30V-1V Buck Power Supply (Digital Interface)

Final Design Report with Diagrams EEL 4924: Electrical Engineering Design II (Senior Design) 23 April, 2013

Team Members: RYAN BLANCHARD & EMANUEL MANCO

Table of Contents: Table of Contents……………………………Page 2 Abstract............................................Page 4 Table of Figures……………………………….Page 3 Project Features/Objectives……………Page 7 Concept/Technology……………………….Page 7 Flowcharts & Diagrams……………………Page 9 Responsibilities………………………………..Page 10 Gantt Chart………………………………………Page 10

Table of Figures Typical Buck Output ………………………Page 4 Possible LCD Interfaces....................Page 5 Analog Component ……………………….Page 6 LT1339……………………………..……………Page 8 PIC24FJ256DA210 Diagram…..……….Page 9 PIC24F Clocks & Graphics Module….Page 9 Block Diagram……………………………….Page 10 Gantt Chart……………………………………Page 10

Project Abstract: Our project consists of making a variable power supply that utilizes a buck converter to increase efficiency. Our design is at least 85% efficient but we have recorded up to 95% efficiency. We also implemented a current limiter, restricting the current to 3 amps. This was achieved through the use of parts with low resistances (such as the MOSFETS that we will use for switching which have an on resistance from the drain to source of 12.5mΩ and capacitors with very low ESR [Equivalent Series Resistance] to minimize ripple in the output). Our design also called for the use of pulse width modulation. A transformer was used to step down the input voltage coming from the wall at 120V to around 55V, mainly for safety reasons and for easier part selection. The current limiting was done with a closed loop controller implemented with an op amp and a setup of capacitors and resistors determined by a transfer function that we must come up with. We used a LT1339 to help with voltage control. Our power supply has an interactive menu on a 4.3” (WQVGA) TFT resistive touch LCD display. The user is able to type, using the on screen keyboard, the voltage he/she wants the power supply to output and is also be able to see the value he entered on the RGB screen. A second screen with an on-screen round dial is also available, allowing the user to steadily increase or decrease the output voltage. Each screen is also supplied with a digital voltmeter, allowing the user to check the output in case of errors. The meters are controlled using the analog-to-digital features of the microcontroller while the output of the digital interface uses a digital-to-analog chip to send the correct voltages to the analog part of the project. Output:

Figure 1: Typical Buck Output

Introduction: Variable power supplies have a wide range of uses. At school we use them in the lab to supply power to whatever project is being worked on. Having a variable supply with a current limiter allows it to be connected to a wide array of projects and supply them with the proper current and voltage for what they are trying to achieve. Power supplies are used in a wide assortment of products, because the power and current being used needs to be controlled in some way, you can’t just plug a product into the wall and hope everything works right. You need to be able to control the voltage and current, and a power supply does this. The power supply will also always provide the constant voltage and current desired, even if the load changes. Using a graphic touch screen puts a new spin to power supplies which normally use a segment display. With better software, new features can be added to this product.

Technical Objectives: - The main problem associated with the analog component of the power supply was coming up with the transfer functions for the control of the voltage and current, and then implementing it with physical parts (resistors, caps, op amps, etc.) - The main problem associated with the digital component of our project was choosing the simplest and most cost effective interface. We considered two possible designs since our LCD has embedded drivers. The first one includes a microcontroller with an external LCD controller and external ram. This design is cheaper but involves finding a microcontroller and an LCD controller that can communicate with each other. The second design involves using a microprocessor with a built in LCD controller. The PIC24FJ256DA210, with built in graphics controller was the best choice since it was made to be used with VGA type LCDs. - Our CPU needed to be fast enough to take in a digital value, send a reference voltage to the analog component, as well as continually write data to the graphic screen. The PIC24F solved this problem with the built in graphics accelerator. Although the primary clock on the chip is running at 32MHz, the graphics controller is running at 96MHz. This allows the screen to be clear even when other peripherals on the chip are being used.

