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Original question:

UCC15241-Q1: BLDC driver design

UCC15241-Q1: Single supply for BLDC driver using bootstrap diode+capacitor not recommended. Why?

Florian Sonnenberger
Prodigy 40 points
Part Number: UCC15241-Q1

Tool/software:

Hello,

I've read in a different thread that using this part to power all 6 transistor gates in a BLDC driver is not recommended if the power for the high-side FETs is level-adjusted using a bootstrap diode+capacitor.

https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/1457611/ucc15241-q1-bldc-driver-design

Unfortunately no reason was given why this is not recommended, other than using dedicated DCDC-converters is "much simpler".

However it also takes up a lot more board space and the part cost is also quite a bit more than the diode+capacitor solution.

Could somebody please give reasons why this is not recommended for this part and what problems one would run into using this part with a bootstrap circuit for the high-side FETs?

I guess one downside would be that the part can only be used in single output configuration (VDD-VEE)?

  • +1
    TI__Mastermind 33115 points

    Florian,

    Thanks for the question and I understand the desire to bootstrap the bias for the high-side using a single UCC15241-Q1 dedicated to the low-side only. Most of the reasons for not using a bootstrap circuit are related to dual, +VDD/-VEE output and the challenges that come as a result. For single output only, the bootstrap implementation becomes more feasible but still is not without it's issues, mostly related to the UCC15241-Q1 itself

    1. During startup, the time it takes to charge the upper bootstrap cap could exceed the soft start timeout of the UCC15241, resulting in soft start time out fault. The bootstrap circuit needs some time for both voltage rails to settle
    2. When the bootstrap cap is dynamically loaded or during transient conditions, the capacitor dV/dt can result in UVP protection enabled
    3. The bootstrap circuit needs a low-side pulse to charge the high-side bootstrap cap. Considering the minimum duty cycle and minimum on time, is the primary side PWM a reliable gate pulse to charging the bootstrap cap? 
    4. The UCC15241 has several protections related to time (like soft-start timeout) and voltage (like voltage regulation OVP/UVP) - if these can be dealt with appropriately, along with 1-3 above, the single output bootstrap circuit can be made to work.

    Please let us know?

    Regards,

    Steve