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UCC21520: Bootstrapping and Gate turn on off Resistors

Part Number: UCC21520

 I want to use 

UCC21520DW

for a typical halfbridge application. Maximum Gate charge for my MOSFETs is 45nC. Max. Duty cycle is 95% and minimum is 5%. VCCI is 12 V, INA and INB is 3.3 V, for MOSFETs side i am using 12V 2W isolated dc dc converter. After calculation and considering in voltage ripple  Bootstrap cap value come up with 800nF. i have decided to choose 1uF cap for this application.

Question:  1- will there be any kind of duty cycle limitation issue regarding bootstrapping for high side MOSFETs?

2- I have chosen 18 ohm turn on resistance and 9 ohm for turn off. is that fine?

MOSFETs Blocking voltage is 600v. 
I can provide further information if needed.

  • Hi Muhammad,

    Thank you for your interest in UCC21520DW. I'm an applications engineer with TI High Power Drivers group and can help review this.

    Can you tell me the desired switching frequency of your system? This will help me to better answer the following questions.

    1. Will there be any kind of duty cycle limitation issue regarding bootstrapping for high side MOSFETs?

    The bootstrap capacitor charges during the low-side's "on" period, when the switch node is brought to ground. This means that the worst case, minimum, low-side pulse width should be adequate for bootstrap charging. This is a minimum low-side pulse width is product of duty cycle and switching period. 

    2. I have chosen 18 ohm turn on resistance and 9 ohm for turn off. is that fine?

    Output resistors are sized to dampen output overshoot and undershoot by limiting the dV/dt on the output, however, larger output resistance will slow down the on/off transition time of the power FETs. These on/off times should be considered when designing for the efficiency of your system, and should be small compared to the switching period of your system. The 18 ohm-on and 9 ohm-off resistors should allow for fast switching, though the driver is capable of supplying and sinking more current if the design allows - which would minimize losses during on/off transitions.

    Regards,

    John

  • Hello Mr. John,

    Switching frequency is 30kHz.
    Low side MOSFET minimum duty cycle is 5 %.
  • Hi Muhammad,

    Thanks for the information.

    My estimates show that CBT should charge to approximately 8.8V considering the worst case initial low side pulse. This should be adequate to switch the high side FET before the next low-side cycle. If you find need a faster charge time for CBT, consider lowering the RBT value to shrink the time constant of the system.

    The Ron and Roff resistors should also be adequate in this case. My estimates show the high side output t-rise to be about 100ns, and high side output t-fall to be about 25 ns. Once the system is up and running, these values can be decreased and tuned to allow faster switching and a more efficient system overall.

    If this answered your questions, could you please press the green button?

    Regards,

    John