Design a proper USB Charger - Page 1
Forum section: electronics / projects

author: Pedram
author_rank: Regular Contributor
timestamp: July 19, 2014, 08:13:04 pm
content: Hi, i'm planning to design a proper USB charger this summer for use in my car . i'm some how a beginner.some requirement/facts :2A output (nominal). 2.5A Current Limited.5.1V output.Stable and reliable over temperature/timeAmbient temperature is about 50 Degree (circuit will be inside the dashboard. so no airflow and it's beside the heater).startup delay .( 5Sec )Sleep mode( On for 1 Hour after turning off the car)(detect using accessory power)Low noise.Undervoltage and Overvoltage(Is necessary ?) lockoutno overshootAlso i don't have easy access to fancy parts such as USB Current limiters or new regulators.I only have access tolocallyavailable parts, including: LM2576 , LM2596 and LM2678. ( i can order other parts but i have to wait a month which i don't want. )also it's more fun to use basic parts to do the jobAny suggestion on design ?i will update this post with my design.thanks for your help/comment/suggestion in advance.(sorry for bad english)

author: rob77
author_rank: Super Contributor
timestamp: July 19, 2014, 08:24:29 pm
content: considering the requirements... either you have to use the fancy new parts you don't have access to, or you'll end up with a relatively complex circuit.if you need a usb charger for your car, then just buy one. if you want to have some fun with a project, then go ahead

author: Pedram
author_rank: Regular Contributor
timestamp: July 20, 2014, 04:37:55 am
content: i think an attiny13a for startup delay and sleep mode is the simplest solution.opamp on regulator's feedback pin with a shunt resistor for current limiting.also a couple of opamps for under/over voltage lockout.

author: rob77
author_rank: Super Contributor
timestamp: July 20, 2014, 08:51:30 am
content: as i mentioned - a relatively complex circuitbut such a project is good for learning and gaining real world experience, and you can have a lot of fun with thatbtw. don't forget the input protection, 90V spikes/transients are nothing unusual in a car - you need to protect the input of your regulator against those.

author: Pedram
author_rank: Regular Contributor
timestamp: July 20, 2014, 11:22:54 am
content: My design so far.TODO: Current Limiter and also some protection on attiny and VACC sensing

author: Pedram
author_rank: Regular Contributor
timestamp: July 20, 2014, 11:49:51 am
content: Added some protection and VACC SensingTODO: Current Limiting. More protection.Any Idea on how to limit current ?

author: Odysseus
author_rank: Regular Contributor
timestamp: July 21, 2014, 07:09:08 am
content: Funny. I just killed a music player of mine that I had for 7+ years ( RIP) while simultaneously using a cheap cigarette charger and the AUX port of my stereo. I was expecting a ground loop issue to cause noise, as the same charger had done with my GF's iPhone, but instead the charger died, putting 12V into my now deceased music player.It wasn't really the charger's fault, however. It was a Sansa E200, which I now know uses a "phantom ground" to eliminate output DC blocking capacitors, meaning the common terminal of the headphone amplifier is at half the supply voltage with respect to USB ground. Woops.Incidentally, I tried it out in order to convince myself to build an isolated DC-DC converter for my car. The answer was yes. I'd say that if you are serious about reducing noise, I would suggest an unregulated, isolated DC-DC converter followed by a conventional linear regulator. At the very least use a common mode choke EMI filter the input, which serves to reduce incoming noise, as well as noise produced by ground bounce caused by noisy charging currents.

author: Pedram
author_rank: Regular Contributor
timestamp: July 21, 2014, 08:08:55 am
content: Thanks for sharing your experience. My current charger ( which use LM338 ) have this ground loop problem and i could hear Alternator noise on my stereo. i added a common mode filter on the input and an inductor on output. less noise but still i can hear it. Isolation is good but 10W isolator is expensive and not available locally. so i think i can live with a little noise. to limit the ground noise i will move shunt resistor to positive side . still i have no idea how implement a current limiter.

author: Pedram
author_rank: Regular Contributor
timestamp: July 21, 2014, 11:04:02 am
content: Added temp sensor.some cleanupadded common mode choke on input.added USB Connectormoved shunt to high sideTODO: current limiting

author: Pedram
author_rank: Regular Contributor
timestamp: July 23, 2014, 12:34:43 pm
content: i added current sensing circuit. ( 1:1 copy from uSupply schematic )should i stop the circuit when currents exceed 2.5A or i should lower the output voltage ?rev 4.