January 27, 2025 - Laser Driver Wire Inductance

This script is authored by BNC electrical engineer Cameron Simmons. It is an excerpt from a video that is linked below:

We recently began testing transistors for a new laser driver board. During the prototyping stage something came up that I thought would be valuable to share with users. 

We were testing a new MOSFET on an existing driver board and the footprint did not line up with the chip’s pins. So, we needed to use some kind of conductive material to connect the pins to the footprint. The obvious choice was wires and originally we attached wires that were a similar size as the pins. Check out the video linked below to see the size difference between the two - one is definitely better than the other.

We were getting extremely low rise times and lots of ringing and distortion, despite the MOSFET being faster than the original. This was not going to meet the specs we needed it to. It was clear that something in the setup was slowing the signal down. Instead of wires we used copper tape - pictured in the video.

This completely fixed the problem. The issue was the self inductance of the wires. I used some online calculators to compare the self inductance of the two methods and the results were very surprising. If I were to use stranded wire instead of copper tape, I would need to use 4 AWG cable to achieve the same results. Obviously this is not practical or even possible. This is a great illustration of why standard wire is a bad choice for attaching a laser diode to a driver and why we recommend using our stripline cables and output PCBs instead. The wide flat cable will facilitate a clean, fast pulse even at longer distances.

Questions? The Berkeley Nucleonics team is proud to be able to speak to our existing and new customers one on one. Call in or email us with questions - you may even get to speak with Cameron!