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bq76940 High Side FET or Low Side FET?

Eric Hsu34
Prodigy 310 points
Other Parts Discussed in Thread: BQ76940, BQ78350

I'm recently design in a bq76940 + bq78350 bundle solution

although i know there's a low side FET driver integrated in bq76940

but the customer concern about the short GND issue of the low side FET solution

can someone help explain the major difference and effect of High side FET and Low side FET?

and also the pros and cons please

Thanks!

  • 0
    TI__Guru 54715 points
    Low side protection FETs are generally easy to implement with N-channel devices since the control voltage is within the range of the battery voltage (between + and -). The charge FET control does need to be able to go below ground to keep the FET off. Low side switching is a challenge for ground based communication to the battery though because of the communication. When PACK- moves from Battery- the reference for the communication signals is lost and you can't talk to the battery. Additionally the communication lines are a leakage path for both charge and discharge current. These are likely the customer concerns you mention. Solutions might include switching the signal lines on fault such as figure 10 of www.ti.com/.../slua726 if it is OK not to talk during fault (be careful with /PRES so you don't create a latch), some sort of isolation in the communication lines or power, or high side switching. Isolators add a cost and current requirement, changing from SMBus on the bq78350 to an communication bus with isolation may require an additional processor.

    High side switching is relatively easy to implement with P-channel FETs since again the control voltage is within the range of the battery voltage, and the charge FET drive must allow the FET to stay off at higher voltage. With the high side switching the ground path is maintained. One of the challenges of high side P-channel switching is turning off the discharge FET quickly with control from what is normally a ground-referenced signal output. One solution is Q5 on page 43 of www.ti.com/.../sluu474. Other solutions may be used to pull the gate off quickly when needed. P-channel FETs are often not as attractive as N-channel since there may be fewer choices with higher Rdson and/or at a higher cost.

    N-channel FETs in a high side switching add a level of complexity in that the control voltage for the FET must be above the battery voltage. This requires a power supply above the battery and pack, drivers for the FETs, and level shifters for the control. For lower voltage devices, these functions may be integrated with the gauge, for higher voltage systems it may be difficult to find these functions in a high voltage device. A discrete implementation may be desired when the extra circuitry is acceptable and the performance of the N-channel FETs is needed for the system.