Experiment 8 Design and Experimentation of a SEPIC Converter for LED Lighting

Transcription

1 Experiment 8 Design and Experimentation of a SEPIC Converter for LED Lighting 8.0 Objective The objective of this experiment is to familiarize the students with the basics of Solid State Lighting (SSL) and applications of DC-DC converters in SSL. Students will design a SEPIC converter for PWM dimming of LEDs and use the TI LED Lighting kit for verification. 8.1 Pre-laboratory Assignment Given the following specifications: An input voltage of =12, =20, 0< <0.6, and =200. Fig. 1: Circuit diagram of a SEPIC converter a) Determine an analytical expression for the converter voltage conversion ratio. b) Determine the steady-state duty ratio for the given specifications. c) Draw both current and voltage waveforms for all inductors, capacitors, the switch, and the diode. d) Design the inductors (L 1 & L 2 ) and capacitors (C 1 and C 2 ). The current ripple i and i should be below 10% of their average currents I and I at the maximum load, respectively. The steady state ripple and should be below 0.5 % of the steady-state value of the output voltage (the converter should operate in CCM under all operating conditions). 8.2 In-laboratory Simulation Using SimPowerSystems toolbox of MATLAB/SIMULINK software, simulate the circuit for the modified SEPIC converter with the components determined in Pre-lab assignments.

2 Fig. 2: Circuit diagram of a SEPIC converter for interleaved and simultaneous switching. In addition to the inductors and capacitors used from the Pre-Lab assignment the modified SEPIC converter shown above uses R 1 =R 2 =R 3 =200Ω as the output loads. Switches Q 2 and Q 3 are both operate with a switching frequency of 20kHz. 1) Set the duty ratio of both Q 2 and Q 3 to 0 (i.e. keep them turned off). Plot I L1, I L2, I out, V C1, and V out as well as the voltage and current waveforms of the diodes and MOSFETs. Compare simulation results with the analytical ones found in the pre-lab assignment. 2) Set the duty cycle of both Q 2 and Q 3 to 50%. a. Simultaneous Switching: Set both Q 2 and Q 3 to switch on and off simultaneously, that is, when Q 2 is on so is Q 3. Plot I L1, I L2, I out, V C1, and V out. b. Interleaved Switching: Set Q 2 and Q 3 to switch complementary to each other, that is, when Q 2 is on Q 3 is switched off and vice versa. Plot I L1, I L2, I out, V C1, and V out. c. Compare the waveforms for the two different strategies mentioned above. Explain briefly any differences you seen in your simulation results. If so, which strategy is better with respect to component sizing? 8.3 Implementation Requirements: 1. Software a. Texas Instrument LED Backlight EVM 2. Hardware a. Blackhawk USB 2000 emulator b. DC/DC LED Lighting power board and Piccolo F28035 controlcard c. TI LED Panel EVM

3 Hardware Setup [M1]-J4 [M3] Q1 [M1] JTAG connector Fig. 3: DC/DC LED lighting power board 1. Startup Check On the Piccolo F28035 controlcard ensure SW1 is in the off position and SW2 (position 1 and position 2) is in the On position. 2. Controller Card Ensure that the F28035 control card is into the socket on the LED Lighting board and connect a cable from the USB connector ([M2]-JP1 USB Emulator connector) on the board to the computer. [M2]- LD1 near the LED Lighting board s USB connector should turn on. Connect the Blackhawk USB 2000 emulator to the DC/DC LED panel board via the [M1]-JTAG connector. 3. Jumper Connections Ensure that [M1]-J1 and [M2]-J4 have a jumper connection on them. Remove any jumper placed on [Main]-J6. 4. Connecting the SEPIC Converter to the LED Panel Ensure that a banana-to-banana plug connector is attached to the SEPIC out connection ([Main]-BS2) and the LED Bus connector ([Main]-BS3). This connects the DC-DC converter to the parallel LED strings.

4 Software Setup 1. Launching GUI C:\TEMP\ti In the folder (given by the link above) double click on the Lighting_DCDC (shown below) to launch the GUI. Accept any licensing prompt. 2. GUI Interface

5 The GUI developed for the LED lab is able to provide control over both the output voltage level and independent control of the average current through each string. The sliders for each string allow for the user to set the desired current through each string and the slider for the SEPIC Output Voltage allows for the control of the output voltage of the DC-DC converter. Alternately, the user can manually enter the desired SEPIC output voltage and LED string currents into the boxes to the right. 3. Connection Setup The GUI must communicate with the emulator for the user to be able to control the SEPIC converter as well as the MOSFET in each individual string and hence the average current through the string. I. Click the Setup Connection button on the bottom right hand side of the GUI.

