Design and Development of Digital Power Supplies and Power Conversion Electronics
Ed Wettlaufer
Technical Manager – Mechatronics
Jacob Hammitte
Project Engineer - Mechatronics
Background
The mobility of a society improves quality of life by providing access to people, places, and experiences. It also enables economic advancement by providing access to goods and markets. These are considered self-evident truths here in the developed world, but mobility is limited for the world’s poorest people, such as in rural Africa.
Imagine a solar-powered electric vehicle (EV) that is operated and maintained by a village entrepreneur. High isolation loads in Africa can permit a low-speed, lightweight vehicle to travel daily between adjacent villages. This could facilitate access to distribution centers, markets, and healthcare not available in the village. Users would fund the business as it enables them to sell their goods, therefore increasing revenue and trade. It may also be financed by government education subsidies for women and children. When not providing mobility, it could be the village power source for pumping water, grinding grain, and charging cellphones. This could also help empower women especially. An EV can fill the need for many women to collect wood or water while saving time and energy for education or making goods to generate income.
Challenges
Designing a vehicle with off the shelf components can present issues with system integration, especially when cost and availability drive architectural decisions. This version of the AFREECAR EV is a powered cart with solar charge capability and on-board DC power storage and conversion. Since cost is a primary consideration, an inexpensive brushless direct current (BLDC) motor was selected. The motor/controller is a 48-volt machine, so 4 12-volt lead acid batteries were connected in series to provide power storage. An onboard AC charger for 48-volt batteries was readily available and a similar inverter selected to convert the battery DC power to 120VAC power. The solar panels were arranged in a series-parallel connection to provide 24Vdc at 8 amps – the limit of the off the shelf charge controller. A parallel arrangement would be more efficient, and with the option of lithium-Ion batteries, the engineering team would need to design a charge controller and battery management system to work with the unique power I/O.
Digital Power Designer
Altair Embed Digital Power Designer is an add on to Altair’s product for controls development, Embed. Digital Power Designer contains a library of control blocks for code generation and power conversion blocks for simulation. The code generation features, and simulation of microcontroller peripherals are specifically based upon the complete TI C2000 range of microcontrollers, many of which are optimized for digital power and power conversion applications. Digital Power Designer includes numerous ready-made control diagrams as a platform for starting new designs.
Digital Power Designer blocks efficiently model digital (or analogue) power supply configurations, along with control loops, to simulate and verify performance without the need for hardware. This paradigm allows you to develop your control routines, simulate, verify, and make any needed corrections before sending out printed circuit board images to a hardware manufacturer. When your hardware arrives, you can use Digital Power Designer to compile and download the diagram to the microcontroller and test your design utilizing hardware/software in the loop (HIL/SIL) simulation, if desired.
A switch mode regulator can act as a current device or a voltage device, by altering the duty cycle based on either the current output or the voltage output. Altering the load observed by the photovoltaic cells, a switch mode regulator can also track the maximum power point (MPPT). The addition of MPPT allows the solar panels to be used at their maximum efficiency for a given sunlight intensity.
To start the design in Embed using the Digital Power Designer Library an interleaved buck converter was inserted into the model. In this block almost every value of the circuit can be modified to correspond with real components. This allows for quick modifications of a component in the buck converter by altering its values. Next the control for the buck converter was created. To create the controls for the buck converter, compensators and simulated microcontroller peripherals were used. In order to get feedback from the buck converter; current shunts, amplifiers, voltage dividers, and analog to digital converters were utilized. To control the buck converter a compensator and control mode block provided by the Digital Power Designer Library was used. For the buck converter there are two different control modes. One control mode housed an average current control mode block, and the other a voltage control mode block. A control mode is chosen by sampling the voltage of the battery. If the battery voltage is indicating the battery is near full, then constant voltage control will be used. These control blocks created for the simulation can be used to control hardware with minor adjustments to its inputs, outputs, and compensators. This allows for debugging of the hardware and real-time tuning of the compensators. All these features provide quicker development and debugging of the solar charge controller.
In order to evaluate the controllers charge mode (voltage or current) and the MPPT capability, experiments were conducted focusing on; ambient light intensity, and cell arrangement as design variables and MPPT “follow point” and charge efficiency used as responses.
To optimize the MPPT algorithm, ambient light intensity was varied between 200 and 1000 kW/m^2. Adjustments to the algorithm, across the range of ambient intensity, created a controller which would closely follow the maximum power point during daily variations in sunlight.
To maximize the efficiency of the controller output, various arrangements of panels were modeled. Compensation values in the buck converter were optimized to provide maximum charge output across a broad range of input powers.
Conclusion
The environment in rural Africa is diverse and the AFREECAR EV operational duty cycle demanding. These conditions necessitate the need for a purpose-built solar charge controller. Altair Embed with the Digital Power Designer add-in provides Designers the tools required to design and optimize controllers based on specific operating requirements. Creating systems models with Embed, provides the ability to conduct experiments around design points quickly reducing design cycle time and costly errors in performance.