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UCC27211: Bootstrap supply questions

Mark Barnes
Prodigy 70 points
Part Number: UCC27211
Other Parts Discussed in Thread: SN6505B

Good morning!

Please could you help me with a couple of questions about the UCC27211:

1. Is there an equation to determine the minimum switching duration & frequency required to keep the bootstrap capacitor charged for a given gate capacitance?

2. I have an application where the bridge output will be static for long durations of time. I'm intending to feed the HB pin with an isolated 12V supply (ground referenced off the bridge supply voltage). I believe the internal blocking diode in the UCC27211 makes this safe to do without any additional external parts?
(I've removed the bootstrap cap and added a decoupling cap on the HB pin)

Kind regards,

Mark

  • 0
    TI__Expert 3355 points
    Hi Mark,

    Thank you for your question. I'm an applications engineer in the high power drivers group and will help you with this question.

    1. There are two portions of the Cboot charging cycle which should be taken into consideration: the initial charge up, and the recharging between individual cycles. Typically, we recommend a value where Cboot>10*Cgate, so ensure proper gate switch and hold up during high side cycles. The initial low side switching cycle should be long enough to charge Cboot, the time being dependent on Rboot*Cboot time constant. A safe value for initial low side pulse would be 5*Rboot*Cboot.

    Minimum switching duration and frequency are highly dependent on leakage current from the output during high side switching cycles. Typical designs will have a high value (~5k) HO-HS resistor which will slowly drain Cboot. The maximum on time will be set by the UVLO limit for HB-HS (recommended min = 8V). Cboot should be replenished with a low-side cycle before this level is reached to avoid UVLO kicking in and disabling HO.
    And example calculation assuming VHB-HS=12V, Cboot=0.1uF, RHO-HS=4.7k would be as follows.
    Vuvlo=VHB-HS*e^(-Ton(max)/(RHO-HS*Cboot))
    8V=12V*e^(-Ton(max)/(4.7k*0.1uF))
    Ton(max)=190us
    If there are other sources of leakage current, then this maximum on time will go down, so it is best to derate the value for a robust system.

    2. If I'm understanding this correctly, you intend to supply HB with an isolated supply, raising it +12V in reference to Vbus. This method won't work for this device. Maximum HB-HS voltage = 20V as seen in section 7.1. If HB pin is held +12V in reference to Vbus, this spec will be violated when HS is pulled to GND, as HB-HS will equal Vbus+12V. If Vbus>8V, then Vbus+12V>20V, and the device will be damaged.
    An alternative method would be to place the isolated supply between HB-HS and keep Cboot in place. There are two things to be aware of when using this method. The floating supply should be capable of maintaining regulation with its negative pole being referenced to HS, which quickly switches between Vbus and PGND. Also, if VDD > Vfloat, the floating supply should have a reverse current blocking capability to prevent VDD from shorting through the supply. This could be as simple as a diode with its cathode tied to HB, anode tied to the Vfloat.

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

    If not, feel free to ask more follow-up questions.

    Best Regards,

    John
  • 0
    TI__Expert 3355 points

    Also, please reference this thread and response from my coworker, Jeff, on a recommendation for 100% duty cycle operation on a similar half bridge driver.

    https://e2e.ti.com/support/interface/industrial_interface/f/142/t/701944

    Thanks,

    John

  • 0 in reply to John Geiger
    Prodigy 70 points

    John,

    Thank you for your extremely informative response, you've just saved me a lot of frustration and saved some UC27211's from death with your comment about the isolated supply! Schematic has now been changed to the new reference point and a diode added.

    It's particularly handy that I can keep the bootstrap capacitor in place, as per reference design, as some of my boards will be switched at 500kHz (hence no supplementary isolated supply needs to be populated on these boards) and the others will be switched at variable DC-400Hz (hence I'd populate the supply and keep the bootstrap cap in place).

    I'm intending to use a NXJ1S1212MC-R7 for my isolated supply. I'm not particularly experienced with that part so I'll have to evaluate it's performance when it's attached to a quickly switching reference. I'll have to have a think about the SN6505B solution as it's a surprisingly expensive IC (also slightly overkill in current capability) and it'll only supply an unregulated 5V it seems whereas I really want to be driving 12V for the small gain in overall efficiency.

    If you have any comments about my approach, please let me know, in any case I'll certainly be accepting your excellent answer as the solution.

  • 0 in reply to Mark Barnes
    TI__Expert 3355 points
    Hi Mark,

    Glad I could be of assistance!

    I would think that module should be okay as the decoupling caps should handle the high transient current during switching. As a side note, however, I think the SN6505B solution allows you to use a higher turns ratio transformer to achieve a higher voltage output.

    I think you're using the correct approach thus far, please feel free to come back and ask more questions any time.

    B/r,
    John
  • 0 in reply to John Geiger
    Prodigy 70 points

    The vote of confidence is appreciated John, thank you.