making a 5V – 3/4 A power supply for car

car_alimFor one of my project, I needed to have a >3A power supply to put in my car, delivering stable 5V. For this I tried different power supply from ebay used for phone & tablets. One of them was not so bad if you connect it after engine start. The main issue with these power supply is the poor quality of the filtering and engine start spike cancellation. As my circuit is sensitive to this spike, I decided to build my own power supply.

Here is the summary of this experience … click next !

Linear Low DropOut Circuit

The easiest way do transform a DC voltage to another DC voltage is to use a linear regulator. I was first expected to use a such circuit as it is simple and low cost. The problem is that basically, we can consider it have to transform voltage into heat. To convert 13V to 5V we need to decrease 8V, with 3A = 24Watt. For this reason, I tried to help them with a large metal heat sink.

ldo_car_alim2

The first of my power supply was based on a Micrel 29500-5, delivering up to 5A for a unit cost of 6€. For sure, even with the large heat sink, this was not sustainable for a long term use as the temperature grown up to 80-100°C after 2 minutes of intensive usage.

 

ldo_car_alimThe second one was based on Fairchild KA378R05, this is a LDO regulator working up to 3A. Same technology, same consequences, I had to mount a really large heat sink to support 2 minutes full load. The big advantages of this component to me was to be low cost 1€10 and to have a on/off pin allowing to control switch on with a delay (based on a simple RC circuit) to avoid the engine start spike. For my purpose I’m able to split power into 2 lines of 1.5A eventually. By the way, my opinion is that heat sink is too large for my design.

Switching Circuit

switching_car_alimWhen looking for high current and low heat generation, the only way looking good seems to use a switching circuit. Usually switching is complex and that’s the reason why I was not looking that way first. As a consequence of complexity they are also more expensive. That’s not a reason for not looking at them. I finally found an interesting circuit, because it is not so complex, based on a LM2679T This circuit provides 5V – 5A for a cost of 4€ (alone), I later list the components and total price.

The circuit, from the datasheet is that one :

lm2679 switching regulator circuit
lm2679 switching regulator circuit

 

 

 

 

 

 

By taking a look to the photo, you will see that a really small heat sink is enough. I load this circuit with 3A for 3 minutes and get something like a ambiant+ heat… This makes a big difference.

What about total price ?

  • LM2679 -5 (4€)
  • 6TQ4S (2€)
  • Inductance 22uH (0,8€)
  • C 0.01uF (0,20€)
  • C 0,47uF (0,07€)
  • C 15uF x 3 (0,20€)
  • C 180uF x 2 (0,34€)
  • C 1nF ( 0,10€ )
  • C 10nF ( 0,10€)
  • R 5.6K (0,06€)
  • Total : < 8€ HT

Compared to a power supply found online on ebay, this is a little bit more expensive, but I expect the quality to be better and the life of my component, getting power from this system longer.

One of the advantages of this system is that you can change the SoftStart Capacity to delay the regulator on startup. This can be useful to avoid the start spike. In my design, I used a 10uF capacity instead of the 1nF one. The system is start providing 5V after 2 seconds.

This was a good idea but not a solution : in fact, the system is delivering not 0V but 1.3V during softstart. This did not correct my issues. After multiple test, I finally used the 1nF capacity. The main solution to have my system running correctly I put in place is to have a really long reset for my micro-controller. 5s Reset gives a good answer to my starting issues. Another thing I’ve done is to pullup my I/O : as during this starting time the I/O are not yet configured as input or output they seems to be really noisy and this corrupt some of my digital devices. With the pullup they seems to work better.

I actually did not put protection in front of the circuit. The regulator is accepting up to 40V which seems to be good enough but I’ve read sometime the voltage can be higher on engine startup. Some are using Zener for protection, this looks durty (even if it is the solution you have in the previous schema for 5V output) In this case, it is recommended to add a PPTC in the input to ensure no short cut on battery. I’ll test this later I think.

This circuit is in test, I will let you know !

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