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Topology selection advices.

Perico Eldelospalotes
Intellectual 505 points
Other Parts Discussed in Thread: BQ24075

Hello everybody.

              My question is about the topology selection of the unity that will charge the battery and will power my system. I have been looking at the Texas products for battery management and it seems that the two main topologies are DIRECT CONNECTION and PATH SELECTION. We are designing new devices and we plan to add a battery charger that will be able to charge the battery and power the entire system.

              The advantages of DIRECT CONNECTION topology are more efficiency in case of the system being powered by only the battery, as any element is in the way between the battery and the system being powered and typically less components. On the other hand, the PATH SELECTION topology offers less thermal issues and isolation from the battery charger, wich means that the system is not affected by the changing voltage while the battery is being charged.

              Appart from these general considerations, I will post my system requeriments and maybe some could help or give some advice:

-Current during normal operation: <1A.

-Current peaks: 2 A (during GSM transmissions).

-Battery chemistry: Lithium ion-Lithium polymer (1 cel).

-Battery capacity: 1~2 Ah.

                                                       Thank you in advance!!

              

  • 0
    TI__Expert 7730 points

    We have chargers that only have one BAT/OUT pin and the system load is connected directly to battery. In this case, the input power will provide current to both system load and the battery. When there's no input, battery will supplement the power to the system. There're two disadvantages in this topology: 1. If the battery is deeply discharged, when the input power is applied, the system cannot be turned on instantly. Instead, the system will need to wait the battery to be charged to a certain value as the system and battery is shorted together. 2. The battery charging termination is not accurate because the IC has no idea how much current is flowing into battery.

    For the chargers that have power path control, which is the second topology in your post, the system and battery are connected to separate pins of the IC and there's a FET between those two pins. In this case, when the input power is applied, the system can be turned on immediately and the charging termination can also be accurately controlled. Normally the Rdson of the FET between sys and bat is very small so the power loss is not very critical either.

    Bq24075 is a charger with power path, the BATFET Rdson is 50 mohm and it can support up to 1.5A charge current.

    Thanks,

    Wenjia