Signal Processing of DC/DC converter inductor current measurement By Yinjia Li Zakir Hussain Ranizai This thesis is presented as part of Degree of Bachelor of Science in Electrical Engineering Blekinge Institute of Technology September 2012 Blekinge Institute of Technology School of Engineering Department of Electrical Engineering Supervisor: Anders Hultgren Examiner: Sven Johansson 1 2 Acknowledge First of all, we would like to express our grateful thanks to our thesis supervisor Anders Hultgren, not only thanks for his patient guidance during this thesis report writing, but more important also great thanks to his constantly encourage to let us keep thinking, courage us to face difficulties and help us to growth the ability of work independently, without his help this thesis would not have been complete. And also, we would like to thanks to Electrical engineering department of Blekinge institute of technology sets lab sections for different subjects and project courses, so we gained more knowledge from practice work and also be able to relate to theoretical. These provide us good preparation for today’s thesis work. At last we would like to thanks to our family, friends, provided to us great care and concern so we would be able to complete this bachelor education. 3 Abstract This thesis is one part of ongoing research project of Ericsson AB together with Blekinge institute of technology on the aspect of Ericsson Power modules. The aim of this project is to apply signal processing method on the measured current signal of inductor component of Ericsson BMR 450 Buck converter by using software Matlab, the method should be able to remove the noisy part which exists on the measured signal, so the signal can be undulated within normal range. A set of solutions have presented in this thesis report. The solutions include the Analog and discrete filter design, the algorithm of reconstruct signal, method of finding the signal slopes and algorithm of finding current average values of signal, also presented in the last chapter of thesis report. The result of this thesis project provides possible valuable reference for other research part of Power modules and helps the future development technology of DC/DC converter. Keywords: Signal processing, Buck converter, filter design, slope, average value, DC/DC converter 4 Table of Contents Chapter 1 ........................................................................................................................................... 9 1.1 Introduction ........................................................................................................................ 9 1.2 Buck converter circuit topology .......................................................................................... 9 1.2.1 Theory of Process of a basic buck converter ............................................................ 9 1.2.2 Circuit Analysis ....................................................................................................... 10 1.3 Thesis Objectives ............................................................................................................... 11 1.4 Scope of thesis work ......................................................................................................... 11 1.5 Outline of the thesis .......................................................................................................... 11 Chapter 2. FFT signal frequency analysis ........................................................................................ 13 2.1 Introduction ...................................................................................................................... 13 Chapter 3 Filter Design .................................................................................................................... 17 3.1 Introduction ...................................................................................................................... 17 3.2 Analog filter design based on FFT ..................................................................................... 18 3.2.1 Simulation comparison .......................................................................................... 21 3.3 Discrete filter design ......................................................................................................... 27 3.3.1 Anti-aliasing analog low pass filtering .................................................................... 27 3.3.2 Signal down sampling ............................................................................................. 31 3.3.3 Discrete filter design on the down sampled signal ................................................ 32 Chapter 4 Algorithm Design ............................................................................................................ 39 4.1 Introduction ...................................................................................................................... 39 4.2 Signal re-construction algorithm ....................................................................................... 39 4.3 Algorithm of derives the signal slope ................................................................................ 44 4.4 The average value algorithm ............................................................................................. 47 Chapter 5 Conclusion ...................................................................................................................... 51 Appendix A-Simulink Model and parameter settings description for Analog filter design ............. 52 Appendix B-Basic Fitting tool to verify the slope function .............................................................. 56 Appendix C-Matlab Code ................................................................................................................ 59 Appendix D-Refferences .................................................................................................................. 72 5 List of Table Table 1 Filter order corresponding to three different bandwidths ......................................... 25 List of Figures Figure 1 Ericsson BMR 450 Buck converter ............................................................................... 9 Figure 2 Circuit diagram of buck converter ............................................................................. 10 Figure 3 Buck converter in On-state ........................................................................................ 10 Figure 4 Buck converter in Off-state ....................................................................................... 10 Figure 5 The measurement current signal .............................................................................. 13 Figure 6 FFT result of measured signal tell where the Nyquist frequency is .......................... 14 Figure 7 Top figure: log scale on both x and y axis. Down figure: log x scale only .................. 15 Figure 8 Useful signal frequency range ................................................................................... 16 Figure 9 Signal disturbances frequency range ........................................................................ 16 Figure 10 Magnitude characteristics of physically realizable filters ........................................ 17 Figure 11 Passband and stopband cutoff frequency ............................................................... 18 Figure 12 Four types of filters comparison (All are fifth-order filters) .................................... 19 Figure 13 Frequency and phase response of analog filter ...................................................... 20 Figure 14 Frequency response convert into Hz of analog filter .............................................. 20 Figure 15 Frequency response convert into Hz of analog filter in log scale ............................ 21 Figure 16 measurement signal by using ‘sim’ ......................................................................... 21 Figure 17 Filtered signal by using ‘sim’.................................................................................... 22 Figure 18 Zoom in the signals .................................................................................................. 22 Figure 19 Timing compare between 'sim' and 'lsim' ............................................................... 23 Figure 20 FFT spectrum compare between measurement signal (Left) and filtered signal (Right) ......................................................................................................................................... 23 Figure 21 Poles of analog filter ................................................................................................ 24 Figure 22 Filtered signal and FFT result when set bandwidth in 3.81*106 Hz. ...................... 25 Figure 23 Filtered signal and FFT result when set bandwidth in 7.914*106 Hz. .................... 25 Figure 24 Filtered signal and FFT result when set bandwidth in 1.329*107 Hz. ................... 26 Figure 25 Stopband and pass band cut off frequency of Anti-aliasing filter ........................... 28 Figure 26 Frequency response of anti-aliasing filter ............................................................... 28 Figure 27 The result signal after passing through the analog anti-aliasing Nyquist filtering .. 29 Figure 28 FFT spectrum of the signal after passing through the anti-aliasing filter ................ 29 Figure 29 Output signals between frequency content filtering (Left)