BQ79640 DSG-FETs and CHG-FET parallel instead of serial

If you are using the low side protection FETs you can take a discharge path from the common drain node, shown on the datasheet, figure 9 of www.ti.com/lit/slua749 , or net PFD of the EVM figure 33 of www.ti.com/lit/slvu925  When you do that there are some considerations:

The discharge path does not have overcharge protection, so you system design must manage that situation.  The charge path continues to have discharge protection in case the charger malfunctions.

The load detect function of the bq76940 will not be available since it comes from the CHG pin and that will be blocked by the CHG FET off.  Some sensing of the discharge terminal would be needed through an additional circuit for voltage based recovery.  Time based recovery may be an option.

The FET drive of the bq76940 is limited, be sure to use a sufficient driver for your larger FET or FET array.

If using the companion bq78350-R1 gauge, it is designed for the series FET configuration.  Check your expected system use for any unexpected behaviors.

Thanks for your fast answer!

At first I want to say: yes I plan to use the bq78350-R1 as campanion for the bq76940 and yes I also want to use low-side-fets. Does it mean I cant take a discharge path from the commen drain with bq78350-R1? Your last sentence sounds like that.

What about using one DSG and one CHG-Fet in serial (common drain) and for example 4 DSG-Fets parallel to this serial connection. With that the drains of these 4 parallel fets are directly connectet with the pack-. The load detection feature should work in this case. My only one drawback will be that there is no overcharge detection.  For discharge I could use only these 4 DSG-FETs and for charge I could use all Fets. Is that right?

I know if there are big fets the switching time will be longer without a driver. I will check this. I hope I dont need a driver and can use the slow switching fets for precharging my dc-capacitors (of my inverter) instead of a seperate precharge-cicuit.

Armin

The gauge team does not recommend bq78350-R1 with split power paths.  Checking system behaviors is easier if you are designing the complete system rather than just the battery without knowledge of how the system behaves.  With a series FET configuration the battery has only the single port for charge and discharge, and the system designer usually recognizes that they must manage current flow between the charger, load and battery.  In systems where the battery is removable this may be handled mechanically with the battery plugged into the charger or the load, never both.  In systems where the battery is embedded, the system must decide how will the current be routed if there is a load and charger simultaneously connected. When the charger and load terminals are separate paths to the battery, there is a tendancy to connect both the charger and load simultaneously.  The bq78350 has a body diode protection feature for the FETs but does not have a power path decision or control mechanism for the system.  Consider a situation where the high current load depletes the battery to an under voltage condition where the discharge FET turns off.  The charger then starts to charge the battery.  The bq78350 sees the charge current and turns on the discharge FET to protect its body diode although the battery has not yet recovered from the UV condition. Since the charger can't support the full load current the battery will start delivering load current again, the bq78350 will see the discharge current and turn off the DSG FET again.  This could cycle unless the system has some other control.  Overcurrent conditions may behave similarly. You may want to analyze system behaviors and check with the gauge experts on the gas gauge forum if you have questions.

The loss of load detect is from the blocking action of the charge FET from the common drain connection.  Load detect is through the CHG pin and enabled when the CHG output is disabled.  With the CHG output disabled and the charge FET off, the CHG pin will detect the voltage on the charger or the load of the charger rather than the load of the discharge path.  A separate discharge FET won't help since it is the charge FET which blocks the pull up.

ok. thanks for these advices. I will check this up  with the gas gauge team as well.

I have one more question about your last section:

In my opinion the load detection in a serial fet connection is working because the dsg-fet and its body diode  is seperating the positive voltage on pack minus (with load). if there is no dsg-fet the detection feature cant work because the body diode of the chg-fet pull and the shunt Rsns pull down the positive voltage on pack minus. Please tell me when i am wrong with that.

thanks a lot

armin

You are correct, with series FETs and the discharge FET enabled but charge disabled (CHG_ON low) the load detection circuit will be enabled but will not indicate a load since the PACK- (and CHG pin through the connection resistances) remains low.  If you were to connect parallel FETs directly to the sense resistor load detect would not work either.

As described in the datasheet the load detection is a comparator on the CHG pin, it will sense PACK- voltage when the FETs are off through the normal circuit path. It does not determine current flow or potential for current flow.

okay. thank you!

i will have some other questions about what load detection is exactly for and how the bq76940 detects a connected charger but i ask them in a new thread to keep the order of this forum.

armin