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LTC1778 voltage spikes destroy switching MOSFETs / Diodes; Similar measured at DC385A application

Hallo,

I have problems with existing LTC1778 circuits on our PCBs.

Circuit is designed for 24V Input, Output 3.3V 5A.

Basic functions are OK but we have complaints that synchonous MOSFETs (IRF7470 40V) or Diode (SD2114S040S5R0 40V) in parallel to bottom MOSFET often get defect. 

PCB layout and component Dimension should be done well from our point of view.

But to get sure we ordered the DC385A now.

There we mesasured very similar peaks of over 40V at the switching MOSFETs (FDS6982S 30V):  

Schematic: https://www.analog.com/media/en/technical-documentation/eval-board-schematic/385asch.pdf

Test results:

FDS6982S M1A

FDS6982S M1B

Demo Board loaded with 1.2V 2A

M1A measurement at DC385A

FDS6982S M1A

FDS6982S M1B

Demo Board loaded with 1.2V 2A

M1B measurement at DC385A

FDS6982S M1A

FDS6982S M1B

Demo Board loaded with 1.2V 6A

 

 

 

Measurement was done with tip and barrel method with a 2Gs 200Mhz Oscilloscope direct at the diodes or MOSFETs.

     

Do you have any idea how to reduce switching peaks at MOSFETs?

We checked with RC snubber circuit (6.2R/680pF) and similar. Peaks getting little lower but not below 30V. Disadvantage is the heating of snubber parts.

I checked circuit with LTSpice with your LCT1778.asc. In simulation there are no spikes with ideal leads.

But in real life spikes appear and it is hard to prevent or reduce them below 30V.

Any ideas for improvement?