The TI E2E™ design support forums will undergo maintenance from July 11 to July 13. If you need design support during this time, open a new support request with our customer support center.

  • State TI Thinks Resolved
  • Locked Locked
  • Replies 7 replies
  • Answers 2 answers
  • Subscribers 30 subscribers
  • Views 424 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

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.

DRV8770: Bootstrap capacitor above 1uF

Joshua Coe
Prodigy 70 points
Part Number: DRV8770

We are trying to design a system with one high frequency leg and one low frequency leg. Using the calculations we are getting about 3uF boostrap capacitor for the 60Hz leg. I know that the bootstrap diode is internal and that it has a dynamic resistance, but it also has a disclaimer in the gate drive circuit to limit current if cap is above 1uF. I am wondering if I was to add a bootsrap resistor where would it go. Other circuits appears to be between the diode and VDD, however for the integrated diode I would have to put it between VDD and the gate drive chip. I was not sure if this would effect the other gates as I do not have the simulation for this driver. I was also thinking it might be able to go between the diode and the bootstrap capacitor, but I'm not sure how this would affect the performance of the gate/switching as it would be in the discharge path as well as the charge path. 

  • 0
    Genius 5040 points
    Joshua Coe said:
    Using the calculations we are getting about 3uF boostrap capacitor for the 60Hz leg. I know that the bootstrap diode is internal and that it has a dynamic resistance, but it also has a disclaimer in the gate drive circuit to limit current if cap is above 1uF. I am wondering if I was to add a bootsrap resistor where would it go.

    Hi Joshue,

    Do you need 3uF cap for driving large power FETs with high gate current? If yes, then why thinking of adding resistor to limit the diode current which charge the 3uF cap? You want more output but doesn't want more input current? 

    With 3uF cap, you cannot use the internal diode and forget about the series resistor to limit the diode current.

    What is your GVDD? If it is less than 20v then I would use an external Schottky diode to charge the 3uF cap with GVDD. Note: this drive spec to have bootstrap up to 20v max, but most FETs have max gate voltage of 19v or less, so be careful. 

    Brian

  • 0 in reply to Brian Dang
    TI__Mastermind 37683 points

    Hi Joshua,

    I agree with Brian's comments. 

    Joshua Coe said:
    We are trying to design a system with one high frequency leg and one low frequency leg

    I may not know this terminology but what do you mean high frequency and low frequency leg? Why the need for the 3uF capacitor.

    Regards,

    Pablo Armet

  • 0 in reply to Pablo Armet
    Prodigy 70 points

    We are trying to design one leg to be switching at 10-20Khz, while the other will be switching at 60Hz. Our gate charge is small on the mosfets so we don't necessarily need a lot of current, however for slow switching frequency we needed a larger capacitor. The gate driver has a disclaimer that over 1uF precautions need to be taken to limit the current (I believe through the diode). Would adding the entire diode and Rboot externally be a better choice then? I was thinking just adding an Rboot would fix the issue, however I don't really know where to add it as the diode was internal.

  • 0 in reply to Pablo Armet
    Prodigy 70 points

    Since the bootstrap calculation in the application notes is dividing by FSW we needed a larger cap due to being only 60Hz.The Gate drive VDD is 9V.

  • 0 in reply to Joshua Coe
    TI__Mastermind 37683 points

    Hi Joshua,

    The image below is the appropriate placement for the RBOOT. The resistor can be placed AFTER the CBST (near BST pin).

    Regards,

    Pablo Armet

  • 0 in reply to Joshua Coe
    Genius 5040 points
    Joshua Coe said:
    The gate driver has a disclaimer that over 1uF precautions need to be taken to limit the current (I believe through the diode). Would adding the entire diode and Rboot externally be a better choice then? I was thinking just adding an Rboot would fix the issue, however I don't really know where to add it as the diode was internal.

    Adding a resistor in series with the boot cap will limit the diode current, but it also invalidate the Equation 3 for Qtotal as the ILBS_TRAN is is no longer 220uA.

    With the resistor, I think the cap will not be fully charged at pwm higher duty cylce -- too little time during GLx=H to charge the cap with series resistor. You might need to use external diode for this application.

    Btw, the datasheet should point out that PWM must be less than 100% duty cycle, but it says nothing on this. A boobytrap. 

    Brian

  • 0 in reply to Brian Dang
    Prodigy 70 points

    Thank you for your help! We are planning on doing 50% duty cycle for the slow speed which is the leg that needs the higher cap. The fast speed will be 5%-95% at around 20Khz. We were using .54nF for that so should still have plenty of time to charge. I tried putting the Rboot at the GVDD with the evaluation board as I didn't see Pablo's response at the time, but it seems to be working well with the eval board. Our mosfets are different though so not exactly apples to apples comparison. My co-workers liked the idea of an external diode so I think that is what we are doing. Again thank you both for your help!