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DRV8432EVM: Capacitive loads

Michael Hollerer
Prodigy 40 points

Part Number: DRV8432EVM

I'm rather new to this field and want to apologize in advance for possibly stupid questions.
I want to use the motor driver on a capacitive load at low frequencies (~100Hz) in (R&D in BioTech).

- In full-bridge mode the manual states that two PWM signals control each full bridge - meaning that I can switch from positive to negative polarity and to a 0V state (Both PWM signals low)? Would the capacitive load discharge in this state or would?
- Will the initial current peak be also limited to 5A (one full bridge) for transient pulses (~ns-charging of capacitance)?    (would a big bulk-capacitor effect this in any way?)
- Will the quality of the flank deteriorate in parallel mode?

Currently we are using a self-assembled H-Bridge with an HIP4081A Bridge driver, a 470uF bulk capacitor and IRF520N FETs.
Unfortunately, the rise time is rather slow ~50ns when changing polarity and we observe a strong oscillation (~10% of V, period~150ns) at the voltage build-up within this setup. It seems that both features (rise-time and signal quality) are crucial to our application. After extensive search in the web I concluded that motor-drivers like the DRV8432EVM Evaluation board should fit our purpose, but as i'm a layman in that topic I might be going down the wrong road.

So I'd be greatful for any comment on whether this Evaluation board meets my criterias and whetherit is suitable for such an application.

Thanks,
Michael

  • 0
    TI__Mastermind 37683 points

    Hi Michael,

    Thank you for posting to the motor drives forum.

    Let me research this and I will get back to you by 10/04 or earlier.

  • 0 in reply to Pablo Armet
    TI__Mastermind 37683 points

    Hi Michael,

    If you are driving a purely capacitive load, then the driver will most likely trigger an over-current protection (OCP) fault due to the large inrush current. If the inrush current is 9.7A for around 250ns, then the OCP will be triggered. 

    It is recommended to drive an LC filter rather than a purely resistive load to reduce the high inrush current (similar to the application diagram shown in Figure 17 of the DRV8432 datasheet). Will driving an LC filter work for your application?

    If you require a higher current rated device, then the DRV8874EVM is another option you can take a look at.  

  • 0 in reply to Pablo Armet
    Prodigy 40 points

    Thank you for your response Pablo.

    At the moment we don't wanna apply an LC filter, though it will become interesting in the near future.
    Is there an alternative to the DRV8874EVM featuring a higher output voltage then 37V? We would need smthg in the range of 50+V.

    In general a low rise time, high transient current rating and high output voltage are priority for our application.

    Thanks a lot,
    Michael

  • 0 in reply to Michael Hollerer
    TI__Mastermind 37683 points

    Hi Michael,

    The only 50+V driver options are the DRV8412 and DRV8432. 

    What is the transient current that you need? I think that will be the biggest decision factor on whether  these two drivers can be used in your application.

  • 0 in reply to Pablo Armet
    Prodigy 40 points

    Hi Pablo,

    thanks for the suggestions. We will now start out with the higher current rated device - the DRV8876 module - and implore the other possibilities later.

    One last question regarding the operation of the DDRV8876 Evaluation Module, I would like to apply a bipolar Square Wave to one load.
    Can this be done by directly adressing IN1 and IN2 (with a signal generator) and bypassing the input of the MSP430™ microcontroller which is controlled by the potentiometers? Will there be a delay in transition between positive and negative polarity?

    I didn't find anythin on that in the user manual.

    Thanks for all your effort,
    Michael

  • 0 in reply to Michael Hollerer
    TI__Mastermind 37683 points

    Hi Michael,

    Did you mean the DRV8874? The DRV8876 is the lower current device.

    You can connect an external signal generator to the IN1 and IN2 pins on the EVM and bypass the MSP430 micro-controller. However, you need to remove the 0ohm resistors tying the IN1 and IN2 signals to the microcontroller.

  • 0 in reply to Pablo Armet
    Prodigy 40 points

    Thanks Pablo,
    this helped a lot!

    Michael