SIMULATION WITH THE BOOST TOPOLOGY ECE562: Power Electronics I COLORADO STATE UNIVERSITY Modified in Fall 2011 ECE 562 Boost Converter (NL5 Simulation) Laboratory 2 Page 1
PURPOSE: The purpose of this lab is to simulate the Buck converter using NL5to better familiarize the student with some of its operating characteristics. This lab will explore some of the following aspects of the buck converter: Discontinuous Conduction Mode Inductor sizing Differential voltage across the inductor Time it takes for the converter to reach steady state Output Ripple voltage and selection of the capacitor. Ripple current through the capacitor Equivalent Series Resistance (ESR) of the output capacitor. Effects of changing and removing load resistance Effects of the ON resistance of the switch Efficiency Effects of changing frequency NOTE: The simulations that follow are intended to be completed with NL5. It is assumed that the student has a fundamental understanding of the operation of NL5. NL5provides tutorials for users that are not experienced with its functions. http://nl5.sidelinesoft.com/downloads/manual.pdf Figure 1 - Boost.nl5 "Stock" Demo ECE 562 Boost Converter (NL5 Simulation) Laboratory 2 Page 2
V1 is a DC voltage source (VDC) from the source library. It needs to be set for 24 volts. L is an ideal inductor from the library. Set to 10 µh. R is an ideal resistor from the library. Set to 1 kω. D1 is an ideal diode from the library. Set to 700 mv (diode drop). C is an ideal capacitor from the library. Set to 100 µf. O1 is an ideal comparator used to turn the switch S1 on and off. By varying the width of V3 below, its output will act as a Pulse Width Modulator. S1 is a voltage controlled switch, a standard component in the library. V2 is 0.5 volt reference for the Schmitt trigger comparator O1. Set V2 to 500 mv. V3 is Pulsing source. Set to values listed below using the components editing window. This sets it to a switching frequency of 25 khz with a 50% duty cycle. Set the transient simulation parameters to End time: 1e-3 Calculation step: 10e-9 Figure 2 - Transient analysis of schematic above, showing voltage across output capacitor and at the output of the inductor. Remove the voltage markers and use a current measurement to measure the inductor L current. ECE 562 Boost Converter (NL5 Simulation) Laboratory 2 Page 3
QUESTION 1: What is the peak operating current, and what is the operating mode of the converter? Verify mathematically the mode and the peak current. Figure 3 Current through inductor. QUESTION 2: What is the output voltage of the converter at steady state? Verify your results mathematically. ECE 562 Boost Converter (NL5 Simulation) Laboratory 2 Page 4
Figure 4 Voltage across output capacitor. Now change L1 from 10 µh to 1 mh and rerun the simulation. Remember you can adjust the Screen time under Transient Settings. QUESTION 3: What is the peak operating current now? What is the operating mode of the converter (remember that you can observe this by zooming in)? Also, verify the mode mathematically. Figure 5 Current through 1mH inductor. ECE 562 Boost Converter (NL5 Simulation) Laboratory 2 Page 5
QUESTION 4: How long does it take for the converter to reach steady state? What is the peak inductor current during steady state? Verify peak current result mathematically. Figure 6 Converter reaching steady state. QUESTION 5: Calculate the size of the inductor required to put this converter in CCM. QUESTION 6: What can be said about the differential voltage measurement across L1? Add a data point to look at the voltage across the inductor. Return to the simulation screen, right click, and select Data. Under the Traces tab, select Function, and in the Add new trace window, type V(V1)-V(VL). Run the simulation to 3 ms. ECE 562 Boost Converter (NL5 Simulation) Laboratory 2 Page 6
Figure 7 Differential voltage across inductor. Change the simulation end time to 5 ms and display the voltage across the output capacitor. From this simulation we can see the output voltage stair step up to its final value. QUESTION 7: How long does it take for the output voltage to reach its peak? Figure 8 Output voltage ramp to peak. QUESTION 8: How long does it take for the output voltage to reach its final value? Note that you should increase the calculation step size to 50 us to keep the calculation time reasonable. What is the output voltage? Prove your simulation results mathematically (Vout). ECE 562 Boost Converter (NL5 Simulation) Laboratory 2 Page 7
Figure 9 Output voltage reaching final value. Now run the simulation for 100 μsec at a time greater than 2000 μsec. QUESTION 9: What is the peak-to-peak ripple voltage? Re-adjust the step size accordingly to get enough resolution. Figure 10 Peak to peak ripple voltage. QUESTION 10: With everything else left as is, what is the minimum output capacitance required to limit the output voltage ripple to 2 volts peak to peak? ECE 562 Boost Converter (NL5 Simulation) Laboratory 2 Page 8
Figure 11 Ripple when capacitor is reduced for 2 Vpp max. Now, place a current marker on one of the pins of the capacitor. Run the simulation for 200 μsec at a time greater than 10 msec. QUESTION 11: What can be said about the current through the capacitor? Figure 12 Current through capacitor. QUESTION 12: If the ESR of the capacitor is modeled by a 10 Ω resistor in series with the capacitor. What happens to the output voltage ripple and the capacitor current? ECE 562 Boost Converter (NL5 Simulation) Laboratory 2 Page 9
Figure 13 Circuit with ESR modeled by 10 Ω resistance. Figure 14 Capacitor current with 10 Ω ESR. ECE 562 Boost Converter (NL5 Simulation) Laboratory 2 Page 10
Figure 15 Output voltage ripple with 10 Ω ESR. Change the load resistance from 1 kω to 100 Ω with C=1μF and L=1mH. QUESTION 13: What happens to the inductor ripple current, capacitor ripple voltage, and capacitor current with respect to the original values? Figure 16 Inductor ripple current with R out = 100 Ω. ECE 562 Boost Converter (NL5 Simulation) Laboratory 2 Page 11
Figure 17 Capacitor ripple voltage with R out = 100 Ω. Figure 18 Capacitor current with R out = 100 Ω. QUESTION 14: What operating mode is the converter in now? ECE 562 Boost Converter (NL5 Simulation) Laboratory 2 Page 12
QUESTION 15: What happens if the load resistance is removed? Figure 19 Inductor ripple current with R out removed. Figure 20 Output voltage with R out removed. ECE 562 Boost Converter (NL5 Simulation) Laboratory 2 Page 13
Figure 21 - Capacitor current with R out removed. Replace the load resistance and set to 100 Ω. Insert a resistor as shown to model the R DSon value for a FET. Figure 22 Circuit configuration with R DSon modeled as a separate resistor. ECE 562 Boost Converter (NL5 Simulation) Laboratory 2 Page 14
QUESTION 16: What observations can be made from increasing the on resistance of the switch? Remember that the on resistance value of the switch will provide different results as compared to your classmates. QUESTION 17: What can be said about the efficiency of the converter? (Comment on the different configurations of the circuit used throughout this lab.) η = Pout Pin η = (Vo Vg)*(1 D) QUESTION 18: What can be observed by increasing the switching frequency to 100 khz? Hints: With everything else left as it is, change the pulse source V3 to have a period of 10 μs and a pulse width of 5 μs. Figure 23 Output voltage with a switching frequency of 100 khz. ECE 562 Boost Converter (NL5 Simulation) Laboratory 2 Page 15