Introduction

• September 12, 2022
• 1 hour to complete
• Workbench v1.7
• • 
  • 

Introduction

Review, Safety considerations when working in power electronics, before proceeding.

There are three major components to the Workbench based power electronics system which will be used to perform all the experiments in this course. These are as follows:

1. Workbench simulation and rapid prototyping software platform.

2. GaN/Si power module with on-board controller and data logger.

3. Magnetics and snubber card.

In this experiment, a pre-built Buck converter open-loop control model is run in real-time, as an introduction to all the components mentioned above. The following section gives a brief overview of the various components which will be used to run the Buck converter model. This will be followed by running the system in real-time using the GaN/Si power electronics platform.

Overall system

The overall system is shown below and consists of the following components:

1. Workbench software platform: A numerical simulation platform that supports both model and code-based design. The toolbox within the platform consists of various pre-built commonly used arithmetic, logical, conditional, and storage operators that can be used to model complex systems by simply dragging and dropping these tools and interconnecting them. In addition to being able to simulate a system, the platform can generate digital controller specific code that can be programmed into a micro-controller/digital signal processor to do real-time control.

2. Isolated DC power supply: This is used to power the converter DC bus as well as other auxilary power stages for gate driver, on-board controller, etc.

   Note

   The communication channel between the computer and the three-inverter module is not isolated. Hence, the DC power supply must be isolated to avoid common mode current, which could otherwise potentially destroy the whole system.

3. USB COM channel: Code and data transfer happens via USB 2.0. Under ideal conditions, the maximum data transfer is 480 kbps, which translates to 15,000 samples per second of real-world variable such as average input and output currents, voltages, etc.

4. GaN/Si power-pole board: The hardware module contains all the power electronics, gate drivers, current and voltage sensors, digital signal processor, data-logger, and programmer necessary for design, deployment, and real-time control of common DC-DC and DC-AC converters. There are two GaN power poles and one Si power pole, each rated 40 V, 5 A. Forced cooling using two frame-mounted fans allows for converter to be switched safely up to 250 kHz at rated conditions. Each power-pole is independently controlled by the on-board TI TMS320F280049 digital signal processor (DSP).

   

   In addition to PWM control of the power-poles, the DSP reads feedback signals from current and voltage sensors on the DC bus and at the switching node, and the switch-heatsink junction temperature. The measured data as well as other model variables can be logged and viewed in real-time on Workbench software platform. This is made feasible by a dedictated on-board data transfer link between the computer and the DSP. The same channel is also used to transfer program from the computer to the DSP.

5. Plug-in magnetics card: Based on the converter under consideration, different magnetics cards are connected to the main board. There are 4 cards: Buck/Boost/Buck-boost, Flyback, Forward, and Full bridge. Depending on the jumper selection, the magnetics can connect to either the Si or the GaN power pole.

   

   No magnetics card is needed for DC-AC experiments.

6. Rheostat load: A 50 Ω, 5 A, 100 W rheostat is used as the load for all the experiments.

7. Digital signal oscilloscope (DSO): The on-board data logger is limited in bandwidth and can thus only log average signals varying at lower than 100 Hz. To measure switching frequency components, which are typically around the order of 100 kHz, a DSO is required. Review, Instructions on using DSO, for instructions on how to use a DSO.

8. Non-isolated probes: The power-pole and the magnetics boards have spring-tip probe coupling points to measure various signals as listed in the table below. Spring-tip termination offers the least parasitic inductive loop, thus significantly reducing the effect of radiated noise as well as ringing in the measured signal.

   Warning

   Do not make probe connections when the board is powered on. Make sure that the spring tip is inserted into the proper port marked with the return symbol (⏚)

Power-pole board familiarization

The basic block diagram of the power-pole board is shown below

The simplified version of the actual board is shown below. The locations of the various components on the board are indictated in the following table.

Power terminals:

Terminal designation	Location	Comments
Vin-	E2	Negative DC bus terminal.
Earth	D2	Safety earth termination. Enclosure is tied to this terminal.
Vin+	C2	Positive DC bus terminal.
Vo-	I9	Magnetics output/input negative terminal.
Vo+	G9	Magnetics output/input positive terminal.
SWA	F6	Si power-pole mid-point.
SWB	F7	GaN power-pole mid-point (with external free-wheeling diode).
SWC	F9	GaN power-pole mid-point (without external free-wheeling diode).

