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WEBENCH® Tools/LM5118: Design for 8A battery charger between two 48v batteries?

Part Number: LM5118

Tool/software: WEBENCH® Design Tools

I'm looking to create a buck-boost voltage and current-limited converter for the purpose of charging a 48vDC (nominal) battery pack from a larger battery bank of the same voltage.  The challenge is due to the fact that the voltage ranges of both packs are basically the same, and could potentially cross in all four ways (both low, both high, one high, one low, etc.) during a charge cycle.  The desired maximum output current is between 8-10A.

Say the main pack is at "float" (57.6v), and the smaller pack is empty (~46v): the controller will have to BUCK until the smaller pack charges up, going into BOOST to reach the full charge voltage of 58.8vDC (different chemistries).  If the main pack is not on float (~51.2v), it'll just have to use the BOOST range for a larger part of the charge.

It looks to me that the LM5118 could be capable of doing this.  However, when I put the design requirements into WEBENCH (42-60vIN, 58.8vOUT/8A), I get the error, "Can't find suitable FET for the design. Try reducing load current." 

My question really comes down to the following: as an electronics hobbyist with no intentions of a commercial application, is it even feasible for me to solder one of these chips to a carrier board (can't do much with the exposed pad on the underside, which could be a problem for thermal dissipation), and expect it to work?  I've read several other posts, and it seems that this IC is very sensitive to board layout, which a carrier board won't help.  (I do have experience doing tight SMT soldering on a standard 0.1" protoboard, but nothing finer than SOIC/SOT-23.)

My alternative is to purchase a Chinese-made boost board, and follow that with a current-limited buck board ($30 total, probably a resulting efficiency of ~85%).  From a cost effectiveness and efficiency standpoint, which is more viable?

  • HI Sid,

    It is likely that WEBENCH cannot find a suitable FET because it takes a relatively conseravative approach to selection the FETs. Also if you are truly going to charge the 48V battery with 8A that is quite a high power (384W) range the LM5118 since it is a non-synchronous topology. With the power level it could possibly be better to do a cascaded boost-boost configuration with multiple phases.

    As for using the LM5118 on development board I would not recommend it due to noise issues are you pointed out. As with any switching converter it is better to have multi-layer PCB so that the effect of noise can be minimized. Furthermore at 384W of power I would recommend using pretty heavy copper (>=2oz) on the PCB to help minimize conduction losses.

    Thanks,

    Garrett