I'm using the DRV8711 with IRLR2905Z Mosfets at 42V/4,0Arms. The board dimensions are 71*77mm, 4 layer using large GND copper planes.
After adding series resistors to the lowside FETs the boards work OK even at high voltage. But the power losses are quite high compared to the BOOST_DRV8711 evaluation board. I measured >93°C on top of the DRV8711 with a temperature measuring sticker, board temperature reaches 72°C.
Comparing the FET parameters, they look similar. The only difference (appart from the package) is the higher Gate charge for the IRLR2905 which is typical 23nC compared to 14nC for the FETs on the BOOST_DRV. I tried to increase both IDRIVEN (91=200mA) and IDRIVEP (10=150mA), but that doesn't make any difference.
Other parameters:
R(sense)=0,04R
R(DS,on)=11,0mR
TORQUE=105, ISGAIN=00 (=factor 5), should result in 4,0Arms
Stepmode is set to 1/16 microstep. All other values set to default.
Can you give me a hint were / what to measure to see if the MOSFET timing is OK? I suspect a short time short-circuit between low and high side during transition from high to low or vice a versa.
And how do I select the proper Rise-Time? Maybe the default gate drive times are to fast for these FETs? PWM frequency is now 34kHz, that seems OK to me.
I did a similar design some years ago with the obsolete A3986 on a 2-layer board, using IRLR024N (75mR) and had no trouble with high power losses. So I think the FETs themself shouldn't be the problem here?!
With kind regards,
Thorsten Ostermann