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enter image description hereI am currently working on one of the BLDC drive applications.Due to higher current requirements it is planned to use multiple MOSFETs in parallel to distribute the current among themselves.

Here I wanted to know whether the gate-drive current from the driver I am using is sufficient or not. Could somebody give hints or procedure to make sure the driver you have chosen is right one? (You may consider example values as Vdriver(max)=12V, Vdriver(min)=0V, I_driver(max)=2A, MOSFET gate charge=150nC, No. of MOSFETs in parallel=3, Switching frequency = 16kHz.

I hope these are the basic parameters to do the analysis. Assume if you need anything else.

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    \$\begingroup\$ Individual gate resistors! See: electronics.stackexchange.com/questions/274557/parallel-mosfets/… \$\endgroup\$
    – winny
    Commented Mar 23, 2017 at 14:15
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    \$\begingroup\$ 16 kHz and all customers less that 50 years old will return your product and ask for a refund... why not 22kHz?... \$\endgroup\$
    – bobflux
    Commented Mar 23, 2017 at 14:45
  • \$\begingroup\$ @peufeu An industrial servo drive, top quality, top price runs at 16kHz very well. The garbage you can buy on ebay can run also at 50kHz. The PWM frequency is not a measure of quality for this kind of product. \$\endgroup\$
    – Marko Buršič
    Commented Mar 23, 2017 at 15:05
  • \$\begingroup\$ Well, I guess if the people in the vicinity all wear ear protection due to machine noise, no-one will notice the 16kHz noise... \$\endgroup\$
    – bobflux
    Commented Mar 23, 2017 at 20:03

2 Answers 2

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Generally when paralleling MOSFETs, it's best to use a separate driver for each FET. Each FET adds more capacitive load, which slows down the edges at the fixed current the gate driver can provide. Since dissipation is a issue, else you wouldn't be using multiple FETs in parallel in the first place, fast gate transitions are probably important.

Each FET will also have a little different gate threshold voltage, so the capacitive coupling from the drain will effect each a little differently. It is better to let each FET do its own thing, working against its own series gate resistor.

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  • \$\begingroup\$ How important is the driver current parameter here? How do we consider for the analysis? \$\endgroup\$
    – IamDp
    Commented Mar 23, 2017 at 15:04
  • \$\begingroup\$ I cannot use different driver pins for different mosfets since all the three mosfets should be turned ON at the same time without significant time difference. So I must drive them from a single gate-drive pin. \$\endgroup\$
    – IamDp
    Commented Mar 23, 2017 at 15:52
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    \$\begingroup\$ @Durg: You look at the total equivalent gate charge, and compute how much current it takes to switch that charge in the time you want the gate to slew the full range. Gate charge tends to go up with higher voltage capability and lower Rdson. It can be significant with power FETs. \$\endgroup\$
    – Olin Lathrop
    Commented Mar 23, 2017 at 15:52
  • \$\begingroup\$ @Durg: you use different FET drivers, but drive them all from the same digital signal. In other words, you tie the inputs of the FET drivers together. \$\endgroup\$
    – Olin Lathrop
    Commented Mar 23, 2017 at 15:53
  • \$\begingroup\$ Clear. But I am afraid the propagation delays involved here could be significant and damage the FETs. \$\endgroup\$
    – IamDp
    Commented Mar 23, 2017 at 15:56
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You would better put a gate resistor for each MOSFET separately. It also depends of the type of the MOSFET used if it has the parallel operation capability. Other things is up to you, for example if the rise/fall switching time is within your specifications. It could be that using small power gate driver with those MOSFETS in parallel you'll get worse result that using a single MOSFET. A switching loss is determined by fall/rise switching time, therefore bigger time, bigger the loss is.

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  • \$\begingroup\$ I am looking for more objective solution to suffice my requirements. And I am not very sure of your point which mentions on the usability of FETs for parallel operation. \$\endgroup\$
    – IamDp
    Commented Mar 23, 2017 at 16:03
  • \$\begingroup\$ @Durgaprasad Then you should expand your question with specification of MOSFETS and gate drivers. \$\endgroup\$
    – Marko Buršič
    Commented Mar 23, 2017 at 20:32

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