Tuesday, 2 August 2016

Car USB charger / 12V to 5V DC-DC converter

If you happened to buy this piece of chinese crap
Do not use it!!! Really, don't!
The thing is that it almost killed my smartphone. For a while it worked well, but suddenly, it burned out and all 12 volts went through straight to my phone! Luckily, the phone has a protection, which might have not worked and my phone could've passed away. I assume, not all of such devices have overvoltage protection, especially cheap ones. So, I consider myself lucky.

What next? Buy another one? And hope it won't burn out and take my smartphone with it! No, I've decided to do this the hard way. Why not build my own DC-DC converter, it should be cheaper and more reliable and I've already got a casing for it.

The original one was built on AD85063 chip. A few minutes of browsing the net gave me only chinese version of the datasheet, but it is all clear to me. Link to the datasheet

Do you see why it just couldn't  work for long while? I've marked red the key parameters. 1.2A is a maximum peak current through the chip, and 700mA is a maximum normal working current the chip can deliver for a long period of time. It doesn't correspond to what it says on the USB sockets 1A and/or 2.1A. I've gotta say to chinese guys: "you have fooled me again"!

Okay, it's time to make it right. First thing you've gotta decide what chip you're gonna use. I have some experience with very popular MC34063, but it's not powerful enough (only 500mA), and frankly, it is very old, it is time to find some modern solution. Once again, a few minutes of browsing the net and I've found a perfect chip for this job - NCP3170A. See the datasheet here

Key features
• 4.5 V to 18 V Operating Input Voltage Range
• 90 m High-Side, 25 m Low-Side Switch
• 3 A Continuous Output Current
• Fixed 500 kHz 
• Short-Circuit Protection
• Thermal Shutdown

My schematic diagram
Car USB charger / 12V to 5V DC-DC converter


This is how it looks

And it works perfectly! Now I can charge two energy-hungry devices, because my charger is capable to deliver 5V at 3A of continuous current. By the way, both USB sockets are connected in parallel, so it doesn't metter which one to use.

Hope this article will be useful to someone. Despite almost all modern cars already have USB socket, most of cars on the streets are a bit older and have only 12V socket. This charger allows you to charge your smartphone, tablet, navigator etc from 12V.

Any questions? Put it in the comments.