AFE for BMS Design 2 - Schematic Capture

In part one of this series I looked at the inital concept of what an AFE is and a basic outline. In the previous post I presented the follow ideas for the design of the system :

  • 9-15S LiPo battery (BQ76940)
  • 100mA balance current
  • 40A continuous duty current shunt, 50A peak
  • PCB area less than 10x10cm panel
  • LED SOC indication
  • Ability to communicate to an external display to show detailed information
  • Temperature sensing inside the battery pack
  • Track the health of the pack

Looking at this design, I selected to use a companion controller to work with the AFE for the inital design of this BMS. This is not technically required, and if you wish to impliment your own the schematic is almost identical except swapping the companion controller out for your selected device.

Laying out the schematic

I’m using KiCad for this design, and so I will use some of the features such as sub scheets. These are extremely powerful for the layout of the design and help to reduce the complexity on each stage of the design.

Cell Interface

For the interface to each of the battery cells, we need to have the measurement path. This can be a relatively high (~1k) impedance path that allows the AFE to sample the voltage of each of the cells.

We also require the mosfet to allow us to discharge the batteries during the balancing cycle. Internally to the AFE, it uses a moset to short the selected cell’s terminals to discharge the cell. Because of the high impedance of the measurement path, this instead connects the negative sampling line to the batteries positive. This is what is used to drive the gate on the N-FET.

Current Shunt

We will be using a large resistor package for our current shunt, as it will dissapate heat during heavy loads. Because resistor values change under heat (Temperature Co-efficient) we want to minimise the termperature change experienced by the resistor. By using multiple resistors in paralell we are able to reduce the thermal disappation in each resistor.

Decoupling, Power & Misc

The AFE requires an external pass transistor that can be regulated to drop the incoming voltage to meet the input range of the IC. Using an external pass transistor is both low cost and