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UCC27524: Max number of Mosfets that can driven

Part Number: UCC27524
Other Parts Discussed in Thread: BQ76952, , UCC27624, UCC27614

Hi There,

We are developing a BMS using BQ76952 Chip and we need to drive at least 40 Mosfets using "UCC27524DR" gate driver. Here, one output of UCC27524DR is used to drive 20 parallel mosfets and other output to drive another 20 parallel Mosfets.

I just want to ask, if it would be fine to drive this many mosfets using a single gate driver. HERE is the Mosfet datasheet.

As its a BMS, we expect the mosfets turn off times in nano seconds in case of any faults.

Thanks..!!!

  • Hello Alex,

    Thanks for reaching out!  And thank you for including a link for the MOSFET being used!

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    This is a heavy load for the driver... so you will likely have a slow rise-time.

    For a rough calculation, we can use the capacitor equation:  C = Q/V

    Q = 60 nC  [Remember that gate charge varies with gate-to-source voltage, so this is a worst case]

    V = 12 V   (Assumed VDD = 12V)

    --->    C = 5 nF    (Per MOSFET)

    Times (40 FETs)  --->  200 nF

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    To answer your question, using one driver will not accomplish the turn on/off times you want with 40 MOSFETs.

    You could test this out with an EVM board with our device on it, assuming you have a board with this amount of FETs on it; and externally connect to it.

    UCC27624EVM

    UCC27614EVM

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    You could also reference this Battery application note (page 16), where they use one 5A channel to drive 4 MOSFETs.  This may be a good baseline, but will obviously vary based on what turn-on/off times you are requiring.  You will likely want to calculate or simulate your circuit to see if you have enough drivers for your application.

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    I have a few notes you may want to consider:

    1. How fast do these MOSFETs need to be on?  (how many nano seconds)
      1. Keep this in mind and calculate rise times of your MOSFET gates; or run simulations to figure this out.
    2. With this being a BMS project, I assume this will not be switching often, so thermal concerns should be minimal.

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    Regarding device selection, I would suggest using one of the following (you will likely need several drivers to achieve the turn on/off times you want):

    • UCC27624  -  This is our new generation of the UCC27524.  I suggest using this part, as it is newer and has more robust specifications.  It also has 5A sink/source for each of its two channels.
    • UCC27614 -  This is our new single-channel driver with 10A Source / 10A Sink capability.

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    I hope this helps!

    Thanks,

    Aaron Grgurich

  • Hi Aaron,

    Thank you for the kind and detailed response..!!!

    Based on the total capacitance from you, I have calculated rise time RC = 2.1 x 200nF = 420nS. The value of R = 2.1ohm is taken from datasheet as Gate resistance. Please correct if I am wrong.

    I am fine with the switching times of 400-600nS. The important thing is Mosfets should turn off fast in case of any fault in BMS. So do we have any calculation for fall times just like rise times ?

    You are correct, as its a BMS application the switching won't be too often.

    Thanks..!!!

  • Hi Alex,

    The dynamic pull-up resistance of the UCC27624 is 1.04 Ohms during the 5A peak.  Otherwise the DC pull-up resistance is 5 Ohm (but this is not really relevant here).  The UCC27614 has even lower pull-up resistance with its 10A drive strength.

    Otherwise, yes, that is how you would calculate your RC time constant.

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    In terms of RC time constants, you can assume similar discharge times.  Although, based on RC time constant math, a capacitor initially discharges faster than it charges.  So, in reality, discharge should initially happen a bit faster (assuming same current strength, which it would be for both of the devices I proposed).  I believe Wikipedia (and other websites) have a nice quick summary of calculations and time constants needed for 80% rise times, etc.

    For a rough calculation, you could use:  i = C * (dv/dt)

    To figure out the current needed.  Or you can move variables around to figure out what rise times you can achieve with a given amount of drive current strength. 

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    Also consider using at least some gate resistance (1 to 5 Ohms) on each MOSFET gate to help with potential gate ringing.  Or at least place a short so that other gate resistors can experimented with later.

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    When you get to the layout portion of this project, ideally you will want to keep trace length as close to the same as possible to minimize timing differences so that all the FETs feel the load at the same time.

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    I hope this helps!  Again, I suggest using either the UCC27624 or the UCC27614.

    -Aaron Grgurich

  • Hi Aaron,

    I understood your point..!!!

    I will move further as per your suggestions.

    Thank You..!!!