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UCC21222: Gate resistor power calculation

Suresh R
Intellectual 260 points
Part Number: UCC21222

Hi,

I want to clarify below two points.

1) I want to know how to calculate the the power dissipation for gate series resistor.

I did as below.

Gate Voltage(V): 12V

Gate series resistor(R): 16.5R

Mosfet Total Gate charge (Qg): 331nC

Switching Frequency(F): 12kHz

Gate current: 

V/R = 12/16.5 = 0.727 A

Gate voltage rise time: 

Q=I*t

Gate voltage Rise Time (tr) =331nC/0.727 = 455nS

R16.5 (power dissipation) = V*I = 12*0.727 = 8.7W for 42uS for 50% duty cycle of 12kHz

Is this correct?

2) Can i consider the pulse power limitation for meeting the power requirement in above case. But its withstanding limit only for short period of time 8000Hrs mentioned in datasheet (refer below link, page 5 continuous pulse power)

http://www.vishay.com/docs/20043/crcwhpe3.pdf

Please advice how to go about with the Gate resistor selection.

Thanks & Regards,

Suresh.

  • 0
    TI__Expert 3355 points

    Hi Suresh,

    Thank you for your question. Good to talk to you again.

    It is also important to consider the effect of the gate driver source/sink impedance. You can find these values in the electrical characteristics table of the datasheet. These will further limit the peak current. Since the gate driver is charging up a capacitive load, it will not necessarily source 0.717A continuously for the full 455ns of the charge time, either.

    Typically, these resistors can handle the pulse power rating of a gate driver just fine, since it is for durations <1us. I would primarily focus on making sure the resistor can handle the rms power dissipation that the system requires. Based on my calculations, the entire gate driver system will only dissipate ~6mW since it is switching at such slow speeds. This should be fine with any of the package sizes in that resistor datasheet. If necessary, you can always add parallel resistors to spread the power dissipation out across multiple resistors.

    If this answered your question, could you please press the green button? If not, feel free to ask more questions.

    Thanks and best regards,

    John

  • 0 in reply to John Geiger
    Intellectual 260 points

    Hi John,

    Nice taking to you.

    Can you tell the formula by which we can calculate the gate driver power dissipation and Gate resistor power dissipation so that I can calculate for my application. Its very general in your description above.

    Regards,

    Suresh.

  • 0 in reply to Suresh R
    TI__Expert 3355 points

    Hi Suresh,

    Please refer to section 9.2.2.6 in the UCC21222-Q1 datasheet for formulas to calculate power dissipation in the gate driver. You can use the calculations in this section to infer the power dissipation in the gate resistance as well. Simply subtract the gate driver power dissipation from the total power dissipation.

    If this answered your question, could you please press the green button? If not, feel free to ask more questions.

    Thanks and best regards,

    John