N channel fet failure - dv/dt figures?

[kitkat]

hi,

i have a failures of  a full bridge circuit, using 4 off IPB70N12S3-11, additionally protected by Fredfets,  VS-12EWH06FN-M3..

it is the backbone to a HV pulse driver, producing  ~+/- 400v pulse @ 200W max @ 100% duty, by stepping up a 250W 48V rail,and dilivery into a tuneable piezo transducer.

its switching at a varying frequency, 12-25khz, with varying duty cycle from 10 to 100% and a generous 1us deadtime.

  im getting some failures of these fets, and i suspect dv/dt, and the deatime is more than sufficient, and there not even warm. the transformer is wound ~ 47 turns giving an inductance of ~ 10mH. there's rinining on the secondary, when open circuit, due to its massive 680mh inductance, but i dont this thats a contributor (can be easly cleaned up with a snubber load, i dont think it ringing can feed back to primary?). there is no feedback between input power and output power, its simply a driver.

Not being a PSU designer, more of a hv pulse forming background, is there something else i should consider apart from a possible dv/dt failure? - i can see no reference to a maximum dv/dt/ permitted slew rate for the FET. IPB70N12S3-11 on its datasheet.

the drivers are LTC4444 devices, half bridge devices. No failure or damage seen in these.

thanks,

Kitkat.

[Meghana]

Hello @kitkat ,

Thank you for posting on Infineon Community.

Can you please share us the schematic of your design?

Regards

Meghana

[kitkat]

Hi Meghana,

thanks for getting back.. The primary of the transformer is between nodes P1- & P1 +..

[Meghana]

Hello @kitkat ,

MOSFETs are capable of operating at switching frequency upto few hundreds of kHz. Considering the gate resistance used in your design and the switching frequency, i can say that dv/dt might not be the cause of failure. 

Here i suspect the Vds overshoot to be causing the failure. Leakage inductance of the transformer connected across the bridge would be one of the major contributor for this overshoot. Can you please share the datasheet of the transformer to estimate this overshoot. 

Also please measure the Vds waveform across any of these four MOSFETs in the working board and share us the data for further evaluation. Meanwhile i will try to get you the value of maximum dv/dt for this device for your reference.

Regards

Meghana R

[kitkat]

Hi meghana,

i pm'd you..

[Meghana · Accepted solution]

Hello @kitkat ,

As discussed, a case has been created under My Case system and hence will be closing this thread here in community. 

Regards

Meghana R