-DC boost converter for automobile electronics. Considering the key requirements, it should be emphasized that the actual aim of the converter is to offer stable 10V current, and the basis of the circuit should be UC 3843 controller. The key aim of the converter is to prevent the engine electronics from the DC peaks that are essentially below the nominal during the engine start, and above the nominal after the start. The UC 3843 controller was selected due to its endurance capacity for the current leaps, and availability of the datasheets with clear and detailed calculations. Introduction The project was developed on the basis of the UC 3843 controller due to the increased stability of this chip, as well as high adaptability of this chip type for most circuit solutions. Moreover, the selected low cost PWM controller is featured with the clearly modulated square wave that is required for proper current measurements, and further adjustments of the circuit. Its parameters are the most suitable for the technical assignment, since it works within the given frequency range, and the operating frequency will be controlled by a capacitor. The DC-DC boost controllers are regarded as the common solutions for electronic devices powered by alkaline batteries. Therefore, the DC-DC converter will help to avoid the possible voltage declines. The boost converter, that is needed for the current project, is a converter that provides the step-up conversion of the input voltage. Hence, the power supply source may be regulated, and provide varying output voltages. Objective The key aim of this paper is to define the key functional features of the DC-DC converter, as well as reveal the detailed project development and implementation. Specifications of the design will be explained from the perspective of circuit usability, as well as the specific applications of the circuit components. System Description The basic principle of the boost converter is explained by the switch position that regulates the current flows, and operates the inductor that stores the necessary energy. Therefore, thee inductor resists the DC changes, and the input voltage is boosted. The switch is controlled by the UC 3843 chip, and it switches on an off the DC flow. Hence, the energy is either stored in the inductor, or collapses, which causes polarity changes that boost the input voltage. The DC-DC boost converter projected has two distinct stages of work, (similarly to the simplest) boosters: The On-state, when the switch is closed, and the inductor current grows (Baha, 2006) The Off-state. The switch is open, and the DC flows through the semi-conductor, charging the capacitor, and powering the load. (Baha, 2006) The continuous mode of the converter is featured with the DC flow through the inductor, and its stable rate (it does not fall to zero) When the switch is closed, the input voltage can be found on the inductor. This provides the changes in the DC, and the figure is as follows: The current increase during the On-state will be calculated as follows: The Off-state is featured with the zero voltage drop in the semi-conductor (diode) when the capacitor is large enough for the circuit voltage. The current will be calculated as follows: In discontinuous mode the current amplitude may be too high, and the inductor becomes totally discharged at the end of the cycle. This often happens if the load is light, and the current across the inductor falls to zero (Jalbrzykowski, 2008) Since the current across the inductor is zero, the maximum DC value will be: The current will fall to zero while the off-state lasts Voltage Mode Control The complete V control function is as follows: Where: Gdv (s) – transfer function of power stage Gfb_ffc (s) – transfer function of the feedback network G error (s) – function of the error Current Mode Control The load current control will be equal to the diode DC, and the current mode control can be figured during the off-state of the booster. The I max can be replaced with the following expression: Component Selection The components will be selected considering the design requirements of the boost converter, and including the required output voltage 10 V. Considering the system properties, the voltage supplied will be 8 Volts, and the circuit load is 1.7 mk amps. = = Therefore, D= D = = 0.2 Fsw=120 khz (9.7-9.9) = (0.2) I ripple=0.2*(I Load) (0.34 amp) MOSFET selection Requiremens 10V input, 1.7 amp load, D = 0.2, Trise = Tfall = 55 ns, Fsw = 120 KHz Select P-Channel MOSFET for ease of driving gate. cost = $1 The MOSFET should be able to bear the max voltage Vsw max = Vin + Vout + Vdiode Considering the project requirements and parameters, the V max value will be 16 V approximately. The maximum current that flows through the MOSFET will be Isw max = IL1avg + Iout + (IL1 + IL2)/2 The current for the circuit will be 0,4 A (IRFZ44 MOSFET is selected) (Krein, 2008) Boost converter inductor selection As it is stated by Benqassmi and Ferrieux (2004): “The DC-DC converter is an RLC circuit and the inductance value has to be considered in the design of the system.” The formula for