bq77908A bq77910 high current cell balancing

This shows N-ch devices, it could be done with P-ch. Certainly the designer must select the balance resistors for current desired & power, also the FET.

.2475.910A_5S_ser_ext_bal.pdf

Thank you for the info, I want to make sure if I understand this example schematic correctly.

1. R7,R10,R15,R23,R27,R29 limit the current flow into internal FETs, and divided the single cell voltage in half to drive external FETs on while internal FETs are on.

2. R8,R11,R17,R24,R28 limit the current flow into external FETs.

3. If I wish to use P-ch external FETs, I should:

  a. connect source to high voltage point

  b. drain to low voltage point

  c. gate to C3,C6,C10,C14,C15's high voltage point

And may I ask why is D4 and D6 TVS diode needed?

Thank you for your help. :)

1. Yes

2. Yes

3. Yes, but you will need some drain resistor for each

D4 and D6 are TVS diodes due to the type of surge encountered.  It is basically an impulse at turn off of a heavy current either from the load switching or IC protection (like SC).  A zener may be suitable here also.  Other inputs may need protection against transients during cell connection, depending on you connection method.

With the external balancing circuit, watch the gate voltage during short circuit.  A high gate-source voltage FET is desirable for the short circuit transient but may not be available with a low Vgsth for balancing.  If protecting the balance FET gate with a zener it may be best to connect the gate to Cn with a resistor to maintain input filtering.

Correct, the switching for balancing is not particularly fast and there is some time before measurement is completed.  A FET with a larger Qg should work OK.

Sorry, I stated the placement poorly.  The suggested resistor with the gate zener would be between the gate and VCn pin of the IC where the schematic above showed a wire.  With the bi-directional TVS  on the gate and without the gate resistor, the TVS basically goes across the input resistor (for example across R10).  This means that the input filter works up to ~ 5V across the input resistor, then the diode conducts and the inputs basically have no filter.   That may be OK in your system since 5V across the resistor is a large transient, perhaps larger than your system could experience.

The cell count, the resistance and inductance of the cells and pack construction as well as the load switching speed will all impact the voltage transients your pack experiences.   It can be very helpful to test thoroughly to be sure you are not over designing or under designing  your pack.

You've been very helpful! Thanks a lot!

Dear TI,

we want to use the BQ77908A for only 4 cells in connection with BQ34Z100 Fuel Gauge.

Is it possible to connect all unused cell inputs of the BQ77908 directly to +Batt, without low pass?

And are there application examples for use of both parts together (BQ34Z100 and BQ7798A)?

Thanks a lot.

Best Regards

Roland Stark

The bq77908A unused inputs should be tied to the top most used input as shown in the datasheet to avoid differential voltages which could occur between the different filters on the cell input and the BAT pin or unfiltered BATT+.  Excessive voltage could damage the IC.

For 4 cells you might consider one of the 4 cell gas gauges which may give you a simpler implementation.  If the high current balancing of this thread's topic is of interest, see www.ti.com/lit/slua420

There is no simple application example schematic since there are a variety of implementations possible.

Sharing the sense resistor with the gauge connected at the protector requires ground or communication isolation.

Placing the gauge outside the protection FETs requires an additional sense resistor.

Using a high side switch such as shown in the z100 datasheet diagram will require an appropriate current fault recovery system.

These options let you customize the solution to your needs, but where the cell count is supported in a gas gauge, a gauging device with protection may be a simpler implementation.

Hello,

thank you very much for the answer.

The project is a protection pcb with fuel gauge for a 4S battery pack of Li Ion and LiFePo4 batteries.

Charging and discharging current is 2 A. Protection FETs are on board. But it must have an I2C Interface.

These options let you customize the solution to your needs, but where the cell count is supported in a gas gauge, a gauging device with protection may be a simpler implementation.

Don you prefer a TI IC what is suitable for this use?

Thanks a lot in advance.

Best Regards

Roland Stark

The 4 cell gauges are SMBus interface, so unless your host can talk SMBus, you may have the best approach.  If the host can talk SMBus, a 4 cell impedance track gauge might be the bq20z45.  There are other SMBus gauges, see the choices in the Battery Fuel Gauge product selection matrix on the TI website if they may be suitable.

Roland

The bq20z45 has been used in some LiFePO4 applications, but it has not been optimizedd to work with this chemistry. We will release the bq30z554 this summer and it has some enhanced features to support LiFePO4 cells.Will your development schedule allow you to wait for this device?

Regards
Tom

Dear Tom,

thank you very much for the information.

But I2C is needed. So i would prefer to use two current shunts one for BQ77908, one for BQ34Z100.

Can i send you the schemtic to hear your opinion?

Regards

Roland

Roland,

You can send the schematic and we will review.

Tom

Dear Tom,

i tried to find a application for using BQ77908A with MOSFET in plus line, but i cannot find one.

Do you have any idea how to use it with FET in plus line?