Possible Design 1: Microcontroller with external LCD controller and external RAM

 THE CHOSEN DEIGN Possible design 2: Microprocessor with an embedded LCD controller and external RAM Figure 2: Possible LCD Interfaces

Figure 3: The analog component of the power supply (Buck Converter)

Cost Objectives A ball park estimate for the cost of our power supply would be around $400. We were able to procure a decent amount of expensive parts for free from companies, so the main costs came from from the interface (SRAM, PCBs, LCD etc). Free parts included sample parts such as the Microchip processors.

Project Features/Objects Objectives The main objective of our project was to create a variable power supply that is able to put out a user defined voltage between 1 and 30V. The power supply is also able to limit the current to 3A. We gad a goal of achieving an output efficiency of 85%. The power supply has an interactive menu. The user will have the ability to enter the input values through an on screen keypad or an onscreen round dial. The user is able to see the input and output values on a 4.3” LCD screen. As time allows, more features may be added to the user interactive menu.

Features When it comes to features, our power supply will utilize what is known as a buck converter (see Figure 3, Page 4). We’ll be using a 15uH inductor, MOSFETS with an RDS (ON) equal to 12.5mΩ, and an LT1339 DC controller analog IC chip, and capacitors with very low ESR values. We also have heat syncs, however, the parts we have so far can handle much higher voltages than we will be using so we may not need them. On the interface front, the 4.3” TFT LCD display has a 480x272 resolution and 16 bits-per-pixel interface color Red, Blue, and Green. Due to memory space and controller limitations, we used an external SPI Flash chip to save bitmap images. The Inputs to the supply are read through resistive touchpad which interacts with the LCD.

Concept/Technology ANALOG - I’m really interested in analog design and power, so that’s what led me to this project idea. In doing background research I found that the most efficient power supply for stepping down voltage from the 120V from the wall is a buck converter. A buck converter is efficient because of its use of MOSFETS with low RDS (ON) values as switches. A variable switching power supply doesn’t constantly supply power. It switches on and off at high frequencies, and has a duty cycle that we can also control. Some types of power supplies only output specific values; we wanted to make something that would be useful for a wide range of applications, which is why we selected a variable supply. Then there are power supplies like the one we made in junior design that use a full bridge rectifier paired with zener diodes across the load, which are very inefficient. In the end, the buck converter was by far the best choice for attacking this design. Below is a figure showing the inside of the LT1339 controller I used in implementing our power supply.

Figure4: LT1339

DIGITAL – The PIC24FJ256DA210, 100 pin package TQFP, was made for use with LCD interfaces. I chose this processor because it has an internal LCD controller, multiple Analog–toDigital modules needed to read the supply outputs, and three serial peripheral interface (SPI) modules used to send data to digital to analog converter as well as the 16Mbit SPI flash and the LCD itself. The processor also comes with 96k Bytes of SRAM. However, this is not enough to support 480x272 pixels at 16bpp RGB, so a parallel 256k x 16 SRAM is used to buffer each screen. Buffering the screens allows the chip to do much less work once a particular screen is entered. The SPI flash chip is used to store images such as bitmap which require more space as well as the calibration data for the touch screen X and Y coordinates. Another tool which was very useful for this project was the Graphic Display Designer X which is compatible with MPLabX’s program environment. GDDx allows users to generate graphical user interface (GUIs) code. I was then able to modify and add to this code to be able to create desired user interface and integrate the use of the ADC as well as the DAC serial modules to communicate between the analog component and the digital interface part of the project. A 3.3V and a 5V regulator are used to power up the board. 3.3V is used to power up the PIC24F, SRAM, and the SPI Flash while the 5V is used to power up the LCD.

Figure 5: PIC24FJ256DA210 Diagram

Figure 6: PIC24F Clocks & Graphics Module

Figure 7: Block Diagram

Separation of Work Ryan – Will be working on the analog design of the supply Emanuel – Will be working on the digital design of the supply (user interface)

Schedule

Conclusion We were very successful in implementing our design the way we planned. In fact, part of our project turned out better than we even better than we expected in the beginning. We would like to thank the staff of Electrical Engineering Design II as well as the two TAs who were very helpful throughout the semester and made sure we were on track 100% of the time.