This post is part 2 of the In-Car USB Power series. You may want to read part 1 first.
The USB 2.0 standard power output is 500mA, but modern smartphones, tablets, music players and other devices take a long time to charge at this rate. The wall adaptors for these products provide something closer to 1 amp at 5 volts. With two USB jacks, I wanted to be able to provide 2 amps total power so that two devices could charge quickly at the same time. To make sure that the system could handle this comfortably, I decided to set the design requirement at 3 amps.
A car’s electrical system is nominally about 12 volts. The alternator output voltage is more like 13-15 volts while the engine is running to charge the battery. USB power provides 5 volts, so we need a way to convert from a variable input voltage of 10-15 volts to a fixed output of 5 volts.
The simplest way to convert to a lower voltage would be to use a linear regulator such as the 7805. However, linear regulators drop the voltage by converting extra power to heat, which just wastes it. To provide our design requirement of 3 amps at 5 volts, we would be drawing 3 amps at 12 volts and burning off the extra power. 3 amps at 5 volts is 15 watts, and 3 amps at 12 volts is 36 watts. This means that we are burning off 21 watts of power for a terrible efficiency of 40%. Not only is this horribly wasteful, but it’s going to be almost impossible to dissipate this much power with a passive heatsink. We need to do something smarter.
A switching regulator is a much better choice for this task. A buck converter with a regulated output of 5 volts will handle the job quite nicely. Using Digikey’s parametric search, I found the Micrel LM2576, which takes a wide range of input voltages and generates a fixed output voltage of 5 volts at 3 amps with a minimal number of external components. The datasheet claims 82% efficiency for the 5v fixed output version with an input voltage of 12 volts. This isn’t amazing for a switching regulator, some of which can do much better than 90%. However, for a DIY project like this simplicity and cost are more important than excellent performance, and 80% is sufficient for this purpose. To generate 15 watts at 80% efficiency, we will be drawing 18.75 watts. The 2576 will be dissipating the extra 3.75 watts, a much more manageable number.
We now have to figure out how to get rid of those extra 3.75 watts. The maximum operating temperature for the chip is 150°C, and the thermal resistance to ambient air is 65°C per watt. This means that when the TO-220 package of the chip is dissipating heat to the surrounding air without being connected to a heatsink, PCB copper plane, or any other heat diffuser, the chip temperature will rise 65°C above the ambient air temperature for every watt dissipated. It is not unreasonable to estimate that a car that has been parked in the sun in the summer where I live could reach 50°C. The 3.5 watts we need to dissipate will raise the temperature 65 * 3.5 = 227.5°C above ambient, for a total of 277.5°C, much higher than the maximum temperature for this chip. We’re going to need a heatsink.
There are all sorts of heatsinks you could choose for this purpose. I found a pretty simple one for a TO-220 package that has a thermal resistance of 25.9°C/W without moving air. The 2576 has a thermal resistance of just 2°C/W to its own case, for a total of 27.9°C/W. This solution rises to about 98°C above ambient, for a total of 148°C. This is very close to the maximum operating temperature, and if I actually intended to operate the device at 3 amps, I would want to find a better solution. But since the design requirements were set artificially high (3 amps instead of 2 amps) to make sure the design could easily manage the typical use case, this will do fine. (The actual intended draw of 2A at 5V is 10 watts, dissipating 2.5 watts at 80% efficiency for a temperature rise of 75°C above ambient. On that hypothetical hot summer day, the chip will reach 125°C, which is well within its limits).
The rest of the components I selected using the typical application circuit shown in the 2576 datasheet. I placed my Digikey order, and my parts were on their way! Next up: test and assembly.