Regards,
I have a pair of DRV8711 controllers and a pair of hexFET CSD88537ND drivers to design a stepper motor control application. based on BOOST-DRV8711 board design,
i would like to know what the current limitation is at maximum 4.5A since the hexFET can handle up to 8A current.
Could it be for not having a heatsink in the hexFETs?
Thanks for your help.
Hi Luis,
Thanks for the inquiry. The maximum continuous current specified in the FET datasheet is calculated based on temperature, thermal impedance and rds(on). Below you will find a link to a blog on how TI specifies maximum continuous current for our FETs. The 8A specified for the CSD88537ND assumes Tambient = 25C, TJ = TJmax = 150C and RthetaJA = 60degC/W. Under these conditions, the FET would be operating at TJmax which, for reliability, is not recommended. Typically, the max operating junction temperature is derated from TJmax by 20C to 25C (more or less depending on your requirements).I am also including a link to a technical article on how much power TI MOSFET packages are capable of dissipating. For the dual SO8, the practical limit is about 2W maximum. It can be more or less based on your PCB design and stackup and ambient conditions.
I was not involved in the design of the DRV8711 EVM and I don't know what thermal data was collected. I will forward this e2e to the DRV8711 apps team to get your some more information on this design.
Hi John.
After reading this insteresting documentation about selecting MOSFET and doing some research on the design of this BOOST-DRV8711, I found a test document "Boost-DRV8711 Stepper Motor Booster Pack - Test Data", and I have the following questions:
The maximum current specification of BOOST-DRV8711 at 4.5A, is RMS or PEAK current?
beacuse the test shown at 6.23A Full Scale Current or 4.4A RMS, and the maximum temperature under these conditions is 138°C in the nexFET, and this temperature is below the maximum operating temperature,150°C for nexFET CSD88537ND.
Is safe to work the BOOST-DRV8711 under these conditions?
The CSD88537ND is a double nexFET, and how can I take the thermal data presented in the datasheet into account to do the power dissipation calculations in this stepper motor application conditions? I mean if the thermal values are for a single FET or for the entire package?
Best Regards,
Luis
Hi Luis,
Nice to hear from you again. I was not directly involved in the BOOST-DRV8711 design. Hopefully, I can adequately answer your questions. If not, I can forward this to the DRV apps team. Typically, the current is rms since the motor is driven with an AC waveform: Ipeak = Irms x sqrt(2) = 4.4A x sqrt(2) = 6.22A. Based on the test results, it is safe to operate the device at TJ = 138degC with an rms current of 4.4A. The CSD88537ND is specified for TJmax = 150degC and usually you want to derate the maximum operating junction temperature by 15-25 degC for reliability purposes. It is a dual FET and the maximum power dissipation of 2.1W is for the entire package which is split between the two FETs. The maximum specified continuous current of 8A is for a single FET. If both FETs are carrying equal current, then because of the package power dissipation limit the current would have to be half or 4A (continuous or rms) per device. As I pointed out in my previous response, the continuous power dissipation and current are calculated quantities using the thermal impedance measured on a standard, single layer PCB as specified in the datasheet and explained in the blog below. Your actual results will be dependent on your PCB layout and stackup. Let me know if you have any additional questions.
