• State Resolved
  • Locked Locked
  • Replies 7 replies
  • Subscribers 29 subscribers
  • Views 891 views
  • Users 0 members are here
Support feedback
Options
Options
Related

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.

DRV8312 bootstrap diode current limit

Andrew Kuklinski
Prodigy 60 points
Other Parts Discussed in Thread: DRV8312, DRV8301, DRV8313

Hello,

We are using the DRV8312 in a circuit where it is not possible to charge the bootstrap caps fully during power up using the "internal circuit to charge the external bootstrap capacitors by enabling a weak pulldown of the half-bridge output" as described in the datasheet. As a result, when the low side FET is first turned on, there is a 5V difference between GVDD and bootstrap cap voltage, giving us an instantaneous current approaching 1 Amp through the 5 ohm current limiting resistors that we have put on the GVDD inputs as recommended in the datasheet. I can't find a spec in the datasheet for the maximum allowable current in the internal bootstrap charging diodes. Is there a problem with this amount of instantaneous current through the diodes occurring once every few seconds.

Also, we are running with large bootstrap caps (22uF) because of low PWM frequency requirements. Is the 5 ohm resistor large enough to limit the bootstrap charging current? I ask because the 22uF cap is significantly larger than the threshold of "larger than 220nF" given in the datasheet for using the 5 ohm resistors, exposing the internal diodes to this current for a longer duration.

thank you,

Andrew Kuklinski

  • 0
    TI__Guru 55615 points

    Andrew,

    The diode is not the limiting factor...it is the metal traces in series with the internal diode that are not sized large enough to handle capacitors larger than the recommended 220nF.  

    We have only tested and verified the reliability up to 1uF with the series 5 ohm resistors.  I can't recommend anything greater than this.  

    What frequencies are you trying to support?  What are your current requirements?  

    We have a couple of other options like the DRV8313 and DRV8301 that support lower frequency operation through the use of charge pump circuitry. 

  • 0 in reply to Ryan Kehr
    Prodigy 60 points

    Thanks for the response Ryan,

    We are trying to support typical frequencies during normal operation as low as 1400 RPM with 2 electrical cycles/rotation, resulting in the PWM_x (46.6Hz) signals being held high for over 7 milliseconds. We are designing to a steady state current of 3 Amps, but it can be as high as 7 Amps during startup.

    We are tryign to avoid a complete replacement of this FET driver part for schedule reasons, so I implemented a workaround today: I added an external dual schottky diode part (BAT54S) to one of the the bootstrap capacitors. One of the diodes charges the bootstrap cap from GVDD when the lower FET is on, just like the internal diode. The other external diode prevents the internal diode from charging the bootstrap cap. There is an additional diode drop when the bootstrap cap is holding the upper FET on. However, I noticed that the external diode charges the bootstrap cap up closer to GVDD than the internal diode did. So I actually see a higher voltage on the BST_x pin with the external dual diode solution. Now we can specify the external diode, current limiting resistor and traces to handle any expected current So, if we bypass the internal charging diode completely in this manner, do you see any other problems with running the DRV8312 in this manner?

    Best regards,

    Andrew

  • 0 in reply to Andrew Kuklinski
    TI__Guru 55615 points

    Andrew,

    Interesting solution.  We are currently considering any possible negative effects of this solution and building it up in our lab.  

    By when will you need a final response on the viability of this?

  • 0 in reply to Andrew Kuklinski
    TI__Guru 55615 points

    Andrew,

    I forgot to ask you what your motor voltage is and the voltage you are measuring on the BST_x pin under the external diode conditions.  Please let me know.

  • 0 in reply to Ryan Kehr
    Prodigy 60 points

    Hello Ryan,

    Our motor voltage varies anywhere between 0V and 13.5V.

    Using the external diode, I observe that the votlage on the BST pin charges to about 50 mV of GVDD (+12V) with the external diode. When the bootstrap cap is holding the upper FET on, I see the BST pin is about 300mV below the bootstrap cap voltage.

    We need to know by around Tuesday of next week if there are any potential issues with this solution. 

    Thanks for your help.

    Andrew

  • 0 in reply to Andrew Kuklinski
    TI__Guru 55615 points

    Andrew,

    We have verified with design and tested in our lab using series diode and 5 ohm resistor between GVDD and bootstrap cap.  As did you, we then added a series diode between cap and BSTx pin (opposing direction of internal diode).

    All looks good. 

  • 0 in reply to Ryan Kehr
    Prodigy 60 points

    Excellent news Ryan. Thanks for your support on this issue.

    Best regards, Andrew Kuklinski