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UCC27526: Driving two CSD19538Q2 with UCC27526 - finding gate resistor

Andrej Pajnic25
Prodigy 60 points
Part Number: UCC27526
Other Parts Discussed in Thread: CSD19538Q2,

Hello,

I need to drive two CSD19538Q2 which need to make switch ON and OFF as fast as possible with pulse duration of 60ns. Pulse duty cycle is around 0,1% and MOSFET Vgs is 38V with pulsed Ids is max 30A.

Now, I want to drive those MOSFETs with UCC27526 but have no idea, what Rgate should be (resistors between driver output and MOSFET gate).

Will you be so kind and help me with this issue? Thank you.

With best regards,

Andrej

  • 0
    TI__Genius 13445 points
    Hi Andrej,

    Thanks for reaching to the High Power Drivers group, my name is Mamadou Diallo, I am the AE supporting low-side drivers.
    I think you mean the MOSFET Vgs = 3.8V (max) from looking at the datasheet.

    Please refer to the attached app note discussing how to properly select the gate resistance to achieve the desired rise/fall times.

    www.tij.co.jp/.../slla385.pdf

    Please let us know if you have further questions or press the green button if this addresses your question.

    Thanks.

    Regards,

    -Mamadou
  • 0 in reply to Mamadou Diallo
    Prodigy 60 points
    Dear Mamadou,

    I am aware of this application note but must admit, that it is written too complicated. And on the other hand, it is with lot of "if it is small" or "too big" and so on, but without clear example.

    Let me start with the sentence on page two: "External gate resistors may not be required if a MOSFET or IGBT's internal gate resistance is large enough." OK. I see. But what does it means "large enough"? Which number represent "large enough"? And on the other hand, datasheet for MOSFET says, that Rg is typically 4.6Ω and max 9.2Ω. Should I use 4.6Ω or number, that is as twice as high?

    I think that it would be fine to add this design to TI's Webench or to have application note with exact path explained how to do this calculation. I guess that excel sheet can do the job too. And for sure I am not the only one who is lost with this AN... :-)

    With best regards,
    Andrej
  • 0 in reply to Andrej Pajnic25
    TI__Genius 13445 points
    Hi Andrej,

    Thanks for the follow up questions.

    The value of the external gate resistance will vary application by application reason why we don't specify any specific numbers. Some applications (switch mode power supply for example) where fast turn-on/off is required, these applications would want to minimize this resistance anywhere between 1-20-Ohms (Rule of thumb). A smaller gate resistor value increases the switching speed, and helps improve efficiency, while the trade-off is possible EMI the circuit.

    It looks like the total gate charge is 5.6nC/FET (11.2nC for both parallelled FETs) which is small load. For your specific application needs, assuming you're using a single turn-on/off path, I would suggest starting off anywhere between 2-20 Ohms gate resistance and fine tuning the gate resistor to your desired rise/fall times. (The tech note I shared with you uses 153nC max gate charge).

    If you're using separate turn-on and turn-off paths, you would want to size both Ron (resistance on turn-on path) and Roff(turn-off path) separately to achieve your targeted rise and fall times. In this case, you would want to reduce Roff as much as possible (0 to 5 Ohms) while tuning the Ron portion. (1 to 20 Ohms). A separate turn off path is provided using a diode in series with the smaller value resistor Roff, to get asymmetric turn on/off speeds.

    Please let us know if you have further questions or press the green button if this addressed your question.

    Thanks.

    Regards,

    -Mamadou