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DRV8825: Power dissipation

Part Number: DRV8825
Other Parts Discussed in Thread: DRV8886, DRV8886EVM, DRV8886ATEVM, DRV8886AT

Hello, I'm having an issue with DRV8825 power dissipation and I would like to know if it is a normal operation or maybe we are doing something wrong.

VMA and VMB input voltage is 11.5V, and the value of sense resistors is 0.1ohms.

We are working in full step mode to move a nema 8 motor (winding resistor: 6.5ohms, current: 0.6A)

The VREF is set so the current per coil is 0.4A. The voltage and current measured per coil is (V = 2.94V and I = 0.4A) and the driver input current is 0.23A.

With these values the power delivered to the motor coils is 2*2.94*0.4 = 2.352W and the power at the input of the driver is 11.5V*0.23A = 2.654W

The power dissipated by the driver is 0.293W which differs a little from the thoretical value 4*0.3*0.4² = 0.192W calculated with the formula 4*RdsON*(Iout(RMS))² 

If we change the VREF to get a current per coil of 0.05A the voltage and current measured per coil is (V = 0.375V and I = 0.047A) and the driver input current is 0.026A.

With these values the power delivered to the motor coils is 2*0.375*0.047 = 0.03525W and the power at the input of the driver is 11.5V*0.026A = 0.299W

The theoretical dissipation value by the driver in this case is 4*0.3*0.047² = 0.0026W and the measured dissipated power is 0.299W - 0.0325W = 0.264W.

Measurements of voltage and current are made using a multimeter. When the current to the motor is dissabled the current at the input of the driver is 4mA.

Is this a normal operation of the DRV8825? Does the efficiency decrease so much when the output current is reduced? 

I'm almost sure that something is not correct in our calcs but I can`t figure out what.

Thanks

  • Hi Marcos,

    I don't think using a multimeter is appropriate for understanding the electrical power delivered to the motor windings. It is useful for measuring the voltage and current directly from the supply, but this is a DC voltage and current. When you measure the voltage and current from the driver outputs, these are essentially AC voltages and currents. This is because the outputs form an H-bridge with the motor coils. To make the motor change position with each step, the H-bridge changes the direction of the motor terminal voltage and current. If you want to measure at the outputs, you will need to use an oscilloscope to calculate RMS current and voltage from waveforms. For this, you will need a differential probe to measure voltage and a current probe.
  • Hello James,

    Thanks for your fast reply. The measurements over the motor winding were always made without steps to change position so the voltage and current were close to a DC value.

    It is not 100% accurate, for sure. Anyway, the power dissipatted by the driver for a holding current over the motor windings (50mA) is 70 times more than the theoretical power.

    I don't think that our measurement method is so bad. Just to be sure. To calculate the power dissipation on the driver we should apply the next formula:  4*RdsON*(Iout(RMS))²  , right??

    Regards

  • Hi Marcos,

    If you are using this to drive a stepper, and the stepper is holding (not spinning), then the power dissipation of the IC cannot be calculated by 4*RdsON*(Iout(RMS))². This is because the current in one coil is different than the current in the other coil, so you will need to measure current in both coils and calculate P_DRV = 2*RdsON*(Iout_A(RMS))²+2*RdsON*(Iout_B(RMS))².
  • Hello James,

    Thanks again for your reply. We are working in full-step mode so the current is almost the same in both coils for every angle position.

    Regards

  • Marcos, thanks for reminding me. I was thinking of microstepping.

    I recalculated the power dissipation below.

    In your first case, you have
    Measured output power = 2*2.94*0.4 = 2.352W
    Calculated power dissipation by the FETs = 4*0.3*0.4² = 0.192W
    Power dissipation due to sense resistors = 0.1*0.4²*(2.94/11.5)*2 = 0.008W
    Subtotal power dissipation = 2.552W
    Measured input power = 11.5V*0.23A = 2.654W

    This leaves 0.102W of dissipated power unaccounted.

    Now you said that you measured the current your board draws to be 4 mA when the the current to the motor is disabled. How do you disable it? Do you use nSLEEP, nRESET, or nENBL?

    I'm asking because the IC draws between 5 mA and 8 mA when it is awake. If the IC draws 5 mA, then the power dissipation is due to operating current = 11.5*.005 = 0.0575W. Now we only have 0.0445W unaccounted.

    If you measured the 4 mA with the DRV8825 in sleep mode, then the IC draws only 10 uA. That would mean that the 4 mA is drawn by something else on your board. If that is the case, the the power dissipated by some other component on your board is 0.046W, which would account for all the power supplied and dissipated according to your measurements.
  • Hi James,
    We are disabling the H-bridges using the nENBL pin. (nENBL=1, DRV8825 supply current ≈ 4mA)
    I repeated the measurements today with the same results. When the current in the coils is 50mA the input current to the driver is 26mA which is much more than the expected value.
  • Marcos,

    I am not sure if the DRV8825 can properly regulate currents as low as 50 mA since that will be 0.05*0.1 = 5mV on your sense resistor. Also, VREF would need to be set to 5*0.1*0.05 = 25 mV. The datasheet recommends VREF values between 1 V and 3.5 V. Below 1 V, accuracy is degraded.

    To debug further, can you please provide a scope capture of the current through the motor or the voltage on the ISENSx pins? I do not trust the multimeter measurements when the device might not be regulating current properly.

    Some of our newer devices like the DRV8886 use a current mirror architecture and can achieve current regulation this low.
  • Hi James,

    Sorry, I can't make new tests until wednesday. I will send you the ISENS voltage captures as soon as possible. 

    Anyway, I didn't realize that VREF value should be between 1V and 3.5V. If we want to work in the recommended window we should increase the sense resistors to an unacceptable value.

    It seems that we can't use the DRV8825 in our design.

    Just one question about the DRV8886 driver. As it is indicated in the datasheet, we plan to adjust the Ifs using a resistor (around 20k-30k) and a DAC.

    Do you know how quick will the driver follow the changes on DAC voltage?

    Thank you very much for your support.

    Regards

  • Marcos,

    I am not sure how quickly it responds, but I know that it is fairly responsive. We do that exact implementation on the DRV8886EVM and DRV8886ATEVM.

    There are two variants of the DRV8886. The DRV8886AT variant uses our smart tuning decay modes, so I would recommend trying that one to avoid any tuning issues.
  • Hi James,

    We will try the DRV8886AT and I will let you know.

    Thank you,

    Marcos

  • Marcos,

    Good luck! I'm going to mark this thread as "TI Thinks Resolved". If you have questions about the DRV8886AT, please create a new thread.