Figure 1. Typical laboratory setup for the DC-DC Converters lab.

1 Introduction

The DC-DC converter is commonly used as a switching regulator, the main advantages of this type of regulator are its ability to step voltage up or down and its high consistent efficiency. Another advantage of the switching regulator is that it can easily be isolated by using a transformer for its storage component. In this lab you will do experiments on three different DC-DC converter topologies: The buck, the boost and the inverting buck-boost. You will observe the operating characteristics of these three topologies and compare them.

Below is a summary of what you will be working on during this laboratory.

Equipment Familiarization:

  • Fluke 43B setup.
  • DC-DC Converter Box
  • Load Box Setup

Circuit 1: Buck Converter

Experiment with the Buck Converter topology to observe its typical operation and compare it to the other topologies.

Circuit 2: Boost Converter

Experiment with the Boost Converter topology to demonstrate typical operation and compare it to the other topologies.

Circuit 3: Inverting Buck-Boost

Experiment with the Inverting Buck-boost Converter topology to demonstrate typical operation and compare it to the other topologies.

1.2 Safety

  • Please watch the Safety Video before attending your lab session.

  • Remember the voltages (300VDC and 208VAC) that you are working with can cause serious harm to you if not respected. Please be careful with your hands and fingers around the circuits to avoid electrocution. If you run into any problems during your experiment disconnect power immediately.

  • Do not leave banana leads connected at only one end of the circuit with the other end floating around. This free end of the cable can potentially have voltage on it and create a dangerous hazard.

  • While working with these voltages that can possibly be exposed it is a good idea to remove any metal watches, rings, bracelets etc…

  • Understand the ratings and limitations of the equipment you are operating. Monitor your circuits closely and try to operate the equipment within specifications at all times.

  • The capacitor in the Diode Rectifier Box has a resistor in series with it. The resistors purpose is to limit the destructive inrush current that is associated with the charging of a capacitor. Therefore the switch must be in an open state to allow the current to be limited by the resistor at start-up. After the capacitor is charged the resistor must be shorted via the switch because the resistor is unable to handle the continuous power flow. Leaving the resistor in will cause errors in your results because you’re limiting the current available to the capacitor. Failure to do either step will damage the equipment!!!

  • Do not make changes to the circuit while power is applied. This doesn’t include changing load switches or moving your voltage/current probes around to make different measurements.

  • Make sure that all large capacitors are discharged before making changes to your circuit. Mainly this is the DC-Link Capacitor that needs to have a significant load on it to make sure it discharges in a reasonable amount of time.

  • Please keep your work area tidy while working on experiments.

  • Always have an Instructor or TA check your circuit changes before you apply power.

2 Pre-lab

Each student must complete a pre-lab to hand in at the beginning of your laboratory section. You must have completed all actions of the pre-lab before being allowed to participate in the lab. The laboratory is usually completed in pairs, so please try and find a partner in the same lab section as you. See the laboratory schedule to make sure you show up to the correct time and place.

2.1 Pre-lab Reading

Familiarize yourself with the lab equipment, procedures, documents and results sheet.

  1. Look at equipment pages to familiarize yourself with the equipment listed below that is used in Lab 4. Note that there are links to the lab equipment pages available.

  2. Read over the entire lab manual so you understand what you will be undertaking during the lab.

  3. Look at the provided ECE401 - Lab 4 - Results sheet to see what you will be recording as results during the laboratory.

  4. Print off a ECE401 - Lab 4 - Sign-off sheet . You only need one per group.

2.2 Pre-lab Questions

  1. Using an input voltage of 13V, an inductance of 0.82mH and switching frequency of 4kHz. Calculate the items below for the three DC-DC converter topologies used in this lab: A Buck, a boost and a inverting buck-boost. Do the calculations for all of the duty cycles listed in tables 1-3 in the results section of this lab. Place your results in the appropriate cells in the tables. On a separate sheet of paper show all of your work for a duty cycle of 40% for all three topologies. (Assume continuous conduction)

    1. Calculate the theoretical ideal output voltage.

    2. Calculate the change in inductor current (ΔIL).

3 Experimental Procedure

Danger!

The power used in this lab can cause severe electrocution that can lead to serious injury or even death. So please follow instruction carefully and be cautious with your experiments.

Have an Instructor or TA verify your circuit connections before you apply power!!!

