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BQ76200: FET choice

Filippo Piccardi
Prodigy 40 points
Part Number: BQ76200

Hello, I'm using the BQ76200 to drive the bidirectional switch composed by two N-FET (Q1 and Q2) for the high-side protection of a battery pack.

My idea is to keep the two protection FETs always on in normal mode and turn off Q1 to inhibit battery charge and turn off Q2 to inhibit battery discharge.

Consider now the case where both FETs are on and I am discharging the battery.

My question is: how the discharging current overcomes the CHG FET Q1? Does it flows through Q1 body diode or it flows in the Q1 channel from source to drain?

I know that an N-channel FET is designed to conduct current from drain to source but I'm asking my-self if it is a bidirectional device able also to conduct current from source to drain (when it is on) bypassing the body diode.

This is an important aspect in my design because if the discharge current flows only through Q1 body diode I have to choose a protection FET with a body diode able to conduct a large amount of current!

  • 0
    TI__Guru 54715 points
    Hi Filippo,
    Yes the MOSFET will flow current through the body diode. When the channel is enhanced current can flow either direction. When the charge FET is off for example and discharge current flows, it will flow in the body diode and heat the FET. You may choose to supplement the body diode with external components to avoid the heating if necessary, or implement a body diode protection in your controller to turn the charge FET back on when discharge current is present. The BQ76200 does not provide the current sensing or automatic body diode protection.
    You might refer to you favorite FET suppliers documentation or other MOSFET resources for a better description of the MOSFET characteristics.