Design
The charger included with the Eee is rated at 9.5v, 2.315A. There isn’t a fixed voltage regulator available for this exact voltage, so the circuit needed to be designed around an adjustable regulator.
I decided to design the charger around the LM2576 “Simple Switcher” IC from National Semiconductor. There are tons of ICs like this available, many of which are a bit more efficient, however I selected this one because it is readily available and relatively cheap. It also has a lower drop-out voltage (~2V) than many other chips I looked at which is important when powering the device from a car or 12v SLA battery.
This circuit could have used a standard three pin regulator IC such as the LM317, however most types require an external transistor when handling so much current and not to mention the fact that they are very inefficient; they draw the same amount of current from the input as the load and the difference in power is dissipated as heat.
The main problem with using the LM2576 is the fact it needs quite a large inductor due to its somewhat low switching frequency. The inductor I used is made by Pulse Engineering, part number PE92108KNL. I’d prefer a smaller one, however I couldn’t find one capable of supplying the required current that I could purchase in single units. Besides the PE92108KNL is apparently designed specifically to work with the LM257x series.
The circuit also includes a low voltage cut-out based on a 9.1v Zener diode and BC337 transistor that will shut down the regulator if the input voltage is below 11.5V. This prevents unstable operation of the regulator at lower input voltages, and also helps prevent accidental flattening of the supply battery. Substituting this transistor for similar type may affect the cut-out voltage; the Vbe of the transistor should be 1.2v.
All of the components used should be pretty readily available in most areas. I got everything from Farnell. Jaycar also sells everything except the inductor. Make sure you specify high temperature, low ESR capacitors as these help result in more stable operation and better efficiency of the charger.
Unfortunately the end result is a charger that is slightly bulkier than I would really like. I attempted to fit this inside an old mobile phone charger case so the whole thing could hang out of the cigarette lighter, however I ran into trouble making the circuit stable enough and dissipating all the heat. Due to the high current involved compared to a mobile phone charger the components are much bulkier so it’s pretty tricky to get all to fit! If I do get it finished I’ll add an update.
Circuit Diagram
Parts List
- 2x 10k resistor (R1 & R4)
- 2x 22k resistor (R2 & R3)
- 1x 1.5k resistor (R5)
- 1x 120μF 25v electrolytic capacitor (C1)
- 1x 2200μF 16v electrolytic capacitor (C2)
- 1x 1N5822 Schottky diode (or equivalent)
- 1x 9.1v 0.5W Zener diode
- 1x BC337 NPN transistor
- 1x LM2576T-ADJ IC
- 1x 100uH, 3A inductor (e.g. Pulse PE92108KNL)
- 25°C/W or better minature heatsink (e.g. Thermalloy 6073)
- Cigarette lighter plug with 3A fuse and 2.1mm DC plug (e.g. DSE P1692)
- 2.1mm DC chassis mount socket
- 1.7mm x 4.75mm (ID x OD) DC plug and cable
- Small plastic enclosure
PCB Design
Source : http://www.marlwifi.org.nz/projects/asus-eee-car-charger
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