3.1 Equipment Familiarization

It is good practice when working with equipment that has a danger associated with it that you familiarize yourself with the use, function and safety precautions necessary to operate the equipment in a manner which will keep the equipment in good working order and the user safe.

3.1.1 Fluke 43B

Setup the Fluke 43B and the current probe in the same manner that was done in the previous labs.

Figure 2. Current Probe, Fluke 43B Power Quality Analyser, Test Leads and the Power Adaptor for the Fluke 43B.

  1. Take the Fluke 43B out of the case and connect the following:

    1. The power supply to the device and an electrical outlet.

    2. The test leads with the 4mm probe tips to channel 1.

      figure 3. 4mm style probe tips

    3. The current probe to channel 2 using the supplied BNC adaptor.

  2. Turn on the Fluke 43B and configure the current probe by:

    1. Turn on the current probe to 100mV/A and make sure the Fluke 43B is setup to use the same by going to ‘Instrument Setup/Probes’ in the ‘Main Menu’.

      Note

      Note that the current probe setting used for this lab is now 100mV/A instead of 10mV/A that was used in the first 2 labs. This is because the maximum current in this lab does not exceed 10 Amps and this setting will allow us to increase the accurancy of our measurements.

      figure 4. Current probe set to 100mV/A.

    2. Zero the current probe using the dial on the current probe and also note the arrow on the current probe to specify the currents polarity.

3.1.2 DC-DC Converter Box

figure 5. DC-DC converter box

The DC-DC Converter Box is used to create the 3 DC-DC converter topologies that are experimented on during this laboratory. The 3 topologies are the Buck, Boost and Inverting Buck-boost. These 3 typologies, which are shown below, can easily be created one at a time in the adjusted form by using the DC-DC converter box.

figure 6. Buck circuit

Note

The normal and adjusted forms of the circuits above are functionally the same however due to the use of a low-side switch in the DC-DC Converter Box the circuits need to be adjusted to account for both this and to easily create the 3 different topologies.

figure 7. Labelled components of the DC-DC Converter Box

Above is an image of the DC-DC Converter Box with the main power components that make it up labelled. The box is made up of the following components.

DC Power Supply - Internal to the DC-DC Converter Box is a ~14V DC Power Supply. It can be turned off and on with the switch and has a indicting light notifying the user if the power is on. The Power supply output is available on the red and black banana jacks on the front of the box.

MOSFET (datasheet ) - The Mosfet is connected to the heatsink on the PC board which also contains the mosfets gate control. Its drain and source terminals are available on the blue screw terminal block which are then connected to to the blue and black banana jacks on the front of the box. A schematic symbol is shown on the front to indicate its location.

Diode (datasheet ) - The Diode is also on the PC board and also has its terminals connected to the blue screw terminal block which are then connected to the white on the left and blue banana jacks on the front of the box. A schematic symbol is shown on the front to indicate its location.

Capacitor (datasheet ) - The capactor is located inside the box and its terminals are available on the white banana jacks on the front of the box.

Inductor (datasheet ) - The inductor is located inside the box and its terminals are available on the blue banana jacks on the front of the box.

RV1 - The potentiometer that controls the MOSFET’s duty cycle of the PWM switching. - The Velleman K8004 PWM gate control board controls the switching of the mosfet. The potentiometer labeled VR1 controls the duty cycle of the mosfet from zero to maximum (~90%). You will require a small screwdriver to turn the potentiometer. The duty cycle is zero when VR1 is fully counter-clockwise. Always start your experiments with the potentiometer in this position.

Note

  • For all converter types the output voltage is always the voltage across the capacitor.

  • ΔIL is the peak to peak current in the inductor in one switching cycle.

  • All of your measurements should be made with the meter in scope mode.

    • Measure the input voltage
    • Measure the output voltage
    • Measure the inductor current

  • Make sure to use standard banana leads as shown below to make the connections to the DC-DC Converter Box as it is not equiped with safety banana jacks.

    figure 8. Standard banana leads

  1. To power the DC-DC Converter Box use the 15amp cable to connect the receptacle on the side of the box to a standard 120Vac receptacle on the wall.

Figure 9. 15 A cable required to power the DC-DC Converter Box’s DC Power Supply

3.1.3 Load Box Setup

For all 3 experiments the load box should be configured with the 3 banks connected in parallel (as shown below) to increase the amount of load connected to the DC-DC Converter. During the experiments pay attention to the number of load switches required to be on as noted in the results table.

figure 10. Load box setup

3.2 Buck Converter

  1. Connect the circuit as shown in circuit 1.

    1. Use 4 short shielded banana leads to configure the load box as required.

    2. Use 2 short banana leads to create the Buck converter topology as shown.

    3. Use 2 long banana leads to connect the DC-DC Converters output capacitor to the load box and turn on the required number of load switches.

