TIDA-010208: BMS Design

Umang Butani

Part Number: TIDA-010208
Other Parts Discussed in Thread: BQ76972, TIDA-010247, XCSD19536KTT, CSD19536KTT

Tool/software:

Hello,
As we are designing on BMS system using BQ76972 IC with taking reference from TIDA-010208.

Can anyone help us to solve some of below query!
1. For Number of MOSFET, how one can identify how many mosfet will require for charging as well as discharging?
For our application we are using ~40V battery, charging current is ~45A and Discharging current is approx 90A.
Can you help to provide any calculations or question for this?
2. For DSG signal, it's understandable that, Q28 will be provide PACK+ at gate to turn on, but can anyone help to identify what will be the application of Q29 and Its supporting circuit?

Best Regards!
Umang

over 2 years ago

0 Kian Lin over 2 years ago

TI__Expert 4500 points

Hi Umang,

See my answers below:

You should consider the thermal and voltage stress when you design the protection FETs circuit. Your pack is 40V and max continuous current is ~90A. So you may need >80V Vds MOS and calculation the conduction loss based on the Rdson you choose.
this is reverse protection cicuit you can find more details at https://www.ti.com/lit/an/sluaa09a/sluaa09a.pdf?ts=1725266468701&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FBQ76952%253FkeyMatch%253DBQ76952%2526tisearch%253Duniversal_search%2526usecase%253DGPN-ALT

BRs

Kian

0 Umang Butani over 2 years ago in reply to Kian Lin

Prodigy 160 points

Hello, Kian
Thanks for your answer, sluaao9a is good read.

Based on figure 5.1, there are connection of 12 FET on single CHG and DSG pins.
Do you suggest to use any mosfet driver IC or BQ76972 IC's pin will be sufficient to drive all the mosfets?

Also, We will use BQ7697202 part, Based on datasheet it will come with REG1 LDO config as 3.3VDC @ 45mA,
So when BAT pin will have supply voltage from 12V to 40V, it will provide 3.3VDC at REG1 Pin. We do not have to do any configuration for this. Is it right interpretation?

Best,
Umang Butani

0 Kian Lin over 2 years ago in reply to Umang Butani

TI__Expert 4500 points

Hi Umang,

Umang Butani said:

Based on figure 5.1, there are connection of 12 FET on single CHG and DSG pins.
Do you suggest to use any mosfet driver IC or BQ76972 IC's pin will be sufficient to drive all the mosfets?

It is based on the ciss of MOSFET you select. We ever tested 8*9600pf MOSFET and only used BQ76972 IC's pin. But should add some circuit for fast turn on. Pls refer to TIDA-010247.

Umang Butani said:

Also, We will use BQ7697202 part, Based on datasheet it will come with REG1 LDO config as 3.3VDC @ 45mA,
So when BAT pin will have supply voltage from 12V to 40V, it will provide 3.3VDC at REG1 Pin. We do not have to do any configuration for this. Is it right interpretation?

Correct.

BRs

Kian

0 Umang Butani over 2 years ago in reply to Kian Lin

Prodigy 160 points

Hello Kian,
Appreciate your quick response,

As we are planning same to use 8 xCSD19536KTT FETs for each channel, Do you recommend to add these type of fast turn on circuit for both CHG and DSG pins?
Also, If we have to add provision for driver, Any driver part or series will you recommend?

Thanks,
Umang

0 Kian Lin over 2 years ago in reply to Umang Butani

TI__Expert 4500 points

Hi Umang,

This circuit is recommended if use 8*CSD19536KTT.

About gate driver, I think isolated gate driver can be used. You can find similar question here: https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/1372378/ucc23511-q1-using-isolated-gate-driver-to-drive-fet-s-in-high-side-back-to-back-configuration?tisearch=e2e-sitesearch&keymatch=back%252525252520to%252525252520back%252525252520FET#

BRs

Kian

0 Umang Butani over 2 years ago in reply to Kian Lin

Prodigy 160 points

Hello Kian,

Can you help us to provide current sense resistor calculation guide?

Based on datasheet, maximum differential voltage at SRP and SRN can reach upto 1.8V only.
Based on this,
V = 1.8V, Differential voltage at ICs pin

I = 90A, maximum discharge current

results will be R = V/I ~ 0.02 ohm and P ~ 162W?

I am messing sothering here! Can you please guide us to properly calculate Rsens value!

Regards,
Umang Butani

0 Kian Lin over 2 years ago in reply to Umang Butani

TI__Expert 4500 points

Hi Umang,

Sorry for the late reply. I was on timebank last week.

You don't have to consider the max voltage on SRN/SRP pins. the max value shows you the transient voltage it can withstand.

The recommended range is:

But we do not use full scale range when we need large discharging current. You should calculate the power with equation P=I^2 * R. If you need 90A max discharge current, you may need choose 0.3-0.5mohm shunt based on your thermal design. It will generate 2.43-4 watt power loss and can also guarantee good accuracy.

BRs

Kian

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TIDA-010208: BMS Design