calculating the inductance is given as follows h The maximum inductance value can be obtained with the maximum filling factor, which is close to 0.8. With the current circuit project it will be 5.333 h. Considering the I max that flows through the inductor, the nominal resistance of this component should not exceed 0.47 Ohm Diode Selection Diode should be selected considering the V max value of the circuit. Hence, the voltage of electric lakage for the diode should be Vds > 1,15 (Vout + Vd + Vin) = 23 V Vr > 1,15 (Vout + Vin) = 22.5 V Capacitor Selection The input capacitor should ease the current impulses that flow through diode. That is why the Cout should be of high quality. Hence, the ceramic ESR capacitors are regarded as the best alternative for the project. The minimum capacity is defined considering the  Vout ripple. Cout >= Amin Iout min T / Vout The Cout may be of a larger capacity, especially if the load current ripples. Circuit Simulation The circuit was simulated in the PLECS software, therefore, the calculated voltages and currents were checked practically. The circuit, given below represents the voltage source for the project, and its key aim is to offer stable power of a limited current, and the use of such alternative voltage source can be explained by the fact that the project circuit requires properly controlled power source for examining its effectiveness, as well as checking the calculated parameters of the resistors, capacitors, and inductors. Additionally, as it is stated by Basso (2008), this power source can power the circuit with the modified I and V parameters, which is achieved by controlling the relay hysteretic limits. The figures below are the output waveforms of the output current and voltage. And this is the DC-DC converter circuit with voltage controller, Figure The output current is given as follows Detailed Practical Design UC 3843 controller was selected as the basic element for the circuit. The selection is explained by the fact, that it suits the given parameters properly. Therefore, the lower voltage limit for UC 3843 is 8V, and it can bear the breakdown voltage of the other components (including diode and MOSFET with 23V and 20V respectively). Circuit Diagram System Evaluation The circuit behavior with the loading of 2, 4, and 10 ohm is given on the figure below: – 2 ohm – 4 ohm – 10 ohm As it can be seen on the figure below, the output voltage is close to the calculated, and is about 10-11V. It is seen, that the current impulse which is higher than 2 Amp will be eased by the current pulse clipper in the UC 3843 controller. Power losses. The power losses are not essential. As it is shown on the graph, the current falls with the increase of the load. In general, the calculated power of the circuit pulsations do not exceed 10mW, and the simulation shows that the circuit works in the regime of unbreakable currents, as the current that flows through any inductor is close to its maximum value (up to 40 mAmp) The overall system efficiency is low, as the loads decrease the output current essentially. Moreover, the V/A parameters of the circuit are not satisfactory because of the absence of proper galvanic isolation. In fact, it is not essential for the stable work of the circuit, however, such isolation will help to prevent serious power losses in the inductors, as well as protect the elements from breakups. Moreover, it is not clear, how the MOSFET will stay open, as the source potential will equal with drain current, while the gate current can not be higher than 7 lead current on the controller. Therefore, the current-measuring resistor should be also included int the circuit for measuring current across the inductors. Overall Performance Evaluation Considering the basic principles of the DC boost converters, it should be stated that when the capacitor is loaded in the previous cycle, it powers the controller, when the switch is open. However, the current-measuring resistor is not suitable for proper measuring, as the controller can work only in regime of the interrupting currents of the inductors. This means, that the current will depend on the input voltage. Considering the possible suggestions, the overall scheme will be as given below Another diode will stabilize the ripple currents of the circuit, and the output current will be essentially higher in comparison with the previous scheme (the measurements show 300-400 mA, with 8-15 V input). References Baha, B. (2006) Modelling of resonant switched-mode converters. IEE Proceedings-Electric Power Applications, Vol. 145 Issue 3, pp. 159 -163, May 2006. Basso, Christophe (2008). Switch Mode Power Supplies: SPICE Simulations and Practical Designs. New-York: McGraw-Hill. Benqassmi, H. Ferrieux, J (2004) Current-Source Resonant Converter in Power Factor Correction, IEE Proceedings-Electric Power Applications, Vol. 136 Issue 4, pp. 129 -263, June Jalbrzykowski, S. (2008) Current-Fed Resonant Full-Bridge Boost DC /DC Converter, IEEE Trans. Industrial Electronics, vol. 55, no. 3, March 2008, pp. 1198-1205 Krein, P. T. (2008) Elements of Power Electronics. New York: Oxford. Read More