bi-directional buck needed

I moved this post to the non-isolated forum since you are asking for a buck converter, I think.

I'm not quite sure what you need though.  Does the system use both battery packs?  Does it use them at the same time or does it just use one or the other?  What battery charger are you using?

It sounds like you want a buck converter that would step down your higher voltage pack to around the level of the lower voltage pack.  Then, you would charge either battery pack by applying a voltage on the output of the buck with the current going through it to the battery?

Chris,

Thank you for the prompt reply.  This is a customer request and I have asked for more details about the block diagram.  However here is what I know:

The customer does not control the smart batteries that have integrated chargers.  They get a pack that is either 11.1 volts nominal (12.6 max) or a pack that is 14.3 volts nominal (16.8 max).  They then apply that to a power supply board that at present time only handles 16V max.  This power supply board is responsible for charging and discharging the battery through SMBUS commands.  The smart pack has an integrated charger built in.  They are worried because the 16.8 max battery pack will charge beyond 16V which is the limit of the power supply board.  It sounds to me like they need a charger around their prepackaged charger if they want to protect the battery from going above 16V.  That is why I placed it in battery chargers.  Let me know your thoughts. 

I suggested the BQ24725 as a host controlled battery charger for this application.

-Jason

Well, if the batteries have chargers built in, would another charger help?  The bq24725 and other SMBUS host controlled chargers charge to a maximum charge voltage that is told to them by the host.  So, one option is to simply have their system only charge the batteries to 16V.  They can do this with the bq24725 and possibly with the charger that is built into their packs.  Placing a charger in front of the charger for their pack is not recommended.

Another option is to get higher voltage caps.

The last option is to use a circuit that would step down the 16V to say 12V but allow reverse current to flow and charge the battery.  I thought that is what your post was about originally which is why I moved it to this forum.

Am I on the right track for what you need?  Do you see any other options?

Chris,

Thank you for your comments.  The charger option and higher caps are not options for this customer. 

So the only option is a circuit, like you state, that would buck down and then allow reverse current to flow and charge the battery.  After discussing with the customer, they mentioned:

"Maybe I can rig up something with a few diodes or op-amp+fet to implement this back drive ability.  Do you know of any buck converters where the switching Fet can be turned on when the converter is disabled basically shorting the input to the output? "

Do you have any thoughts?

-Jason  

What is the maximum current that will be flowing through this circuit?

What is the maximum voltage at this point?  To charge the batteries up to 16.8V, this point in the circuit will likely need to be 20V or higher to give the smart charger enough voltage to work with.

Chris,

Is there a family of parts that I could suggest?  I can have the engineer navigate through the various options.

I appreciate your help,

Jason

That's what I'm trying to determine by asking for the voltage and current.  It could be as simple as essentially an LDO circuit with NEXFETs and an op amp.  Or it may need to be a switcher, such as the TPS62110 or a TPS54xxx IC.  Your voltage and current requirements will direct you to one or the other families.  You might also look at a SWIFT IC with a NEXFET in parallel to allow the reverse current.

Chris,

I determined the requirements and was researching different options.  Do you have recommendations for this?

Vin Min = 12

Vin Max = 18

Vout = 10.5

Iout = 1.5A

Sincerely,

Jason

I was going to suggest something, but I do not see how the numbers above correlate to the previous posts (if at all).  If all you need is a buck meeting the above requirements, TPS54140 or TPS54231 should work, but I am not sure of the other requirements in the earlier posts.

Hmm, I would expect your 3 cell pack voltage to go down to at least 9V maybe lower.  If this were the case, the output of the TPS54231 would be lower than that.  This should be considered for your system's input voltage needs.