Part Number: DRV8323R
We have a design using the DRV8323 that has been working great but every once in a while the DRV8323 fails with all the fet gate drives shorted. The Mosfets are fine! The etches to pins 7 (VDRAIN), 8(GHA) and 9 (SHA) get blown off the PCB making the board un-repairable. The PCB layout was taken from the App notes and Data sheet. The pcb copper is 2oz. The board temperature by the DRV8323 is 50C. We use it in PWM x 1 mode, supplying a 15KHz signal. The supply voltage is 48Volts and has a large snubber to keep the supply to less than 52 volts (measured) during a back EMF event. The motor is rated for 500 watts. The internal switching power supply continues to operate properly! Please advise
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In reply to Wang5577:
I don't have an EVM. Your analysis sounds correct but I have been unable to capture the event on a scope. There is a bulk capacitance of 3 x 6800uF caps on the 48V supply about 50mm from the IC. There is a 1uF (C18) on the VM pin to The VCP <3mm from the IC. There is a .1uF 100V (C23) ceramic cap < 6mm from the VM pin to the power ground plane which is also connects directly to the power pad of the DRV8323. There is a 10uF (C25) 100V ceramic about 25mm from the VM pin to the power ground plane also. The VDRAIN pin connects directly to the DRAIN of one of the mosfets just like it's shown in Figure 64 Layout example in the data sheet with NO capacitance. Should there be?. The design uses a split plane as shown in SLVA951 "Figure 9. DRV8323R Split Ground Plane". The two grounds are tied about 7mm from pin 1 of the IC as shown in Fig 9. When the motor is "free wheeled" I get a peak voltage of about 52 volts on the 48V buss.
Top Layer (Pink) and 2nd layer showing connection from VM pin to drain of MOSFET and VDRAIN to Mosfet Q27Top layer with ground plane, power ground to logic ground is tied on the green layer between the C20 and C19 reference designation silkscreen underneath the pink etch.
In reply to David Cassidy:
I have a 0.047uF 100V ceramic from CPL to CPH. I have 2.2uF 100V ceramic in parallel with 0.1uF 100V cermaic size 1812's in locations C6, C26 and C4(not in view) stacked on top of each other from each of the mosfet drains to ground right by the Rsense resistors R23, R27 and R5(not in view). Are you suggesting they should go to the mosfet source / Rsense junction not ground?
I have only monitored the 48 volt supply at the bulk capacitors. It goes up to 52 volts when the motor free wheels. I will monitor VM and Vdrain right at the pins. Should I use a diff amp to monitor the GHA to SHA or are you looking fo the absolute voltages? What should I use for a ground pin reference for the scope? pin 2? Thanks
I started off by monitoring the VDrain and SHA pins. I found when I quickly change the PWM from say 60% to 10% duty I get a spike of about 58V at SHA when commuting occurs.
The spike is very short.
I have a snubber from the drain / source junction of the fets of 4.7ohm in series with 4700pF. This spike only occurs when I change the PWM quickly. The VDrain pin has no spikes on it that I've been able to detect.
Please advise. Thank you!
Thanks for your quick reply
The scope has an offset of 45 volts putting the center line at 45 volts. The trigger point (upper right hand corner) is 57.1 volts. This is still below the 65 volts Absolute max but I'm guessing there must be some point it shoots higher causing the failure. It typically takes days before I get a failure. I added 10uF 100V Ceramic SMT across location C26 which is the closest I can get physically. The ground is right at the current sense resistor and right at the drain tab of the high side mosfet. The spike amplitude did not change. I used one of our "solder inplace" scope probes to eliminate ringing to make the measurements. Is it possible this could be turn on time of the fet body diode? I always thought it was in the order of ns and nobody ever gives a spec for it. Is there another way you could suggest? Would a small rc in series with the SHA pin help? TVS?
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