[FAQ] BQ25750: What do I need to know about the BQ25750 power path switchover?

Tool/software:

     By default, the BQ25750 has an automatic power path switchover. When the VAC power is out the of range set by the ACUV/ACOV resistors, system power will change to VBAT. When VAC is in range again, system power will change to VAC.

     When this happens, there will be an in-rush current through ACFETs or BATFETs to charge the system capacitors. If there's is no system load, you can estimate the current into the system capacitors with the following equation:

i = C Δv/Δt

     Where i is the current through the capacitor, C is the capacitance, Δv/Δt is the change in capacitor voltage with respect to time. If the capacitance is large or the change-in voltage is large, the in-rush current can be enormous. To prevent damage to the ACFETs/BATFETs, the BQ25750 has a built-in protection system to monitor the current through ACFETs and BATFETs during the power path switchover. The BQ25750 will turn-off the ACFETs or BATFETs if the current through the FETs is greater than 8A. This current is measured through the input and output current sense resistors, respectively.

     Here’s a few conditions that may cause the ACFET and BATFET to change state:

     However, if the protection system triggers, power to the system load will stop and the system voltage will fall. There are a few methods to reduce the in-rush current during the power path switchover and prevent the protection system from triggering:

1. Make sure that the ACUV/ACOV levels are set appropriately. This effectively limits the Δv in the equation above. It is recommended to limit the Δv < 30V.
   
   
2. If you can control the system load, reducing the system load current will reduce the current through the ACFETs/BATFETs during the power path switchover.
   
   
3. You can install an RC filter on the gate of the ACFETs and BATFETs to slow down the FET turn-on time. Going back to our equation, this increases Δt which therefore decreases the current into the capacitors. This RC filter will need be to sized experimentally depending on your application.
   
   
4. Limit the SYS capacitor value, C, in the equation above. A typical value of 80uF~200uF is enough for most applications.

     There are also a few methods to work around the protection system:

1. You can use the BQ25750's auto reverse mode to provide power to the system when VAC is out of range. Auto reverse mode uses the converter to supply power to the system from the battery. In this setup, power only flows through the converter and not the ACFET or BATFET. This can be used as a soft-start method, bringing the SYS voltage close to the battery voltage during the transient, and then transitioning back to turning on the BATFET with very small dV at the time. The method would work as follows:
   
   
   1. When valid VAC is present, SYS is powered via the ACFET. Host MCU programs the VSYS_REV register to a value close to the battery voltage (say 48V for a 12s battery), and enables the AUTO_REV mode (EN_AUTO_REV = 1). ACUV is programmed according to customer specification (say 38V for a 48V supply).
      
      
   2. When the VAC falls below ACUV, the device automatically shuts off the ACFETs and turns on the converter in reverse mode to regulate VSYS to VSYS_REV (48V in this case).
      
      
   3. After VSYS reaches VSYS_REV, customer can disable the reverse mode, which will automatically turn on the BATFET to power the system (EN_REV = EN_AUTO_REV = 0).
      
      
   4. Once VAC rises above the ACUV voltage, the device will automatically turn off the BATFET and turn on the ACFET – MCU host can then re-enable the AUTO_REV mode to start the cycle again.
      
      
2. To disable the protection on the BATFETs, you can connect the BATFETs from the system to the SRN side of the sense resistor instead of from the system to the SRP side of the sense resistor. Keep in mind, the IC will no longer be able to monitor the current into the system because the system current no longer flows through the battery current sense resistor.
   
   
3. To disable the protection on the ACFETs, you can short across the AC sense resistor. With this setup, the IC will no longer be able to monitor the input current and the IAC_DPM will no longer work.

     If the protection on the ACFETs/BATFETs is disabled, damage can occur to the ACFETs/BATFETs from the in-rush current spike. You will need to make sure the FETs are sized appropriately and have a high SOA to handle the current of your application. Please see this application note for more information on how to read the FET SOA charts.