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MCT8316Z: Issues with Configuration, Overheat and Waveform

liran vigdor
Prodigy 90 points
Part Number: MCT8316Z
Other Parts Discussed in Thread: DRV8317

Tool/software:

HI I'm using MCT8316Z to drive 5 different types of BLDC motors.

1) Overheating - 

We use to drive it with 18Khz, 32V  ( drive current is limited by Ilim to 0.2 -2 depending on motor)

we now switched to 50Khz and we get Overtemperature failures intermittently (basically under load, so I assume more current is used and then it fails)

we tried 18-30Khz- and didn't see the failure.

The Slew rate is 25V/usec. - could this be the issue ? I assume going to faster switching will make the FET more efficient ?

However I can't seem to find the link to the current draw.. It happens on motors with 1Amp and 1.5Amp limit.

2) Device configuration:

-Delay compensation- Other then the table of recommended values (linked to Slew rate)- is there any way to measure\decide which delay to use ? 

I don't really understand what does the delay compensation do ?

-ADVANCE_LVL  - how can I measure the motors phase to hall sync so that I can update this register ?

3) I took some image from one of the motors and have some questions- 

1- the Beck EMF (marked as 1) in front of the phase doesn't look like the section after the Phase ON section (marked as 2) - any idea ?

2- I would assume the commutation switching (marked as 3)  would accrue in the middle of the ON phase, but it's seems to be out of center by ~20Deg ? 

 

  • 0
    TI__Mastermind 24540 points

    Hi liran,

    liran vigdor said:
    1) Overheating

    There are multiple factors that play into thermal generation. We do have a thermal calculator you can use if you would like to see the main contributors.

    Using a faster switching slew rate will improve the thermal performance by shortening the thermal losses due to higher Rds of the MOSFET while the MOSFET is switching. Another setting that can help to improve the thermal performance is to lower the PWM switching frequency to 30 or 20 kHz.

    liran vigdor said:
    2) Device configuration

    Delay compensation is used to account for variation in deadtime and propagation delay to reduce this variation and improve the stability of the motor. The DLY_TARGET recommended in table 8-6 of the datasheet are selected based on the typical propagation delay (t_PD) and deadtime (t_DEAD) specification for the given slew rate. The recommended values should be sufficient but to measure, you would measure the time it takes from the PWM switches to when OUTx switches and use a DLY_TARGET value slightly higher than the measured value.

    For the ADVACE_LVL setting, we recommend giving a specific speed command and adjusting ADVANCE_LVL until the motor speed at the given speed command no longer increases.

    liran vigdor said:
    3) I took some image from one of the motors and have some questions

    1. Can you provide a capture of all the phase voltages? Likely the phase voltage during the Hi-Z stage (1) is not being pulled high strongly, this shouldn't lead to any issues but seeing all the phase voltages will help to determine if there is an issue occurring.

    2. For the MCT8316Z the commutation switching point is dependent on the hall inputs. Ideally we do expect the commutation switching point to occur in the middle but due to load conditions or other factors this could be delayed or advanced. Please provide a capture of all the hall signals (assuming you are using digital all) plus the phase current.

    Regards,

    Joshua

  • 0 in reply to Joshua Horlander Cruz
    Prodigy 90 points

    Hi Joshue.

    thanks for the answers.

    1- The thermal calculator is for DRV8317 and not MCT8316, i tried using it but the Vm is limited to 20V.

    2- I don't really understand the Advance_lvl and how you suggested to measure it, the Driver doesn't have speed control. my controller PIV, will make sure the Velocity is reaching the requested. also It's not clear to me how does the driver moves the output if no Incremental encoder is connected ? does it assume constant speed and meaure the previous Hall duration ? otherwise how does he know to delay by Deg ? 

    3- i'll try to provide images later this week.

    thanks

  • 0 in reply to liran vigdor
    TI__Mastermind 24540 points

    Hi Iiran,

    liran vigdor said:
    1- The thermal calculator is for DRV8317 and not MCT8316, i tried using it but the Vm is limited to 20V.

    In the BLDC Thermal Calculator, the MCT8316Z specific calculator sheet can be selected at the bottom of Excel.

    liran vigdor said:
    It's not clear to me how does the driver moves the output if no Incremental encoder is connected ?

    Please allow me some time to look into this. I will aim to get back with you by 7/30.

    Regards,

    Joshua