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BQ77915: Multiple mosfets question

Hugo Feng
Prodigy 20 points
Part Number: BQ77915
Other Parts Discussed in Thread: CSD18502KCS, CSD18535KCS

Hi,

I'm trying to design a BMS using the BQ77915 for a 3S lithium ion pack with an intended 30A continuous current.

Is it possible to substitute CSD18535KCS for the CSD18502KCS in figure 6. of http://www.ti.com/lit/an/slua773/slua773.pdf without any modifications?

Can I use the hibernate feature tied to a switch for general power off / emergency shutdown?

Also with 4 mosfets for discharge, would heat be an issue inside a submerged aluminium housing?

Kind regards

Hugo

 

  • 0
    TI__Guru 54715 points
    Hi Hugo,
    1. The CSD18535KCS should work very much like the example in the application note, it will have some different characteristics so switching may vary but the basic function should be similar.
    2. Hibernate is a slow operating function with the 4.5 s typical tPRES_DEG_ENT delay. It should be fine for a general power off but you may want something faster for an emergency shutoff.
    3. 4 MOSFETs for discharge will spread the heat between the devices. It looks like you are considering low RDSON devices to minimize the heating in the FETs. A submerged aluminum housing sounds like it would have good heatsinking properties, but you will need some study and design to get heat transfer from the FETs to the housing.
  • 0 in reply to WM5295
    Prodigy 20 points

    Thank you for your reply.

    Just a couple of things before I order the all the parts.

    There shouldn't be any issue with the hibernate switching the full current off? 4.5 seconds is acceptable.

    How much heat will there be? I assume it's ((current² x rdson / total fets ) since it's just switching on and off.

    (30² x 0.002) / 4 = 0.45w is that correct?

    Should be fine even with no heatsink and just convection to the housing?

    Kind regards

    Hugo

  • 0 in reply to Hugo Feng
    TI__Guru 54715 points
    Hi Hugo,
    The normal expectation is that the batteries are used within their acceptable range and protection never trips. If current shares evenly your calculation looks correct. MOSFETs tend to adjust RDSON to share current, so it should be a good assumption, check your FET design references. You must do a thermal analysis to see how much the housing will heat.