Thank you very much in advance

Roland

Hi Roland,

You might consider something like the attached concept.  The FET drive will need optimized for your cell count.  CHGCTL (CHGST) will be required for recovery from OC/SC faults and the part must be configured for this.  8780.bq77910A-Pch.pdf

Hello,

thank you very much for the information.

Best Regards

Roland

Can you clarify the 'ground or communication isolation' required to use a single shunt resistor with the BQ34Z100 and the BQ77908A? GND, in these systems, is typically BATT- (and not PACK-, the GND that the load sees) - can the BQ34Z100 and the BQ77908A not share BATT- as GND?

Moving the BQ34Z100 outside of the FETs (thus requiring an additional sense) is undesirable because of the added overhead, but does it make communicating with the IC easier? It wouldnt seem that the BQ34Z100 and an external microcontroller share a GND if the BQ is 'inside' the FETs and shunt and the microntroller is 'outside'.

Best

Sam

Here is the document showing balancing with external fets.0143.bq77910 external fet balancing 2475.910A_5S_ser_ext_bal.pdf

In this figure from page 4 of the BQ34Z100 datasheet, it shows the BQ34Z100 VSS and the BQ77908 both connected to the negative of the bottom battery.

The voltage range allowed on SRN is only 0.125V, so this would the be the offset on the I2C or HDQ signals going to the system.
The I2C and HDQ are referenced to the top of the sense resistor, but the small SRN voltage does not affect communication.

For the bq77908A it is intended to be a STAND ALONE device, not needing communication once its programmed.
So its programmed in the factory, with no system load, then it runs by itself.

Under operation its SCLK SDATA lines would be offset as shown on the chart on page 11 of its datasheet, during typical operating loads.
However again, you shouldn't need to talk to the protector once its programmed.

I agree that on page 4 it does have them hooked up with no isolation. My question was with regards to the post that WM5295 made on June 6 in which he said "Sharing the sense resistor with the gauge connected at the protector requires ground or communication isolation." Is the isolation that he is referring to in this case external to this system altogether? Thank you.

Best 

Sam

The isolation would be in addition to the circuits in figure 2 (from the bq34z100 datasheet) above.  One side of the isolator would connect to VSS (both ICs, ~ battery-), CLK, DAT and an appropriate power of the gauge.  With the protection FETs in the low side switching between the sense resistor and PACK-, the other side of the isolator operates with PACK-, the external world clock & data lines, and an appropriate external power supply.

This lets the external clock & data go above & below the protector and gauge VSS (battery- potential) when the pack is protected without damaging the gauge.

The gauge and isolator (internal side circuitry) remain as a load on the battery after the battery protects.

That answers my question, thank you.

Hi

I have been reviewing Bq77908A or Bq77910A model.

There are a few questions.

1) LiFePO4  is applied Bq77908A or Bq77910A model ?

2) How much is possible detection about the maximum of Over Current  in Bq77908A or Bq77910A model ?

3) Are there any concerns about the future discontinued of Bq77908A or Bq77910A model ?

Please, I want a quick answer if possible.

Hi

I want to get too a reply about three questions for the bellow.

Please send  to the reply quickly if possible.

I have been reviewing Bq77908A or Bq77910A model.

There are a few questions.

1) LiFePO4  is applied  in Bq77908A or Bq77910A model ?

2) How much is possible detection about the maximum of Over Current  in Bq77908A or Bq77910A model ?

3) Are there any concerns about the future discontinued of Bq77908A or Bq77910A model ?

1. Yes, if you look at the datasheet OV and UV ranges, these will support normal values used for LiFePO4.  See the ('910A) datasheet pages 41 and 43.

2. Overcurrent threshold can be set as high as 500 mV in specific steps, the range will also affect the SC choices.  See the datasheet page 47.  Current will depend on the sense resistor used, and be aware of tolerance in the threshold and sense resistor. Be aware of the recommended range and be sure to stay within the absolute maximum values for the sense pins.

3. No.

Hi

Thank you very much for your response

There are additional questions about BQ77910A.

1)   Shunt power consumption of a model in development progress is about 90W.

      So, we configured about 90W in parallel combination of 14ea with Shunt 2mOhm_7W.

               (Total resister is 0.14Ohm.)

      To detect over_current 300A, The BQ77910A detected 300A currents by a combination of the above possible?

2)    The USB Interface Adapter of BQ77910A EVM B/D with USB Interface is used for the Bq78pl116 equal?

   Thank

1. 0.14mOhm x 300A gives 42 mV.  This is in range of the bq77910A, however:

The available OCD selections near this value are 35, 40 and 45mV.  Be sure to consider tolerances.

The losses in such a large array of resistors and high currents may be significant.  Route your sense signals from a point where they do not carry load current, and test the effectiveness of the design.

2. Yes, the bq77910A uses the same USB-to-GPIO adapater and 2.0.19 firmware version as the bq78PL116.