Thermal Design 2

Over the last lab experiments you learned:

1. How to modify component footprints and schematic symbols in OrCAD
2. How to capture the circuit schematic of an astable multivibrator, simulate the circuit¹, netlist it with provided footprints and draw a trivial PCB layout

In this project, students are expected to apply the information provided about thermal design in the lectures to the following industrial exemplar using OrCAD skills obtained in the previous lab sessions.

In an industrial project you have been asked to design an alarm indicator with LED blinkers that must be visible from a far distance while it should be installed on a fuel tank. Super high bright LEDs can be used for this purpose, however, the high power consumption increases their temperature. In order to supply enough current you might need to use power transistors (MOSFETS). There is a mandatory maximum temperature regulation by mechanical design team to avoid any possible hazards due to close distance from the fuel tank. Furthermore, mechanical specifications limit the size of alarm indicator i.e. the PCB area.

Project manager asks you to provide certain amount of luminance by LEDs for which you need to use at least 10 LEDs on each side of the blinker. In this design power transistors should supply around 20A. The diodes occupy inevitably large area on the board, hence they don’t need special care regarding temperature increase. However, the temperature of transistors and board might increase due to its high power dissipation. Calculation of transistor power dissipation might be complicated, however, for this example you could assume, it is 5W for each transistor. Following the example DC-DC converter example explained in lecture 10, find the required thermal resistance and choose appropriate transistor packages with reference to the information provided in transistor datasheets. Based on the transistor you choose, provide calculations that guarantee the temperature of transistors remain below 70°C assuming that maximum ambient temperature is around 30°C.

Apply the information you obtain during thermal design lectures to cool down the circuit by PCB design techniques and appropriate component footprint selection, as much as possible.

Redesign your board with the new footprints that can be found online or from your standard OrCAD library. Lab reports should include the following items in three pages excluding datasheets:

• Initial Layout
• Discussion about thermal design trade-offs
• Calculations
• Selection of Components (Transistor) with link to datasheet
• Redesigned Layout
• Heatsink (if needed)