Automatic Switching Between Power Supplies

Hello,

I'm looking for direction in how I would go about switching between two power sources automatically without a drop off. My application is a small engine EFI system that I would like to perform the power on start up from a lithium battery pack and then switch power draw over to an on engine generator after the engine has started and the generator is making power. I would like the battery to always be available to provide power if the generator fails, but not to be charged by the generator. I would consider the battery portion a total loss system where the only time I draw power from it is during the start up and I would externally charge the battery after several uses.

I have discovered ORing controllers and hot swap controllers, but I'm not enough of an electronics guy to decipher whether I need something that fancy or not.

Thanks,

Casey

You're on the right track with diode ORing (steering diode) and hot swap controllers.

If the engine generator when operating will always output at least 1 volt more than the maximum terminal potential of the battery a simple steering diode circuit will work. Additionally, the circuit will have to be ok with the 1 volt diode drop. If the diode voltage drop and power loss are unacceptable then a Hot Swap type of circuit is the solution.

If you'd like more information post the voltage and current involved.

David,

Thank you for your response. I'm not sure on the voltage difference yet. It is a three phase generator. For now I'm going to use an ATV type rectifier/regulator so that voltage will be ~14.4. I'm looking at a max current draw of 2.5A. The battery voltage with a full charge could either be 12.6V for a fully charged 3 cell lipo or 14.8V for a fully charged 4 cell. I wouldn't think either of those voltages would be at that level for very long.

Do you have any sources I could research for the Hot Swap circuit?

Thanks,

Casey

Something like this?
http://www.hobbyprojects.com/the_diode/steering_diode.html

If the diode forward drop is not a problem the circuit in your link will work. I would use Schottky diodes for minimum voltage drop and minimum power dissipation. The disadvantage of a Schottky diode is the reverse leakage.

A suitable diode is the VISHAY V30100S-E3/4W available at Digikey for $1.73. This diode has a reverse breakdown of 100 volts and that is more than high enough to withstand automobile voltage spikes.

http://www.vishay.com/docs/88941/v30100s.pdf

Looking at Fig. 3 in the datasheet we see that the forward drop at 125 deg C is 0.3 volts. The power dissipated is then 0.75 watts which will cause the diode (with no heatsink) to be 50 deg C above the ambient temperature.

And looking at Fig. 4 we see that the reverse leakage current is roughly 1 mA at the highest temperature to be expected under the hood. Will your battery be ok leaking off a few mA-hours per day?

David,

Let me do a little more research on my end on the voltage requirements for the system, but I think the forward drop won't be an issue. I'll rig up a bench test with the VISHAY parts you recommended and see how it works.

The battery will only be connected during use, so the leakage is not significant.

If I wanted to take this to the next level and use something fancier than diodes, where should I start looking?

Thanks,

Casey

The Linear Technology LTC4359 Ideal Diode controller is something to look at for your application.

http://cds.linear.com/docs/en/datasheet/4359fa.pdf

How about this guy to be able to beard board my test circuit?

http://www.intersil.com/content/dam/Intersil/documents/fn31/fn3183.pdf

That part is a low current part and it is good to only 15 volts. For a vehicle I think a part rated for automobile service should be used. To meet that requirement it must handle spikes to 60 volts, 1 minute at 24 volts (for a double battery jump start), and a reversed battery.

Ok. I did find some MSOP-8 prototyping boards that convert it to .1" header spacing. I'll give those a shot.

Hi, can anybody, Elaborate this topic because I am thinking to give a seminar on it in my college. so any one suggest me more about it.