    4. Ensure the duty cycle control potentiometer (RV1) is fully counter-clockwise which is the required starting position.

    5. Prepare the Fluke 43B so the required measurements can be made.

Warning

Get a lab instructor or TA to check your circuit before you energize.

Circuit 1. Buck Converter Circuit
Click here to see a simulation demo of this circuit.

  1. With the circuit on, the input voltage should be in approximately the 13-14V range. After you verify this move your voltage probes to measure the voltage across the mosfet. Using a screwdriver slowly turn up RV1 by about 1/10 of a revolution, as you do this you should start to see the mosfet switching at approximately 4kHz. Make sure that the range of the meter is adjusted to see approximately 3 switching cycles. Make sure that your voltage and current are going positive if not flip your probes so they are. If you didn’t turn up your duty cycle too high you should see that the converter is operating in discontinuous mode.

  2. To measure the duty cycle (δ) use the Fluke 43B in the scope mode and connect the voltage probes across the mosfet. Change the voltage channel reading to measure duty- (inverted duty cycle), inverted because the voltage across the mosfet is zero when the mosfet is on.

Note

Be careful with all of your measurements. The automatic measurements feature of the meter is handy, but can lead you to get large errors in your results. Always confirm your results by counting divisions on the meter to determine if results are good. If there is a discrepancy always use the value by counting divisions. This is particularly important for measuring ΔIL and duty cycle.

  1. Now adjust the duty cycle to approximately 10% and record the actual value you measure in Table 1 of the results section.

  2. Determine if the circuit is operating in continuous or discontinuous mode and place a C for continuous or D for discontinuous in the appropriate cell in table 1.

Note

When the circuit is in discontinuous mode to make your waveforms not jump all over the screen you should be triggering on the current channel because it will be more stable. However, when it is in continuous mode you should trigger on the voltage.

  1. Measure VIN,DC and VOUT,DC using the Fluke 43B in scope mode with the automatic measurement set to DC.

Note

When you get to higher duty cycles you will notice that the input ripple (120Hz) will become more and more significant due to loading. This will make your measurements harder to obtain because it is difficult to separate the switching ripple which is riding on top of this rectifier ripple. To obtain good measurements for VIN,DC and VOUT,DC make sure that you adjust your time base so that you see at least 3 cycles of this rectifier ripple.

  1. To measure the peak-to-peak inductor current (ΔIL) use the Fluke 43B in scope mode with approximately 3 switching cycles showing. You can use the automatic peak-peak measurement if the current is stable. But, it will tend to jump up and down when it is in continuous conduction. When this is the case it is best to count divisions over one switching cycle when it is at the one of the peaks of the rectifier ripple to obtain an appropriate value. This means that from one minimum of the switching cycle to the next minimum the current magnitude should not change.

Note

Something’s you might want to experiment with to obtain these results are the following: Try triggering on the current and playing with the trigger level to try and get the current to stay somewhat stationary on the screen. You can try using the hold button to pause the screen to make it easier to count divisions.

  1. To measure the switching frequency you can use the automatic measurement of the scope but it will depend again on if the circuit is operating in continuous or discontinuous conduction. If it is in discontinuous conduction it is better to measure the switching frequency of the current and therefore when in continuous conduction it is better to measure the switching frequency by measuring the voltage across the mosfet.

  2. For each duty cycle mentioned in the buck converter section repeat steps 5 – 9 to complete the measurement section of table 1.

  3. When you are finished obtaining all the measurements for this section turn off the power and return RV1 to a 0% duty cycle (fully counter-clockwise).

  4. Before you move on to the next section verify your results with an Instructor or TA and get them to sign your results sheet.

3.3 Boost Converter

  1. Adjust the previous circuit to create the circuit as shown in circuit 2.

    1. The input voltage is still from V+ to GND

    2. The output voltage is still the voltage across the output capacitor and load box.

    3. For the boost converter use a smaller load as indicated in table 2.

    4. Ensure the duty cycle control potentiometer (RV1) is fully counter-clockwise which is the required starting position.

Warning

Get a lab instructor or TA to check your circuit before you energize.

