We are using six instances of the BQ24735 chip to charge six lithium-ion batteries (minimum 10V, maximum 16.8V). The input to all six chargers is a regulated 24V source from an AC-to-DC power supply. The outputs of all six chargers are connected to a single node (DC_SUM in the schematic below) which powers the load.
When AC is present (ie, 24V is powered) the power from the AC-to-DC power supply should flow to the load and to all 6 batteries to charge them. When AC is turned off (24V not present), the batteries should supply power to the load.
The schematic below (X02) shows a single instance of the charger circuit. There are six identical copies of this schematic of this in our design, with all instances of 24V tied together, and all instances of DC_SUM tied together.
BQ24735_X02.pdf
Our initial prototype showed that when AC is applied, each instance of the charger chip switches from AC to battery power at a slightly different time. As a result, certain instances of the ACFET (Q2) turn on before other instances of the BATFET (Q5) have turned off. This causes a surge current to flow from the already-on ACFETs through the not-yet-off BATFETs and destroys these BATFETs.
The new design (X03, below) seeks to correct this problem by adding an ideal diode (MAX16171 or LTC4357 are alternative population options and will NOT be used simultaneously).
In this new design, ACDRV and BATDRV are now disconnected to avoid this timing issue. Also, the 24V output of the AC-to-DC power supply is connected via another ideal diode (not shown) to the DC_SUM node. Since this 24V is higher than the battery max voltage (16.8V), the load will always be powered by the AC-to-DC power supply so long as AC is applied.
BQ24735_X03.pdf
We believe this new design (X03) could solve the timing issue that occurs in the original (X02) design where one instance of the ACFET turns on before all other instances of the BATFET turn off, causing any of such instances of the BATFET to be destroyed by the surge current.
Is this a reasonable way to solve the timing issue inherent in connecting these six battery charging circuits together?
