Hello TI FAE,
Here we have met DRV8432 short circuit issue on our tank filament board. Detailed schematic is below, also we have a PPT for initial analysis
Tank_Filament_Board_Failure_Analysis_20230909.pptx
Hello Eric,
Filaments of X-ray tubes are similar to incandescent light bulbs whose resistance is very low when not energized (cold). So when the power is applied there will be a very high inrush current almost similar to a short circuit on the load. With a motor load the inductance of the motor does a natural limiting to the rise time of this current. However in this circuit only the series 15uH inductors L1 t o L4 contributes to the inductance that will slow down the current rise time. We need to make sure if that will be enough for this application. See page-13 of the datasheet. It depends on the VIN and the cold resistance of the filament. What is the VIN voltage supplied to the PVDD pins? I am unable see this value in the PPT. Can we get a capture of the current waveform when first time power up is done after the filament is at ambient room temp of the application? Especially we need to look at the current profile during the first 250 ns. We can calculate if the inductors values are sufficient to guarantee safe operation of the driver at first startup of the filament drive if we know the VIN voltage and filament cold resistance. Current rise during Toc_delay or IOCT of 250ns (typ) could be higher than safe limit before the Ioc limit kicks in. If this is not mitigated the driver will be damaged.
Looking at the below picture, it appears that pins have been damaged due to excessive current flow. If it was a moisture damage due to washing with water and moisture absorption by the IC package, usually the package gets busted. It doesn't appear to be the case based on this image. I assume proper DI/DM water was used and adequate drying process was followed.
Lastly, what is the failure rate? How many boards were tested and how many failed with damaged device? Did the failure happen always with the first power on with the filament connected and driven?
Regards, Murugavel
Till now, at least 5 pieces of DRV8432 failed, not at the first time for sure, the total failure rate is around 15%
Below listed the test steps related with DRV8432.
[Step1] DRV8432 will be soldered on our Tank Filament Board for Function Test in the air, we called PCBA level function test
[Step2] Install Tank Filament Board into HV Module for subsystem function test, no oil in this step, called subsystem function test
[Step3] Fill in insulation oil into HV Module, then put this HV Module into CT gantry with X-ray tube and rotation, called system test, 5 pieces failed at this step
Total capacitors for VIN is about 16 pieces of 10uF, about 160uF, due to the PCB dimension, these are the maximum capacitors we can used for VIN of DRV8432.
Power input schematic
DRV8432 peripheral circuit
For inrush current, I will do measurement later this week and check if serial inductances are enough or not ?
For DRV8412 has different PCB footprint with DRV8432, right ? Do TI has the same PCB footprint IC for DRV8432 replacement ?
During inrush test, I have captured below waves, which is under load unconnected, the PID loop control the output to maximum current for several seconds, the maximum current is about 19A with PWM A& B in parallel mode.
Is OC_ADJ pin aimed to limit the output current ? I would like to limit output current.
For inrush current, i try to measure many times, no larger current.
Zoomed waves have been uploaded below.
As you known, we have used thermal pad between DRV8432 hot slug and heat sink in the current design. now we have removed the thermal pad and directly installed heat sink on DRV8432 with optimized stand off height, once this Tank filament board put into oil, it will have much better thermal dissipation.
For over current risk, we will limit PWM duty cycle to avoid any over current risk, the firmware is under coding now