Circuit 2. Boost Converter Circuit
Click here to see a simulation demo of this circuit.

  1. Once it is checked, apply power and follow a similar procedure as before to complete the boost converter table.

Warning

Make sure to not exceed a 70% duty cycle for the boost converter because its nature is to increase the output voltage and at high duty cycles the output voltage and current get very large. These voltages and currents can damage or destroy the circuit. To protect the equipment please be cautious with this!

  1. Before you move on to the next section verify your results with an Instructor or TA and get them to sign your results sheet.

3.4 Inverting Buck-Boost

  1. Adjust the previous circuit to create the circuit as shown in circuit 3.

    1. The input voltage is still from V+ to GND.

    2. The output voltage is still the voltage across the output capacitor and load box.

    3. For the buck-boost converter it is important that you change the load to try and keep the converter as close as possible to the border region between continuous and discontinuous conduction. Use the load settings as indicated in the table 3 under the switches row.

    4. Ensure the duty cycle control potentiometer (RV1) is fully counter-clockwise which is the required starting position.

Warning

Get a lab instructor or TA to check your circuit before you energize.

Circuit 3. Inverting Buck-Boost Converter Circuit
Click here to see a simulation demo of this circuit.

Warning

Make sure again that you don’t increase the duty cycle past 70% for the buck-boost converter because the nature of this converter is to increase the output voltage rapidly at high duty cycles. These high voltages by themselves can destroy the circuit however they also produce high currents which can also damage the circuit. Please be cautious with these experiments.

Warning

It is also important that you have an appropriate load for each duty cycle in this section. Remember that as the duty cycle goes up you want to decrease the number of switches at the output which increases the resistance of the load, decreasing the current at higher voltage levels.

  1. Once it is checked, apply power and follow a similar procedure as the previous two to complete table 3.

  2. When you are finished obtaining all the measurements for the inverting buck-boost converter turn off the power and return RV1 to a 0% duty cycle (fully counter-clockwise).

  3. Before you move on to the next section verify your results with an Instructor or TA and get them to sign your results sheet.

3.5 Clean-Up

  1. Return the safety banana leads neatly to the wall and place the probe tips in the appropriate box which should be available on the counter somewhere. Put the Fluke 43B away in its case as shown.

Note

Please make sure that the Fluke current probe” is turned OFF, leaving the probe on will drain the 9V battery with-in a day and will then need to be changed before the next class.

Figure 11. Return safety banana lead and the test leads to the wall, place the Fluke 43B back in the case as shown and return the 4mm test probes to the box.

  1. Place the remaining equipment tidy on the workbench.

Figure 12. Leave the remaining equipment tidy on the workbench.

  1. Remember to back-up and share any computer files you may have created during the laboratory and that you log-off or shutdown the laboratory desktop.

  2. Have a lab instructor or TA sign your Sign-off sheet to ensure that you have cleaned everything up properly before you leave.

4 Post-lab

The following is what you are expected to hand-in approximately one week after completion of the lab, check eClass for the exact time and date. All reports need to be submitted to the appropriate link on eClass. You only have to hand-in one copy per group. Please have your pages in a single pdf file in the following order:

  • Use a scanned/picture of your ECE401 - Lab 3 - Sign-off sheet as your cover sheet. Make sure that you have obtained the required lab sign-off signatures at the bottom of the page. Also make sure that all of your group members names, CCID’s and lab section are visible in the table at the top of the page.

  • The completed ECE401 - Lab 4 - Results sheet .

  • The answers to the post-lab questions.

4.1 Questions

  1. For the graph DC-DC converter gains vs. Duty cycle how do your results compare to the ideal values? Do your results perform as expected? Try and account for any differences between them? (5 marks)

  2. When the output voltage is less than the input, which circuit minimizes the inductor peak-to-peak ripple? When the output voltage is more than the input, which circuit minimizes the inductor peak to peak ripple? What is an advantage of having a smaller peak to peak ripple? (10 marks)

  3. Referring to your calculations, does the change in current in the inductor for when the mosfet is on approximately equal the change in current for when the mosfet is off? Are these values approximately equal to your peak-to-peak inductor current measurements? If not, explain why there could be some errors in your results. (5 marks)

  4. What affects would changing the switching frequency of the mosfet have on the circuits’ performance? Explain. (5 marks)

  5. What affects would changing the inductors’ inductance have on the circuits’ performance? Explain. (5 marks)