EECS 100 Boost Converter Laboratory L Chua University of California Berkeley Department of Electrical Engineering and Computer Sciences EECS 100 Professor Leon Chua LABORATORY 7 v3 BOOST CONVERTER In many situations circuits require a different supply voltage than that provided by the power supply In battery operated systems it is often necessary to boost the voltage to the circuit s needs Examples are circuits that require 110V but must be run from a car battery Many hybrid cars use electric motors needing several hundred volts these motors are smaller and more efficient more than the battery or generator supply In this laboratory we will design and test a boost converter to produce 15V from a 5V input Here is the schematic diagram To analyze the circuit we assume first that it is working correctly in particular that the output voltage is 15V We will later verify of course that this is indeed the case The voltage Vc is a pulse train and changes between 0V and 5V For Vc 5V the transistor IRF510 is on i e essentially a short circuit Then Vboost 0V and Vdiode Vboost Vout 15V Since Vdiode is negative the diode does not conduct any current i e it behaves like an open circuit With Vc 0V the situation reverses now the transistor is off and the diode conducts The diagram below illustrates the two situations In situation a Vc 5V the supply voltage Vin appears across the inductor From the differential equation for inductance we observe that inductors integrate voltage Therefore the inductor current IL is a ramp with slope determined by Vin and L In situation b the inductor again integrates the voltage Page 1 EECS 100 L Chua Boost Converter Laboratory Vin Vout 10V that appears across it In steady state the current increase and decrease must be identical as otherwise the average current would continually increase or decrease Since it is negative the current through the inductor decreases as shown in the following timing diagram Vc Ton Toff 5V 0V T 1 f time IL IL IL0 time Since voltage is proportional to the slope of the current we note intuitively that reducing the ratio of Toff Ton results in higher output voltage Vout This is because the positive slope is proportional to Vin and the negative slope of the decreasing current is proportional to Vout Vin In the laboratory we will analyze this relationship quantitatively If you want more information about Boost Converters please check out the website http en wikipedia org wiki Boost converter Page 2 EECS 100 L Chua Boost Converter Laboratory LAB REPORT Lab Session Name 1 SID Name 2 SID Let s first derive an expression for the voltage boost factor Vout Vin We start by writing expressions for IL during Ton and Toff Hint set up the differential equation for current and voltage in the inductor during the two phases During Ton IL of 2 P During Toff IL of 2 P From the timing diagram shown in the guide we know that the magnitude of IL is the same during Ton and Toff Equate the equations above and solve for the voltage boost factor Vout Vin Vout Vin of 2 P Remarkably this result depends only on Ton and Toff and is independent of the value of the inductance Calculate Ton Toff for Vout 15V and Vin 5V Ton Toff of 2 P To finalize the design of the boost converter we must determine the operating frequency f 1 T with T Ton Toff and the values of L and Cfilt We will pick f 100kHz to account for the frequency limitation of solderless breadboards From this we can calculate Ton and Toff and then solve for L from one of the equations for IL 6mA Round L to the nearest available value use the resistor scale i e multiples of 10 12 15 etc L mH of 2 P During Ton the diode is not conducting and the entire current to the load comes from Cfilt Because of this the output voltage will drop Keeping this drop to Vout 100mV for RL 1k determines the value of Cfilt Realizing that Vout Vout we conclude that the current through the is approximately constant IR Vout RL From this we can calculate Vout and solve for Cfilt Page 3 EECS 100 Boost Converter Laboratory Cfilt F L Chua of 2 P Verify your result with Multisim For simulation only add a 6 resistor in series with the inductor to account for the winding resistance do not add this resistor in the actual circuit you will be building Hand in a transient simulation showing Vc Vboost Vdiode Vout and the current through the inductor note it s the same as the current through Vin and computed automatically by Multisim for 3 cycles in steady state Simulation result V of 10 P Now you are ready to test the boost converter in the laboratory Although it is designed to generate only 15V it can produce voltages in excess of 25V e g when the input voltage is chosen higher than 5V Because of this exert extra caution and touch circuit nodes only after having determined e g with the oscilloscope that voltage levels agree with your simulation results and are below 20V Also complete the entire circuit before turning on power Especially do not omit the diode and load resistor Measure Vc Vboost Vdiode Vout with the oscilloscope and compare your result to Multisim Comment on any discrepancies hint consider the assumptions made for the calculations Oscilloscope printout of 10 M Explanation of differences of 5 M Page 4 EECS 100 Boost Converter Laboratory L Chua In Multisim and the actual circuit vary the load resistor RL from 100 to 20k and graph your result Label the axes Simulation of 5 P Experiment of 5 M Ideally the voltage should be independent of the current IRL through the resistor In practice it drops because of the series resistance of the inductor and diode and the finite on resistance of the transistor Practical implementations of boost converter contain additional circuitry that monitor the output voltage and dynamically adjust Ton and Toff to ensure a constant Vout Page 5 EECS 100 Boost Converter Laboratory L Chua SUGGESTIONS AND FEEDBACK Time for completing prelab Time for completing lab Please explain difficulties you had and suggestions for improving this laboratory Be specific e g refer to paragraphs or figures in the write up Explain what experiments should be added modified how or dropped Page 6 EECS 100 L Chua Boost Converter Laboratory PRELAB SUMMARY Lab Session Name 1 SID Summarize your prelab P results here and turn this in at the beginning of the lab session Turn in a copy of the pages with prelab results at the beginning of the lab During Ton IL of 2 P During Toff IL of 2 P Vout Vin of 2 P Ton Toff of 2 P L of 2 P Cfilt of 2
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