DSO measurement terminals:

Terminal designation	Location	Scaling	Offset	Comments
Vin	A3 (gnd) & A4 (signal)	1 V ≘ 1 V	0 V	DC bus voltage.
Io	A4 (gnd) & A5 (signal)	1 V ≘ 5 A	1.5 V or 7.5 A	Magnetic circuit output current. Current from the terminal into the external power supply/load is negative.
Ao	A5 (gnd) & A6 (signal)	1 V ≘ 1 V	0 V	Analog output synthesized by the controller. Used for observing digitally generated control signals.
Iin	A7 (gnd) & A8 (signal)	1 V ≘ 5 A	1.5 V or 7.5 A	DC bus current. Current from the external power supply/load into the terminal is negative.
Vo	A8 (gnd) & A9 (signal)	1 V ≘ 1 V	0 V	Magnetic circuit output voltage.
SWA	E5 (gnd) & E6 (signal)	1 V ≘ 1 V	0 V	Si power-pole mid-point voltage.
SWB	E7 (gnd) & E8 (signal)	1 V ≘ 1 V	0 V	GaN power-pole mid-point voltage (with external free-wheeling diode).
SWC	E8 (gnd) & E9 (signal)	1 V ≘ 1 V	0 V	GaN power-pole mid-point voltage (without external free-wheeling diode).
V1	H5 (gnd) & H4 (signal)	1 V ≘ 1 V	0 V	Transformer primary/secondary voltage for Flyback, Forward, and Full bridge magnetics card. The measurment point is indictated by 📍 (test-point symbol) in the magnetics card.

Jumpers:

Jumper designation	Location	Comments
Vdc	E5	If the jumper is inserted, then it connects the top switches of the power-pole to the positive DC bus (Vin+). Otherwise, leaves these switches hanging.
Diode	D7	If the jumper is inserted, then it connects the external anti-parallel freewheeling diode across the bottom switch of the SWB GaN power-pole. Otherwise, the reverse current freewheels through the GaN device, leading to significant losses.
Si	G6	If the jumper is inserted, and it is solely within G6 grid, then it connects the Si power pole to the magnetics card.
GaN	G6 & G7	If the jumper is inserted, and it is across G6 and G7 grid, then it connects the GaN power pole (with external free-wheeling diode) to the magnetics card.
Buck magnetics card	G3 (bottom)	If the jumper is inserted, then it connects the 4.7 nF capacitor across the switches for soft-switching. This is typically not connected, except for the soft switching experiment.
Flyback magnetics card	G3 (bottom)	If the jumper is inserted, then it connects the snubber circuit for the transformer primary side. Otherwise, the snubber is disconnected.
Forward magnetics card	G3 (bottom)	If the jumper is inserted, then it connects the snubber circuit for the transformer primary side. Otherwise, the snubber is disconnected.

Other connections:

Location	Comments
A1 (bottom)	USB C terminal for programming and data-logging.
C5 (bottom)	Two 12 V header for cooling fans. Fan ON/OFF is controlled from the DSP based on heatsink-device junction temperature.
B4 (bottom)	Active MOSFET-based load control terminals. Not needed if the load is a rheostat. Active load allows for remote control of the load.

Indicators:

Location	Comments
C7	Buzzer. Beeps with gradually increasing frequency as the input voltage exceeds 24 V. If over- current, voltage or temperature, or DC bus under-voltage fault occurs the buzzer beeps five times. Once this occurs, the controller must be programmed again from Workbench to clear the fault.
B8	Green LED. If blinking, indicates insufficient input voltage, anything below 14 V. If solid green input voltage is sufficient (14 V - 22 V) or exceeds the safe limit of less than 24 V.
B8	Red LED. If ON, indicates Vo terminal over voltage. By default this is anything over 24 V.
B9	Red LED. If ON, indicates magnetics card output terminal over current. By default this is anything over 4 A.
C9	Red LED. If ON, indicates DC bus over current. By default this is anything over 4 A.
C8	Red LED. If ON, indicates DC bus under/over over. By default this is anything below 14 V or above 24 V.
B8/B9/C8/C9	All Red LED. If all are ON, indicates heatsink-device junction over temperature. By default this is anything above 85 °C.

Preparing the Workbench model

1. Copy the folder where pre-built example project for this experiment is present, usually in C:\Program Files (x86)\Sciamble\WorkBench v1\Examples\CUSPLab\BasicPowerElectronics\Experiment1 and paste it in a location where the user has permission to edit and save files, like the Desktop folder.

2. Click on the icon on desktop to launch the application.

3. Workbench has tabs on the left, right and bottom that represents docked windows, each of which convey or control different pieces of information to and from the user.

   

   Clicking on these tabs reveal their respective docked windows. The content within these docks change based on the context as will be seen later.

4. Click on the Toolbox tab on the left tabs section and pin the window by clicking on the icon on the top right corner. Similarly pin the Explorer dock on the right. The tools in the Toolbox are grouped into categories based on their operation. Click on the drop-down list: to navigate between the tool classes.

5. Click on the icon in the Toolbox dock to hide the toolbox for now.

6. The Explorer/Solution Explorer dock acts as both file browser to navigate to and, open, add, or remove files from project, as well as, a container to display the project structure. The toolbar within the Explorer dock: consists of the following buttons:

   1. : Creates a new project.

   2. : Opens an existing project.

   3. : Creates a new model or script file and adds it to the selected project.

   4. : Adds an existing model or script file to the selected project.

   5. : Removes selected project, model, or script file. This does not delete it from the physical folder, simply removes it from the current project.

   6. : Permanently deletes (sends to Recycle Bin) the selected model or script file.