6 II. Once the Setup window appears ensure that the Baud Rate is set to and the Boot on Connect box is unchecked. Follow the path below to select the COM Port that is used by the emulator to communicate with the GUI. Control Panel -> System -> Hardware tab -> Device Manager -> Ports (COM & LPT) Look for the comport that is named USB Serial Port or similar and then select this comport in the Setup Connection window. Once all of these requirements have been satisfied click the OK button to finish setting up the connection. 4. Powering up the SEPIC I. At this point check with the TA to ensure that all hardware and software requirements have been satisfied to run the experiment. Once the TA is satisfied with these proceed to connect the 12V adaptor to the [M1]-JP1 connector on the LED control board and turn SW1 to the on position. [M1]-LD1 and [M1]-LD2 should turn on at this time. II. On the main GUI window click the Connect button on the bottom right and wait for the status bar at the bottom left to change from Disconnected to Connected.

7 5. Voltage Reference Move the SEPIC Output slider to 20V which sets the reference voltage for the output to be 20V. The controller uses a feedback loop to change the duty cycle of the MOSFET to ensure the output of the SEPIC converter is approximately 20V at all times. 6. Experimental Data I. Record the waveform for the gating signal for the MOSFET of the SEPIC converter by connecting the oscilloscope to the gate of the MOSFET located on sector [M3]-Q1 of the PCB hardware (as shown in the diagram). Measure the duty cycle of this MOSFET and compare the duty cycle of the experimental value with those found analytically and through simulation. II. Vary the current in LED string 1 from 0.025A to 0.075A in increments of 0.025A and record the value of the duty cycle of the SEPIC converter and the LED-k1 gate driver. Comment on the effect of the increase in current in the LED string on the duty cycle of the SEPIC converter. Record the duty cycle. III. Characterizing LEDs: a) Set all LED strings Target Currents to 0A.

8 b) Then set the output voltage of the SEPIC converter to 16V. c) Set the Target Current on String 1 to the maximum (0.1A). d) Record, in Excel or MATLAB, the average current that is actually flowing through the LED string from the GUI provided and measure the duty cycle of the LED switch using the oscilloscope. e) Keeping the Target Current of the String 1 to the maximum (0.1A) change the output voltage of the SEPIC converter in the range of 16V to 22V by increments of 1V. For each, measure and record the actual average current flowing through the string and the duty cycle of the switch in series with the LED string. f) Calculate the voltage drop across each LED (assume all LEDs in each individual string are identical) and maximum current flowing through the each LED using: %&' " #$! = ()*+,-./ ! = 8:; 6! 1)4< =<>?,! g) Repeat these measurements for LED strings 2 and 3. h) Plot the I MAX vs V LED curves for all three strings. i) Assuming that all the LEDs on the TI kit are identical, plot a graph of the average I MAX vs V LED curve from the data collected. To do this, take the average of maximum current at each voltage level through each of the LEDs in the three strings at each of the voltage measurements. Plot a graph of the average I MAX vs V LED. Compare your results for the average I MAX vs V LED with the following LEDs that can be found on DIGIKEY. Indicate the most likely LED that has been used in the TI kit. DIGIKEY Part # ND; ND; ND; VLMW51P2Q3- GS08DKR-ND; and MX6SWT-A AE8DKR-ND IV. Record the waveforms of the PWM signal for the LED string 1 and 2 by varying the reference current in LED string 1 from 0.025A to 0.075A in increments of 0.025A while maintaining an average current of 0.04A in LED string 2. To record the waveforms connect the two pins of the oscilloscope to the PWM ports PWM-1 and PWM-2 located on the M4 sector of the PCB layout. Comment on the effect of the change in reference current on the duty cycle of the MOSFET in series with the LED strings. Are both duty ratios the same? Which of the two PWM strategies explored in the simulation part of this lab do these switches follow?

9 V. Record the waveforms of the PWM signal for the LED string 1 and 6 by varying the reference current in LED string 1 from 0.025A to 0.075A in increments of 0.025A while maintaining an average current of 0.04A in LED string 6. To record the waveforms connect the two pins of the oscilloscope to the corresponding PWM ports (PWM-1 in sector M4 and PWM-2 in sector M6) of the PCB layout. Comment on the effect of the change in reference current on the duty cycle of the MOSFET in series with the LED strings. Are both duty ratios the same? Which of the two PWM strategies explored in the simulation part of this lab do these switches follow? 7. Powering Down the SEPIC After finishing with your experiment set all the LED String Target Currents to 0A and then set the SEPIC Output to 0V. Once the Voltage reaches 0V click the disconnect button. Next move the [M1]-SW1 switch on the main LED board to the off position and then disconnect the power adaptor from the board.