Other Parts Discussed in Thread: TPS2024, , CSD18537NKCS
I have the circuit shown in the attached schematic. It shows one of the three motor phase drivers that wil be used to drive a brushless D.C. motor. I am currently using an external D.C. power supply set to 40V and a current limit of approximately 4A.
MOSFETs are FDP61N20
U9 is isolation ISO7730FDWR
U8 is the UCC27714
D26 is standard diode VS-3EMU06-M3/5AT
Other diodes are shottky DSS120UTR
Note that the R22 through R27 have been changed to 100R.
The problem I am getting is that the low-side 3 MOSFETs are all getting hot when the bridge is operated with a high-side drive of 5% PWM and the low-side is disabled. I am using a single channel of a BLDC motor as a load connected between the bridge output and GND, so not currently using the low-side drive for this test. This causes the current shown on the D.C. power supply to increase by about 250mA.
I have tried connecting the gate and source for each of the low-side MOSFET's to ensure that they cannot be being switched on, but still they are getting hot.
I do not have a differential scope probe but can use two channels of a scope (TPS2024) to effect the same thing using difference measurements if this would be of help.