   7. : Download project files.

   Click the second icon, to open an example project. This opens the file bowser within the Explorer dock.

7. Navigate to the folder where the pre-built example project was pasted in Step 1. Double click the BuckConverter_Si.project node within that folder to open the project.

   

8. Click the icon to explore the files within the project. To display the model file, double click the Buck node in the Explorer dock.

   

9. Double click the slider block labelled Slider to open its properties in Property dock on the left.

   

   In real-time as the slider is incremented, the converter duty cycle varies from 0 to 1, in steps of 0.05.

Preparing the setup

Warning

Before proceeding, ensure that the isolated power supply is powered down and that the USB cable is disconnected.

1. Magnetics card connection: Unscrew any existing magnetics card and replace it with the Buck/Boost/Buck-boost magnetics card. Prior to connecting the card, ensure that the jumper on the bottom of the magnetics card is removed. Make sure that all the 6-pins of the magnetics card are making solid contact with the power-pole board.

   Warning

   Never leave the magnetics card unscrewed. If contact is lost while the converter is running, it has the potential to cause very high voltage due to interruption of inductor current, potentially damaging the converter, or worse, could lead to a safety hazard.

2. Rheostat setting: Set the slider such that the resistance across the two closest rheostat terminals is 10 Ω.

3. Power connections:

   1. Connect the Vin terminals to the isolated power supply:

      1. DC +ve: ● Vin+ (Red)

      2. Ground: ● Earth (Green)

      3. DC −ve: ● Vin− (Black)

      
      

   2. Connect the Vo terminals to the rheostat load:

      1. Rheostat rail: ● Vo+ (Red)

      2. Rheostat's other terminal that is closest to the rail: ● Vo− (Black)

      
      

4. DSO connections:

   1. Connect DSO channel 1 probe to Io. Set the following options, if they are supported by the DSO:

      1. Set the probe to 1x.

      2. Set the measurement type as Current.

      3. Set the scaling factor to 5x.

      4. Set the offset at 1.5 V or 7.5 A.

      5. Set termination to 1 MHz.

      6. Set the channel as inverted.

   2. Connect DSO channel 2 probe to SWA. Set the following options, if they are supported by the DSO:

      1. Set the probe to 1x.

      2. Set the measurement type as Voltage.

      3. Set the scaling factor to 1x.

      4. Set the offset at 0 V.

      5. Set termination to 1 MHz.

      6. Set the channel as default/non-inverted.

   3. Connect DSO channel 3 probe to Iin. Use the same setting as that of Channel 1.

   4. Connect DSO channel 4 probe to Vo. Use the same setting as that of Channel 2.

5. Jumper settings:

   1. Insert the Vdc jumper, that connects the drain of the top switches to the positive DC bus, Vin+.

   2. Insert the GaN/Si jumper to the Si side. This connects the buck converter inductor to the mid point of the Si power pole.

   3. The jumper on the bottom of the magnetics card was removed in Step 1, while inserting the card.

   4. The jumper for GaN switch's external diode, labelled "Diode", can be left inserted or can be removed, since this experiment does not use the GaN power-pole.

6. Connect the USB cable to the power-pole board and the computer.

7. DC power supply settings:

   1. Make sure that the DC power supply is fully turned down to 0 V prior to turning on the supply.

   2. Turn on the power supply and gradually ramp up the voltage from 0 V to 15 V.

   3. If the option is available, set the power supply current limit at 4.5 A.

The final wiring should look similar to this:

Real-time open loop duty cycle control

1. Click on the icon in the top dock of Workbench to transition from the simulation mode to the real-time mode, where the control algorithm is executed in real-time on the DSP. The generated gate signals, based on the duty cycle, is relayed to the gate drivers. The measured average feedback signals are read by the ADC and relayed back to Workbench where it is displaed on the Scopes.

   

   Note

   When in real-time mode the top dock changes color from light-blue to light-salmon. In this mode, pressing Run will immediately download the program to the connected real-time controller and begin execution. Any previously programmed code will be lost forever. All necessary laboratory safety precautions must be taken.

2. Click to run the control algorithm in real-time. The following message will be displayed if code has been transferred successfully:

   

   If programming fails, check the USB connection and try again.

3. Once programmed, increase the duty cycle by clicking on the slider "+" button. Increase gradually since sudden increases could lead to high transient current that could trigger an over-current fault. If that occurs, stop the model by clicking on button and re-run the model.

4. Vary the duty cycle from 0 to 0.9 and observe the switching cycle waveforms on the DSO:

   

   Also observe the average values displayed on Workbench:

   

5. Click on to stop the model.

6. Turn OFF the power supply and disconnect the USB cable.

This concludes the experiment. This experiment was a cursory introduction to various concepts that will be continuously re-visited in subsequent experiments.

Lab report and reading assignment

1. Attach screenshot of the Workbench model display for different duty cycles.

2. Attach DSO waveforms for different duty cycles.

3. Go through further resource material on various features of Workbench available here.