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UCC28950: Source Resistor Value in Figure 40 of SLUA618A

Conor
Guru 12050 points
Part Number: UCC28950

Tool/software:

Hi,

Please tell me Figure 40 in SLUA618A, specifically regarding the role of the source resistors connected to each FET.

https://www.ti.com/lit/ml/slua618a/slua618a.pdf

Based on my understanding, these resistors are used for ringing suppression, dead-time control, current sensing, and gate drive current limiting

Also could you please provide guidance on the typical resistor values recommended for this circuit?

Thanks,

Conor

  • 0
    TI__Guru* 85616 points

    Hello,

     

    In the past to protect FET drivers from over current resistor were place in series with the gate driver.  Later resistors were used in series with the gate of resistors in series of the gate to reduce ringing at FET turn-on and turn-off.  Q = 1/R*(L/C)^0.5.  The larger the R the smaller the Q.

     

    When I look at the circuit the author drew, he uses a fast turn-off PNP transistor for each FET and put a resistor connected to the FET’s sources and PNPs collectors to the gate drive transformer.  This should reduce both gate driver ringing at FET turn-on and turn-off.  This could also be a current limit the FET turn-on and turn-off current.

    Since most gate drivers today are current limited you would not have size this resistor to protect the gate driver.  However, if you are using an older gate driver without current limiting you would size the R based on the maximum gate drive.  This equation estimates the forward diode drop and Vbe of the transistor is roughly 0.6V.

    R = (Vgate-0.6V)/(Idrive max)

    So how do you size this gate drive resistor now.  Most engineers will typically add roughly 2 to 10 ohms of resistance to dampen possible ringing possible ringing.

    If they see ringing they will increase the R.  If there is not ringing they will not adjust the R.

    Thank you for interest in Texas Instruments (TI) products.  If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

     

    Regards,