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TPSI3050: Isolated Gate Drive for BLDC Braking

Dave Kearney
Prodigy 85 points
Part Number: TPSI3050
Other Parts Discussed in Thread: TPSI3052-Q1,

Hi, apologies if any of my questions have been answered previously but I couldn't see any direct links.

I'd like to drive 3x FETs to short the phases of a BLDC with the TPSI3050. The combined gate capacitance of all three is 12nF and combined gate charge is ~120nC (at the TPSI3050 10V drive voltage). They will be controlled via duty cycle of a 1kHz PWM and I would expect to see currents up to 25Apk-pk through the motor when braking the FETs fully ON.

First off does this sound reasonable? Using a basic t=CV/I calc, can I estimate that the rise time (based on the TPSI peak drive current of 1.5A) will be ~12nF*10V/1.5A = ~80ns? (Best case most likely due to any losses in the circuit which might limit I) And that given a PWM of 1kHz (and thus a cycle time of 1ms), I should have plenty of time to recharge the VDDH/VDDM caps between each switch of the FETs? The calculator tool suggests that with Cdiv1, Cdiv2 = 500nF each, I would see 0.492V sag max on VDDH and that even at the lowest PXFR setting (7.32kR), the rail would still recover fully after 1.51.5us (allowing a max swithing speed of 6.6kHz). Am I interpreting all that correctly?

Is there any advantage of increasing the PXFR value then in this case? It's not going to give me any faster switching times right, it's just going to allow the circuit to recover more quickly?

Is there a reason why it drives 10V and not 5V? When FETs tend to have a Vgs(th) lower than 5? Is it to minimise Rds(on)? Does this increase risk of overshoot damaging say a FET with VGSMAX of +/-12V?

Also, this seems to be the only dedicated isolated gate driver with integrated isolated supply on the market and seems like a great solution for space constrained designs, any idea why all the other vendors require an external isolated supply?!

Many Thanks in advance,
Cheers,
Dave

  • +1
    TI__Expert 6526 points

    Hello Dave,

    Thanks for reaching out to our team on E2E! Your math and interpretations all look correct to me, it seems you've put a lot of thought into this design. 

    Dave Kearney said:
    Is there any advantage of increasing the PXFR value then in this case? It's not going to give me any faster switching times right, it's just going to allow the circuit to recover more quickly?

    This is correct. The only benefit I can think of is if you needed a faster tSTART, which is only relevant for the initial power-on. The actual time to turn the FETs on or off does not improve with higher RPXFR.

    Dave Kearney said:
    Is there a reason why it drives 10V and not 5V? When FETs tend to have a Vgs(th) lower than 5? Is it to minimise Rds(on)? Does this increase risk of overshoot damaging say a FET with VGSMAX of +/-12V?

    I think our systems team looked at the MOSFET market and selected VDRVs (10-V with TPSI3050-Q1 and 15-V with TPSI3052-Q1) to provide the most coverage. You're correct, the primary benefit of driving a MOSFET at or near VGS_MAX is to maximize enhancement which minimizes RDSON power loss. Our TPSI305x-Q1 devices have a shunt to clamp VDDH and prevent overvoltage.

    Dave Kearney said:
    Also, this seems to be the only dedicated isolated gate driver with integrated isolated supply on the market and seems like a great solution for space constrained designs, any idea why all the other vendors require an external isolated supply?!

    I'll share your kind words with our systems team! I imagine it was simpler to initially create an isolated gate driver without integrating the power transfer so many vendors did that. After receiving feedback from customers, we saw the demand for an isolated gate driver with integrated power transfer, and developed the TPSI305x-Q1. 

    Also noticed your company provides a drone delivery service. I'm under the impression this is for a drone project - please keep us updated on this development. I think drones are very cool  Slight smile. 

    Best regards,
    Tilden Chen

    Solid State Relays | Applications Engineer